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Generic Microphone Calibration

This calibrates a "generic" microphone — one that doesn't need to be a precision measurement mic (e.g., an inexpensive electret used just to record verification tones in a booth) — by playing a broadband stimulus through a speaker and comparing what it records against a co-located, already-calibrated measurement microphone.

What you'll need

  • A calibrated measurement microphone (see Measurement Microphone Calibration) and its preamp, positioned alongside the generic microphone.
  • The generic microphone being calibrated.
  • A speaker to play the stimulus.
  • An analog to digital converter and a digital to analog converter.

Opening the workspace

Launch CFTSCal and select the Generic Microphone Calibration workspace.

Settings

Field What it means
Test → Input Which input channel the generic microphone under test is wired to.
Test → Device / Ref. A free-form label for identifying the generic microphone you're calibrating (e.g., product ID, serial number, asset tag, etc.). Click + to add a new one to the drop-down list.
Test → Target folder Organizes calibrations into folders.
Ref. → Input Which input channel the reference measurement microphone is wired to.
Ref. → Device / Ref. Which calibrated measurement microphone to use as the reference, and its gain. To add a new reference microphone, use Measurement Microphone Calibration — it can't be added from here.
Speaker Which output the speaker playing the stimulus is wired to.

Running the calibration

Click Golay or Chirp — both are always available once a generic microphone, a reference microphone, and a speaker output are selected.

  • Golay plays a pair of complementary Golay-code sequences, several times each, and cross-correlates the recorded response against them. More robust to background noise, at the cost of taking longer.
  • Chirp plays a single frequency sweep. Much faster, but somewhat more sensitive to noise.

Both runs expose a Smoothing window parameter (default 10) — the width, in frequency bins, of a Hamming-weighted moving average applied to the computed sensitivity curve, which trades fine frequency detail for a less noisy curve. Set it to 0 to see the raw measurement. See Speaker Calibration for the other run parameters, which are the same.

How the calibration is computed

Both microphones record the same stimulus at the same time. The reference microphone's known sensitivity \(S_{cal}\) tells you the sound pressure that was actually present, so whatever the generic microphone did differently must be a property of the generic microphone:

\[ S_{exp}(f) = \frac{V_{exp}(f)}{O(f)} = \frac{V_{exp}(f) \times S_{cal}}{V_{cal}(f)} \]

Writing a quantity as a function of \(f\) (frequency, in Hz) just means it takes a different value at each frequency — which is exactly the point for a generic microphone, whose sensitivity is a curve rather than the single number \(S_{cal}\).

Or, in the dB form cftscal actually uses:

\[ S_{exp_{dB}}(f) = 20 \times log_{10}(V_{exp}) + 20 \times log_{10}(S_{cal}) - 20 \times log_{10}(V_{cal}) \]

Unlike a measurement microphone, the result is a curve rather than a single number, because a non-precision microphone's sensitivity varies substantially with frequency — that's the whole reason this workspace exists. See Calibration Math for the derivation.

The arithmetic can't distinguish the microphone from its position

The equation above attributes every difference between the two recordings to the generic microphone. If the two microphones aren't at the same distance and angle from the speaker, that positioning difference is silently folded into \(S_{exp}\) as though it were the microphone's own frequency response.

Reviewing the results

Generic Microphone Sensitivity plots the frequency response (in dB re 1 Vrms) of every calibration currently selected in the list below.

Generic Microphone Calibrations (the list) shows every calibration ever run for this workspace, with the following columns:

Column Meaning
Name Which generic microphone was calibrated (organizes the list; see Device below if you've filed calibrations into folders that don't match the device).
Date When the calibration was run.
Device The device label recorded at calibration time, independent of which folder the calibration is filed under. Usually matches Name — compare the two if you've reorganized calibrations into folders.
Input Which input channel the generic microphone was wired to.
Gain The generic microphone's preamp gain, in dB, that was in effect.
Microphone Which reference measurement microphone was used.
Mic. Channel Which input channel the reference microphone was wired to.
Speaker Which speaker played the stimulus.
Speaker Channel Which output channel the speaker was wired to.
Max. Freq. The highest frequency the calibration covers.

Sanity-checking a calibration

  • Does the response look like previous calibrations of the same device? A sudden change usually means the microphone moved relative to the speaker, or is damaged.
  • Are the two microphones actually co-located? If the generic and reference mics are at noticeably different distances or angles from the speaker, the computed sensitivity will reflect that mismatch, not the generic mic's real response.
  • Is Max. Freq. as high as you need? It's limited by both the speaker and the reference microphone's usable bandwidth.

Troubleshooting

Common pitfalls

  • Reusing a device label for a different physical microphone breaks your ability to track a specific unit over time — give each physical mic its own label.
  • Wrong reference microphone selected, or that microphone's own calibration is stale, propagates straight into the generic mic's measured response.
  • Microphones not close together is a common, easy-to-miss source of a distorted-looking response.