A

TECHNICAL REPORT

on the

THE BEHAVIOUR OF SOUND IN AN ENCLOSED SPACE.

Compiled by,

ANTHONY Olatubosun E. Arc – 01 – 9211

JONES Abimbola O. Arc – 01 – 9226

OYEDELE Olu’toyin A. Arc – 03 – 1952

In partial fulfilment of ARC 507; Environmental Control III (Acoustics and Noise Control)

April, 2007.

TABLE OF CONTENT

1.0 Introduction

2.0 Physical properties of sound

3.0 Behaviour of sound in an enclosure

3.1 Reflection

3.2 Absorption

3.3 Refraction

3.4 Diffusion

3.5 Diffraction

3.6 Transmission

3.7 Reverberation and echo

4.0 Factors that affect the behaviour of sound in an enclosed space.

5.0 General Requirements for Speech Intelligibility

5.1 Some standard architectural-acoustical measures

6.0 Conclusion

7.0 References

1.0 INTRODUCTION

Sound can be described as a disturbance or turbulence which passes through a physical

medium in the form of longitudinal waves from a source to a receiver causing a sensation

of hearing. This medium could be solid, fluid or gas. The speed of sound through these

different media differs due to their molecular composition.

Bearing in mind the properties of sound, this paper attempts to study the behaviour of

sound within an enclosure, factors that stimulate this behaviour and general requirement

for speech intelligibility within the space.

1.1 Physical properties of sound

Some of the physical properties of sound are:

• Wavelength of sound – This the distance between two pressure peaks or

valleys, measured in metres (m) and represented with the Greek alphabet ‘l’

(lambda).

• Period – This is the time taken for on complete oscillation. This is measured

in seconds(s) and represented with the letter ‘T’.

• Frequency – This is the

number of oscillations

per second. This is

represented with ‘f’ and

measured in Hertz

(Hz).

T

Amplitude

Wavelength

• Velocity of sound – This is the rate at which a sound wave travels from a

source through a medium to the receiver. The unit is m/s.

• Amplitude – This is the distance between a crest (the highest point) and a

valley (the lowest point)

• Pitch – it is the highness or lowness of a tone determined by the rapidity of

the oscillations producing it.

The above mentioned properties of sound determines the behaviour of sound genearally

either in an open or enclosed space. These factors are used to describe sound especially in

music, in its study, arrangement of sounds, the music scale, tunes and others.

3.0 BEHAVIOUR OF SOUND IN AN ENCLOSURE

An enclosed space is a room or area bounded on every of its sides. The materials for

enclosure may be classified into two:

• Those that allow sound rays to pass through and

• Those that do not allow sound rays to pass through.

Areas bounded with materials that allow sound rays to pass through tend to enjoy good

acoustic as the effect of indirect sound from reflection is reduced within the space. In this

space, sound from external source can pass through the material into the enclosure as

background noise. The vibration of some of the materials can be a source of noise within

the space and this can be a bane to the achievement of clear and audible speech and music.

The effect of indirect sound may be pronounced in spaces enclosed with materials that do

not allow the passage of sound rays through them. Adjustments to the material to aid

diffusion will improve the acoustic of the space. Some of these materials can absorb sound,

reducing the effect of indirect sound.

On encountering barriers posed by the enclosure, sound waves are likely to behave in the

following ways:

• Reflection

• Absorption

• Refraction

• Diffusion

• Diffraction

• Transmission

3.1 Reflection

This occurs when the wavelength of a sound wave

is smaller than the surface of an obstacle. In the

case of an enclosed space, the sound waves hit

every side of the enclosure continuously until the

sound energy reduces to zero. The amount of

waves reflected depends on the smoothness, size, and softness of the materials of enclosure.

The angle of incidence of sound rays is equal to that of the reflected rays only if the surface

of the reflector is flat. But when it is curved, the angles are different.

3.2 Absorption

When sound waves hit the surface of an obstacle,

some of its energy is reflected while some are lost

through its transfer to the molecules of the barrier.

The lost sound energy is said to have been

absorbed by the barrier. The thickness and nature

of the material as regards its softness and hardness

influences the amount of sound energy absorbed.

