System Performance Analysis for Microphone Test

All the measurement results in the following figures come from the Audio and Acoustics Test Software.

THD

Measure the speaker THD and compare the DUT THD measured during a microphone test with the speaker THD. If the measured DUT THD is higher than the speaker THD, the measurement result is valid.

Measure the speaker THD at 94 dB SPL and 100 dB SPL using a GRAS 40PP as the reference microphone. The following figure shows the measurement results.

The maximum speaker THD is lower than 1%, which is consistent with the speaker specifications provided by the speaker vendor. The measured speaker THD means that test conditions, such as chamber isolation and speaker performance, are well enough to test microphones.

Measure the THD of a DUT at 94 dB SPL and 100 dB SPL. If the DUT THD is higher than the speaker THD, the measurement result is valid.

Frequency Response

If you use a reference microphone to equalize the speaker and then replace the reference microphone with a DUT, you need to verify that the speaker and the amplifier generate enough energy at all the tested frequencies after equalization.

Measure the frequency response of the speaker at the beginning and end of equalization and compare the results with your target speaker frequency response, for example 94 dB SPL. If the speaker frequency response is close to the target value, the speaker generates enough energy.

The following figures show the measurement results.

After equalization, the speaker frequency response deviates from the target value 94 dB SPL within ± 0.06 dB SPL across the tested frequencies.

If you do not equalize the speaker, the speaker may not generate enough energy at certain frequencies. As a result, signals are more sensitive to noises and measurement results may be unstable.

Carefully consider the balance between the frequency response and THD when equalizing the speaker. During equalization, the speaker SPL increases at certain frequencies where the speaker THD becomes higher.

The following figures show the speaker THD at the beginning and end of equalization.

If you use a reference microphone during testing, you need to verify that the distance between the reference microphone and DUT is close enough to ensure that the reference microphone and DUT receive the same signals.

Place two reference microphones close to each other and measure their frequency responses. The following figure shows the measurement results.

The following figure shows the difference between the frequency responses of the reference microphones.

The maximum difference is 2 dB, which is consistent with the microphone specifications provided by the microphone vendor. The distance between the reference microphones is close enough.

In a microphone test, replace one reference microphone with the DUT and measure the DUT frequency response. Check whether the frequency response difference between the DUT and the other reference microphone meets test requirements.

Noise Floor

An audio and acoustics test system can measure only the microphone noise floor higher than the system noise floor.

The acoustic test chamber, test system instruments, and environmental conditions affect the system noise floor. For example, an amplifier connected to the speaker may increase noises.

The microphone specifications provided by the microphone vendor describe the noise floor of GRAS 40PP to be 32 dBA. The noise floor measured using the example CompactDAQ-based audio and acoustics test system is 35.1 dBA, which is higher than 32 dBA. Therefore, the measured noise floor originates from the test system instead of the microphone.

The following noise spectrum shows that the chamber’s low isolation at low frequencies causes this issue.