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FREEFIELD vs. DIFFUSEFIELD USE

Only a small percentage of all acoustical measurements are performed in a well defined and/or well controlled
environment of an e.g. acoustical laboratory – on the contrary most acoustical measurements are done
under not really controlled conditions. Here are some hints on how to use our microphone.

Sound Fields:
Free field: There are no reflecting objects, only the microphone disturbs the sound field.
Diffuse field: There are many reflecting surfaces or sound sources so that the sound waves arrive from all
directions.
Pressure field: This is found in small confined spaces like sound calibrators.

Depending on the nature of the sound field an appropriate microphone, which is optimized for the sound field
could be selected. Unfortunately there are many practical situations where the sound field is not really of a well
defined type. This application note should give you an idea on how to measure with a free field response
microphone.

The free field microphone is the most common in use, chosen on tradition but we should know about the sound
field.
The following picture shows both the free field and the diffuse field response of a free field microphone.

The diffuse field response is not easy to measure, because it is not easy to generate a truly diffuse sound field
over a wide frequency range but there is a known procedure to estimate the diffuse frequency behavior of a
free field microphone.
From literature we know, that a microphone’s random (diffuse) incidence response can be approximated by
measuring the 90 deg incidence response relative to a single sound source.

While it is an approximation only iSEMcon has measured the 90deg response of many EMX-7150
th
microphones and used the averaged data to generate a 19 order polynomial. This is now used to
approximate the “diffuse field” response from the microphones free field response data.

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