part 1

Unit -2 Introduction and theory of soundIntroduction and theory of sound

Acoustics – definitions, terms related to acoustics. Theory of sound : generation, propagation, transmission, reception of sound, sound waves, frequency, intensity, wavelength, sound pressure, measurement of sound, scales- decibel scale ,

• Characteristics of speech• Music and hearing• Music and hearing• Distribution of energy in speech• Music frequencies• Intelligibility of speech, high fidelity reproduction of music• Human ear characteristics- making of sound • Binomial hearingg• Behavior of sound in enclosed spaces.

STUDY OF SOUND ‐ ACOUSTICS HANDLED BY G.YOGAPRIYA

INTRODUCTIONINTRODUCTION AND THEORY OFAND THEORY OF

SOUNDSOUND


DEFINITIONSDEFINITIONS• What is Acoustics ?

– Acoustics is defined as thescience that deals with theproduction controlproduction, control,transmission, reception, andeffects of sound.

– The physical principles ofthis science are utilized inthis science are utilized inarchitecture to attaindistinct hearing conditionsin enclosed spaces.


p

• The study of acoustics helpsto

– To appreciate and analyse To appreciate and analyse the problems involved in the propagation of sound in these enclosures.in these enclosures.

– To suggest ways and means of producing means of producing optimum conditions of hearing

– To obtain both subjective and objective assessments


jof the results achieved.

DEFINITIONSDEFINITIONS• What is resonance ?

• resounding or reechoing

i i th i t it f d b th ti • increasing the intensity of sounds by sympathetic vibration

– Eg) Put two tuning forks of equalfrequencies side by side, but nottouching. Strike one tuning fork so thatg gyou can hear its tone, and then suddenlysilence it. You can still hear a faint tone.This is because the second tuning fork


ghas started vibrating sympathetically.

DEFINITIONSDEFINITIONS• What is Reverberation?

– It is the persistence of sound in aparticular space after the original soundis removed.

– A reverberation, or reverb, is createdwhen a sound is produced in anenclosed space causing a large numberof echoes to build up and then slowlydecay as the sound is absorbed by thewalls and air.

– This is most noticeable when the soundsource stops but the reflections


continue, decreasing in amplitude, untilthey can no longer be heard.

DEFINITIONSDEFINITIONS• What is Reverberation time?

– Standard reverberation time has beendefined as the time for the sound to diea a to a le el 60 decibels belo itsaway to a level 60 decibels below itsoriginal level. The reverberation time canbe modeled to permit an approximatebe modeled to permit an approximatecalculation.


DEFINITIONSDEFINITIONS• The reverberant sound in an auditorium dies away y

with time as the sound energy is absorbed by multiple interactions with the surfaces of the room. In a more reflective room, it will take longer for the gsound to die away and the room is said to be 'live'.

• In a very absorbent room the sound will die away • In a very absorbent room, the sound will die away quickly and the room will be described as acoustically 'dead'. But the time for reverberation to completely die away will depend upon how loud the sound was die away will depend upon how loud the sound was to begin with, and will also depend upon the acuity of the hearing of the observer


DEFINITIONS• What is echo?DEFINITIONS– In audio signal

processing and acoustics processing and acoustics, an echo (plural echoes) is a reflection of sound, arriving at the listener arriving at the listener some time after the direct sound.

• Typical examples are the echo produced by the bottom of a well by a bottom of a well, by a building, or by the walls of an enclosed room. A true echo is a single reflection


gof the sound source.

SOUND

1. A sound is a vibration in an elastic medium, which may be ya solid, liquid, or gas, which can be registered by the ear.

2. It can be pleasant or unpleasant, loud or soft, high

l S d i d b or low. Sound is made by vibrations, something moving back and forth. Stretch a rubber band tightly between your fingers, and pluck it. You can see it quiver and hear the can see it quiver and hear the sound it makes.


CHARACTERISTICS OF SOUND WAVESSounds are generally audible to Sounds are generally audible to the human ear if their frequency (number of vibrations per second) li b t 20 d 20 000 lies between 20 and 20,000 vibrations per second, but the range varies considerably with the individual.

