ABSOLUTE

SOUND

PRESSURE

[dBPa]

-94dB

SENSITIVITY

[dBV/Pa]

SOUND

PRESSURE

[dB SPL]

VOLTAGE

[dBV]

A-WEIGHTED FILTER

5pF

LMV1032

SNAS233G – DECEMBER 2003 – REVISED MAY 2013

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MEASURING NOISE AND SNR

The overall noise of the LMV1032 is measured within the frequency band from 10 Hz to 22 kHz using an A-

weighted filter. The input of the LMV1032 is connected to ground with a 5 pF capacitor.

Figure 22. Noise Measurement Setup

The signal-to-noise ratio (SNR) is measured with a 1 kHz input signal of 18 mVPP using an A-weighted filter. This

represents a sound pressure level of 94 dB SPL. No input capacitor is connected.

SOUND PRESSURE LEVEL

The volume of sound applied to a microphone is usually stated as the pressure level with respect to the threshold

of hearing of the human ear. The sound pressure level (SPL) in decibels is defined by:

Sound pressure level (dB) = 20 log Pm/PO

Where,

Pm is the measured sound pressure

PO is the threshold of hearing (20μPa)

In order to be able to calculate the resulting output voltage of the microphone for a given SPL, the sound

pressure in dB SPL needs to be converted to the absolute sound pressure in dBPa. This is the sound pressure

level in decibels which is referred to as 1 Pascal (Pa).

The conversion is given by:

dBPa = dB SPL + 20*log 20

μPa

dBPa = dB SPL - 94 dB

Translation from absolute sound pressure level to a voltage is specified by the sensitivity of the microphone. A

conventional microphone has a sensitivity of

−44 dBV/Pa.

Figure 23. dB SPL to dBV Conversion

8

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