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A Free Field, a Random Incidence and a Pressure Response
microphone can all be used for the same application and produce the same test
result, at low frequencies (<1 kHz).
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Below 1 kHz, the Free Field, Random Incidence (diffuse Field) and Pressure Response microphones will all provide the same results and any can be used in the same application, because the test data output will be the same (within 0.2 dB uncertainty of the system.) As the frequency of interest for the test gets higher, the actual sound pressure waves become smaller, causing smaller objects to now have an impact on the sound field. As the waves approach a similar size to the microphone mass, the output between the responses will deviate. As frequencies become higher, it becomes more important to choose the correct sound field, so that you have the most accurate test results. In the below graph you can see how the separation starts to occur after 1 kHz. For example, at 20 kHz a ¼” pressure response microphone will output close to a 6 dB incorrect result, if a pressure response microphone is used incorrectly in a Free-Field application. A ½” microphone will be out close to 10 dB at 20 kHz due to effects of the larger mass. Below 1 kHz, they all exhibit the same test result.
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