BASIC CONCEPTS IN ARCHITECTURAL ACOUSTICS ENVIRONMENTAL CONTROL III (ACOUSTICS AND NOISE CONTROL)...

ARC 507: Environmental Control III (Acoustics and Noise Control)Department of Architecture, Federal University of Technology, Akure, Nigeria

BASIC CONCEPTS IN ARCHITECTURAL ACOUSTICS

ENVIRONMENTAL CONTROL III

(ACOUSTICS AND NOISE CONTROL)

DEPARTMENT OF ARCHITECTURE.

FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE

ONDO STATE.


Outline• Introduction• The Nature of Sounds• Properties of Sound• Propagation of Sound• Sound Power and Intensity• Effects of Barrier on Sound• Conclusion


Acoustics is the science of sound and it

covers two areas, those of room acoustics

and control of noise.

Noise is unwanted or damaging sound

which interferes with what people are trying

to do, or sound which has an adverse effect

on health and safety.

Introduction


Introduction

This lecture covers basic

architectural

acoustics including the properties

and nature of

sound, the terms used to describe

sound waves,

and the relationship between sound

pressure,

sound intensity and sound power.


The Nature of Sound

Sound is a disturbance, or wave

which

moves through a physical

medium (such as

air, water, or metal) from a source

to cause

the sensation of hearing in

animals.


Sound waves

Sound waves are longitudinal

waves

originating from a source and

conveyed by

a medium. They are characterized

by

velocity (v), frequency (f),

wavelength (٨),

and amplitude (a).


Sound waves

Compression in sound waves is

a region

of raised pressure.

Rarefaction in sound waves is a

region of

lowered pressure.


Sound waves

Figure 1: Compression And Rarefaction Of Sound By A Vibrating

Tuning Fork.


Sound waves

Figure 2: Visual ization of sound rarefaction and compression in a

coi led spring.


Sound waves

Figure 3: Sound wave i l lustration.


Frequency range of sound

Sounds produced by various sources can

range from frequencies below 20Hz to

20,000Hz and above.

Infrasound are sounds with frequencies

below 20Hz.Ultrasound are sounds with

frequencies above 20,000Hz.


The Audible range of sound

Audible sounds range from the threshold

of audibility to the threshold of pain.

The threshold of audibility is the lower

Limit of hearing and it has a standard value

of 1 picowatt per metre square (1pW/m²).


The threshold of pain is the upper

Limit of hearing and it has a standard value

of 1 watt per metre square (1W/m²).

Sounds below the lower limit of hearing

are inaudible while the those above the

upper limit may cause pain or even damage

the human ear.




Figure 4: Audible range of sound.



Figure 5: Audible range of sound.



The sound level or decibel scale is the logarithm

of the ratio of measured sound intensity to the

intensity at the threshold

of audibility.



The loudness of a sound is determined by referring

to the loudness or phon scale which shows sounds of

various levels and frequencies which are perceived as

of the same sound loudness.



Figure 6: Equal loudness contours.



Figure 7: Psychological and physiological eff ects of sounds


Properties of Sound

•Wave length

This is the distance between two

successive pressure peaks. Its

symbol is ٨ and it is measured in

units of metres (m).


Properties of Sound

•Period

This is the time taken for one

vibration cycle. Its symbol is T

and it is measured in units of

seconds (s).


Properties of Sound

•Frequency

This is the number of vibration cycles per

seconds. Its symbol is F and it is measured in

units of Hertz (Hz).

For the relationship between frequency and

period,

F=1/T


Properties of Sound

•Speed or wave velocity

This is the speed with which sound travels

through a medium. Its symbol is C and it is

measured in units of metres per seconds

(m/s).

For the relationship between the speed

(C), frequency (F) and wave length (٨ ),

C=F ٨


Properties of Sound

Figure 8: Variation of speed of sound with the medium of transmission. .


