architectural acoustics123 (2)

8/13/2019 Architectural Acoustics123 (2)

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ARCHITECTURAL ACOUSTICS

Arch i tectural acous t icsis the science ofcontro l l ing sound w i th in bui ld ings. The fi rs tapp l icat ion of archi tectura l acoust ics was in

the design of opera housesand then concer thalls.

Archi tectura l acoust icsinc ludes room

acoust ics, the design of record ing andbroadcast studios, home theaters, andl is ten ing rooms fo r media playback.


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BASIC ACOUSTICAL CONCEPT

Audible range:The normal ear in young adults detects sound having

frequencies in the region 20Hz to 20kHz, although it is possible forsome people to detect frequencies outside these limits.

The musical scale is logarithmic and that the highest note on a piano isabout 4kHz. The lowest note(27.5Hz) comes close to the lower limit of

hearing.

Acoustic shadow:Screening or barriers (of walls, earth mounds,vegetation, etc.) can create an Acoustical Shadow if the sound is ofhigh frequency.

Sound shadow:A phenomenon caused by the ABSORPTION orOBSTRUCTION of a SOUND WAVE by an object in its path. The effectproduced is perceived as a reduction in LOUDNESS depending on theobservers position with respect to the sound source and obstructingobject and is greatest when the three are aligned.


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SOUND PROPAGATION

Sound propagates through air as a

longitudinal wave. The speed of sound

is determined by the properties of the

air, and not by the frequency or

amplitude of the sound. Sound waves,

as well as most other types of waves,

can be described in terms of thefollowing basic wave phenomena.


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SOUND PROPAGATION


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BEHAVIOUR OF SOUND

Boundary Behavior:As a sound wave travels through a

medium, it will often reach the end of the medium andencounter an obstacle or perhaps another medium throughwhich it could travel. When one medium ends, another mediumbegins; the interface of the two media is referred to as theboundary and the behaviour of a wave at that boundary is

described as its boundary behaviour. There are essentially fourpossible behaviours which a wave could exhibit at a boundary:

Reflection (the bouncing off of the boundary),

Diffraction (the bending around the obstacle without crossingover the boundary),

Transmission (the crossing of the boundary into the new

material or obstacle), and Refraction (occurs along with transmission and is

characterized by the subsequent change in speed anddirection).


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MECHANICS OF SOUND

TRANSMISSIONWhat is sound and what is a sound wave?

Sound is the movement of air particles created by a vibrating source.

Air particles are in constant random motion, exerting very small pressure variations

around the steady-state atmospheric pressure.

Each particle is subject to both an inertial force (due to its mass and acceleration) and a

force which tends to restore the particle to its resting position (due to the elasticity of the

medium).When an object - a sound source - is set into vibration, each air particle moves to and fro

about its average position along an axis parallel to the direction in which the wave

propagates.

Air particles themselves do no move very far, they simply transfer pressure changes by

what is referred to as sound propagation.

This constitutes what we call a 'sound wave' which moves away from the sound source at a

velocity determined by the medium.The velocity of propagation of a sound wave in air is about 344 meters per second, while in

water it is 1437 m/s.


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Sound waves move out spherically from a point source of sound, and as

they do so they become less intense.

Sound pressure is inversely proportional to distance from the source as

long as the sound does not encounter obstacles, like the head and external

ears for example.

Obstacles, which create a change in the medium, impede or resist the

propagation of sound.

When a sound waved encounters an obstacle or change in medium, a

portion of the sound wave is reflected from the surface.

That portion of a sound wave not reflected from an obstacle is absorbed

and continues to be propagated through the new medium.

Reflectance of a sound is at the heart of our understanding of the action ofthe middle ear, whose purpose is to overcome the impedance mismatch at

the interface of air and fluid of the inner ear.

Reflected sound may encounter the original sound wave and, depending on

the relative timing of the two, they may either reinforce or cancel one

another.

Sound waves may also be diffracted, which means that, depending on the

frequency of the sound, they are able to wrap around small or medium-sizeobjects.

Reflectance and diffraction are two principle ways that sound waves are

altered by the head.


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REFLECTION OF SOUND WAVE

The amount of energy which becomes reflected isdependent upon the dissimilarity of the two medium.The more similar that the two medium on each sideof the boundary are, the less reflection which occursand the more transmission which occurs.