3.3 Refraction

This is the bending of sound when it travels from one medium

into another medium. The difference in the composition of the

two different media bends the sound i.e. the angle of incidence

changes into an angle of refraction as it travels into the new

medium.

3.4 Diffusion

This is the scattering of waves from a surface. It

occurs as a result of the texture and hardness of the

obstacle is comparable to the wavelength of the sound.

The direction of the incident ray changes when it

strikes the surface of the obstacle. Satisfaction is

achieved when sound is heard in all direction at equal

level.

3.5 Diffraction

When the wavelength of a sound wave is smaller or equal to the

size of the obstacle, the sound rays tend to bend round the edge

of the obstacle thereby turning the edge to a sound source.

3.6 Transmission

In this phenomenon, sound wave is carried by

molecules of the obstacle through vibration and re-

emitted at the other side irrespective of the

medium. It can be structure borne, air borne or

impact sound.

3.7 Reverberation and Echo

Reverberation: This is the persistence of sound in an enclosed space as a result of

continuous reflection or scattering of sound after the source has stopped. It is one the most

prominent behaviours of sound in an enclosure. It occurs when sound waves hits a surface

and are reflected toward another surface which also reflects it. Some of the sound is

absorbed with this continuous reflection which gradually reduces the energy of the sound

to zero. The phenomenon can affect the audibility of sound in an enclosure, especially if

the reverberation time, which is the time taken for the sound pressure level to diminish to

60 dB below its initial value is considerably long.

Echo: this occurs when the reverberation time is long enough to cause a distinct repetition

of the direct sound. This condition is an advanced form of reverberation where the sound

is heard clearly and repeatedly after some time until it fades.

4.0 FACTORS THAT AFFECT THE BEHAVIOUR OF SOUND IN AN

ENCLOSED SPACE.

The way in which sound behaves in an enclosed space depends on many factors which

include:

• Reduction in its intensity of sound – This can results due to the distance between its

source and the receiver.

• Absorption of direct sound by the audience – The listeners of the sound absorb some

of the sound in the process of hearing.

• Absorption of direct and reflected sound by surfaces – The walls, ceiling and floor of

the enclosure absorbs and reflect sound waves thereby controlling the way the

sounds behave.

• Reflection of sounds from right-angled corners - Sound incident to a right-angled

corner of room will be reflected back towards source if surfaces are acoustically

reflective. This can in turn produce echoes especially in large spaces.

• Dispersion of the sides of an enclosure - Reflections can be controlled by making one

surface dispersive i.e. not at right angle to each. This would have affected the

reflection of the sound thereby affecting its behaviour.

• Edge diffraction of sound - Edge diffraction results in the curvature of part of a

sound wave around the edge of a barrier. This causes the obstacles to scatter the

sound waves making it behave like a source of sound.

• Sound shadow - Any barrier interrupting a sound wave will create a shadow,

synonymous to light rays. However, because of edge diffraction some sound will

creep into this but such penetration is frequency dependent - high frequencies are

less diffracted than low frequencies. Such problems can occur in auditorium with

balconies.

• Primary reflection – This depends on the angle of incidence which is equal to the

angle of reflection. Also, the nature of sound reflector is important.

• Panel resonance - Sound waves can propagate "through" a solid material by panel

vibration. The sound does not actually penetrate the material but rather causes this

to vibrate and act as a sound source itself. The panel will be vibrated by both direct

and reflected sound waves.

Fig.1 Potential apparent changes in direction of sound reflection distance from sources

When a sound is encountered in the open air, in a horizontal plane, the sound that reaches

a listener proceeds by a direct path from the sound source. But for sounds in an enclosed

space, the listener

will receive not

only the direct

sound but also

the sound energy

that is reflected

from the surfaces

of the enclosed space. The boundaries of the space will have three principal effects on the

experience of sound; sound intensity is likely to increase, sound duration is likely to

increase, and apparent sound direction may be altered. This can be seen in Fig. 1 above

showing the effect of reflection, the distance between the source and the listener and the

change in intensity. It can be seen that there is only one direct sound wave experienced by

the listener while indirect sound waves of varying intensities are as a result of reflection by

the boundary of the enclosure which are at different distances from the source and the

listener.