The range of audible sound is also differentiated into 3 main categories. Subsonic or low frequency sound is defined in the range of 20Hz to about 500Hz. Midrange frequencies inhabit the realm of 500Hz to g q6KHz (6000Hz) With high frequency sound defined in the remaining 6KHz to 20KHz6KHz to 20KHz.


Those with frequencies above the audible range are called ultrasonic.A sound wave is usually represented graphically by a wavy, horizontal line; the

f h ( h ) i di upper part of the wave (the crest) indicates a condensation and the lower part (the trough) indicates a rarefaction.


GENERATION OF SOUND WAVES

GENERATION OF SOUND WAVES

Sound waves are generated by anyvibrating body. For example, when a

i li i ib b iviolin string vibrates upon beingbowed or plucked, its movement inone direction pushes the molecules ofthe air before it crowding themthe air before it, crowding themtogether in its path.

When it moves back again past its original position and on to theother side, it leaves behind it a nearly empty space, i.e., a space withrelatively few molecules in it In the meantime however the moleculesrelatively few molecules in it. In the meantime, however, the moleculeswhich were at first crowded together have transmitted some of theirenergy of motion to other molecules still farther on and are returningto fill again the space originally occupied and now left empty by theg p g y p p y yretreating violin string.


•In other words, the vibratory motion set up byth i li t i lt t l i ithe violin string causes alternately in a givenspace a crowding together of the molecules ofair (a condensation) and a thinning out of theair (a condensation) and a thinning out of themolecules (a rarefaction).


•Taken together a condensation and ararefaction make up a sound wave; such awave is called longitudinal, orcompressional because the vibratorycompressional, because the vibratorymotion is forward and backward along thedirection that the wave is following.direction that the wave is following.

•Because such a wave travels by disturbingthe particles of a material medium, soundwaves cannot travel through a vacuum.


THE PROPAGATION OF SOUND

1. Sound is propagated in air, much like blowing up a large balloon which expands equally balloon, which expands equally in all directions. For sound to be generated and heard it must ghave a source, a medium through which to pass and a receiver.

2 Propagation means "movement 2. Propagation means movement through“ Sound will propagate through air and water.through air and water.


All media have three properties which affect the behavior of sound propagation:

A relationship between density and pressure. This relationship, affected by temperature,

i f i idetermines the speed of sound within the medium.

The motion of the medium itself, e.g., wind. Independent of the motion of sound through the medium if the medium is moving the sound is medium, if the medium is moving, the sound is further transported.

Th i i f h di Thi d i The viscosity of the medium. This determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.


TRANSMISSION OF SOUNDTRANSMISSION OF SOUND

There are three requirements for sound to "occur" in an environment:

(1) A vibrating source to initiate sound sound, (2) A medium to transmit sound vibrations throughout the environment and(3) A receiver to hear or record sound vibrations. vibrations. Sound is initiated in an environment by a vibrating source.


Water is a more efficient transmitter of sound compared to air as sound travels faster and further in water.air as sound travels faster and further in water.

The movement of the gas or liquid medium is identical to surface waves found on any large body of water surface waves found on any large body of water.

The wavelengths of speech are of the size of ordinary objects, unlike light, whose wavelengths are extremely small .Because of this, sound does not ordinarily cast "acoustic shadows" but, because its wavelengths are so large, can be shadows but, because its wavelengths are so large, can be transmitted around ordinary objects


For example, if a light is shining on a person, and a book is placed directly between them the person will book is placed directly between them, the person will no longer be able to see the light (a shadow is cast by the book on the eyes of the observer).

However, if one person is speaking to another, then placing a book between them will hardly affect the p g ysounds heard at all; the sound waves are able to go around the book to the observer's ears.

On the other hand, placing a high wall between a highway and houses can greatly decrease the sounds of h ffi i if h di i f h ll (h i h the traffic noises if the dimensions of the wall (height

and length) are large compared with the wavelength of the traffic sounds.


