2.behaviour of sound

8/4/2019 2.Behaviour of Sound

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ITS BEHAVIOUR

SOUND

PRESENTED BY:-

SUBHRANSHU PANDA

SUKRUTI PHATAK

T. PRATIMA

RUCHI SHARDUL

PUSHPENDRA SAHU

AVINASH PATRA

RASHMI SAI

SARANSH SHRIVASTAVA


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A BRIEF ON THE

BEHAVIOUR OF SOUND We'll be taking a look at the way sound behaves when it interacts with

obstacles on its path. Generally the nature of these interactions depend asmuch on the material the obstacle and its dimensions as the soundfrequency content does.

The behaviors we'll be looking at mostly relate to waves in general butwe'll concentrate on sound waves. These are:

Reflection

Diffraction

Refraction

Absorption

Transmission

Echo Reverberation

Resonance

Insulation


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Reflection A wave hitting a flat surface with an incidence

angle of (between the normal to the surfaceand the direction of the wave) is reflected at a

reflection angle of degrees. In the figure we

observe an example of a flat surface and then a

concave surface in which all the reflected rays

converge in the focus point of the curved surface.


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Dependency on the shape & form

Concave surfaces are avoided in acoustics as theytend to focus sound in a precise point creating badsound distribution. However they are used for theconstruction of directional microphones, as theyallow signals (very weak ones too) to be picked up.

While, convex surfaces diffuse sound and are thusgreatly used to improve the acoustics ofenvironments.

When a wave reflects off a convex surface, thereflected wave's virtual extension passes through thesurface's focus point.


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Reflections inside a room

When a sound gets diffused in a room, it reaches thelistener in different ways. The first signal that reaches

the listener is the strongest one and is the direct one,in other words, the signal that has taken the shortestroute between the source of the sound and thelistener. After the direct signal, arrive the signals thathave undergone one reflection only, and that have

therefore a smaller amplitude compared to the directsignal.


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This is because of the loss of energy

that occurs with reflection. We call

these signals earlyreflections ("precocious sound").

After a further delay come all the

signals that have undergone more

than one reflection, with an

amplitude that is yet inferior to the

early reflections. These are

called reverb cluster, taking their

name from the fact that they are not

considered individually but as asingle body. The figure below shows

us the distribution of these signals in

time, and their amplitudes


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REFRACTION


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Refraction

This term refers to the phenomenon by which a wave thatcrosses two media of different densities changes directionas it passes from one to the other. This behavior is easilyexplainable if we recall what we said about the speed ofsound in media of different densities.

We know that sound travels faster in denser media.

Walls have a greater density than air and therefore wavefronts that begin to penetrate the wall are faster than thosethat are still outside it. So, as it enters the wall the verysame wave front has a faster part (the one already insidethe wall) and a slower part (the one still outside the wall).

When the whole wave front has fully penetrated the wall,its direction has changed angle. Exiting the wall the samephenomenon occurs but inversely, and the wave returns toits original direction.


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Open-air refraction

In the morning the upper layer (cold air) has a greater densitycompared to the lower layer (warm air) and so sound tends tomove upwards .

In the evening we have the opposite situation and the denser layer(cold air) becomes the inferior one. This causes sound to movedownwards. This has to be taken into careful consideration whenorganizing an open-air concert [Live sound ] seeing that the longsetup process takes place many hours before the concert begins

and therefore the atmospheric conditions will inevitably change bythe time it starts.


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DIFFRACTION


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Diffraction

The best and the most direct way to describe thisphenomenon is to say that it takes place when a soundcircumvents an obstacle. This greatly depends uponfrequency content seeing that sounds with a greatwavelength (and thus a low frequency) easily overrideobstacles that are smaller than the sound's wavelength.

This is one of the reasons why the first frequencies to beattenuated are the high ones whilst low ones are diffusedover far longer distances.


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ABSORPTION


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Absorption

Absorption can be described as the conversion of

acoustic energy into thermal energy by a surface.

