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Microphone Compensation

An ideal microphone for acoustic measurements would have a perfectly flat frequency response (all frequencies detected with equal amplitude).  In practice, most microphones do not meet this criteria.  Microphone compensation curves solve this problem.  

Many microphone manufacturers provide a frequency response curve for their products.  This curve shows both the total frequency range covered and the frequency response of the microphone within this range.  By creating a simple text file containing this information, we can compensate for its response.

Building a microphone compensation file:
Run the NOTEPAD.EXE application, spreadsheet or word processor that can produce a standard ASCII text file.
Enter two columns of numbers representing the microphone response curve.
The left column contains the frequency in Hertz. 
The right column contains the offset in decibels (this offset is subtracted from the logarithmic spectrum value).
Enter the information in ascending frequency order.
Separate the columns with a tab character.

Example:
---------------------------
100.0        -10.0
500.0          -5.0
1000.0         0.0
2000.0         2.0
10000.0      -5.0
15000.0     -15.0


Enter as many as32,768 pairs of numbers; at least 3 pair are required.  The analyzer will use a cubic spline algorithm to interpolate between these data points for each frequency bin in the spectrum.

Any line with a semicolon in the first column will be treated as a comment line and ignored.

By convention, microphone compensation files have an extension of .MIC.

Note:
Microphone Compensation files and Spectral Overlay files have identical formats and can be interchanged.  This is useful if you want to verify that you have entered the microphone response table correctly.  To do this, simply select the microphone compensation file as an overlay.

Interpolation: When manually creating a compensation file the results can have unexpected excursions due to the cubic spline interpolation algorithm.  You can turn off this algorithm by adding a line to the beginning of the file as follows:

;SPLINE OFF



See also:  Standard Frequency Weighting