Conversely, the less similar that the two medium oneach side of the boundary are, the more reflectionwhich occurs and the less transmission whichoccurs. So if a heavy rope is attached to a light rope(two very dissimilar medium), little transmission and

mostly reflection occurs. And if a heavy rope isattached to another heavy rope (two very similarmedium), little reflection and mostly transmissionoccurs


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Reflection of sound waves off of surfaces can lead toone of two phenomenon - an echo or a reverberation.

But reflection of sound waves in auditoriums and

concert halls do not always lead to displeasingresults, especially if the reflections are designedr ight. Smooth walls have a tendency to direct soundwaves in a specific direction. Rough walls tend todiffuse sound, reflecting it in a variety of directions.This allows a spectator to perceive sounds from

every part of the room, making it seem lively and full.For this reason, auditorium and concert halldesigners prefer construction materials which arerough rather than smooth.


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DIFFRACTION OF SOUND

WAVES Diffraction involves a change in direction of

waves as they pass through an opening oraround a barrier in their path.

The amount of diffraction (the sharpness of

the bending) increases with increasingwavelength and decreases with decreasingwavelength.

Diffraction of sound waves is commonly

observed; we notice sound diffractingaround corners or through door openings,allowing us to hear others who are speakingto us from adjacent rooms.


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REFRACTION OF SOUND

WAVESRefractionof waves involves a change in thedirection of waves as they pass from one medium toanother. Refraction, or bending of the path of thewaves, is accompanied by a change in speed and

wavelength of the waves. So if the medium (and itsproperties) are changed, the speed of the waves arechanged. Thus, waves passing from one medium toanother will undergo refraction. Refraction of soundwaves is most evident in situations in which the

sound wave passes through a medium withgradually varying properties.


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Diffraction is the change in the direction of the propagation of

sound waves passing the edge of the obstacle as illustrated in the

following figure: Diffraction

phenomenon dependssignificantly on theratio of the wavelength

of the sound to the sizeof the obstacle. Thelonger the wavelengththe stronger the sounddiffraction. Diffraction

effect happens to thesound transmittedthrough openings aswell.


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PROPOGATION OF SOUND

WAVES SOUND DIFFUSION:Ideally a diffuse sound field is one in which the

sound level is of similar quality everywhere. It is one of the majorcharacteristic of many acoustically designed spaces for hearing.

The comparative distribution of SOUND PRESSUREvariationsthroughout a given space or the process by which a SOUND WAVE is

distributed in the space. If sound pressure is uniformly distributedthroughout the space, the sound is said to be well diffused.

SOUND DIFFUSION FIELD:Repeated REFLECTIONsandDIFFRACTIONsof sound within a space result in good DIFFUSION anda uniform distribution of sound energy. A diffuse sound field istypically created in gymnasia, swimming pools and interior spaces

with marble, concrete or glass walls. But it also can occur outdoorswith sounds coming from many directions, such as in urban streetslined with high-rise buildings.


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SURFACE EFFECTS

1. GROUND ABSORPTION:If sound is propagating over ground,ATTENTUATION will occur due to acoustics energy losses onreflection. These losses will depend on the surface. Smooth, hardsurfaces will produce little ABSORPTION whereas thick grass mayresult in sound levels being reduced by up to about 10dB per100meters at 2000Hz. High frequencies are generally attenuationmore than low frequencies.

2. ATTENUATION DUE TO BARRIERS AND TREES:A band of treesseveral hundreds of feet deep is required in order to achievesignificant attenuation. Significant attenuation can be achieved bythe use of solid barriers. A barriers should be at least high enoughto obscure the line of sight between the noise source andreceiver. A barrier is most effective for high frequencies since lowfrequencies are diffracted around the edge of a barrier more easily.

The maximum performance of a barrier is limited to about 40dB, dueto scattering by the atmosphere. A barrier is most effective whenplaced either very close to the source or to the receiver.


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BUILDINGS INTERNAL

ACOUSTICS

Positioning or orienting the major

openings away from the noise source.

Buildings can be planned so as to

accommodate screens (such as noisebreaking projections, wing walls, etc)

for noise protection.