Properties of Sound

Factors that affect the speed of sound

through a medium

•Elasticity of the medium

•Density of the medium

•Temperature of the medium


Properties of Sound

•Amplitude

This indicates the intensity of sound. Its

symbol is I and it is measured in units of

watts per metres square (W/m²).


Properties of Sound

Figure 9: Amplitude i l lustration.


Properties of Sound

The inverse square law of sound states that the

intensity of sound in a free field is indirectly

proportional to the square of the distance from the

source.

This infers that there is a decrease in the intensity

of sound the farther the observer is from the source.


Properties of Sound

Figure 10: The inverse square law.


Properties of Sound

Figure 11: Amplitude i l lustration.


Properties of Sound

•Pitch

This is the property of sound that is perceived as highness and lowness depending on the rapidity of the vibrations producing it . It is measured in cycles per second

(cps).


Properties of Sound

Figure 12: Pitch i l lustration.


Properties of Sound

•Sound pressure

This is the force per unit area and it gives the magnitude of the sound wave. Its symbol is p and it is measured in

units of Pascal (Pa).


Properties of Sound

The pressure changes produced by a sound wave are also known as sound pressure.

Compared with atmospheric pressure on which they are superimposed (about 100,000 pascals), they are very small (between 20

micropascals and 200 pascals).


Properties of Sound

Figure 13:Changes in sound pressure over t ime.


Properties of Sound

Figure 14:Sound pressure superimposed on atmospheric

pressure.


Properties of Sound

Figure 15:Relationship between sound pressure and sound frequency

in a pure tone.


Properties of Sound

Figure 16:The characteristics of Machine noise.


Propagation of Sound

Inside a room, close to a source like a machine, the direct sound dominates, and the sound pressure may vary significantly with just small changes in

position.



This area is called the near field and its extent is about twice the dimension of the machine or one wavelength of the sound.



The area beyond the near field is called the far field made up of two sections,

•The free field

•The reverberant field



In the free field the direct sound still dominates and the sound pressure level decreases by 6 dB for each doubling of distance.



In the reverberant field the reflected sound adds to the direct sound and the decrease per doubling of distance of the sound pressure level will be less

than 6 dB.



Figure 17: The near fi eld and far fi eld of sound. Source: National Institute for Occupational Health and Safety

(1988).



Figure 18: Decrease in sound intensity for a point source with

doubling of distance.


Properties of Sound

•Spherical wave fronts

These are produced when sound spreads out from a point source in a free space. The wave fronts are spherical and the sound

pressure level decreases 6 dB for each doubling of distance.



Figure 19: Decrease in sound intensity for an omnidirectional

point source.source.


Properties of Sound

•Cylindrical wave fronts

These are produced when sound spreads out from a line source (such as a road with constant traffic or a pipe carrying fluid). The

waves are cylindrical and the sound pressure level decreases 3 dB for each doubling of distance.



Figure 20: Decrease in sound intensity for a l ine source with

doubling of distance.



Figure 21: Decrease in sound pressure level for a l ine source.


Properties of Sound

•Perpendicular wave fronts

These are produced when sound spreads out from a plane source (such as close to a large vibrating panel or sound travelling down

a duct). The waves are perpendicular and the sound pressure does not decrease with distance.



Figure 22: Perpendicular wave fronts.


Properties of Sound

The above relationships hold true only in ideal conditions. Decrease in sound levels depends on

•Absorption by air and moisture ;

•Wind and temperature gradients;

•Absorption of the ground; and

•Reflection and absorption by obstacles .


Sound Power and Sound Intensity

•Sound Power

This is the fundamental property of the source of sound that depicts the energy emitted by a sound source per unit time. Its symbol

is W and it is measured in units of Watts (w).



A source that emits power equally in all directions is called an omnidirectional source. Any other source is called a directional source.


Sound Power and Sound

Intensity

Figure 23: Decrease in sound pressure level for an omnidirectional

point source .



•Sound Intensity

Sound intensity at a point in the surrounding medium is the power passing through a unit area. Its symbol is I and it is measured in

units of watts per metre square (W/m²).



