Lesson 13

Basic Concepts in

Architectural Acoustics

Lesson 13

Basic Concepts in

Architectural Acoustics

1

13.1 Introduction.13.2 The nature of sound.

13.2.1. Sound waves.

13.3 Properties of sound.Wavelength of sound.

Sound pressure and frequency.Pitch.

13.4 Propagation of sound.

Spherical, cylindrical and perpendicular wave fronts.13.4.3. Other factors

13.5

13.6 Effect of barriers on sound.13.6.1. Reflection of sound.13.6.2. Transmission and absorption of sound.13.6.3. Diffusion of sound.13.6.4. Masking and diffraction of sound.

Sound insulation.Reverberation.Echoes.

Tests and Exercises.References.

13.2.2. Frequency range of sound.13.2.3. The audible range of sound.

13.3.1.13.3.2. Period and frequency of sound.13.3.3. The inverse square law of sound.13.3.4. Speed, frequency and wavelength of sound.13.3.5. Speed of sound and the medium of transmission.13.3.6. Sound pressure.13.3.7.13.3.8.

13.4.1. Sound fields.13.4.2.

Sound power and sound intensity.13.5.1. Measurement of sound.13.5.2. Sound power.13.5.3. Decrease of sound intensity with distance from source.13.5.4. Relationship between sound intensity and sound pressure.

and refraction

13.6.5.13.6.6.13.6.7.

13.1. IntroductionThis lecture covers basic architectural acoustics including the properties and nature of sound, theterms used to describe sound waves; and the relationship between sound pressure, sound intensityand sound power. Sound is measured on the decibel or phon scale. Audible sounds range from thethreshold of audibility to the threshold of pain. Basic concepts in acoustics include absorption,diffraction, echo, insulation, masking, reflection, refraction, reverberation and transmission of sound.

This lecture covers basic architectural acoustics including the properties and nature of sound, theterms used to describe sound waves; and the relationship between sound pressure, sound intensityand sound power. Sound is measured on the decibel or phon scale. Audible sounds range from thethreshold of audibility to the threshold of pain. Basic concepts in acoustics include absorption,diffraction, echo, insulation, masking, reflection, refraction, reverberation and transmission of sound.

2

Figure x: Visualization of sound rarefaction and compression in acoiled spring.

Figure x: Compression and rarefaction of sound by a vibratingtuning fork.

Figure x: Changes in sound pressure over time.

Figure x: Sound pressure superimposed on atmospheric pressure.

13.2. The Nature of Sound

Acoustics is the science of sound. Noise is anyunwanted sound, and is a subjective concept.Acoustics covers two areas, those of roomacoustics and control of noise.

Noise is defined as unwanted or damagingsound, that is, sound which interferes with whatpeople are trying to do, or sound which has anadverse effect on health or safety. To be able todeal with the problems of noise we must firsthave an understanding of basic architecturalacoustics - the nature of sound and its physicalproperties.

An understanding of the nature of sound wavesis essential to discussion on acoustics. Soundwaves are longitudinal waves originating from asource and conveyed by a medium. Sound is adisturbance, or wave, which moves through aphysical medium (such as air, water or metal)from a source to cause the sensation of hearingin animals. Sound is the sensation of the mediumacting on the ear. The source can be a vibratingsolid body such as the string of a guitar or themembrane of a drum, but it can also be avibrating gaseous medium, such as air in awhistle. The medium may be either a fluid or asolid.

For example, consider a vibrating tuning fork asthe source and the air as the medium. As a prongof the fork moves outward, air molecules incontact with it are also moved and cause aregion of raised pressure (called a compression).When the prong moves inward, the air pressureon its outer edge is lowered (called ararefaction), and the air molecules move back.This motion is passed on to adjacent moleculesand in this manner a sound wave propagates andenergy is transferred, even though eachmolecule only oscillates around a centralposition. This process can be visualized if youcan get hold of a coiled spring. Vibrate one endand watch the coils compress and stretch out.

A sound wave is characterized by its velocity,frequency, wavelength and amplitude. Thefrequency is the number of waves per unit timewhile the velocity is the product of thewavelength and the frequency. See figure x. The

13.2.1. Sound waves

Acoustics is the science of sound. Noise is anyunwanted sound, and is a subjective concept.Acoustics covers two areas, those of roomacoustics and control of noise.

Noise is defined as unwanted or damagingsound, that is, sound which interferes with whatpeople are trying to do, or sound which has anadverse effect on health or safety. To be able todeal with the problems of noise we must firsthave an understanding of basic architecturalacoustics - the nature of sound and its physicalproperties.

An understanding of the nature of sound wavesis essential to discussion on acoustics. Soundwaves are longitudinal waves originating from asource and conveyed by a medium. Sound is adisturbance, or wave, which moves through aphysical medium (such as air, water or metal)from a source to cause the sensation of hearingin animals. Sound is the sensation of the mediumacting on the ear. The source can be a vibratingsolid body such as the string of a guitar or themembrane of a drum, but it can also be avibrating gaseous medium, such as air in awhistle. The medium may be either a fluid or asolid.

For example, consider a vibrating tuning fork asthe source and the air as the medium. As a prongof the fork moves outward, air molecules incontact with it are also moved and cause aregion of raised pressure (called a compression).When the prong moves inward, the air pressureon its outer edge is lowered (called ararefaction), and the air molecules move back.This motion is passed on to adjacent moleculesand in this manner a sound wave propagates andenergy is transferred, even though eachmolecule only oscillates around a centralposition. This process can be visualized if youcan get hold of a coiled spring. Vibrate one endand watch the coils compress and stretch out.

A sound wave is characterized by its velocity,frequency, wavelength and amplitude. Thefrequency is the number of waves per unit timewhile the velocity is the product of thewavelength and the frequency. See figure x. The

13.2.1. Sound waves

amplitude indicates the intensity of the sound.The power of the source is measured in watts(W) while the intensity is measured in watts persquare metre (W/m2).

Sounds over a large range of frequencies can beproduced. The lowest note on the piano is 28Hz, the highest is 4186 Hz, whilst Middle C is262 Hz. Sound with a frequency below 20 Hz iscalled Sound with a frequencyabove 20 000 Hz is called

The average human ear can perceive sound offrequencies between 20 and 16,000Hz. Thisrange varies with individuals, age and othersubjective factors. The range audible to youngpeople with undamaged hearing is about 20Hzto 20,000Hz and for adults 20 to 16,000Hz.Some people can hear sounds of lowerfrequencies. The range is normally reduced withadvancement in age. Some blind people arecredited with hearing sounds inaudible to themajority of people, even to the extent of usingthis ability for echo-location.

The limit of hearing is also affected by theintensity of the sound. The lower limit is the

and it has a standardvalue of 1 pW/m2 (1 picowatt per metresquare). This is equivalent to 0.000 000 000 001W/m2. The upper limit is theand it has a standard value of l W/m2. As thenames suggest, sounds below the lower limit areinaudible while sound above the upper limit maycause pain and even damage the ear. Theaudible range of sound is shown in figure x. Thehuman ear is less sensitive to sounds of higherintensities and this is important in preventingdamage.