In other words, when a sound comes across an

obstacle, it transfers energy to it which is thendissipated as heat.

Generally speaking these four phenomena are all

present when sound meets an obstacle. Thefollowing figure illustrates a typical situation:


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Reflection, diffusion, refraction and

absorption together


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TRANMISSON


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Transmission Acoustic transmission in building design refers to a number of

processes by which sound can be transferred from one part ofa building to another. Typically these are:

AIRBORNE TRANSMISSION IMPACT TRANSMISSION

FLANKING TRANSMISSION


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Airborne transmission - a noise source in one room sends airpressure waves which induce vibration to one side of a wall orelement of structure setting it moving such that the other face ofthe wall vibrates in an adjacent room. Structural isolation thereforebecomes an important consideration in the acoustic design of

buildings. Highly sensitive areas of buildings, for example recordingstudios, may be almost entirely isolated from the rest of a structure.Air tightness also becomes an important control technique. Atightly sealed door might have reasonable sound reductionproperties, but if it is left open only a few millimeters itseffectiveness is reduced to practically nothing. The most important

acoustic control method is adding mass into the structure, such as aheavy dividing wall, which will usually reduce airborne soundtransmission better than a light one.


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Impact transmission - a noise source in one roomresults from an impact of an object onto a separatingsurface, such as a floor and transmits the sound to an

adjacent room. A typical example would be the soundof footsteps in a room being heard in a room below.Acoustic control measures usually include attempts toisolate the source of the impact, or cushioning it. For

example carpets will perform significantly better thanhard floors.

l ki i i l f


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Flanking transmission - a more complex formof noise transmission, where the resultantvibrations from a noise source are transmitted

to other rooms of the building usually byelements of structure within the building. Forexample, in a steel framed building, once theframe itself is set into motion the effective

transmission can be pronounced.


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SECTION PLAN


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Sound Transmission Coefficient (a) Sound transmission coefficient (T):

It is the ratio which the sound energy of a given frequency transmittedthrough a surface to that incident on it.

(b) Sound Reduction Index (SRI):

For composite partitions of n nos. of surface parts, the average transmission

coefficient TAV can be found from the following equation:

where Ti = transmission coefficients of the ith part

Ai = area of the ith part

A = total area of partition =


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Transmission of Sound across Medium Boundaries When an acoustic wave travelling in one medium encounters the

boundary of a second medium, reflected and transmitted waves are

generated. For example, when sound strikes upon a solid partition, part isreflected, part absorbed within the material, and part transmitted to the

other side or to elsewhere in the building.


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Distribution of Energy from Air-borne Sound Striking a Partition

The ratios of the pressure amplitudes and intensities of the reflected and

transmitted waves to those of the incident waves depend on the following

factors:

(a) In angle of incidence, q ,

(b) The densities of the two media, and

(c) The speeds of sound in the two media.

The sound transmission properties of a single leaf solid partition can be

divided into three distinct regions


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Coincidence Effect coincidence effecthas the following characteristics.

(i) The problem is not confined to a single frequency.

(ii) The lowest frequency that the problem occurs is when the velocity of the

bending wave equals the velocity of sound in air. This is called the critical

frequencyand the sound wave is at 0 angle of incidence.

(iii) Above the critical frequency, transmission is dominated by coincidence.


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ECHO

E h


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Echo An echo is a reflection of sound, arriving at the listener some time

after the direct sound. Typical examples are the echo produced by

the bottom of a well, by a building, or by the walls of an enclosedroom.

A true echo is a single reflection of the sound wave.

The strength of an echo is measured in dB . Echoes may be desirable

(as in sonar) or undesirable (as in telephone systems).


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REVERBERATION


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Reverberation

If so many reflections arrive at a listener that they are unable to

distinguish between them, the proper term is reverberation. An echo canbe explained as a wave that has been reflected by a discontinuity in thepropagation medium, and returns with sufficient magnitude and delay to beperceived.

