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per wiki, an acoustic anechoic chamber is

a room designed to absorb as much sound as possible. The walls consist of a number of baffles with highly absorptive material arranged in such a way that the fraction of sound they do reflect is directed towards another baffle instead of back into the room. This makes the chamber almost devoid of echos which is useful for measuring the sound pressure level of a source and for various other experiments and measurements.

Does an anechoic chamber take mainly effect on the sound from inside or outside the chamber? Are there some measurements about the effect that an anechoic chamber takes on the sound from inside or outside?

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The term anechoic itself refers only to the sound from within the chamber. As the name suggests, it produces no echo, and an echo can only occur as a reaction of a sound in the room.

However, most (if not almost all) of the time anechoic chambers are also acoustically isolated from outside noise, depending on their specific use case.

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  • $\begingroup$ Thanks for your answer. Is there a term to refer a chamber that is mainly acoustically isolated from outside noise and does not care the echo within the chamber? $\endgroup$ – peterpanai Jan 26 at 22:37
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    $\begingroup$ One would probably call that a soundproof room, but even then they often have measures against echoes on the inside. $\endgroup$ – noah Jan 26 at 22:40
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In a purpose-built anechoic chamber, the outermost wall will be concrete typically 8 inches thick and the foundation slab poured separately from that of the surrounding structure. The thick concrete walls will reflect back almost all of the sound coming towards the chamber from outside, and the separate slab will isolate the chamber from vibrations transmitted through the floor from other activities in the building.

In this case, the primary source of sound leakage entering the chamber will be via the air conditioning ducts. These will always be heavily lined with absorptive material and baffled into a labyrinth shape, but ~60Hz noise from the air-moving fans in the system will pass through the ducting with little attenuation. Since even a well-baffled chamber will not be anechoic at very low frequencies (less than or equal to ~60Hz), the sound energy around 60Hz will build up inside the chamber as long as the air-handling system is running.

Accurate low-frequency sound level measurements therefore require that the ventilation system be shut off for the duration of the measurements.

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