The is the logarithm of the ratioof measured sound intensity to the intensity atthe . This scale is alsoknown as the decibel (dB) scale

Different combinations of frequencies and levelsof sound produce the same sensation ofloudness. This is due to the variation of thesensitivity of the ear with frequency. Thusloudness cannot be directly measured byinstruments. Loudness is determined by referringto the loudness or phon scale which shows

13.2.2. Frequency range of sound

13.2.3. The Audible range of sound

infrasound.ultrasound.

threshold of audibility

threshold of pain

sound level scale

threshold of audibility

3

Figure x: Audible range of sound.

Violin

Figure x: Sound wave illustration.

Figure x: Audible range of sound.

Acoustics is the science of sound. Noise is anyunwanted sound, and is a subjective concept.Acoustics covers two areas, those of roomacoustics and control of noise.

Noise is defined as unwanted or damagingsound, that is, sound which interferes with whatpeople are trying to do, or sound which has anadverse effect on health or safety. To be able todeal with the problems of noise we must firsthave an understanding of basic architecturalacoustics - the nature of sound and its physicalproperties.

An understanding of the nature of sound wavesis essential to discussion on acoustics. Soundwaves are longitudinal waves originating from asource and conveyed by a medium. Sound is adisturbance, or wave, which moves through aphysical medium (such as air, water or metal)from a source to cause the sensation of hearingin animals. Sound is the sensation of the mediumacting on the ear. The source can be a vibratingsolid body such as the string of a guitar or themembrane of a drum, but it can also be avibrating gaseous medium, such as air in awhistle. The medium may be either a fluid or asolid.

For example, consider a vibrating tuning fork asthe source and the air as the medium. As a prongof the fork moves outward, air molecules incontact with it are also moved and cause aregion of raised pressure (called a compression).When the prong moves inward, the air pressureon its outer edge is lowered (called ararefaction), and the air molecules move back.This motion is passed on to adjacent moleculesand in this manner a sound wave propagates andenergy is transferred, even though eachmolecule only oscillates around a centralposition. This process can be visualized if youcan get hold of a coiled spring. Vibrate one endand watch the coils compress and stretch out.

A sound wave is characterized by its velocity,frequency, wavelength and amplitude. Thefrequency is the number of waves per unit timewhile the velocity is the product of thewavelength and the frequency. See figure x. The

13.2.1. Sound waves

amplitude indicates the intensity of the sound.The power of the source is measured in watts(W) while the intensity is measured in watts persquare metre (W/m2).

Sounds over a large range of frequencies can beproduced. The lowest note on the piano is 28Hz, the highest is 4186 Hz, whilst Middle C is262 Hz. Sound with a frequency below 20 Hz iscalled Sound with a frequencyabove 20 000 Hz is called

The average human ear can perceive sound offrequencies between 20 and 16,000Hz. Thisrange varies with individuals, age and othersubjective factors. The range audible to youngpeople with undamaged hearing is about 20Hzto 20,000Hz and for adults 20 to 16,000Hz.Some people can hear sounds of lowerfrequencies. The range is normally reduced withadvancement in age. Some blind people arecredited with hearing sounds inaudible to themajority of people, even to the extent of usingthis ability for echo-location.

The limit of hearing is also affected by theintensity of the sound. The lower limit is the

and it has a standardvalue of 1 pW/m2 (1 picowatt per metresquare). This is equivalent to 0.000 000 000 001W/m2. The upper limit is theand it has a standard value of l W/m2. As thenames suggest, sounds below the lower limit areinaudible while sound above the upper limit maycause pain and even damage the ear. Theaudible range of sound is shown in figure x. Thehuman ear is less sensitive to sounds of higherintensities and this is important in preventingdamage.

The is the logarithm of the ratioof measured sound intensity to the intensity atthe . This scale is alsoknown as the decibel (dB) scale

Different combinations of frequencies and levelsof sound produce the same sensation ofloudness. This is due to the variation of thesensitivity of the ear with frequency. Thusloudness cannot be directly measured byinstruments. Loudness is determined by referringto the loudness or phon scale which shows

13.2.2. Frequency range of sound

13.2.3. The Audible range of sound

infrasound.ultrasound.

threshold of audibility

threshold of pain

sound level scale

threshold of audibility

amplitude indicates the intensity of the sound.The power of the source is measured in watts(W) while the intensity is measured in watts persquare metre (W/m2).

Sounds over a large range of frequencies can beproduced. The lowest note on the piano is 28Hz, the highest is 4186 Hz, whilst Middle C is262 Hz. Sound with a frequency below 20 Hz iscalled Sound with a frequencyabove 20 000 Hz is called

The average human ear can perceive sound offrequencies between 20 and 16,000Hz. Thisrange varies with individuals, age and othersubjective factors. The range audible to youngpeople with undamaged hearing is about 20Hzto 20,000Hz and for adults 20 to 16,000Hz.Some people can hear sounds of lowerfrequencies. The range is normally reduced withadvancement in age. Some blind people arecredited with hearing sounds inaudible to themajority of people, even to the extent of usingthis ability for echo-location.

The limit of hearing is also affected by theintensity of the sound. The lower limit is the

and it has a standardvalue of 1 pW/m2 (1 picowatt per metresquare). This is equivalent to 0.000 000 000 001W/m2. The upper limit is theand it has a standard value of l W/m2. As thenames suggest, sounds below the lower limit areinaudible while sound above the upper limit maycause pain and even damage the ear. Theaudible range of sound is shown in figure x. Thehuman ear is less sensitive to sounds of higherintensities and this is important in preventingdamage.

The is the logarithm of the ratioof measured sound intensity to the intensity atthe . This scale is alsoknown as the decibel (dB) scale

Different combinations of frequencies and levelsof sound produce the same sensation ofloudness. This is due to the variation of thesensitivity of the ear with frequency. Thusloudness cannot be directly measured byinstruments. Loudness is determined by referringto the loudness or phon scale which shows

13.2.2. Frequency range of sound

13.2.3. The Audible range of sound

infrasound.ultrasound.

threshold of audibility

threshold of pain

sound level scale

threshold of audibility

13.3.1. Wavelength of sound

Wavelength is the distance between two

successive pressure peaks. Its symbol is and itis measured in units of meters (m).

Period is the time taken for one vibration cycle.Its symbol is T and its unit is seconds (s).Frequency is the number of vibration cycles persecond. Its symbol is f and it is measured inunits called hertz (Hz) (named after HeinrichHertz 1857-1894 the German physicist whostudied electromagnetic waves).

Frequency and period are related by

For example, a sound with a period of 0.002s hasa frequency of 500 Hz.

States that the intensity of sound in a free field isindirectly proportional to the square of thedistance from the source. This infers a decreasein the intensity of sound the farther the observeris from the source. See figure x.

Wave velocity is the speed with which soundtravels through the medium. Its symbol is c andits unit meters per second (m/s). It is related to

the frequency (f) and wavelength ( ) by:

c = f

So, if you know the speed and frequency of asound, you can work out the wavelength by:

Similarly, for frequency,

�

�

�

13.3.2. Period and Frequency

13.3.3.

13.3.4. Speed, Frequency and Wavelength

of Sound

The Inverse Square Law of Sound

Figure x: The characteristics of machine noise.