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MAJOR SOURCES OF

ABSORPTION/REFLECTION IN A

ROOM ARE:CEILING:Major sound surface in many rooms. As room size increases so the ceiling increasesin importance.

However, ceiling tiles do not provide a uniform surface e.g. joints between tiles and alsothere are light fittings either recessed or suspended. Parabolic, deep cell diffusers are thebest for sound absorption. Sometimes suspended ceiling baffles in a check board patternare used.

WALLS:These are usually the next most important surface. Their importance increases as

room size decreases. Typically, walls have very poor sound absorbing qualities and this isoften made worse by putting sound reflectors against the walls e.g. filing cabinets.Carpeting the walls will increase sound absorption.

FLOOR:Carpeting the floor will increase the NRC, but only up to about 0.3. Moving to thickercarpeting is often not a cost-effective solution because much of the floor area is coveredby furniture with a worse NRC. Carpeting will reduce impact noise.

FURNITURE:Most furniture is designed to be functional or aesthetically or aesthetic pleasingrather than to be a good sound absorber. In many types of offices, screening has beenpurchased in an effort to improve visual and acoustical privacy. To have any impact at allthe screening needs to be at least 5 feet high and one needs solid mass in this screening toblock sound energy.


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ABSORPTION AND

ABSORPTION COEFFICIENT Absorption:The properties of a material composition to

convert sound energy into heat thereby reducing the amount ofsound energy that can be reflected.

Sound Absorption Coefficient:The fraction of energy striking amaterial or object that is not reflected. For instance, if amaterial reflects70% of the sound energy incident upon itssurface, then its Sound Absorption Coefficient would be 0.30.SAC=absorption/area in sabins per sq. ft.

The absorption coefficient ? is a property of a material. Itdefines the extent to which a material absorbs energy.

Also known as absorption factor; absorption ratio; coefficientof absorption.(acoust ics) The ratio of the sound energy absorbed by asurface of a medium or material to the sound energy incidenton the surface.


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SOUND ABSORPTION

Sound absorption is defined,as the incident sound thatstrikes a material that is notreflected back. An openwindow is an excellent

absorber since the soundspassing through the openwindow are not reflectedback but makes a poorsound barrier. Paintedconcrete block is a good

sound barrier but will reflectabout 97% if the incidentsound striking it.


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RESONANCE

The act of resounding.

A prolongation or increase of any sound,

either by reflection, as in a cavern orapartment the walls of which are not distant

enough to return a distinct echo, or by the

production ofvibrationsin other bodies, as a

sounding-board, or the bodies of musical

instruments.


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RESONATORS

A resonator is a device or system that

exhibits resonanceor resonant

behavior. Resonators are used to either

generate waves of specific frequenciesor to select specific frequencies from a

signal. Musical instruments use

acousticresonators that producesound waves of specific tones.


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SOUND ABSORBING

MATERIALS The materials used for reducing the reflection of sound waves in a

room or hall are known as sound absorbing materials. These materialsmay be in the form of perforated boards, porous materials, heavycurtains, maps, pictures put upon the walls, carpets on the floor etc.

These materials are suitably applied to the walls, ceiling and floor of ahall or a room to reduce reflection of sound waves from their surfaces.

Sound absorbing materials are porous, inelastically flexible,compressible or they may be having combination of two or more ofthese properties.

The materials having porous texture include porous plaster, glass silk,porous plastics and heavy fabric materials. Such materials, in suitablethickness, are used for absorbing sound waves of high frequencies.

Other materials like fibre boards, plywood, oil cloth, paper, etc. can

also be used for sound absorbing. These materials are fixed at somedistance from the solid walls. In such cases, the space between themembranes and the surface of solid walls is often filled with a porousmaterial to provide better sound absorbing effect.


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Broadly speaking, the material having hard, rigidand non-porous surface, provide the leastabsorption, whereas those which are flexible, soft,porous and can vibrate, absorb more sound. Theefficiency of the sound absorption, however,depends upon the porosity of the material usedas sound absorbent. The term used to express thepercentage of the incident sound that can beabsorbed by a material is known as absorption

coefficient of the material. Thus if the absorptionco-efficient is 0.75, this would mean that thematerial is capable of absorbing 75% of theincident sound. The absorption co-efficient differswith the frequency of the incident sound. Ingeneral, low density materials have higherabsorption co-efficient at the higher frequenciesthan at low frequencies.


