Non-Contact Methods for Measuring Front Cavity Depths of Laboratory Standard Microphones Using a Depth-Measuring...

Area of Science:

• Acoustics
• Metrology
• Instrumentation

Background:

• Accurate primary calibration of laboratory standard (LS) microphones at high frequencies necessitates knowing the front cavity depth (lfc).
• Non-contact measurement methods are crucial to prevent damage to the microphone diaphragm during lfc determination.

Purpose of the Study:

• To evaluate and compare non-contact optical measurement methods for determining the front cavity depth (lfc) of LS microphones.
• To identify the most accurate and reliable method for lfc measurement to improve microphone calibration uncertainty.

Main Methods:

• An optical depth-measuring microscope was used to measure lfc on six LS microphones of two types.
• Two primary methods were employed: direct measurement on the microphone annulus (D) and measurement using a gage block (GB) placed on the annulus.
• Plane-fitting data reduction techniques were utilized with the GB method to account for geometric imperfections.

Main Results:

• The gage block (GB) subsampling methods demonstrated excellent agreement with the full GB dataset results.
• GB subsampling methods yielded smaller expanded uncertainties compared to direct measurement (D) subsampling methods.
• Measurements using the GB subsampling method with only nine positions achieved expanded uncertainties within 4 μm (k=2).

Conclusions:

• The gage block (GB) subsampling method is preferred for measuring microphone front cavity depth due to its accuracy and reduced uncertainty.
• This method significantly improves the uncertainty of microphone calibrations compared to using nominal lfc values.
• The optical microscope and GB method offer a robust solution for precise lfc determination in microphone metrology.