Figure x: Relationship between sound pressure and soundfrequency in a pure tone.

4

Figure x: Equal loudness contours.

sounds of various levels and frequencies whichare perceived as of the same loudness. See figurex.

The negative effect of noise on man increaseswith the noise level. The degree of disturbancecaused depends on individuals and subjectivefactors. Urban dwellers are more tolerant ofnoise than rural dwellers while noise levelsacceptable in the day may be quite disturbing atnight. Sudden noises are also more disturbingthan monotonous noises. The effect of noise onany average human being may be psychologicaland physiological and it ranges from annoyanceto permanent and immediate loss of hearing asshown in table 13.1.

sounds of various levels and frequencies whichare perceived as of the same loudness. See figurex.

The negative effect of noise on man increaseswith the noise level. The degree of disturbancecaused depends on individuals and subjectivefactors. Urban dwellers are more tolerant ofnoise than rural dwellers while noise levelsacceptable in the day may be quite disturbing atnight. Sudden noises are also more disturbingthan monotonous noises. The effect of noise onany average human being may be psychologicaland physiological and it ranges from annoyanceto permanent and immediate loss of hearing asshown in table 13.1.

Table 13.1: Psychological and physiological effects ofsounds.

Noiselevel

Possible psychological andphysiological effects.

65 dBA Annoyance, mental and physical fatigue.

90dBAVery long exposure may causepermanent hearing loss.

100dBAShort exposure may cause temporarydamage, long exposure may causepermanent damage.

120 dBA Pain.

150dBA Immediate loss of hearing.

13.3 Properties of sound

13.3.1. Wavelength of sound

Wavelength is the distance between two

successive pressure peaks. Its symbol is and itis measured in units of meters (m).

Period is the time taken for one vibration cycle.Its symbol is T and its unit is seconds (s).Frequency is the number of vibration cycles persecond. Its symbol is f and it is measured inunits called hertz (Hz) (named after HeinrichHertz 1857-1894 the German physicist whostudied electromagnetic waves).

Frequency and period are related by

For example, a sound with a period of 0.002s hasa frequency of 500 Hz.

States that the intensity of sound in a free field isindirectly proportional to the square of thedistance from the source. This infers a decreasein the intensity of sound the farther the observeris from the source. See figure x.

Wave velocity is the speed with which soundtravels through the medium. Its symbol is c andits unit meters per second (m/s). It is related to

the frequency (f) and wavelength ( ) by:

c = f

So, if you know the speed and frequency of asound, you can work out the wavelength by:

Similarly, for frequency,

�

�

�

13.3.2. Period and Frequency

13.3.3.

13.3.4. Speed, Frequency and Wavelength

of Sound

The Inverse Square Law of Sound

13.3.1. Wavelength of sound

Wavelength is the distance between two

successive pressure peaks. Its symbol is and itis measured in units of meters (m).

Period is the time taken for one vibration cycle.Its symbol is T and its unit is seconds (s).Frequency is the number of vibration cycles persecond. Its symbol is f and it is measured inunits called hertz (Hz) (named after HeinrichHertz 1857-1894 the German physicist whostudied electromagnetic waves).

Frequency and period are related by

For example, a sound with a period of 0.002s hasa frequency of 500 Hz.

States that the intensity of sound in a free field isindirectly proportional to the square of thedistance from the source. This infers a decreasein the intensity of sound the farther the observeris from the source. See figure x.

Wave velocity is the speed with which soundtravels through the medium. Its symbol is c andits unit meters per second (m/s). It is related to

the frequency (f) and wavelength ( ) by:

c = f

So, if you know the speed and frequency of asound, you can work out the wavelength by:

Similarly, for frequency,

�

�

�

13.3.2. Period and Frequency

13.3.3.

13.3.4. Speed, Frequency and Wavelength

of Sound

The Inverse Square Law of Sound

Tf

1

Figure x: Variation of speed of sound with medium oftransmission. The cowboy will hear the train noise via the railsbefore he hears it through the air. Source: US Department ofLabour (1980).

5

sounds of various levels and frequencies whichare perceived as of the same loudness. See figurex.

The negative effect of noise on man increaseswith the noise level. The degree of disturbancecaused depends on individuals and subjectivefactors. Urban dwellers are more tolerant ofnoise than rural dwellers while noise levelsacceptable in the day may be quite disturbing atnight. Sudden noises are also more disturbingthan monotonous noises. The effect of noise onany average human being may be psychologicaland physiological and it ranges from annoyanceto permanent and immediate loss of hearing asshown in table 13.1.

Figure x: The inverse square law of sound.

Tf

1

f

c�

�c

f

13.3.5. Speed of Sound and the Medium of

Transmission

13.3.6. Sound Pressure

13.3.7.

13.3.8. Pitch

The speed of sound depends on the medium (itselasticity and density) and its temperature. In airthe relationship is:

where K is the absolute temperature: ie K =temperature in °C + 273. So at 21°C the speed ofsound in air is:

In water the speed of sound is 1470 m/s and insteel, 5050 m/s.

The pressure changes produced by a soundwave are known as the sound pressure.Compared with atmospheric pressure (about100 000 pascals) they are very small (between20 micropascals and 200 pascals) and aresuperimposed on it. The changes in soundpressure at a point over time can be depicted ona graph. See figure x.

A sound may contain waves of only onefrequency, in which case it is called a pure tone.Noise is made up of waves of many differentfrequencies and magnitudes superimposed onone another. Sound Pressure is the force perunit area and gives the magnitude of the wave.Its symbol is p and its unit is pascal (Pa). (Namedafter Blaise Pascal 1623-1662, French physicistand philosopher who was first to measurealtitude by barometric pressure.) A quantityknown as the root-mean-square pressure, prms,is often used in acoustic measurements, toovercome the problem of the average pressurebeing zero.

Pitch is the property of sound that perceived ashighness and lowness. In music, it is the highnessor lowness of a musical tone as determined bythe rapidity of the vibrations producing it.

Changes in pitch are caused by differences in thefrequency at which a sound wave vibrates,measured in cycles per second (cps). A high

c = 20.06 K

20.06 21+273 = 20.06 294 = 344 m/s.

�

� �

Sound Pressure and Frequency

13.3.5. Speed of Sound and the Medium of

Transmission

13.3.6. Sound Pressure

13.3.7.

13.3.8. Pitch

The speed of sound depends on the medium (itselasticity and density) and its temperature. In airthe relationship is:

where K is the absolute temperature: ie K =temperature in °C + 273. So at 21°C the speed ofsound in air is:

In water the speed of sound is 1470 m/s and insteel, 5050 m/s.

The pressure changes produced by a soundwave are known as the sound pressure.Compared with atmospheric pressure (about100 000 pascals) they are very small (between20 micropascals and 200 pascals) and aresuperimposed on it. The changes in soundpressure at a point over time can be depicted ona graph. See figure x.

A sound may contain waves of only onefrequency, in which case it is called a pure tone.Noise is made up of waves of many differentfrequencies and magnitudes superimposed onone another. Sound Pressure is the force perunit area and gives the magnitude of the wave.Its symbol is p and its unit is pascal (Pa). (Namedafter Blaise Pascal 1623-1662, French physicistand philosopher who was first to measurealtitude by barometric pressure.) A quantityknown as the root-mean-square pressure, prms,is often used in acoustic measurements, toovercome the problem of the average pressurebeing zero.