The super-imposition of both

the direct and indirect sounds

creates an increase in sound

level which may result to a

distortion the sound in turn

affecting audibility. Fig. 2

Fig.2 Increase in sound levels arising from the summation of indirect (reflected) sound energy and direct energy from source

It is also noticed that indirect

sound reaches the listener a little later after the listener as experienced the direct sound.

This cause an increase in the duration of the sound caused by the distance between the

source of sound, the listener and the reflector. (Fig. 3)

The above conditions results

into reverberation.

Fig.3 Extension of apparent duration of sound because of a longer length path (hence later) arrival of indirect sound relative to

direct sounds.

The two major factors that affect sound transmissions are:

• Increased weight per unit area of panel decreases sound transmission,

• Increased frequency of incident sound decreases sound transmission.

Besides the mass of the panel, other factors that can affect sound transmission include:

• Panel stiffness - at very low frequencies the stiffness (i.e. resistance to

deformation) may have more effect than its weight. In this part of the frequency

range insulation is termed stiffness controlled.

• Rigid panels - if a rigid panel is struck it will continue to vibrate at frequencies

determined by its size, shape, and thickness - this is its "natural frequency" (natural

mode of vibration).

All subsequent frequencies which produce such vibration are called resonant frequencies

and sound insulation will be reduced. This is termed resonance controlled insulation.

5.0 GENERAL REQUIREMENTS FOR SPEECH INTELLIGIBILITY

The requirements for speech intelligibility are basically the same for un-amplified as for

amplified speech. The most important factors are:

• Speech level versus ambient noise level - Every effort should be made to minimize

noise due to air handling systems and outside interferences. In general, the noise

level should be 25 dB or greater below the lowest speech levels which are expected.

However, for quite high levels of reinforced speech, as may be encountered

outdoors, a noise level 10 to 15 dB below speech levels may be tolerated.

• Reverberation time - Speech syllables occur three or four times per second. For

reverberation times of 1.5 seconds or less, the effect of reverberant overhang on the

clarity of speech will be minimal.

• Direct-to-reverberant ratio - For

reverberation times in excess of

1.5 seconds, the clarity of

speech is a function of both

reverberation time and the ratio

of direct-to-reverberant sound.

Direct-to-reverberation ratio graph

In an important research set forth, a method of estimating speech intelligibility was found

to be considerably applicable in sound system design. These findings were compiled on the

basis of data gathered over a period of years.

The conclusions can are as follows:

• In practice, the articulation loss of consonants can be used as a single indicator of

intelligibility.

Analyzed intelligibility chart

• As would be expected, the researchers found wide variations in both talkers and

listeners. However, a 15% articulation loss of consonants seems to be the maximum

allowable for acceptable speech intelligibility. In other words, if articulation loss of

consonants exceeds 15% for the majority of listeners, most of those people will find

the intelligibility of speech to be unacceptable.

• Articulation loss of consonants can be estimated for typical rooms. Articulation loss

of consonants is a function of reverberation time and the direct-to-reverberant

sound ratio.

• As a listener moves farther from a talker (decreasing the direct-to-reverberant sound

ratio) articulation loss of consonants an increase. That is, intelligibility becomes less

as the direct-to-reverberant ratio decreases. However, this relationship is maintained

only to a certain distance, beyond which no further change takes place. The

boundary corresponds to a direct-to-reverberant ratio of -10 dB.

5.1 Some standard architectural-acoustical measures

The standard measures that may be applied provide information about specific dimensions

of the experience of sound in enclosed spaces (specifically, loudness, duration and spatial

impression). Note that loudness, duration and spatial impression are all psychological

attributes of sound in enclosed spaces.

Standard objective measures address the physical aspects of sounds in spaces that correlate

with these psychological attributes; hence loudness would be assessed by means of

measurements of intensity, duration in terms of measurements of temporal ex-tension and

spatial impression by measurements of the directions from which reflected sound reaches a

listener. These objective measures include: total sound level; objective clarity of speech or

music; reverberation time; early decay time; and objective envelopment.

