The acoustic treatment used on all surfaces of an anechoic chamber. The distance from wedge tip to the surface of the wall needs to be a quarter of a wavelength for the lowest frequency of interest for optimum absorption. The wedge shape offers a much greater surface area for a given wall surface area. An air gap between the wall and the back surface of the wedge allows the wedge to operate the same at low frequencies as per the condition without the gap.
- A schematic showing a section through a semi-anechoic chamber with acoustic wedges lining the walls and ceiling.
- The absorption of low frequency sound increases with the thickness of the absorber. The absorption will be more effective where the particle velocity is high. Close to the boundary of the room the particle velocity will be zero and so this is not an ideal location for sound absorption. The absorption furthest away from the backing surface will be the most effective and this is why thick layers absorb at lower frequencies / longer wavelengths.
- At low frequencies / long wavelengths the absorption must be placed a considerable distance from the wall to reach a point where the particle velocity is significant. As a rule of thumb this distance is 1/10th of a wavelength for any significant absorption and ¼ of a wavelength to absorb all of the incident sound wave. One method used to improve the absorption of a given layer of absorption is to move the absorption away from the surface.
See also: Anechoic Chamber, Semi-Anechoic Chamber, Sound Absorption.
Subjects: Architectural Acoustics Noise & Vibration