lecture24 - Auditorium Acoustics 1. Sound propagation (Free...

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Auditorium Acoustics 1. Sound propagation (Free field) Free field: the sound source is small enough to be considered a point source, located outdoors away from reflecting surfaces Pressure is proportional to 1/r, where r is the distance from the source Pressure is halved as the distance doubles There is a 6 dB drop in sound pressure level as the distance doubles Free fields only exist indoors in anechoic rooms (echo-free )
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Reflection Surfaces Indoors, sound travels only a short distance before encountering walls and other obstacles These obstacles either reflect of absorb sound according to the acoustic properties of the room The reflection patterns are determined by the curvature of the surface 1a. Sound propagation (Indoors)
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2. Direct, early and reverberant sound Direct sound traveling in air (344 m/s) would reach a listener in an auditorium after a time of 20 to 200 ms depending on the distance from the source Shortly after, the same sound would reach the listener from various reflecting surfaces Early sound - any of the first group of reflections arriving within 50 to 80 ms After the early sounds, reflections arrive thick and fast from all directions These reflections are smaller and closer together, merging after a time into reverberant sound
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Interesting: A simple acoustical analysis of a room can be done by examining the nature of the direct, early and reverberant sound produced in that room 2a. Direct sound In auditoriums, some sounds are non-directional: sounds that radiate essentially with the same intensity in all directions Others, such as upper range frequencies of brass instruments, are directional For low frequencies, localization is done based on the slight time difference in the time of arrival For high frequencies, localization occurs through the difference in sound level caused by the sound shadow
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Precedence effect The ear continues to deduce the direction of the source from the arrival of the first sound if successive sounds: arrive within 35 ms have spectra and time envelopes similar to the first sound are not too much louder than the first 2b. Early sound Rapidly varying early sounds are heard as reinforced direct sound if they
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lecture24 - Auditorium Acoustics 1. Sound propagation (Free...

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