UNIT 3ELECTRO-ACOUSTICS

Introduction of Electro-acoustical systems, Unidirectional andStereophonic sound system, Digital and Surround-sound systems,Design criteria for Theatres, Motion picture halls, Multiplexes,Home Theatre System, Conference Room.

OPEN AIR ACOUSTICSFree field propagation of sound, absorption from air and naturalelements, effect of barriers, effect of landscape elements, thermaland wind gradient. Design of open-air theatre and planning ofbuilding. Reduction of noise by screening.

Microphone Types

Podium & Desktop

• For human voice, not music

• Low sensitivity and/or directional so as to prevent feedback

Lavalier Mics

• “Body worn” or “clip-on”

• Originally meant “necklanyard”

• Often refers to mostminiature mics

Pressure Zone Mics

• Aka PLATE, BARRIER, MOUSE mics

• Consists of sensitive lavmounted above hard reflecting plate or surface

• Hemispherical pattern

• May also be pie wedged

Pressure Zone Mics

• Usable film dialog at 3 to 4 feet

• Theatre pickup from floor level (mouse mic)

• Conference table & teleconferencing

• Lavs can be made into giant PZMs

Handheld Performance Mics

• Live vocals

• Feedback suppression

• Risky to use for live recording since talent may not use mic all the time to talk to audience

Studio Music Mics

• High quality, large diaphragm, condenser mic

• Very mellow, rich sound for recording

• Too sensitive for on-stage performance

Narration Mics

• For narration & voice-over

• EQ’d for voice rather than music

• May be studio mic or dynamic

TV/Film Production (boom)

• For dialog, not music

• Very directional, lots of reach

• Usually deployed overhead

ENG Mics

• Electret Condenser

• ENG = Electronic News Gathering

• Medium grade shotgun mics, better than dynamic but not as good as condenser

• Also includes most lavs.

Surveillance Mics

• REACH, not dialog quality

• Super-shotgun, parabolics, sub-miniature

• Voice quality not important

• Not what is depicted in the movies

Sound Reinforcement System

1.1.1.1Electro dynamic-microphones• In this type sound energy striking a diaphram, moves the coilattached to it in a

permanent magnet.• This motion of coil in magnetic field produces current in thecoil (Lenz’s Law).• This type is responsive to sounds impinging from all sides and covers 300-4000 Hz

range.

1.1 Basic Elements

1.1.1 Microphones

1.1.1.2Ribbon Microphones• Sound impinging upon a light, aluminum ribbon (on both frontand back sides),

kept in strong magnetic field, produces motion in the ribbonand in turn current init,

• This type has high directional response.• It is used in broad casting studios for discriminating different sounds.

1.1.1.3Capacitor Microphones• High DC Voltage is applied between the diaphram (which is earthed) and a fixed

plate.• This makes up a charged condenser (q= c x v).• Sound waves on diaphram change the capacity of this condenser, Hence resultant

current changes.• This type has limited use in laboratories as changes in DC voltage current

distribution.

1.1.1.4Carbon Microphones• Carbon granules are packed in non metallic box, called “Carbon Button”.• Two leads from the button are connected to a battery, througha resistor.• Diaphram motion changes the resistance and hence the current.• This type has the advantages of high electrical out put. It isdurable and has low

cost.• It is used in telephone and radio communications

1.1.1.5Crystal Microphones• Peizo electric crystals (like quartz,

Barium titanate etc.) whensubjected to pressure along oneprincipal axis, develop voltagealong other principle axis.

• Sound on diaphram presses the pinon the crystal. This developsvoltage along the electric axis ofthe crystal.

• This type is compact and light.• This is used with portable

instruments.• It is tiny.• Also called “Tie Microphone”.• Bimorphs has many slices of the

crystal, giving large output.• It covers wide frequency range.• It is used for public address,

hearing aid, in sound level meteretc.

• It is low cost and small size.

Electro-acoustics

• To increase the sound level when a sound source is too weak to be heard.

• To provide additional sound to audiences beyond the intended range of the

source.

• To project sound back to the stage for the benefit of the performers.

• To alter the Reverberation Time or other impression of an auditoria.

• To reduce the relative effects of background noise.

• To provide paging, information or warning facilities.

• To reproduce electronic or recorded material.

• Also known as a high level system, this is essentially asingle cluster of loudspeakers located near the source. Such asystemgives maximumrealismas the amplified sound, whilstincreasing loudness and clarity, is still associated with theoriginal source.

Speaker Placement

1. A centrally located system.

A centrally located system.

Basically a number of loudspeakers spaced throughout the auditorium.This is also known as a lowlevel systemas each individual speakeroperates at a lowamplification level to service only a small part of thewhole audience

2. A distributed system.

Whilst it is preferable to use a centrally located system, there aremany situations in which it must be used, for example;

• Where the ceiling height is too lowfor the installation of a centralsystem.

• Where not all of the audience have a direct sightline with the centralloudspeaker.

• When the amplified sound is used to overcome high background noiselevels.

• Where the serviced space may be divided into several smaller spaces.

• In large halls where the source position may vary significantly.

• Whilst realism cannot be expected froma distributed loudspeakersystem, it does provide high intelligibility where the roomis not tooreverberant.

3. A stereophonic system

1. Two or more loudspeaker clusters at strategic positions within theauditorium.

2. Such systems are used when there are a number of different sourcesto be amplified or the source is quite mobile.

3. By using two or more microphones, each connected to their owncluster of speakers, the spatial relationship between the sources ispreserved in the amplified sound.

4. This is achieved because the sound is amplified at intensitiesproportional to the distance between the source and the microphoneand the ear perceives the resultant directional cues.

A stereophonic system

Partha Sarathi MishraAsst. Professor

School of ArchitectureGITAM University

parthaconcept@gmail.com

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