Abstract
The availability of ideal conditions like anechoic chamber to characterize some sound parameters, like sound intensity and sound power necessities the determination of free field and cut off frequency measurements. In this article, full experiment was executed at Wayne State University (Detroit-Michigan), to determine the cut off frequency in all directions; the obtained results showed that the free field can be determined for a specified space. So other tests can take place in this space avoiding regions where reflections and consequently noise can be found. Upon these results tests related to noise abatement in vehicles can be done in such environment.
Highlights
Sound can be defined as any pressure variation which can be detected by human ear
The excel files exported from the software have three readings, the frequency, the sound pressure level (SPL) for microphone 1, SPL for microphone 2, the data were taken for each direction and frequency, the SPL versus the distance was drawn, from which a logarithmic curve fit was drawn to be able to see the dB drop which guide to a free field (5 - 6 dB drop between doubled distances)
It should be noticed here that the cut-off frequency is when the difference in SPL values between two spectra is less than 6 dB, in all figures the graphs show the relationship between sound pressure level versus distance from the source for a specified frequency rang
Summary
Sound can be defined as any pressure variation which can be detected by human ear. It is an important part of our life. 1) Free field describes sound propagation in idealized free space where there is no reflections [5]. This condition holds in an anechoic room where all the sound striking the walls is absorbed. The near field is limited to a distance from the source equal to about a wavelength of sound or equal to three times the largest dimension of the sound source (whichever is the larger) [9]. It is important to find an experimental method to find the different fields of sound in non-ideal conditions depending upon cut off frequency, when this is achieved, it is easy to implement testing of vehicles related to NVH issues
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