Pitch is the property of sound that perceived ashighness and lowness. In music, it is the highnessor lowness of a musical tone as determined bythe rapidity of the vibrations producing it.

Changes in pitch are caused by differences in thefrequency at which a sound wave vibrates,measured in cycles per second (cps). A high

c = 20.06 K

20.06 21+273 = 20.06 294 = 344 m/s.

�

� �

Sound Pressure and Frequency

pitch sound corresponds to a high frequencyand a low pitch sound corresponds to a lowfrequency. Amazingly, many people, especiallythose who have been musically trained, arecapable of detecting a difference in frequencybetween two separate sounds which is as little as2 Hz. When two sounds with a frequencydifference of greater than 7 Hz are playedsimultaneously, most people are capable ofdetecting the presence of a complex wavepattern resulting from the interference andsuperposition of the two sound waves.

Certain sound waves when played simulta-neously will produce a particularly pleasantsensation when heard, are said to be

The ability of humans to perceive pitch isassociated with the frequency of the sound wavewhich impinges upon the ear. Because soundwaves are longitudinal waves which producehigh- and low-pressure disturbances of theparticles of a medium at a given frequency, theear has an ability to detect such frequencies andassociate them with the pitch of the sound. Butpitch is not the only property of a sound wavedetectable by the human ear. Pitch determinesthe placement of a note on a musical scale,corresponding to a standard, specifiedfrequency and intensity. It is often used to tuneboth instruments and voices to one another.Some people have the inborn ability, known as“perfect pitch”, to recognize or sing a given notewithout reference to any other pitch.

Inside a room, sound waves from a source willreflect from the walls, ceiling, floor and otherobjects in the room. Close to a source like amachine, the direct sound dominates and thesound pressure may vary significantly with justsmall changes in position. This area is called the

and its extent is about twice themachine's dimension or one wavelength of thesound.The area beyond the near field is called the

. This is made up of two sections - thewhere the direct sound still dominates and

the sound pressure level decreases 6 dB for eachdoubling of distance, and thewhere the reflected sound adds to the direct

consonant.

near field

farfield freefield

reverberant field

13.4.1 Sound Fields

6

Figure x: Pitch illustration.

Figure x: Intensity illustration.

Figure x: High and low frequency illustration.

Figure x: Amplitude illustration.

13.3.5. Speed of Sound and the Medium of

Transmission

13.3.6. Sound Pressure

13.3.7.

13.3.8. Pitch

The speed of sound depends on the medium (itselasticity and density) and its temperature. In airthe relationship is:

where K is the absolute temperature: ie K =temperature in °C + 273. So at 21°C the speed ofsound in air is:

In water the speed of sound is 1470 m/s and insteel, 5050 m/s.

The pressure changes produced by a soundwave are known as the sound pressure.Compared with atmospheric pressure (about100 000 pascals) they are very small (between20 micropascals and 200 pascals) and aresuperimposed on it. The changes in soundpressure at a point over time can be depicted ona graph. See figure x.

A sound may contain waves of only onefrequency, in which case it is called a pure tone.Noise is made up of waves of many differentfrequencies and magnitudes superimposed onone another. Sound Pressure is the force perunit area and gives the magnitude of the wave.Its symbol is p and its unit is pascal (Pa). (Namedafter Blaise Pascal 1623-1662, French physicistand philosopher who was first to measurealtitude by barometric pressure.) A quantityknown as the root-mean-square pressure, prms,is often used in acoustic measurements, toovercome the problem of the average pressurebeing zero.

Pitch is the property of sound that perceived ashighness and lowness. In music, it is the highnessor lowness of a musical tone as determined bythe rapidity of the vibrations producing it.

Changes in pitch are caused by differences in thefrequency at which a sound wave vibrates,measured in cycles per second (cps). A high

c = 20.06 K

20.06 21+273 = 20.06 294 = 344 m/s.

�

� �

Sound Pressure and Frequency

pitch sound corresponds to a high frequencyand a low pitch sound corresponds to a lowfrequency. Amazingly, many people, especiallythose who have been musically trained, arecapable of detecting a difference in frequencybetween two separate sounds which is as little as2 Hz. When two sounds with a frequencydifference of greater than 7 Hz are playedsimultaneously, most people are capable ofdetecting the presence of a complex wavepattern resulting from the interference andsuperposition of the two sound waves.

Certain sound waves when played simulta-neously will produce a particularly pleasantsensation when heard, are said to be

The ability of humans to perceive pitch isassociated with the frequency of the sound wavewhich impinges upon the ear. Because soundwaves are longitudinal waves which producehigh- and low-pressure disturbances of theparticles of a medium at a given frequency, theear has an ability to detect such frequencies andassociate them with the pitch of the sound. Butpitch is not the only property of a sound wavedetectable by the human ear. Pitch determinesthe placement of a note on a musical scale,corresponding to a standard, specifiedfrequency and intensity. It is often used to tuneboth instruments and voices to one another.Some people have the inborn ability, known as“perfect pitch”, to recognize or sing a given notewithout reference to any other pitch.

Inside a room, sound waves from a source willreflect from the walls, ceiling, floor and otherobjects in the room. Close to a source like amachine, the direct sound dominates and thesound pressure may vary significantly with justsmall changes in position. This area is called the

and its extent is about twice themachine's dimension or one wavelength of thesound.The area beyond the near field is called the

. This is made up of two sections - thewhere the direct sound still dominates and

the sound pressure level decreases 6 dB for eachdoubling of distance, and thewhere the reflected sound adds to the direct

consonant.

near field

farfield freefield

reverberant field

13.4 Propagation of Sound

13.4.1 Sound Fields

pitch sound corresponds to a high frequencyand a low pitch sound corresponds to a lowfrequency. Amazingly, many people, especiallythose who have been musically trained, arecapable of detecting a difference in frequencybetween two separate sounds which is as little as2 Hz. When two sounds with a frequencydifference of greater than 7 Hz are playedsimultaneously, most people are capable ofdetecting the presence of a complex wavepattern resulting from the interference andsuperposition of the two sound waves.

Certain sound waves when played simulta-neously will produce a particularly pleasantsensation when heard, are said to be

The ability of humans to perceive pitch isassociated with the frequency of the sound wavewhich impinges upon the ear. Because soundwaves are longitudinal waves which producehigh- and low-pressure disturbances of theparticles of a medium at a given frequency, theear has an ability to detect such frequencies andassociate them with the pitch of the sound. Butpitch is not the only property of a sound wavedetectable by the human ear. Pitch determinesthe placement of a note on a musical scale,corresponding to a standard, specifiedfrequency and intensity. It is often used to tuneboth instruments and voices to one another.Some people have the inborn ability, known as“perfect pitch”, to recognize or sing a given notewithout reference to any other pitch.