Th i t f th b i

RECEPTION OF SOUND – HEARING MECHANISMS - HUMAN EAR CHARACTERISTICS- MAKING OF SOUND

The ear consists of three basic parts

The outer ear serves to collect and channel sound to the middle ear.

The middle ear serves to transform the energy of a sound wave into the internal vibrations of the bone structure of the of the bone structure of the middle ear and ultimately transform these vibrations into a compression wave in the inner ear.

The inner ear serves to transform the energy of a compression wave within the inner ear fluid into within the inner ear fluid into nerve impulses that can be transmitted to the brain.


Sound waves enter your outer ear andtravel through your ear canal to thetravel through your ear canal to themiddle ear.The ear canal channels the waves toyour eardrum, a thin, sensitive membraneystretched tightly over the entrance to yourmiddle ear. The waves cause your eardrumto vibrate. It passes these vibrations on to thehammer, one of three tiny bones in yourear. The hammer vibrating causes theanvil the small bone touching the

Inside the cochlea,there are hundreds ofspecial cells attached toanvil, the small bone touching the

hammer, to vibrate.The anvil passes these vibrations to thestirrup another small bone which touches

special cells attached tonerve fibers, which cantransmit information tothe brain.stirrup, another small bone which touches

the anvil. From the stirrup, the vibrationspass into the inner ear.The stirrup touches a liquid filled sack

The brain processesthe information from theear and lets usdistinguish betweenp q

and the vibrations travel into the cochlea,which is shaped like a shell.

distinguish betweendifferent types ofsounds.STUDY OF SOUND ‐ ACOUSTICS

HANDLED BY G.YOGAPRIYA

Monaural and Binaural hearing • Monaural‐ Hearing with one ear• Binaural‐Hearing with both the ears.

In open air as well as in enclosures, the hearing is direct and binaural (i.e.with both the ears). This helps to localize sound and obtain a correct idea of the sound perspective.

Binaural hearing could be easily compared to binocular vision. Just as an observer gets an impression of direction and distance or depth with two eyes, similarly his two ears enable him to appreciate the direction of sound and to a certain extent, the distance of sound as well.

It is quite easy to imagine how the two ears enable him to see the direction of sound which is primarily due to the difference in the intensity of the two sounds reaching the two ears It is the depth or intensity of the two sounds reaching the two ears. It is the depth or the perspective of sound.


It may be observed that the reflected sound varies much weakened in intensity as the recede from the speaker Consequently the ratio of in intensity as the recede from the speaker. Consequently the ratio of direct to the reflected sound varies considerably.

It is for this reason that as we go farther from the speaker the direct It is for this reason that as we go farther from the speaker the direct sound is weakened while the reflected or reverberant sound becomes more and more noticeable. It is this factor which permits binaural hearing to appreciate distance or depth.

It is not difficult to appreciate how monaural hearing causes loss of direction. It is common experience to observe that the two ears enable the person consciously to suppress sounds coming from one direction and to concentrate on desired sound from a given direction.

Single ear is unable to do this and consequently the noises and the reverberation present in the room or apparently increased.


Behavior of sound in enclosed placesBehavior of sound in enclosed places


Behavior of sound in enclosed placesBehavior of sound in enclosed places

1. Sound is absorbed in the 5. Sound is emitted by the air also appreciable to reflected sound.

2 S d i fl t d t th

yresonance of the wall in both directions.

6 S d i i t fl t d2. Sound in reflected at the wall surface.

3. Sound is absorbed from

6. Sound is inter‐reflected between bounding surface setting at reverberation.

the wall surface or its surface finished.

4 S d i d d b h

7. Resonance of the enclosed volume of air by direct cross reflection4. Sound is conducted by the

wall to other part of the surface.

reflection.


CHARACTERISTICS OF SPEECH CHARACTERISTICS OF SPEECH

• Any audible sound has three important characteristicscharacteristics1. Frequency(Pitch)

2 Loudness2. Loudness

3. Tonal quality


DEFINITIONS• What is Frequency?q y• The number of cycles per unit of time is called the frequency. For

convenience, frequency is most often measured in cycles perd ( ) th i t h bl H t (H ) (60 60 H )second (cps) or the interchangeable Hertz (Hz) (60 cps = 60 Hz),

named after the 19th physicist.


