Semi anechoic chambers are generally preferred when the object is heavy or large for practical purposes. Sound pressure levels at each microphone position should exceed the background sound pressure levels by 10 dB or more for the object is to be tested. In semi anechoic chamber, a semi anechoic array of 10 microphones is used to measure the precision sound pressure level. The radius of semi anechoic should not be less than the twice the characteristics length of the source. In engineering grade sound power measurement, a rectangular parallelepiped like shoebox measurement surface is required. The distance between the source and array of microphones should be 1 meter, but less than the plan dimensions of the object needs to be tested. However, for tall machines, there is a reduction in size of the chamber. Same as fully anechoic chamber, it is necessary to allow ¼ wavelength between the microphones and the wedge to measure the acoustics measurement, however sometimes in semi anechoic room, microphones and wedge tips need to be positioned closer. The advantages of the semi anechoic rooms are most directivity information of source preserved, especially for small source height relative to wavelength, and high degree of measurement accuracy is achieved. Compared to fully anechoic chamber, semi anechoic chamber is less costly. Semi anechoic chamber requires a large, relatively costly about 20 X 20 X 10 ft room size and large number of microphone positions requires large number of microphones.
The main important factor while designing the anechoic chamber is to determine the cut-off frequency. The internal shape of the anechoic room, sources should not exceed the 0.5% of room volume, depends on the dimension of noise sources. The radius of hemisphere should be at least twice the dimension of the largest source and ?/4 of the lowest frequency of interest which is defined as cut-off frequency.
Generally, cut-off frequency ranges between 70 Hz to 100 Hz, but in some specific cases, it is about 50 Hz. The height and shape of the wedges are designed to set the cut-off frequency. The wedge’s lower cut-off frequency can be calculated by the following expression.
f c ? c / 4h
Where c is the speed of sound in the chamber and h is the height of wedges.
The cut-off frequency of the chamber wedges will be determined by the object to be tested in the chamber. The below wedge performance curve has been taken from the Eckel noise control technologies. Wedges are available in a range of cut-off frequencies from 60 Hz to 250 Hz.