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REQUIREMENTS OF A GOOD

SOUND ABSORBING MATERIALS

A good sound absorbing material should fulfillthe following requirements:

1. It should have sound absorbing efficiency.

2. It should be cheap.

3. It should be easy to fix.

4. It must be durable.

5. It should have good resistance to fire.

6. It should not be subjected to decay.7. It should be light in weight.

8. It should be good in appearance.


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CLASSIFICATION OF

ABSOBENTS

Sound absorbents can be broadly

classified into the following four

categories:

1. Porous absorbents.

2. Resonant absorbents.

3. Cavity resonators.

4. Composite type of absorbents.


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1. POROUS ABSORBENTS

When sound waves strike the surface of porousmaterial, a part of the wave gets reflected while apart enters the pores of the material and is thoughtto be dissipated into heat energy (produced onaccount of the friction developed between the sound

waves in motion in the restricted pores of thematerial). The efficiency of this type of absorbentincreases with the increase in the resistance offeredby the material to air flow, its thickness and theporosity. Slagwool, glasswool, woodwool, asbestosfibre spray, foamed plastic and perforated fibreboards are some of the categories or porousabsorbents. In general, porous materials areselected to absorb sound having high frequency.


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2. RESONANT ABSORBENTS

In this system, the absorbent material is fixed onsound framing (usually timber) with an air space leftout between the framing and the wall at the back.Such an arrangement works most efficient forabsorbing sound waves at low frequency. The

principle of sound absorption in this method is thatsound waves of the appropriate frequency causesympathetic vibration in the panel which acts as adiaphragm. The absorption of sound takes place byvirtue of the dampening of the sympathetic vibrationin the panel by means of the air space behind thepanel. Dampening effect of this system can beimproved appreciably by placing a porous materialin the space.


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3. CAVITY RESONATORS

Cavity resonators essentially consist of a

chamber with a narrow opening (entrance) in

which absorption takes place by resonance

of the air in the chamber which gives loss ofsound energy. Such as arrangement can act

effectively over a single selected frequency.

Application of cavity resonator is normally

restricted to absorption from individualmachine or in similar cases.


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WALL CONSTRUCTION

The sound insulation rating of a wall is generally governed bythe net sound transmission loss it provides and also theefficiency with which it serves as a barrier for speed sound.Weight of the wall is the governing factor in wall insulation. It isseen that a solid one brick thick wall plastered on both sides,proves quiet effective as a sound insulation partition wall. It

has an average reduction of 50dB. It is now, however, possibleto have wall made from a suitable combination of materialswhich are light in weight and yet have high insulation value.

A cavity wall type of construction can be made to haveincreased insulation value by filling the cavity with some

resilient material. In this type of construction, the cavity shouldbe at least 5cm in width and the two wall leaves should be tiedby use of only light butterfly wall ties.


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FLOORS

Transmission of sound takes place more easily through floors.Invariably, sound producing source has actual contact with the floor.Hence, the floor serves as the most common path for thetransmission of impact noise. The ordinary RCC floor weighing lessthan 220kg/sq.m. has a sound reduction of only 45dB. Thus bareconcrete and timber floors do not function effectively as barrieragainst sound. A floating floor resting on a resilient material like

glass wool, mineral wool, quilt, cork, rubber, etc., has an increasedrating for impact sound insulation. There are various types of floatingfloors:

1. Wood raft floating floor: it consists of 50mm*50mm wide woodenbattens, on which 20mm thick resilient quilt is laid over the structuralfloor slab.

2. Concrete floating floor: It consists of a 70mm thick layer of 1:1.5:3concrete screed laid on a 25mm thick resilient layer of mineral woolquilt. The quilt is covered with water proof paper to prevent themoisture from concrete screed traveling below on the structural floorslab.


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DIFFERENT METHODS OF SOUND

INSULATION OF FLOORS


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DIFFERENT METHODS OF SOUND

INSULATION OF TIMBER FLOORS


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NOISE MITIGATION

Noise mitigation is a set of strategies to reducenoise pollution. The main areas of noise mitigationor abatement, are: transportationnoise control,architecturaldesign, and occupational control.

