Acoustics Acoustics-is a science which deals with the

production, control, transmission, reception

and effects of sound in an enclosed

space.

Sound-is the human ears response to

pressure fluctuations in the air caused by

vibrating objects.

wave motion-Sound travels in space

TYPES OF SOUNDS

Speech

Music

Noise

Physical Quantities of Sound

Frequency (f)-the number of sound

ripples generated in unit time.

Frequency of the wave- The number

of cycles that the air particles move

back and forth in one second in a

sound wave

Its unit is cycles per second (c/s)

which is also termed Hertz (Hz)

After physicist Heinrich Hertz

Eight frequency bands, or octaves, are

considered in room acoustics

Speed (c)- The speed of sound in air

has been measured as 344 m/sec

(1,130 ft/sec). This corresponds to

1,240 km/hr (770 mi/hr) which is

extremely small as compared to the

speed of light (300,000 km/sec).

WAVELENGTH () The wavelength and the frequency of sound are

related to each other as shown in

the equation below.

c = speed in meters per time

f = frequency in cycles per time

= meters

INTENSITY - is defined as the amount

of sound power falling on (or passing

through, or crossing) a unit area.

Since the unit of power is watt (W),

the unit of sound intensity is watt per

square meter (W/m2).

10-12 W/m2- The sound intensity

which is just audible, called the

threshold of audibility

10 W/m2- the intensity that

corresponds to the sensation of pain

in the human ear

Characteristics of sound

LOUDNESS is a measure of the intensity of sound and is expressed in

decibels (dB). It is a quantity called

the sound intensity level (IL).

PITCH is the frequency of sound wave perceived by the human ear.

A high-pitched sound means that it

has a high frequency. The female

voice is slightly higher pitched than

the male voice.

Acoustical concerns in

Architecture

Noise Control

the control of air-borne noise

through the insulation of sound or

the shutting-out of unwanted sounds

from the outside.

the control of structure-borne noises

through the isolation of machines

from the rooms or the buildings structure.

Principal Acoustic Defects of

Rooms

Prolonged Reverberation long reverberation time (RT) due to large

amounts of highly reflective surfaces

and/or to large volume of space

which will take considerable time for

reflected sound to die out.

Blurring- Effect of prolonged

reverberation

Echo distinct reflection of original sound which results when the path of

reflected sound is 20 m (65 ft) or

more than the path of direct sound.

If the difference is less than 20 m, the

reflected sound will reinforce the

direct sound which is desirable.

highly reflective- It is recommended

that the surfaces of the front part of

an auditorium

Resonance is the reinforcement of certain sound frequencies due to

sympathetic vibrations. This is

especially the case in enclosed

rooms with highly reflective surfaces.

The effect would be to emphasize

certain frequencies at the expense

of others, which is undesirable for

balance desired in rooms intended

for music.

Flutter Echo a rapid but repetitive succession of sounds caused by

highly reflective parallel surfaces

(wall to wall, or ceiling to floor).

Undue Focusing of Sound is caused by concave surfaces which causes

sound to converge at certain points

with resulting loss of energy in other

parts of the room.

IMPORTANT ACOUSTICAL TERMS

Absorption coefficient the fraction of the incident sound energy absorbed by a

surface.

anechoic chamber a sealed room in which all the surfaces are designed to

completely absorb all sound produced in

the room.

attenuation a reduction in sound level. Sound attenuation in air-conditioning is

specified in terms of dB per meter.

background noise ambient noise

break-in noise transfer of noise from a space surrounding the duct into the duct

through duct walls.

break-out noise transfer of noise from the interior of a duct through duct walls

into a space outside the duct.

dead room a room containing an unusually large amount of sound

absorption..

decibel (dB) a unit of measurement for sound pressure level, sound intensity level

or sound power level.

diffraction a change in the direction of propagation of sound as a result of

bending caused by a barrier in the path of

a sound wave.

diffuse sound (field) a sound field in which the sound comes in equal intensity

from all directions.

direct sound the sound that arrives at a receiver along a direct line from the source

without reflection from any surface.

echo a sound that has been reflected with sufficient time delay.

environmental noise exterior background noise in a neighborhood (ie.

traffic, aircraft).

fidelity faithful reproduction of a sound source.

flutter echo a rapid but repetitive succession of sound from a sound source

usually occurring as a result of multiple

reflections in a space with hard, flat and

parallel walls.

frequency the number of full cycles per second measured.

impact noise noise caused by the collision of two objects.

infrasonic a sound that is below the human audible frequency, below 20 Hz.

insulation see isolation

intermittent sound a sound which is discontinuous or fluctuates to such an

extent that at times its sound pressure level

falls below a measurable level.

inverse square law a law which states that the sound intensity in a free field varies

inversely with the square of the distance

from the source.

isolation a lack of acoustical connection.

leak a small opening in a barrier that allows airborne sound to pass through.

live room a room containing an unusually small amount of sound

absorption.

loudness an auditory sensation that depends on sound pressure level and the

frequency of sound.

masking the increase in the threshold of audibility of a sound that is required so that

the sound can be heard in the presence of

another sound.

noise isolation class (NIC) a single number rating derived from the measured

value of noise reduction between two

rooms.

noise reduction (NR) the reduction in sound pressure level of noise.

noise reduction coefficient (NRC) a single number rating derived from

measured values of sound absorption

coefficients of a material at 250, 500, 1000

and 2000 Hz.

outdoor-indoor transmission class (OITC) a weighted single number rating of the

sound reduction effectiveness of a

partition that separates an indoor space

from the outside.

pitch a listeners perception of the frequency of a pure tone.

reflection coefficient a measure of the sound reflective property of a surface.

resonance the relatively large amplitude of vibration produced when the

frequency of the source of sound is equal

to the natural frequency of a room.

reverberant sound field a sound field created by repeated reflections of sound

from the boundaries in an enclosed space.

reverberation the continuation of sound in an enclosed space after the initial

source has been terminated.

reverberation time (RT) the time it takes for sound intensity to decay by 1 millionth

of its steady state value after the sound

source has been terminated.

sabin a unit of measure of sound absorption.

scattering an irregular diffraction of sound in many directions.

sound insulation the ability of a barrier to prevent sound from reaching a receiver.

sound intensity (SI) the average rate of sound energy flow through a unit area in a

given direction.

sound intensity level (SIL) a quantity expressed in decibels of airborne sound.

sound lock a small space that works as a buffer between a source room and a

receiving room.

sound pressure fluctuating pressure of sound superimposed on the static air

pressure.

sound pressure level see sound intensity level

sound transmission class (STC) a single number rating of the sound insulation

rating of a partition.

structure-borne sound sound propagated through a solid structure.

transmission coefficient the ratio of transmitted sound energy

to incident sound energy

transmission loss (TL) is the measure of sound insulation of a partition.

wavelength distance between two adjacent compressions or rarefactions in a

sound wave.

white noise a noise whose energy is uniform over a wide range of frequencies.

This is analogous to the term white light, which consists of almost equal amount of

light of different wavelength (colors). A

white noise sounds hissy.