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Microphone Sensitivity

Overview

Microphone sensitivity is defined by the mic's output signal with respect to an input to the mic of 94dB SPL (or equivalently, 1 Pa pressure). Typically, the sensitivity is measured with a 1kHz sine wave at the 94dB SPL at the mic.

The definition of the microphone sensitivity is slightly different for the analog microphone and the digital microphone.

Definition

Microphone Sensitivity for Analog Microphone

For analog microphones, the sensitivity is defined as the output root-mean-square (RMS) voltage with respect to 1 Pa pressure. In other words, if the mic output RMS voltage is \(A\) volts given the 94 dB SPL input, then the sensitivity is

\[ S_{V/Pa} = A \]

with the unit of V/Pa.

The sensitivity is usually described in logarithmic unit of dBV. In other words,

\[ S_{dBV} = 20 \cdot log_{10} \left( \frac{S_{V/Pa}}{1} \right) \]

where the "reference" is set to be 1Vrms/Pa.

Microphone Sensitivity for Digital Microphone

For digital microphones, the sensitivity is defined as the percentage of the full-scale output that is generated by a 94 dB SPL. In other words, if the max of the mic output is \(A_{\text{max}}\), and the mic output peak magnitude is \(A\) given the 94dB SPL, 1kHz sine wave input, then the sensitivity is

\[ S_{\%FS} = \frac{A}{ A_{\text{max}} } \]

The sensitivity is usually described in logarithmic unit of dBFS. In other words,

\[ S_{dBFS} = 20 \cdot log_{10} \left( S_{\%FS} \right) \]

Note that

  • The microphone's acoustic input levels in dB SPL are always RMS measurement
  • The digital microphone uses the peak magnitude to define the microphone sensitivity, while the analog microphone uses the RMS voltage to define the microphone sensitivity.
  • For a sinusoidal signal, the RMS level is 3dB below the peak level. If the digital microphone output is connected to the other devices that define their input level in RMS measurement, remember to account for this 3dB difference.

Applications

  • If a digital mic is said to be of mic sensitivity -40 dBFS, it means:
    • The mic can probably take input up to 134 dB SPL (assuming the mapping from input sound pressure to the output magnitude is linear)
    • For a sinusoidal signal of 94 dB SPL at the mic input, the magnitude of the mic output will be \(0.01 * A_{\text{max}}\)
    • For a signal of 94 dB SPL at the mic input, the RMS magnitude of the mic output will be \(\frac{0.01}{\sqrt{2}} *  A_{\text{max}}\)
      • If the signal is human speech, the crest factor for human speech is about 12dB, and hence the peak magnitude of the mic output will be \(\frac{0.04}{\sqrt{2}} *  A_{\text{max}}\)

Reference