Multiple techniques have been developed to addressinterior sound levels. These techniques includedesign of exterior walls, party walls and floor/ceilingassemblies; Many of these techniques rely upon

materials science applications of constructing soundbafflesor using sound absorbing liners for interiorspaces.


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Principles of Noise Mitigation

A noise problem starts with a noise sourcesuch as a stream of traffic on a highway. Thenoise is transmitted through a path and thenarrives at the receiver. The noise will be

perceived as a problem when the noise is sohigh as to be a nuisance to the receiver.

The severity of the problem depends on thestrength of the noise source (such as heavy

or light traffic) or the length of the path, thatis, how large is the separation between thenoise source and the receiver.

To reduce environmental noise one can


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To reduce environmental noise, one can

consider the following methods:

(a) Control at noise sourcesIt is often a primary consideration to reduce noise at its source.

Whenever possible, quieter working methods or technologies should be used.

Enclosing the noise source can also be used to serve both acoustic and other purposes. A noiseenclosure for reducing machine noise is commonly made of an exterior metal skin, an interior perforatedsheet, with some absorptive materials such as fiberglass filled in between.

(b) Noise reduction at the transmission path

An obvious way of reducing noise is to separate the sources of noise from noise sensitive uses. This ishowever often not practical in a compact and high-rise city to rely only on distance attenuation to cutdown the noise such as in the case of tackling road traffic noise. Additional attenuation, which can beprovided through screening by natural landscape (such as earth bunds), structures of noise tolerant uses(such as carpark, commercial blocks or acoustic-insulated office buildings), purposely built podiumdecking, noise barriers or enclosures are often employed. Proper land use planning to avoid busyhighways cutting across residential developments or coming too close to sensitive uses; locating noisetolerant uses to screen noise sensitive developments, and a combination of the different noiseattenuation means can often pre-empt noise problems at the design stage. Options to avoid or minimizenoise, say, through adopting alternative transport such as railway, pedestrian link, cycling path,underground roads can also be considered at the early planning stage.

(c) Protection at the receiver endBy arranging noise sensitive uses such as bedrooms facing away from the noise sources, the impact ofnoise on the receiver can be reduced.

While acoustic insulation by good glazing can cut down noise, its application for residential buildingspractically deprives the receiver of an "open-window" life style and requires the provision of air-conditioning due to the warm and humid climate in Hong Kong. As such, it is often used as last resortonly.


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SOUND BAFFLE

A SOUND BAFFLEis a construction or device which reducesthe intensity of air borne sound. Sound baffles are afundamental tool of noise mitigation, the practice of minimizingnoise pollution or reverberation. An important type of soundbaffle is the noise barrier constructed along highwaystoreduce sound levels at properties in the vicinity. Sound baffles

are also applied to walls and ceilings in building interiors toabsorbsound energy and thus lessen reverberation .

Baffle: A free hanging acoustical sound absorbing unit.Normally suspended vertically in a variety of patterns tointroduce absorption into a space to reduce reverberation and

noise levels. A muffler (or silencer in British English) is a device for reducing

the amount of noiseemitted by a machine.
http://en.wikipedia.org/wiki/Noise_(environmental)http://en.wikipedia.org/wiki/Noise_(environmental)


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SOUND BARRIER

Sound Barrier:A material that whenplaced around a source of noiseinhibits the transmission of that noise

beyond the barrier. Also, anythingphysical or an environment thatinterferes with communication orlistening. For example, a poor

acoustical environment can be a barrierto good listening and especially so forpersons with a hearing impairment


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NOISE REDUCTION

COEFFICIENT(NRC)

Noise Reduction (NR):The amount of noisethat is reduced through the introduction ofsound absorbing materials. The level (indecibels) of sound reduced on a logarithmic

basis. The Noise Reduction Coefficient (NRC)is a

scalar representation of the amount of soundenergyabsorbed upon striking a particular

surface. An NRC of 0 indicates perfectreflection; an NRC of 1 indicates perfectabsorption.


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NOISE REDUCTION

FACTOR(NRF)Noise Reduction Coefficient (NRC):The NRC of anacoustical material is the arithmetic average to thenearest multiple of 0.05 of its absorption coefficientsat 4 octave bands with center frequencies of 250,500, 1000, 2000 Hertz. The NRC rating can be viewed

as a percentage (example: .80 = 80%) of what soundwaves that come in contact with the acousticalmaterial are absorbed by the material and NOTreflected back within the room.