Characteristics of speech Frequency(Pitch)Frequency(Pitch) It is independent of intensity of loudness of sound. Pitch is a psychological phenomenon determined by Pitch is a psychological phenomenon determined by

frequency of a sound wave whereas frequency is aphysical quantity and can be measured .

Sound is called a pure tone when it consists a singlefrequency and when two or more frequencies arepresent it is called a complex tone.present it is called a complex tone.


Pure tone complex tone

Characteristics of speechCharacteristics of speech Loudness:– Loudness is a physiological effect of sensation produced

through the ear and depends on the intensity of sound or amount of energy present in sound waves whileor amount of energy present in sound waves while entering it.

– In oat , Loudness falls down as distance increases“Loudness is inversely proportional the the square of thedistance from source “(Not applicable for semi closedenclosures)


Tonal quality or timbre

Timbre is a general term for the distinguishablecharacteristics of a tone. Timbre is mainly determinedby the harmonic content of a sound.

timbre is what makes a particular musical sound timbre is what makes a particular musical sounddifferent from another, even when they have thesame pitch and loudness.


DEFINITIONSDEFINITIONS

• What is wavelength?g• The wavelength of sound is the

distance between analogouspoints of two successive waves.p

λ = c / fwhereλ = wavelength (m)c = speed of sound (m/s)f = frequency


DEFINITIONS• Sound Intensity.

– The sound intensity in a specified direction is the DEFINITIONSamount of sound energy flowing through a unit area normal to that direction. The sound intensity is normally measured in watt per square metreis normally measured in watt per square metre(W/m2).

– The scale for measuring intensity is the decibel scale.


DEFINITIONSDEFINITIONS

• Sound pressurepSound pressure or acoustic pressure is

the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure(average, or equilibrium) atmospheric pressure caused by a sound wave.

Sound pressure can be measured usingSound pressure can be measured using a microphone in air and a hydrophone in water. The SI unit for sound pressure p is the Pascal.


HIGH FIDELITY REPRODUCTION OF MUSIC

High fidelity—or hi‐fi—reproduction is a term d b h li d hused by home stereo listeners and home

audio enthusiasts (audiophiles) to refer to high‐li d i f d [ iquality reproduction of sound [ or images, to

distinguish it from the poorer quality sound d d b i i di iproduced by inexpensive audio equipment.

Id ll hi h fid li i h i i lIdeally, high‐fidelity equipment has minimal amounts of noise and distortion and an

t faccurate frequency response.STUDY OF SOUND ‐ ACOUSTICS HANDLED BY G.YOGAPRIYA

Frequency RangesFrequency Ranges


THRESHOLD OF AUDIBILITYTHRESHOLD OF AUDIBILITYThe threshold of audibility at any specified frequency isthe minimum value of sound pressure of a pure tone ofthat frequency which is just audible The term is used forcomplex waves such as speech and music . It ismeasures in dynes per sq cm ( dynes – cgs‐force)measures in dynes per sq cm. ( dynes cgs‐force)

THRESHOLD OF FEELINGSound pressure that can cause discomfort and pain. It issituated around 120 dB above the threshold of hearing. It ismeasures in dynes per sq cm and also in watts per sq cm.measures in dynes per sq cm and also in watts per sq cm.


PHONA unit of apparent loudness equal in number to theA unit of apparent loudness, equal in number to theintensity in decibels of a 1,000‐hertz tone perceived tobe as loud as the sound being measured.

EFFECT OF REVEBRATION ON HEARINGThe hang over effect of one syllable on the nextg ydistorts the speech by blending the end of the firstwith the beginning of the second so as to mask the

d ll bl l t l Th b tisecond syllable completely. The reverberationcharacteristics may give an effect of frequencydistortion because of reverberation time beingdifferent for various frequencies. The effect of both willdepend upon position of the observer .


part 1

Technology

sound energy

measurement of sound

sound pressure

effects of sound

sound source

persistence of sound

reverberant sound

sound wasto