When dealing with audible frequencies, the human ear cannot distinguishan echo from the original sound if the delay is less than 1/10 of a second.Thus, since the velocity of sound is approximately 343 m/s at a normalroom temperature of about 20C.

Sound travels approximately 343 meters/s (1100 ft/s). the sound takes halfthe time to get to the object and half the time to return. The distance for anobject with a 2-second echo return would be 1 sec X 343 meters/s .In most

situations with human hearing, echoes are about one-half second or abouthalf this distance, since sounds grow fainter with distance.


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ECHO Vs REVERBERATION

Both echo and reverberation is different thing.Or we can say that reverberation is need while

echo doesn't.


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Sound is a mechanical wave which travels through a

medium from one location to another. This motion through

a medium occurs as one particle of the medium interacts

with its neighboring particle, transmitting the mechanicalmotion and corresponding energy to it. This transport of

mechanical energy through a medium by particle

interaction is what makes a sound wave a mechanical wave.

ECHO

TECHNICAL SPECIFICATION


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As a sound wave reaches the end of its medium, it undergoes certain characteristic

behaviors. Whether the end of the medium is of wall or canyon cliff, there is likely to

be some transmission/refraction, reflection and/or diffraction occurring.

Reflection of sound waves off of barriers result in some observable behaviors whichyou have likely experienced. If you have ever been inside of a large canyon, you have

likely observed an echo resulting from the reflection of sound waves off the canyon

walls. Suppose you are in a canyon and you give a holler. Shortly after the holler, you

would hear the echo of the holler- a faint sound resembling the original sound. This

echo results from the reflection of sound off the distant canyon walls and its ultimatereturn to your ear.

If the canyon wall is more than approximately 17 meters away from where you are

standing, then the sound wave will take more than 0.1 seconds to reflect and return

to you. Since the perception of a sound usually endures in memory for only 0.1seconds, there will be a small time delay between the perception of the original

sound and the perception of the reflected sound. Thus, we call the perception of the

reflected sound wave an echo.


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Left: Reflections that reach the ear after 40ms areperceived as a distinct echo (delay). Right:Reflections that reach the ear within 40ms areperceived as richness and warmth (reverberation).


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A reverberation is quite different

than an echo. The distinction

between an echo and areverberation is depicted in the

animation at the left.

A reverberation is perceivedwhen the reflected sound wave

reaches your ear in less than 0.1

second after the original sound

wave. Since the original sound

wave is still held in memory,

there is no time delay between

the perception of the reflected

sound wave and the original

sound wave.

REVERBERATION


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The two sound waves tend to combine as one very prolonged sound

wave. If you have ever sung in the shower (and we know that you

have), then you have probably experienced a reverberation. The

Pavarotti-like sound which you hear is the result of the reflection of

the sounds you create combining with the original sounds. Because

the shower walls are typically less than 17 meters away, these

reflected sound waves combine with your original sound waves tocreate a prolonged sound - a reverberation.


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INSULATION


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Sound insulation Sound insulation is the process of soundproofing an enclosed space, such

as a room. This type of insulating activity is usually employed when there

is a need to keep sound from filtering into or out of the space. Soundinsulation techniques are often used in business settings, as well as inmulti-family dwellings like duplexes and apartment buildings.

One example of how sound insulation is used is found in a recordingstudio. In order to prevent background noise from interfering with therecording process, singers and musicians create their vocal and

instrumental tracks in a soundproof recording booth. Because the boothprohibits the introduction of sounds from outside the space, there isnothing present to distort or interfere with the quality of the recording.The audio tracks containing vocal performances and the various musictracks are captured exactly as the performers hear them.

In living space, sound insulation normally involves the installation ofinsulation in walls, under floors and above ceilings. This can be especially

important in apartment buildings and other structures where people livein close proximity. The inclusion of the insulation between apartments tothe side, above, and below helps to ensure all the residents enjoy ameasure of peace and quiet, even when others in the building are playingmusic or having a party.


