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Pressure Reciprocity Calibration Coupler with Detailed Moist Air

When high-fidelity measurement microphones are calibrated, a pressure reciprocity calibration method is used.

During calibration, two microphones are connected at each end of a closed cylindrical cavity.



For the calibration procedure, it is important to understand the acoustic field inside such a cavity, including all the thermoviscous acoustic effects, for example, the acoustic boundary layers at higher frequencies and the transition to isothermal behavior at the lower frequencies.


This model sets up a simple calibration coupler model and discusses important considerations when performing a high precision absolute value simulation.


The model results include the acoustic transfer impedance used for reciprocity calibration.

The results are compared with analytical predictions valid at the low frequencies where the system is isothermal.

The model is further extended with precise material property estimation using the Thermodynamics functionality in COMSOL Multiphysics.

This allows setting up a moist air material that depends on the ambient pressure, temperature, and relative humidity.

A Pressure Reciprocity Calibration Coupler with Detailed Moist Air Material Properties models the complex acoustic transfer impedance within a coupler used for calibrating high-fidelity microphones, accounting for the thermoviscous effects of moist air by defining properties based on ambient conditions. 

This approach uses advanced simulation software, like COMSOL, to precisely simulate the sound field, capturing the transition from adiabatic behavior at high frequencies to isothermal behavior at low frequencies, which is crucial for accurate phase sensitivity data in microphone calibrations. 
Key Aspects of the Model
• Complex Acoustic Transfer Impedance: 
The model computes the acoustic transfer impedance, a critical value in microphone reciprocity calibration that describes how sound pressure transfers through the coupler. 
• Thermoviscous Effects: 
It accounts for detailed thermoviscous acoustic effects, including the dynamics of thermal and viscous boundary layers. 
• Frequency-Dependent Behavior: 
The model captures the frequency-dependent transition of the sound field from adiabatic conditions at high frequencies to isothermal conditions at low frequencies. 

• Moist Air Material Properties: 
Detailed properties for moist air are incorporated, allowing for the simulation to be sensitive to ambient pressure, temperature, and relative humidity. 
• Coupler and Microphone Geometry: 
The model considers the geometry of the calibration coupler and the microphones, which significantly influences the acoustic field and the resulting uncertainties, especially at low frequencies. 


Importance in Calibration
• Accurate Phase Sensitivity: 
Analyzing phase sensitivity data, not just modulus data, is vital for modern, high-precision primary pressure reciprocity calibrations. 
• High-Frequency Accuracy: 
The thermoviscous effects become more dominant at higher frequencies, making their accurate modeling essential for reliable calibration results. 
• Low-Frequency Analysis: 
At low frequencies, the system's behavior approaches isothermal conditions, and the model helps in understanding and predicting these low-frequency variations.


The usual primary method of determining the pressure sensitivity of Laboratory Standard Microphones is the pressure reciprocity calibration.

This method uses two microphones, a transmitting one and a receiving one, acoustically connected by a coupler which is an acoustic cylindrical cavity closed at its ends by the diaphragms of the microphones.

The acoustic field inside the cavity must be modeled precisely to obtain the accurate sensitivity of the microphones from electrical measurements.



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** Mahsa Hassankashi **
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