Sound Sources for Qualification of Anechoic Chambers

　Hemi-anechoic chambers are rooms that must be designed to completely absorb direct sound produced by acoustic waves. Furthermore, the emulation of acoustic conditions that are free from the influence effect of the room due to reflections is an essential requirement in order to be able to conduct measurements such as Transmission loss (TL), Diffusion Coefficient, Sound Power, etc. For this reason, general procedures as the ones described in Annex A of ISO 3745:2012 and ISO 26101( an improvement of ISO 3745 Annex A) to ensure the performance assessment of a free field must be followed. Furthermore, the efficiency of the sound absorption in the hemi-anechoic testing rooms should be examined with regard to the lower cut-off frequency.

The main purpose of the inverse square law measurements in the anechoic and hemi-anechoic chamber is to compare the decrease in sound pressure with distance from the source to the expected decrease of sound pressure in a true free-field condition (decrease of 6dB per doubling the distance). Being aware that the different equipment, ventilation ducts, doors, etc. can be the cause of reflections in the chambers, the measurements are focused on this aspect. In order to carry out the inverse the inverse square law measurements, microphone traverses shall be made along at least five straight paths away from the geometric center of the measurement sphere or hemisphere in different directions:

a) One traverse path towards a dihedral corner
b) One traverse path towards a trihedral corner
c) One traverse towards the center of one surface
d) If the plan area is not square, then one traverse path shall be towards the closest boundary surface and one toward the farthest boundary surface.
e) Additional traverses to doors, ventilation openings, sound transmission openings, etc.

 Microphone traverses according to ISO26101

The deviations of measured sound pressure levels from those estimated using the inverse square law obtained shall not exceed the values of Table 1. The deviations in Table 1 also determine the frequency range over which measurements can be made in accordance with this International Standard.

Table 1: Maximum allowable deviation of measured sound pressure levels from theoretical levels using the inverse square law

 　Furthermore, one challenge of a room qualification is finding adequate sound sources. Sources used in the qualification procedure must be omnidirectional, for that reason, to achieve omnidirectionality, different loudspeakers have to be used for different frequency ranges. In ISO 3745, there is a specification of these sound sources. Up to 400 Hz, a 25cm diameter loudspeaker (YAB speaker is 30cm) in a closed 0.02m³ box shall be used. Between 400 to 2000 Hz, two 10cm loudspeakers (YAB speakers are 12cm) mounted face to face and phased shall be used. Finally, at frequencies 2000 through 10000 Hz a small baffled system driving a narrow (< 1.5cm diameter) tube is recommended. In order to ensure the fully conformity with the ISO, YAB has built its own speakers for the qualification of anechoic chambers (Dodecahedron speaker is used in full-anechoic chambers in replacement of Box speaker due to its omnidirectionality).

　Sound sources for qualification of anechoic chambers

YAB measurements of Inverse Square Law in the anechoic chamber of Bandung Institute of Technology, Indonesia 

-References of interest:

ISO 3745: 2012. Determination of sound power levels and sound energy levels of noise sources using sound pressure - Precision methods for anechoic rooms and hemi-anechoic rooms. ISO Geneva, Switzerland, 2012

ISO 26101:2102. Test methods for the qualification of free-field environments. ISO; Geneva, Switzerland, 2012

Qualification of hemi-anechoic rooms for noise emission measurements. In: Proceedings Inter-Noise 1990, Gothenburg, Sweden, INCE Conference Proceedings, 1990-08-13, 223, pp. 685-690

A renovated Anechoic Room: Some Aspects of Requirements and Measurements, Aug 1991. Ove Till and Bjorn Hagerman