Inside a room, sound waves from a source willreflect from the walls, ceiling, floor and otherobjects in the room. Close to a source like amachine, the direct sound dominates and thesound pressure may vary significantly with justsmall changes in position. This area is called the

and its extent is about twice themachine's dimension or one wavelength of thesound.The area beyond the near field is called the

. This is made up of two sections - thewhere the direct sound still dominates and

the sound pressure level decreases 6 dB for eachdoubling of distance, and thewhere the reflected sound adds to the direct

consonant.

near field

farfield freefield

reverberant field

13.4.1 Sound Fields

Figure x: Decrease in sound intensity for an omnidirectional pointsource.

7

Figure x: The near field and far field of sound. Source: NationalInstitute for Occupational Health and Safety (1988).

sound and the decrease per doubling of distancewill be less than 6 dB.

When sound spreads out from a point source ina free space the wave fronts are spherical andthe sound pressure level will decrease 6 dB foreach doubling of distance.

When sound spreads out from a line source(such as a road with constant traffic or a pipecarrying fluid), the wave fronts are cylindricaland the sound pressure level will decrease 3 dBfor each doubling of distance.

When sound spreads out from a plane source(such as close to a large, vibrating panel or soundtraveling down a duct) the wave fronts areperpendicular to the direction of propagationand the sound pressure level does not decreasewith distance.

The above relationships hold true only in idealconditions. In reality the decrease in sound levelswill be affected by:

absorption by the air and moisture in it;

wind and temperature gradients;

absorption of the ground; and

reflection and absorption by obstacles in thepath.

The first three of these are significant over longdistances and are important in the study ofenvironmental noise annoyance. However, theydo not play a significant role in occupationalnoise exposures and so will not be consideredfurther here.

The strength of or loudness perception of sounddepends on the energy content and determinesthe pressure variation produced. The amplitudeof the sound wave have the maximum

13.4.2. Spherical, cylindrical and

perpendicular wave fronts.

13.5.1. Measurement of Sound.

Spherical Wave Fronts

Cylindrical Wave Fronts

Perpendicular Wave Fronts

13.4.3. Other Factors

13.5 Sound Power & Sound Intensity.

sound and the decrease per doubling of distancewill be less than 6 dB.

When sound spreads out from a point source ina free space the wave fronts are spherical andthe sound pressure level will decrease 6 dB foreach doubling of distance.

When sound spreads out from a line source(such as a road with constant traffic or a pipecarrying fluid), the wave fronts are cylindricaland the sound pressure level will decrease 3 dBfor each doubling of distance.

When sound spreads out from a plane source(such as close to a large, vibrating panel or soundtraveling down a duct) the wave fronts areperpendicular to the direction of propagationand the sound pressure level does not decreasewith distance.

The above relationships hold true only in idealconditions. In reality the decrease in sound levelswill be affected by:

absorption by the air and moisture in it;

wind and temperature gradients;

absorption of the ground; and

reflection and absorption by obstacles in thepath.

The first three of these are significant over longdistances and are important in the study ofenvironmental noise annoyance. However, theydo not play a significant role in occupationalnoise exposures and so will not be consideredfurther here.

The strength of or loudness perception of sounddepends on the energy content and determinesthe pressure variation produced. The amplitudeof the sound wave have the maximum

13.4.2. Spherical, cylindrical and

perpendicular wave fronts.

13.5.1. Measurement of Sound.

Spherical Wave Fronts

Cylindrical Wave Fronts

Perpendicular Wave Fronts

13.4.3. Other Factors

8

displacement of each air particle a generatedfrom stronger sounds.

The strength of a sound can be determined bymeasuring some aspect of its energy andpressure. Even though sound does not involvelarge amount of energy and its effects dependsupon high sensitivity of the human hearing.

A sound source can be characterized by thesound power which it emits to the surroundingmedium. This is a fundamental property of thesource and is not affected by the surroundings,such as reflecting surfaces. Hence it is oftenspecified by machine manufacturers so differentsources can be compared. Sound power is theenergy emitted by a sound source per unit time.The symbol for sound power is W and its unit isthe watt. (Named after the Scottish mechanicalengineer James Watt, 1736-1819, of steamengine fame.) A source that emits power equallyin all directions is called an

Any other source is called a

Sound intensity, at a point in the surroundingmedium, is the power passing through a unitarea. Its symbol is I and its unit, watts/m2.

Where:

W is the sound power in watts

and S is the surface area in m2

For an omnidirectional point source, the soundwave spreads out from the source in alldirections. The sound power, W, of the source ishence spread over the surface of a sphere.

So S = 4 r2

And

where r is the radius of the sphere (ie thedistance from source) in meters.

13.5.2. Sound Power.

13.5.3. Decrease of Sound Intensity with

Distance from Source.

omnidirectionalsource. directionalsource.

�

Figure x: Decrease in sound pressure level for an omnidirectionalpoint source.

Figure x: Decrease in sound intensity for a point source withdoubling of distance.

Figure x: Decrease in sound intensity for a line source withdoubling of distance.

Figure x: Decrease in sound pressure level for a line source.

As the distance from the source increases, thesound intensity decreases according to the"inverse square law". In terms of decibels thismeans that when r doubles there will be a dropof 6 dB in sound level.

As most measurements of sound are in terms ofsound pressure (p), it is useful to know therelationship between sound intensity and soundpressure:

Where:

I is the sound intensity in watts/m2

p is the sound pressure in Pa

is the density of medium in kg/m3

C is the speed of sound in m/s

For air at 21°C , = 1.2 kg/m3

and following the equation above:

c = 344 m/s

Therefore, I = = 0.0024 p2

Strictly speaking, this equation is for plane waves(ie waves propagating with parallel wavefronts).However, away from a point source, thespherical waves approximate plane waves.

When a sound wave encounters an obstaclesuch as a barrier or wall, its propagation will beaffected in one of three ways - reflection,diffraction and refraction.

occurs when an obstacle's

13.5.4. Relationship between Sound

Intensity and Sound Pressure.

�

�

13.6.1. Reflection and refraction of sound.

Reflection

sound and the decrease per doubling of distancewill be less than 6 dB.

When sound spreads out from a point source ina free space the wave fronts are spherical andthe sound pressure level will decrease 6 dB foreach doubling of distance.

When sound spreads out from a line source(such as a road with constant traffic or a pipecarrying fluid), the wave fronts are cylindricaland the sound pressure level will decrease 3 dBfor each doubling of distance.

When sound spreads out from a plane source(such as close to a large, vibrating panel or soundtraveling down a duct) the wave fronts areperpendicular to the direction of propagationand the sound pressure level does not decreasewith distance.

The above relationships hold true only in idealconditions. In reality the decrease in sound levelswill be affected by:

absorption by the air and moisture in it;

wind and temperature gradients;

absorption of the ground; and

reflection and absorption by obstacles in thepath.

The first three of these are significant over longdistances and are important in the study ofenvironmental noise annoyance. However, theydo not play a significant role in occupationalnoise exposures and so will not be consideredfurther here.

The strength of or loudness perception of sounddepends on the energy content and determinesthe pressure variation produced. The amplitudeof the sound wave have the maximum

13.4.2. Spherical, cylindrical and

perpendicular wave fronts.

13.5.1. Measurement of Sound.

Spherical Wave Fronts

Cylindrical Wave Fronts

Perpendicular Wave Fronts

13.4.3. Other Factors

9

displacement of each air particle a generatedfrom stronger sounds.