Noise Criteria (NC):Noise criteria curves used to

evaluate existing listening conditions at ear level bymeasuring sound levels at the loudest locations in aroom. NC criteria can be referred to equivalent dBAlevels.


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TRANSMISSION AND COEFFICIENT OF

TRANSMISSION LOSS

Noise insulation:noise insulation propertiesof an element can be described byproportion of sound energy transmittedthrough it. A decimal fraction of total sound

energy transmitted is called itstransmission coefficient (t).

Another form which can describe theinsulating properties of the element is thetransmission loss (Tl)of the sound energy

impinging on it. This is also called soundreduction index. The reduction effect of anelement is expressed in dB.


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SOUND POWER

Sound poweris the acoustical energy emitted by the sound source, and is anabsolute value. It is not affected by the environment.

Sound Pressure: The Sound Pressure is the force (N) of sound ona surface area (m2) perpendicular to the direction of the sound.The SI-units for the Sound Pressure are N/m2or Pa.

Sound Power Level:sou nd pow er level (PWL or Lw),which identifies the totalsound power emitted by a source in all directions. Sound power, like electricalpower, is measured in watts. In the case of sound, the amount of power is verysmall, so the reference selected for comparison is the Pico watt (10-12 watt).The sound power level (in decibels) is defined as :

Sound Pressure Level:The sound pressure level, in decibels, of a sound is 20time the logarithm to the base of 10 of the ratio of the sound pressure to thereference pressure. The reference pressure shall be explicitly stated and isdefined by standard.


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SOUND TRANSMISSION CLASS

Sound Transmission Class (STC): This is a

rating for doors, windows, enclosures, noise

barriers, partitions and other acoustical

products. The rating is in terms of theirrelative ability to provide privacy against

intrusion of speech sounds. This is a one

number rating system, heavily weighted in

the 500Hz to 2000Hz frequency range wherespeech intelligitibility largely occurs.


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SPEECH INTELLIGIBILITY

The ability of a listener to hear and correctly

interpret verbal messages. In a classroom

with high ceilings and hard parallel surfacessuch as glass and tile, speech intelligibility is

a particular problem. Sound bounces off

walls, ceilings and floors, distorting the

teachers instructions and interfering withstudents ability to comprehend.

ACOUSTICS AND SPEECH


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ACOUSTICS AND SPEECH

PRIVACYSpeech Privacy:The degree to which speech isunintelligible between offices. Three ratings areused: Confidential, Normal (Non Obtrusive) andMinimal.

A major problem in the design of many modern office buildings concerns theprovision of adequate levels of speech privacy. This problem is particularlyacute in open-plan offices, libraries etc. Main noise sources in such settingsare:

Telephones

Typewriters

Printers/photocopiers

Environmental services e.g. air conditioning

Basically there are only two ways to overcome the privacy problem:

1. Reducing the signal strength.

2. Increase the noise level.

WAYS TO OVERCOME PRIVACY


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WAYS TO OVERCOME PRIVACY

PROBLEM1. WAYS OF REDUCING SIGNAL STRENGTH

Room interiors provide surfaces that can either absorb orreflect sound.

2. WAYS OF INCREASING THE NOISE LEVEL

Mask or Masking: Sound masking is the addit ionof natural or

artificial sound into an environment to cover-up unwantedsound by using auditory masking. Sound masking reducesor eliminates awareness of pre-existing sounds in a givenarea and can make a work environment more comfortable,while creating speech privacy so workers can betterconcentrate and be more productive. Sound masking canalso be used in the outdoors to restore a more natural

ambient environment.


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SOUND MASKING

Sound masking can be explained by analogy with light. Imagine a darkroom where someone is turning a flashlight on and off. The light isvery obvious and distracting. Now imagine that the room lights areturned on. The flashlight is still being turned on and off, but is nolonger noticeable because it has been "masked". Sound masking is asimilar process of covering a distracting sound with a more soothingor less intrusive sound.

