Acoustic parameters

AN4426

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DocID025704 Rev 1

2.7

Total harmonic distortion

THD is the measurement of the distortion affecting the electrical output signal of the

microphone given an undistorted acoustic signal as input. THD+N is expressed as a ratio of

the integer in a specified band of the power of the harmonics plus the power of noise and

the power of the undistorted signal (fundamental).

Equation 1

Typically ST indicates the THD+N measured in the (50 Hz - 4 kHz) band for a given

undistorted signal 1 kHz @ 100 dBSPL.

2.8

PSRR and PSR

PSRR indicates the capability of the ASIC to reject noise added to the supply voltage. To

in phone applications) is added to the power supply and then the amplitude of the output is

measured. The added noise can be either a square wave or sinusoidal wave. Typically the

square wave is preferred since it is the worst case.

Hz) to the added spurious signal on the supply voltage. It is typically expressed in dB as

given in the equation below:

Equation 2

The capability of the integrated circuit to reject noise added to the supply voltage can also

be expressed with another parameter that is the PSR. Basically it is simply a measurement

of the output when noise of 100 mVpk-pk @ 217 Hz is superposed to the supply voltage.

Consequently expressed in dB as given in the equation below:

Equation 3

To evaluate either the PSRR or PSR, proper sealing of the sound inlet or measurements

performed in an anechoic chamber are recommended to avoid mixing the superimposed

noise with that of the noise floor of the output. Finally, in the microphone datasheets PSR is

commonly given instead of PSRR.