The strength of a sound can be determined bymeasuring some aspect of its energy andpressure. Even though sound does not involvelarge amount of energy and its effects dependsupon high sensitivity of the human hearing.

A sound source can be characterized by thesound power which it emits to the surroundingmedium. This is a fundamental property of thesource and is not affected by the surroundings,such as reflecting surfaces. Hence it is oftenspecified by machine manufacturers so differentsources can be compared. Sound power is theenergy emitted by a sound source per unit time.The symbol for sound power is W and its unit isthe watt. (Named after the Scottish mechanicalengineer James Watt, 1736-1819, of steamengine fame.) A source that emits power equallyin all directions is called an

Any other source is called a

Sound intensity, at a point in the surroundingmedium, is the power passing through a unitarea. Its symbol is I and its unit, watts/m2.

Where:

W is the sound power in watts

and S is the surface area in m2

For an omnidirectional point source, the soundwave spreads out from the source in alldirections. The sound power, W, of the source ishence spread over the surface of a sphere.

So S = 4 r2

And

where r is the radius of the sphere (ie thedistance from source) in meters.

13.5.2. Sound Power.

13.5.3. Decrease of Sound Intensity with

Distance from Source.

omnidirectionalsource. directionalsource.

�

displacement of each air particle a generatedfrom stronger sounds.

The strength of a sound can be determined bymeasuring some aspect of its energy andpressure. Even though sound does not involvelarge amount of energy and its effects dependsupon high sensitivity of the human hearing.

A sound source can be characterized by thesound power which it emits to the surroundingmedium. This is a fundamental property of thesource and is not affected by the surroundings,such as reflecting surfaces. Hence it is oftenspecified by machine manufacturers so differentsources can be compared. Sound power is theenergy emitted by a sound source per unit time.The symbol for sound power is W and its unit isthe watt. (Named after the Scottish mechanicalengineer James Watt, 1736-1819, of steamengine fame.) A source that emits power equallyin all directions is called an

Any other source is called a

Sound intensity, at a point in the surroundingmedium, is the power passing through a unitarea. Its symbol is I and its unit, watts/m2.

Where:

W is the sound power in watts

and S is the surface area in m2

For an omnidirectional point source, the soundwave spreads out from the source in alldirections. The sound power, W, of the source ishence spread over the surface of a sphere.

So S = 4 r2

And

where r is the radius of the sphere (ie thedistance from source) in meters.

13.5.2. Sound Power.

13.5.3. Decrease of Sound Intensity with

Distance from Source.

omnidirectionalsource. directionalsource.

�

Figure x: Perpendicular wave fronts.

S

WI

24 r

WI

�

As the distance from the source increases, thesound intensity decreases according to the"inverse square law". In terms of decibels thismeans that when r doubles there will be a dropof 6 dB in sound level.

As most measurements of sound are in terms ofsound pressure (p), it is useful to know therelationship between sound intensity and soundpressure:

Where:

I is the sound intensity in watts/m2

p is the sound pressure in Pa

is the density of medium in kg/m3

C is the speed of sound in m/s

For air at 21°C , = 1.2 kg/m3

and following the equation above:

c = 344 m/s

Therefore, I = = 0.0024 p2

Strictly speaking, this equation is for plane waves(ie waves propagating with parallel wavefronts).However, away from a point source, thespherical waves approximate plane waves.

When a sound wave encounters an obstaclesuch as a barrier or wall, its propagation will beaffected in one of three ways - reflection,diffraction and refraction.

occurs when an obstacle's

13.5.4. Relationship between Sound

Intensity and Sound Pressure.

�

�

13.6. Effect of barriers on sound.

13.6.1. Reflection and refraction of sound.

Reflection

As the distance from the source increases, thesound intensity decreases according to the"inverse square law". In terms of decibels thismeans that when r doubles there will be a dropof 6 dB in sound level.

As most measurements of sound are in terms ofsound pressure (p), it is useful to know therelationship between sound intensity and soundpressure:

Where:

I is the sound intensity in watts/m2

p is the sound pressure in Pa

is the density of medium in kg/m3

C is the speed of sound in m/s

For air at 21°C , = 1.2 kg/m3

and following the equation above:

c = 344 m/s

Therefore, I = = 0.0024 p2

Strictly speaking, this equation is for plane waves(ie waves propagating with parallel wavefronts).However, away from a point source, thespherical waves approximate plane waves.

When a sound wave encounters an obstaclesuch as a barrier or wall, its propagation will beaffected in one of three ways - reflection,diffraction and refraction.

occurs when an obstacle's

13.5.4. Relationship between Sound

Intensity and Sound Pressure.

�

�

13.6.1. Reflection and refraction of sound.

Reflection

c

pI

�

2

dimensions are larger than the wavelength of thesound. In this case the sound ray behaves like alight ray and, for an obstacle with a flat surface,the reflected ray will leave the surface at thesame angle as the incident ray approached it, sothat the angle of incidence is equal to the angleof reflection.

occurs when a sound ray enters adifferent medium at an angle. Because of thediffering speed of travel of the sound wave in thetwo media, the sound ray will bend. This can bean important consideration in outdoor soundpropagation over long distances. When weatherconditions produce a temperature inversion,sound rays originally propagating upwards canbe bent back to the ground

Some of the energy of sound waves istransmitted to solid barriers in the path of thesound. This energy causes vibration of themolecules of the barrier which may then re-emitthe sound. Sound transmitted in this way isreferred to as structure-borne sound. When it isgenerated by mechanical means it is referred toas impact sound. Sound transmitted through theair is known as airborne sound.

Not all sound incident on a barrier is transmitted.Some is absorbed by the barrier while the rest isreflected. Thus sound may be:

- transmitted (t)

- absorbed (a)

- reflected (r)

The absorption co-efficient is an indication of thesound that is not reflected and is thus anindication of both the sound absorbed andtransmitted.

( ) is the product of the absorptioncoefficient and the area of a given surface. It ismeasured in the `open window unit' which isequivalent to the absorption of a square meteropening with zero reflectance.

When a sound wave strikes an obstacle, part of itis reflected, part is absorbed within the obstacleand part is transmitted through to become asound wave in air again on the other side. The

Refraction

Absorption a

13.6.2.

.

Transmission and Absorption of

Sound

dimensions are larger than the wavelength of thesound. In this case the sound ray behaves like alight ray and, for an obstacle with a flat surface,the reflected ray will leave the surface at thesame angle as the incident ray approached it, sothat the angle of incidence is equal to the angleof reflection.

occurs when a sound ray enters adifferent medium at an angle. Because of thediffering speed of travel of the sound wave in thetwo media, the sound ray will bend. This can bean important consideration in outdoor soundpropagation over long distances. When weatherconditions produce a temperature inversion,sound rays originally propagating upwards canbe bent back to the ground

Some of the energy of sound waves istransmitted to solid barriers in the path of thesound. This energy causes vibration of themolecules of the barrier which may then re-emitthe sound. Sound transmitted in this way isreferred to as structure-borne sound. When it isgenerated by mechanical means it is referred toas impact sound. Sound transmitted through theair is known as airborne sound.