5.1.1 Intensity

Total sound level: is a measure of the increased intensity of the acoustic signal that occurs in

any enclosed space with reflective surfaces. This corresponds to the total sound level at the

measurement position minus the sound level of the direct sound measured at 10m from

the sound source. It is expected that different locations in any given site are likely to yield

different total sound level values, and on each site a range of measurements would have to

be taken.

5.1.2 Temporal Effects

Objective clarity: is a measure of the ratio of direct sound to indirect sound in an enclosed

space; high levels of clarity indicate high levels of direct sound relative to indirect sound. It

can be measured so as to correlate with perceived speech intelligibility, where the direct

sound is defined as including all indirect sound occurring within 50m sec of the direct

sound or with the perceived ‘distinctness’ of music’s structure in time, where the direct

sound incorporates all indirect sound occurring within 80m sec of the direct sound.

Again, clarity can be expected to vary across a space so multiple measurements will be taken

at each site.

Reverberation time: is the classical measure of the effect of an enclosed space on the

perceived duration of a sound, and is defined as the time taken for the acoustic signal to

decay by 60 dB from the cessation of the direct sound. Though similar to clarity, it provides

an index of the ‘liveliness’ of a space, and hence its suitability for types of activities

involving sound that proceed at different paces. Measurements of reverberation time

should be supplemented by measurements of Early decay time, which provides an index

similar to reverberation time but one that appears to correspond more directly to the

subjective judgments of reverberation when the decay in a space is not completely linear (a

feature that is likely to vary between sites).

When reflected sound reaches the listener before 30 – 35m sec of the direct sound, both inputs are likely

to be experienced at the same time.

When reflected sound reaches the listener later than 30 – 35m sec of the direct sound, both inputs will not be integrated and not experienced at the same time.

Spatial Effects

Objective envelopment or early lateral energy fraction: is a measure of the ratio of early

energy arriving at a point from lateral directions to total early energy arriving at that point

(early energy being defined as within 80m sec of the direct sound). This gives an index of

the perceived spatial configuration of a space on the basis of acoustical cues and is likely to

relate to the degree to which a listener can, or cannot, orient themselves in that space on

the basis of sound alone.

From measurement of clarity in its two forms one can legitimately make inferences about

the suitability of a space or site for speech, or for activities involving ‘heightened speech’ (as

in declamation) or music. Such inferences can be buttressed by measures of reverberation

time, as a long RT (or long Early decay time) will seem to ‘smear’ or ‘blend’ sound over time,

making it more suitable for sound sequences that are either slow in pace or that require a

degree of blending it time to achieve their efficacy.

4.2 Effects of Indirect Sound

Some of the effects of indirect sound include:

• Increase in intensity of sound – this is as a result of the super-imposition of both the

direct and indirect sounds.

• Direct sound – indirect sounds are very close to direct sound that it is hard to

differentiate them.

• Termination of direct sound – indirect sound is heard even when the direct sound

has faded away.

• Termination of first reflection

• Equilibrium reached between input power and loss

• First reflection

6.0 CONCLUSION

Sound is characterised by many uniqueness as a wave especially within an enclosed space.

The study of these characteristics is of great importance in architecture. This is more

pronounced in the design of large enclosed spaces designed for speech, music or quiet.

Having carefully studied the behaviour of sound within an enclosed space, it can be said

that the importance of the adequate knowledge of this subject cannot be over-emphasised.

It is imperative that the understanding of the behaviour of sound and related issues within a

confinement is a foundation in achieving acoustically sound spaces by architects and this

must be taken into consideration right from the conception stage of a design to the very

finishing of the construction.

7.0 REFERENCES

M. Barron, Auditorium Acoustics and Architectural Design, London: E. & F. N. Spon, 1993.

W.C. Sabine, Collected Papers on Acoustics, New York: Dover Publications, 1921.

J. Eargle, Electro-acoustical Reference Data, Van Nostrand Reinhold, New York, 1994.

H. Haas, The Influence of a Single Echo on the Audibility of Speech, J. Audio Engineering

Society, volume 20, number 2 (1972).

O.O. Ogunsote, Lecture Notes, Arc 507: Environmental Control III (Acoustics and Noise

Control), Federal University of Technology, Akure, 2007.