Microphone Test System Configuration

An ideal microphone test system has a high-quality, stable sound source and an environment with reproducible conditions.

Generating high-quality, stable acoustic test signals has the following challenges:

  • The performance of a speaker varies and depends on factors, such as temperature around the speaker and age of the speaker.
  • The size of the speaker requires the speaker to move much more air than microphones measure.
  • The speaker membrane is subject to resonance, break-ups, and other non-linear behaviors, resulting in frequency response fluctuations and relatively high distortion.

To mitigate the factors that affect generating acoustic test signals, you need to carefully consider how to set up the hardware and position the speaker and microphones.

Hardware Setup for a Microphone Test

To monitor and control the speaker performance for a microphone test, use one of the following methods.

Method 1: Use a reference microphone to flatten the speaker frequency response and then replace the reference microphone with a DUT.

The following figure shows a typical hardware setup for an analog microphone test where the DUT replaces the reference microphone after equalization.


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The test system consists of the following components:

  • Sound and vibration module—Sends stimulus signals to the mouth simulator, receives and analyzes the microphone signals.
  • Signal conditioner—Amplifies signals and matches impedances between the module and the mouth simulator.
  • Mouth simulator—Sends out sound waves. A point sound source, which radiates sound waves equally in all directions around the mouth simulator, is required.
  • Reference microphone (not shown)—Equalizes the mouth simulator to flatten the frequency response. A DUT replaces the reference microphone after equalization. Techniques, such as laser positioning, can be used to ensure the DUT is in the same place as the reference microphone.
  • Chamber—Provides an environment for accurate and repetitive measurements. The chamber is covered by acoustic absorbent materials from the inside to prevent reflections and reduce the impacts of external noises.

    • Method 2: Use a reference microphone to cancel out frequency response fluctuations of the speaker.

    The following figure shows a typical hardware setup for an analog microphone test with a reference microphone.


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    The test system consists of the following components:

  • Sound and vibration module—Sends stimulus signals to the mouth simulator, receives and analyzes the microphone signals.
  • Signal conditioner—Amplifies signals and matches impedances between the module and the mouth simulator.
  • Mouth simulator—Sends out sound waves. A point sound source, which radiates sound waves equally in all directions around the mouth simulator, is required.
  • DUT and reference microphone—Placed close to each other at the same distance and offset from the speaker, and therefore, exposed to the same sound pressure. This prevents the changes of speaker performance and temperature from affecting test results.
  • Chamber—Provides an environment for accurate and repetitive measurements. The chamber is covered by acoustic absorbent materials from the inside to prevent reflections and reduce the impacts of external noises.

    • For digital microphone tests with or without reference microphones, connect the output signal of the DUT to a connector of the digital reconfigurable I/O module.

    Note Though you can test one or more microphones with one reference microphone, NI recommends that you use a reference microphone for each microphone your DUT has and place the reference microphone close to the microphone for better measurement results.

    Positioning of Speaker and Microphones

    For research and development design tests which require absolute accuracy, the wavefront of signals received by the DUT and reference microphone needs to be plane or near plane. However, the speaker always generates a hemispherical wavefront. To obtain plane waves, you can change the distance between the speaker and the microphones. As described in IEC 60268-4-4:2018, if this distance is at least half of the wavelength at the lowest frequency of measurement, a spherical wave is a practical approximation to a plane wave. Therefore, you can calculate the distance using the following formula:

    Distance ≥ (340 m/s ÷ Lowest Frequency) ÷ 2

    For example, if the lowest frequency is 125 Hz, the distance between the speaker and the microphones should be greater than or equal to 1.36 m.

    Manufacturing tests pursue reproducibility rather than absolute accuracy. If a reference microphone is used to flatten the speaker frequency response and then replaced by a DUT, you can keep the reference microphone at a certain distance from the speaker, such as 10 cm or more, to avoid instabilities and then place the DUT in the same location. If a reference microphone coexists with the DUT in hardware setup, the DUT and reference microphone must receive the same signals and the distance between them should be as short as possible. To calculate the distance between the speaker and the microphones, the previous formula still applies but the highest frequency is used in this case.

    For example, if the highest frequency is 20 kHz and the wavelength is 17 mm, the distance between the microphones should be much less than 17 mm divided by 2. The distance between the speaker and the microphones should be much greater than 17 mm. Therefore, NI recommends you position the speaker and microphones at a distance of 10 cm or more.