Not all sound incident on a barrier is transmitted.Some is absorbed by the barrier while the rest isreflected. Thus sound may be:

- transmitted (t)

- absorbed (a)

- reflected (r)

The absorption co-efficient is an indication of thesound that is not reflected and is thus anindication of both the sound absorbed andtransmitted.

( ) is the product of the absorptioncoefficient and the area of a given surface. It ismeasured in the `open window unit' which isequivalent to the absorption of a square meteropening with zero reflectance.

When a sound wave strikes an obstacle, part of itis reflected, part is absorbed within the obstacleand part is transmitted through to become asound wave in air again on the other side. The

Refraction

Absorption a

13.6.2.

.

Transmission and Absorption of

Sound

10

obstacle's ability to block transmission of sounddepends on its structure and is indicated by itstransmission loss rating. Stiff, heavy materialsstop a lot of sound by reflecting most of it andhence have a high transmission loss. Examplesare sheet metal, timber, bricks and concrete.Soft, porous materials are not good at blockingthe transmission of sound. The fraction of soundenergy which is absorbed by an obstacle iscalled its absorption coefficient. Soft, porousmaterial such as open cell foams and fibrousmaterials are good absorbers of sound and havean absorption coefficient close to 1. Hard, non-porous materials are poor absorbers and havecoefficients as low as 0.02.

is the scattering or randomredistribution of a sound wave from a surface. Itoccurs when the surface depths of hard-surfacematerials are comparable to the wavelength ofthe sound. Diffusion does not break up or absorbsound. However, the direction of the incidentsound wave is changed as it strikes a sound-diffusing material. When satisfactory diffusionhas been achieved in a room, listeners or users ofthe room will have the sensation of soundcoming from all directions at equal levels.

The shadow effect of screens or barriers in thepath of sound differs in accordance with thefrequency and wavelength of the sound and thedimensions of the barrier. This effect disappearswhen the wavelength of the sound wave is morethan the dimension of the barrier in a directionperpendicular to the sound path. At highfrequencies the acoustic shadow is very distinctbut it is somewhat reduced at low frequencies bydiffraction. See figure x.

occurs when an obstacle'sdimensions are of the same order or less than thewavelength of the sound. In this case the edge ofthe obstacle acts like a source of sound itself andthe sound ray appears to bend around the edge.This limits the effectiveness of barriers.

Sound insulation is the reduction of soundtransmission of airborne sounds through walls,floors and partitions.

13.6.3. Diffusion of sound.

Diffusion

Diffraction

13.6.4.

13.6.5. .

Masking and Diffraction of Sound

Sound Insulation

Figure x: Reflection of sound.

Figure x: Refraction of sound with no temperature inversion.

Figure x: Refraction of sound with temperature inversion.

Appropriate sound insulation is achieved byusing elements with an adequate transmissionco-efficient or sound reduction index. Thetransmission co-efficient is a decimal fractionexpressing the proportion of sound energytransmitted. The sound reduction index ortransmission loss defines the reduction effect ofan element and is expressed in decibels.

dimensions are larger than the wavelength of thesound. In this case the sound ray behaves like alight ray and, for an obstacle with a flat surface,the reflected ray will leave the surface at thesame angle as the incident ray approached it, sothat the angle of incidence is equal to the angleof reflection.

occurs when a sound ray enters adifferent medium at an angle. Because of thediffering speed of travel of the sound wave in thetwo media, the sound ray will bend. This can bean important consideration in outdoor soundpropagation over long distances. When weatherconditions produce a temperature inversion,sound rays originally propagating upwards canbe bent back to the ground

Some of the energy of sound waves istransmitted to solid barriers in the path of thesound. This energy causes vibration of themolecules of the barrier which may then re-emitthe sound. Sound transmitted in this way isreferred to as structure-borne sound. When it isgenerated by mechanical means it is referred toas impact sound. Sound transmitted through theair is known as airborne sound.

Not all sound incident on a barrier is transmitted.Some is absorbed by the barrier while the rest isreflected. Thus sound may be:

- transmitted (t)

- absorbed (a)

- reflected (r)

The absorption co-efficient is an indication of thesound that is not reflected and is thus anindication of both the sound absorbed andtransmitted.

( ) is the product of the absorptioncoefficient and the area of a given surface. It ismeasured in the `open window unit' which isequivalent to the absorption of a square meteropening with zero reflectance.

When a sound wave strikes an obstacle, part of itis reflected, part is absorbed within the obstacleand part is transmitted through to become asound wave in air again on the other side. The

Refraction

Absorption a

13.6.2.

.

Transmission and Absorption of

Sound

11

obstacle's ability to block transmission of sounddepends on its structure and is indicated by itstransmission loss rating. Stiff, heavy materialsstop a lot of sound by reflecting most of it andhence have a high transmission loss. Examplesare sheet metal, timber, bricks and concrete.Soft, porous materials are not good at blockingthe transmission of sound. The fraction of soundenergy which is absorbed by an obstacle iscalled its absorption coefficient. Soft, porousmaterial such as open cell foams and fibrousmaterials are good absorbers of sound and havean absorption coefficient close to 1. Hard, non-porous materials are poor absorbers and havecoefficients as low as 0.02.

is the scattering or randomredistribution of a sound wave from a surface. Itoccurs when the surface depths of hard-surfacematerials are comparable to the wavelength ofthe sound. Diffusion does not break up or absorbsound. However, the direction of the incidentsound wave is changed as it strikes a sound-diffusing material. When satisfactory diffusionhas been achieved in a room, listeners or users ofthe room will have the sensation of soundcoming from all directions at equal levels.

The shadow effect of screens or barriers in thepath of sound differs in accordance with thefrequency and wavelength of the sound and thedimensions of the barrier. This effect disappearswhen the wavelength of the sound wave is morethan the dimension of the barrier in a directionperpendicular to the sound path. At highfrequencies the acoustic shadow is very distinctbut it is somewhat reduced at low frequencies bydiffraction. See figure x.

occurs when an obstacle'sdimensions are of the same order or less than thewavelength of the sound. In this case the edge ofthe obstacle acts like a source of sound itself andthe sound ray appears to bend around the edge.This limits the effectiveness of barriers.

Sound insulation is the reduction of soundtransmission of airborne sounds through walls,floors and partitions.

13.6.3. Diffusion of sound.

Diffusion

Diffraction

13.6.4.

13.6.5. .

Masking and Diffraction of Sound

Sound Insulation

obstacle's ability to block transmission of sounddepends on its structure and is indicated by itstransmission loss rating. Stiff, heavy materialsstop a lot of sound by reflecting most of it andhence have a high transmission loss. Examplesare sheet metal, timber, bricks and concrete.Soft, porous materials are not good at blockingthe transmission of sound. The fraction of soundenergy which is absorbed by an obstacle iscalled its absorption coefficient. Soft, porousmaterial such as open cell foams and fibrousmaterials are good absorbers of sound and havean absorption coefficient close to 1. Hard, non-porous materials are poor absorbers and havecoefficients as low as 0.02.

is the scattering or randomredistribution of a sound wave from a surface. Itoccurs when the surface depths of hard-surfacematerials are comparable to the wavelength ofthe sound. Diffusion does not break up or absorbsound. However, the direction of the incidentsound wave is changed as it strikes a sound-diffusing material. When satisfactory diffusionhas been achieved in a room, listeners or users ofthe room will have the sensation of soundcoming from all directions at equal levels.

