Reference Microphone for Microphone and Speaker Tests

In microphone tests, a reference microphone equalizes a speaker to flatten the frequency response or provides reference sound source for the DUT to cancel out frequency response fluctuations of the speaker. In speaker tests, a reference microphone captures the actual acoustic signals from the speaker.

To guarantee test accuracy, a reference microphone satisfying the following requirements is recommended:

Table 3. Reference Microphone Requirements
Requirement Description
Low noise floor In speaker tests, a reference microphone with a lower noise floor than the noise floor of the speaker ensures that the measured noises, hisses, or other imperfections come from the speaker instead of the reference microphone.
Low THD and high dynamic range

In speaker tests, the inherent THD of the reference microphone should be lower than that of the speaker. Otherwise, it is hard to identify where the measured THD comes from.

A high dynamic range with a THD lower than 1% allows a high-quality speaker to be tested even in higher sound pressure level (SPL) situations without overload and damage.
Similar sensitivity over a wide frequency range This prevents the sensitivity from affecting the microphone to measure the real characteristics of the speaker.

In some cases, a high-precision reference microphone may not be the most suitable model to use. For example, the GRAS 1/2’’ 46A series models with high precision may be less suitable than the 1/4’’ 46BE or 46BL models for microelectromechanical system (MEMS) testing. The following table compares these models.

Table 4. Comparison Between 1/2’’ 46A Series and 1/4’’ 46BE or 46BL
1/2’’ 46A series 1/4’’ 46BE or 46BL
High sensitivity Appropriate sensitivity
High dynamic range Appropriate dynamic range
Low noise Smaller size for appropriate placement in a fixture

Besides the previous requirements, NI suggests considering the following factors when choosing a reference microphone:

Table 5. Factors and Considerations for Choosing a Reference Microphone
Factor Consideration
IEPE or constant current power (CCP) support A microphone supporting IEPE or CCP can be directly powered by the sound and vibration input module using a single coax cable.
Transducer electronic data sheet (TEDS) support A microphone supporting TEDS can increase calibration accuracy.