Lab 1: A full description of purpose and "workings" of an ‘Acoustic Anechoic Chamber’

A Description of an Acoustic Anechoic Chamber

The Anechoic Chamber - The term anechoic means non-echoing or echo-free. An Anechoic chamber is a room that is designed to completely absorb all reflections from sound waves. These rooms are also insulated from outside sources of noise. When both of these combine it simulates an extremely quiet open space. This is very useful for working without the impact of exterior influences which in certain tests or examples could hinder someones progress.

These chambers are commonly used in acoustics to conduct certain experiments in essentially well moderated conditions. This is meaning that there are no reflected signals within and that all sound energy will be moving away from the source with almost none reflected back. Two examples of experiments that have taken place in anechoic chambers are measuring the transfer function of a loudspeaker or the directivity of noise radiation from industrial machinery. The interior of an anechoic chamber is extremely quiet, with noise levels around 10 to 20 dBA. We know that the human ear can pick up sounds above 0 dBA so humans hear the chamber as completely void of sound. This leads to some disorientation. 

RAM or Radiation Absorbent Material is designed and shaped to absorb sound radiation. The most common and effective types of RAM uses an array of pyramid shaped pieces that are made from a specific material. In order for this to work all of the surfaces inside the chamber have to be completely coated in this RAM. A popular material for this is a special rubberized foam material with internal mixtures of carbon and iron. The length from the base to tip of the pyramid is based on the lowest expected frequency that the users will be working with. If the frequency is low then the panels are longer whilst the higher frequencies need shorter panels.

These pyramids have their points pointing towards the centre of the room. Pyramidal RAM atenuates signals by two effects, scattering and absorption. Scattering can occur in two ways, coherently when reflected waves are directed away from the receiver or incoherently where waves are picked up by the receiver. Incoherent scattering also occurs inside the foam pyramids. The pyramid shapes are cut at angles the maximize the bounces a waves make within a structure. With each bounce, the waves loses energy to the foam material and as such exits the material with lower signal strength.

Waves that have higher frequencies have shorter wavelengths and as such are higher in energy. Low frequencies have longer wavelengths and are lower in energy according to the calculation where lambda equals velocity divided by frequency, where lambda represents wavelength. To shield against a specific wavelength the cones are made to a specific size to absorb said wavelength. The quality of a chamber is determined by the lowest frequency operation in which measured reflections from the surfaces inside will be the most significant compared to high frequencies.