Acoustic parameters

AN4426

10/18

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2

Acoustic parameters

2.1

Sensitivity

The sensitivity is the electrical signal at the microphone output to a given acoustic pressure

as input. The reference of acoustic pressure is 1 Pa or 94 dBSPL@1kHz. The sound

pressure level, expressed in decibel, dBSPL=20*Log(P/Po) where Po = 20µPa is the

threshold of hearing. 20*Log(1Pa/20µPa) = 94 dBSPL

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For digital microphones the sensitivity is expressed in dBFS

dBV

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2.2

Directionality

The directionality indicates the variation of the sensitivity response with respect to the

direction of the arrival of the sound. MEMS microphones from ST are omnidirectional which

means that there is no sensitivity change to every different position of the source of the

sound in space. The directionality can be indicated in a Cartesian axis as sensitivity drift vs.

angle or in a polar diagram showing the sensitivity pattern response in space.

The following figure depicts the directionality in these two reference systems.

Figure 12. Omnidirectional microphone

2.3

SNR

The signal-to-noise ratio specifies the ratio between a given reference signal to the amount

of residual noise at the microphone output. The reference signal is the standard signal at the

microphone output when the sound pressure is 1Pa @ 1 kHz (microphone sensitivity). The

noise signal (residual noise) is the microphone electrical output at silence.

This parameter includes both the noise of the MEMS element and the ASIC. Concerning

this sum, the main contribution to noise is given by the MEMS sensor, the integrated circuit

contribution can be considered negligible. Typically, the noise level is measured in an

anechoic environment and A-weighting the acquisition. The A-weighted filter corresponds to

the human ear frequency response.