Sound masking may also be used to hide other unwanted noise, such as the intermittentsounds from machinery. In an office this could be sound of elevators. The applications,among others, are in government, military, military contractors, corporate board rooms,and legal offices.Sound masking can be used anywhere to ensure speech privacy orreduce distractions. The most common sound masking installationsare:

Open office plans

Private offices

Public spaces

SOUND MASKING IN BUILDING


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SOUND MASKING IN BUILDING

INTERIORSSound masking can be used anywhere to ensure speech privacy or reducedistractions.The most common sound masking installations are:

Open office plans -open offices can be either too quiet or too noisy. Openoffices can benefit from sound masking because the added sound coversexisting sounds in the area - making workers less distracted and moreproductive.

Private offices -private offices and other enclosed spaces often appear toprovide privacy but actually do not. Many times, walls are lightweight and donot extend to the ceiling deck - only to the ceiling tile. In these cases, soundcan easily travel through partitions or over the walls. Sound masking can beprovided in adjacent private offices, or in hallways outside of private offices, toensure that confidential conversations remain confidential.

Public spaces -sound masking is useful for reception areas, pharmacies,waiting rooms, and financial institutions. Sound masking is provided in thearea where conversations should not be heard - not necessarily in the areawhere the conversation is taking place. For instance, a psychiatrist does notwant those in the waiting room to overhear a private conversation with apatient, so sound masking is provided in the waiting area: not in thepsychiatrist's office.

SOUND AMPLIFICATION


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SOUND AMPLIFICATION

SYSTEMGoing to the movies today is a very different experience from goingto the movies 70 years agothe picture is clearer, most of themovies are in color and the admission price is a lot higher. But thebiggest change is probably the sound experience.

There are many ways to make and present a sound recording. The

simplest method and the one used in the earliest sound movies, iscalled monaural or simply mono.

Two-channel recordings, in which sound is played on speakers oneither side of the listener, are often referred to as stereo. This isntentirely accurate, as stereo (or stereophonic) actual refers to a widerrange of multi-channel recordings.

The simplest two-channel recordings, known as binaural recordings,

are produced with two microphones set up at a live event (a concertfor example) to take the place of a humans two ears. When you listento these two channels on separate speakers, it recreates theexperience of being present at the event.


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SURROUND SOUND

Surround sound encompasses a range of techniques forenriching the sound reproductionquality of an audio sourcewith audio channels reproduced via additional, discretespeakers. The three-dimensional (3D) sphere of human hearingcan be virtually achieved with audio channels above and below

the listener.

Surround sound technology is used in cinema and hometheatresystems, video game consoles, personal computersand other platforms. Commercial surround sound mediainclude video cassettes, Video DVDs, and HDTVbroadcasts

encoded as Dolby Pro Logic, Dolby Digital, or DTS. Othercommercial formats include the competing DVD-Audio(DVD-A)and Super Audio CD(SACD) formats, and MP3 Surround.


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CREATING SURROUND SOUND

Surround sound is created in several ways:

The f i rst and s implest method is using a surround sou ndrecord ing microp hone technique, and/or m ixing-in surroun dsoun d for playback on an audio system us ing speakersencircl ing th e l istener to play audio from dif ferent direct ions .

A second approach is processing the audio w i thpsyc hoacous t ic sound localization methods to simulate a two-dimensional (2-D) sou nd fie ld w ith headphones.

A th i rd approach, based o n Huygens pr incip le, attemptsrecons truct ing the recorded sound f ield wave fronts w i th in thel is ten ing space; an " audio hologram" form.


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SURROUND RECORDINGS

Surround recordings take this idea astep further, adding more audiochannels so sound comes from three

or more directions. While the termsurround sound technically refers tospecific multi-channel systemsdesigned by Dolby Laboratories, it is

more commonly used as generic termfor theater and home theater multi-channel sound systems.


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STEREOPHONIC SOUND

Stereophonic sound, commonly called stereo, is thereproduction of soundusing two or moreindependent audiochannels through a symmetricalconfiguration of loud speakersin such a way as tocreate the impression of sound heard from variousdirections, as in natural hearing.

Stereo recordings are used in FM broadcastingandDigital Audio Broadcasting (DAB)and in severaltelevision systems. To record in stereo, sound

engineers use various methods, including using twodirectional microphones, two parallel omnidirectional microphones, or more complextechniques.

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