Microphone Calibration Using Reduced Coupler Volume
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Solution Overview
Problem
Current methods for calibrating microphones used in hearing diagnostics, such as those in acoustic probes, face challenges in accurately determining sensitivity across a wide frequency range without requiring an acoustic free-field measurement or external transducers, and are limited by the physical size of reference microphones and errors caused by standing waves in couplers.
Innovation Solution
A method that measures the acoustic impedance of an acoustic coupler and estimates its transfer impedance to calculate the sensitivity of a probe microphone, allowing for compensation of errors and accurate pressure measurements in the ear canal, using a Thevenin source parameter calibration and analytical representation of transfer impedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a probe microphone is calibrated in a small coupler assuming equivalent sound pressure at probe and reference microphones, then the calibration procedure becomes simpler and more convenient, but large errors occur towards higher frequencies due to standing waves in the coupler cavity
Solution Approach 1:
The patent changes the physical parameter of the coupler by reducing its volume from the standard 0.4 cc to approximately 0.13 cc using a special insert. This parameter change transitions the standing wave notch (error frequency) from around 4-5 kHz to approximately 17 kHz, significantly improving measurement accuracy in the audible frequency range while maintaining the simplicity of the coupler calibration method
Solution Approach 2:
The patent creates a simplified copy of the calibration setup by using a smaller coupler that replicates the essential function of the standard coupler while eliminating the harmful standing wave effects at lower frequencies. This allows the calibration to be performed with the same reference microphone and procedure but with improved accuracy
2Measurement precision
If the size of the coupler is reduced to transition the standing wave notch towards higher frequencies, then measurement accuracy improves, but the physical size limitations of the reference microphone prevent further reduction
Solution Approach 1:
The patent applies the nesting principle by placing a special insert inside the existing 0.4 cc coupler to create a smaller effective calibration volume of approximately 0.13 cc. This nested structure allows the coupler volume to be reduced without requiring a completely new coupler design and without exceeding the physical size limitations of the reference microphone
3Measurement precision
If an acoustic free-field measurement is used to obtain probe microphone sensitivity, then the most precise calibration is achieved, but the measurement becomes extensive and requires an anechoic chamber or sound box
Solution Approach 1:
The patent extracts the essential calibration function from the complex free-field measurement setup by using a simplified coupler method. By removing the need for anechoic chambers, sound boxes, and extensive noise control measures, the calibration can be performed in a much simpler environment while achieving comparable accuracy through the reduced coupler volume that eliminates standing wave errors
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method enables more accurate microphone calibration without extensive setups, reducing errors and allowing for interchangeable acoustic units, thus improving the precision of sound pressure measurements in hearing diagnostics.
Implementation Method 1
determining the sensitivity of a microphone in an acoustic unit... the sensitivity of the microphone, when evaluating different diagnostic measurements
Implementation Method 2
there are often differences between the sensitivity of different microphones of the same type used in different probes... standing waves in the cavity and the probe being inserted opposite to the reference microphone. The result of these standing waves is a large error towards the 1⁄4 wavelength resonance of the coupler since the sound pressure at the probe cancels out
Data Source
Figure 1
Figure 2A~2B
Figure 3A~3B
AI summary
The present disclosure relates to a method and system for determining the sensitivity of a first microphone (2), the method comprising providing an acoustic coupler (5) having at least one internal cavity configured such that a sound field can be generated within the cavity, the cavity being further in acoustic communication with a reference microphone (6) configured to measure the reference sound pressure (Pref) at a given position in said cavity, the cavity being further provided with an inlet opening configured to establish acoustic communication between the sound inlet of said first microphone (2), the sensitivity of which is to be determined