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It is important to avoid confusion between soundabsorption and sound insulation.

(a) Sound absorption is the prevention of reflection ofsound or alternatively, a reduction in the sound energy

reflected by the surfaces of a room.

(b) Sound insulation is the prevention of transmissionof sound or alternatively, a reduction of sound energytransmitted into an adjoining air space.

Two types of sound insulation are to be dealt with in building


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Two types of sound insulation are to be dealt with in buildingconstruction, as illustrated in Figure.

(a)Airborne Sound Insulation : the insulation against noise originating inair, e.g. voices, music, motor traffic, wind.

(b) Impact Sound Insulation : the insulation against noise originating

directly on a structure by blows or vibration e.g. footsteps above, furniturebeing moved, drilling and hammering the structure.


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Airborne Sound Insulation by Partition

Airborne sound can be transmitted in a receiving room via some or all of the paths (A) to (D) asshown in Figure. It is called thedirect path.

All transmission paths other than path (A) are together termed the indirectorflankingtransmission. This indirect transmission becomes increasingly important when the insulationrequirement of the separating partition is about 35 dB upwards.

The ideal material for good sound insulation has a very high mass and low stiffness but some of themost convenient building material have low mass and relatively high stiffness.

Impact Sound Insulation

Insulation against structure-borne (or impact) noise can be achieved by the use of :

(a) Soft floor finish (carpet, cork, vinyl, rubber, etc.),

(b) Resiliently suspended solid ceiling,

(c) Resilient (anti vibration) mounts, and

(d) Floating floor.

Room to room insulation


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Room-to-room insulation In many cases a continuous suspended ceiling is chosen in order to

achieve maximum flexibility. However these constructions will give lowersound insulation compared to constructions where the partitions areallowed to penetrate the suspended ceiling or reach all the way up to thesoffit.

If partitions do not reach the structural soffit a horizontal transmissionpath for the sound via the void over the suspended ceiling is created.Therefore, traditional acoustic ceilings often provide insufficient soundinsulation. In these cases special acoustic ceiling systems are requiredwhich offer additional sound insulating properties.


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Laboratory value/site value

In practice on site, the room-to-room sound insulation (Rw) can beestimated to be 5-8 dB lower than the lowest value in laboratory forthe suspended ceiling and partition respectively. This is due to thefact that interaction between the suspended ceiling and thepartition considerably reduces sound insulation.

Also flanking transmission might occur and some installation details

might not be perfect.

Room-to-room sound insulation values

Site result will be 5-8 dB lower than laboratory

value.

One way sound insulation


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One-way sound insulation Installations in the void between the structural soffit and the suspended

ceiling, such as piping and ductwork systems, can give rise to noise. Insuch cases, a sound insulating suspended ceiling system can be used toreduce the noise to acceptable levels in the room below.

One-way sound insulation values

In order to determine the noise level generated by a source in the void,you should ascertain the frequency spectra of the suspended ceiling'ssound insulation and the noise source.


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Vertical airborne sound insulation The airborne sound insulation of a floor structure can be improved by means of a

suspended ceiling system. Airborne sound insulation may relate to soundsgenerated in both the room below and the room above. Improvements are always

linked to a specific type of floor structure.

Vertical airborne sound insulation values

The improvements are linked to a homogenous concrete floor of normal thickness(160-200 mm) and with a plastic carpet.


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Impact sound insulation

Impact sound insulation relates to the reduction of footstep sound frompeople walking on a floor structure. It is determined by the impact soundlevel in the room below. A suspended ceiling system can be used to

improve the impact sound insulation and therefore reduce the impactsound level. Improvements are always linked to a specific type of floorstructure.

Impact sound level values

The improvements are linked to a homogenous concrete floor of normalthickness (160-200 mm) and with a plastic carpet.


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THANK YOU

2.behaviour of sound

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