The shadow effect of screens or barriers in thepath of sound differs in accordance with thefrequency and wavelength of the sound and thedimensions of the barrier. This effect disappearswhen the wavelength of the sound wave is morethan the dimension of the barrier in a directionperpendicular to the sound path. At highfrequencies the acoustic shadow is very distinctbut it is somewhat reduced at low frequencies bydiffraction. See figure x.

occurs when an obstacle'sdimensions are of the same order or less than thewavelength of the sound. In this case the edge ofthe obstacle acts like a source of sound itself andthe sound ray appears to bend around the edge.This limits the effectiveness of barriers.

Sound insulation is the reduction of soundtransmission of airborne sounds through walls,floors and partitions.

13.6.3. Diffusion of sound.

Diffusion

Diffraction

13.6.4.

13.6.5. .

Masking and Diffraction of Sound

Sound Insulation

Figure x: Acoustic shadow at high frequencies.

Figure x: Diffraction of sound.

Figure x: Transmission and absorption of sound.

Appropriate sound insulation is achieved byusing elements with an adequate transmissionco-efficient or sound reduction index. Thetransmission co-efficient is a decimal fractionexpressing the proportion of sound energytransmitted. The sound reduction index ortransmission loss defines the reduction effect ofan element and is expressed in decibels.

Appropriate sound insulation is achieved byusing elements with an adequate transmissionco-efficient or sound reduction index. Thetransmission co-efficient is a decimal fractionexpressing the proportion of sound energytransmitted. The sound reduction index ortransmission loss defines the reduction effect ofan element and is expressed in decibels.

13.6.6. Reverberation.

13.6.7. .

When a sound is produced in a room soundwaves spread from the source in sphericalwaves. When these waves strike a surface, somewill be reflected. This reflected sound continuesspreading until it strikes another surface whichreflects it again and so on. This continues evenafter the actual source has ceased producingsound. However, some of the sound is absorbedat every reflection and the sound energy reducesprogressively until the sound becomesinaudible. Reverberation is the persistence ofsound in an enclosed space as a result ofrepeated reflection or scattering, after the soundsource has stopped. Reverberation time is thenumber of seconds required for the energy ofthe reflected sound in a room to diminish to one-millionth of the original energy it had. It can alsobe defined as the number of seconds requiredfor the sound pressure level to diminish to 60decibels below its initial value.

An echo is a distinct repetition of the directsound. This effect may be observed by making ashort sound, such as a sharp clap, in a largeroom.

1. Define architectural acoustics and its rele-vance to the architect.

2. Explain which physical parameter of the waveis affected and how, by each of the followingprocesses:

A. Reverberation

B. Absorption

3. Write concisely on:

A. Masking of sound

B. Sound absorbers

C. Reverberation time.

4. Write a short essay on "Acoustics of Buildings".

5. What is noise? Discuss the harmful effect ofnoise on man.

6. Define the following terms:

A. Echo

Echoes

Tests and Exercises.

13.6.6. Reverberation.

13.6.7. .

When a sound is produced in a room soundwaves spread from the source in sphericalwaves. When these waves strike a surface, somewill be reflected. This reflected sound continuesspreading until it strikes another surface whichreflects it again and so on. This continues evenafter the actual source has ceased producingsound. However, some of the sound is absorbedat every reflection and the sound energy reducesprogressively until the sound becomesinaudible. Reverberation is the persistence ofsound in an enclosed space as a result ofrepeated reflection or scattering, after the soundsource has stopped. Reverberation time is thenumber of seconds required for the energy ofthe reflected sound in a room to diminish to one-millionth of the original energy it had. It can alsobe defined as the number of seconds requiredfor the sound pressure level to diminish to 60decibels below its initial value.

An echo is a distinct repetition of the directsound. This effect may be observed by making ashort sound, such as a sharp clap, in a largeroom.

1. Define architectural acoustics and its rele-vance to the architect.

2. Explain which physical parameter of the waveis affected and how, by each of the followingprocesses:

A. Reverberation

B. Absorption

3. Write concisely on:

A. Masking of sound

B. Sound absorbers

C. Reverberation time.

4. Write a short essay on "Acoustics of Buildings".

5. What is noise? Discuss the harmful effect ofnoise on man.

6. Define the following terms:

A. Echo

Echoes

b. Reverberation

C. Reverberation time

D. Wave frequency

E. Wave velocity

7. Briefly explain the meaning of the following:

A. Inverse square law of sound

B. Structure-borne sound

C. Transmission loss

D. Threshold of audibility.

8. What is meant by reverberation andreverberation time? On what factors does theduration of reverberation depend? Why doesthe magnitude of reverberation time affect thesuitability of a hall for speech and music?

Callender, J.H. (1974). Time-Saver Standards forArchitectural Design Data. McGraw-Hill BookCompany.

Evans, M. (1980). Housing, Climate andComfort. The Architectural Press, London.

Givoni, B. (1976). Man, Climate AndArchitecture. Second Edition. Applied SciencePublishers Ltd., London.

Koenigsberger, O.H., Ingersoll, T.G., Mayhew,A. and Szokolay, S.V. (1974). Manual of TropicalHousing And Building, Part I, Climatic Design.Longman, London.

Markus, T.A. and Morris, E.N. (1980). Buildings,Climate and Energy. Pitman International,London.

National Universities Commission (1977).Standards Guide for Universities. NationalUniversities Commission, Lagos.

Olgyay, V. (1963). Design With Climate -Bioclimatic Approach To ArchitecturalRegionalism. Princeton University Press,Princeton, New Jersey.

United Nations (1971). Design of Low CostHousing and Community Facilities, Volume I,Climate and House Design. Department ofEconomic and Social Affairs, New York.

References.

b. Reverberation

C. Reverberation time

D. Wave frequency

E. Wave velocity

7. Briefly explain the meaning of the following:

A. Inverse square law of sound

B. Structure-borne sound

C. Transmission loss

D. Threshold of audibility.

8. What is meant by reverberation andreverberation time? On what factors does theduration of reverberation depend? Why doesthe magnitude of reverberation time affect thesuitability of a hall for speech and music?

Callender, J.H. (1974). Time-Saver Standards forArchitectural Design Data. McGraw-Hill BookCompany.

Evans, M. (1980). Housing, Climate andComfort. The Architectural Press, London.

Givoni, B. (1976). Man, Climate AndArchitecture. Second Edition. Applied SciencePublishers Ltd., London.

Koenigsberger, O.H., Ingersoll, T.G., Mayhew,A. and Szokolay, S.V. (1974). Manual of TropicalHousing And Building, Part I, Climatic Design.Longman, London.

Markus, T.A. and Morris, E.N. (1980). Buildings,Climate and Energy. Pitman International,London.

National Universities Commission (1977).Standards Guide for Universities. NationalUniversities Commission, Lagos.

Olgyay, V. (1963). Design With Climate -Bioclimatic Approach To ArchitecturalRegionalism. Princeton University Press,Princeton, New Jersey.

United Nations (1971). Design of Low CostHousing and Community Facilities, Volume I,Climate and House Design. Department ofEconomic and Social Affairs, New York.

12