For an omnidirectional point source the sound power is spread over the surface of the sphere.

S=4pr².

Hence the sound intensity is given by

I=W/4r²



The relationship between sound intensity and sound pressure is given as

I=p² / rc

This equation is for plane waves. However, away from a point source, spherical waves approximate plane

waves.


Sound Power and Sound IntensityI is the sound intensity in watts per metre square (w/m²),

p is the sound pressure in pascals (pa),

r is the density of the medium in kilogram per metres cube (kg/m³), and

c is the speed of sound in metres per second

(m/s).


Effects of Barriers on Sound

When a sound wave encounters an obstacle such as a barrier or a wall, its propagation will be affected in one of three ways

•reflection

•diffraction

•refraction



•Reflection

This occurs when the dimensions of an obstacle are larger than the wave length of the sound. In this case, the sound wave behaves like a light

ray and for an obstacle with a flat surface, the reflected ray will leave the surface at the same angle as the incident wave.



Figure 24: Refl ection of sound .



•Refraction

This occurs when a sound wave enters a different medium at an angle. The bending of sound wave is due to the differing speed of travel of the

sound wave in the two media.



Figure 25: Refraction of sound with no temperature inversion .



Figure 26: Refraction of sound with temperature inversion .



•Diffraction

This occurs when the dimensions of an obstacle are of the same order or less than the wavelength of the sound. In case the edge of the

obstacle acts like a source of sound itself and the sound ray appears to bend around the edge.



Figure 27: Diff raction of sound.


Transmission and Absorption of Sound

•Transmission of sound

This refers to the ways in which sound can be transmitted which could be in form of structure borne sound, impact sound, and

airborne sound.



Structure borne sound refers to the transmission of sound involving the re-emission of sound via vibration of the

molecules of a barrier in the path of the sound.



Impact sound refers to the transmission of sound via mechanical means.

Airborne sound simply refers to the transmission of sound through air.



•Absorption of sound

This refers to the product of the absorption coefficient and the area of a given surface. It is measured in the open window unit which is

equivalent to the absorption of a square metre opening with zero reflectance.



The Absorption coefficient is an indication of the sound that is not reflected and is thus an indication of both the sound absorbed and transmitted.



Figure 28: Transmission and absorption of sound.



An ability of an obstacle to block the transmission of sound depends on its structure which indicates its transmission loss rating.

Hence, stiff, heavy materials like concrete have high transmission loss. Soft porous materials like cell foams are not good at

blocking the transmission of sound but are good absorbers.



•Masking of sound

This refers to the acoustic shadow effect of screens or barriers in the path of sound which differs in accordance with the frequency

and wave length of the sound and the dimension of the barriers.



This effect disappears when the wave length of the sound is more than the dimension of the barrier in a direction perpendicular to the sound path.



Figure 29: Acoustic shadow at high frequencies.



•Sound Insulation

This refers to the reduction of sound transmission of airborne sounds through walls, floors, and partitions. It is achieved by using

elements with an adequate transmission coefficient or sound reduction index.



The transmission coefficient is a decimal fraction expressing the proportion of sound energy emitted.

The sound reduction index or transmission loss defines the reduction effect of an element and it is expressed in decibels.



•Reverberation

This is the persistence of sound in an enclosed space as a result of repeated reflection or scattering after the sound source has

stopped.



•Reverberation Time

This is number of seconds required for the energy of the reflected sound in a room to diminish to one-millionth of the original energy it

had. It can also be defined as the number of seconds required for the sound pressure level to diminish to 60 decibels below its initial value.



•Echoes

This is a distinct repetition of the direct sound. Its effect may be observed by making a short sound such as a clap, in a large room.


Conclusion

The basic concepts in architectural acoustics, the nature of sound and its physical properties as discussed in this lesson help in providing an understanding

for dealing with the problems of noise.

BASIC CONCEPTS IN ARCHITECTURAL ACOUSTICS ENVIRONMENTAL CONTROL III (ACOUSTICS AND NOISE CONTROL)...

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