SPL needs to be converted to the absolute sound pressure

in dBPa. This is the sound pressure level in decibels referred

to 1 Pascal (Pa).

The conversion is given by:

dBPa = dB SPL + 20*log 20 µPa

dBPa=dBSPL -94dB

Translation from absolute sound pressure level to a voltage

is specified by the sensitivity of the microphone. A conven-

tional microphone has a sensitivity of -44 dBV/Pa.

Example: Busy traffic is 70 dB SPL

V

This is equivalent to 1.13 mV

PP

Since the LMV1012-15 has a gain of 6 (15.6 dB) over the

JFET, the output voltage of the microphone is 6.78 mV

implementing the LMV1012-15, the sensitivity of the micro-

phone is -28.4 dBV/Pa (−44 + 15.6).

LOW FREQUENCY CUT OFF FILTER

To reduce noise on the output of the microphone a low

frequency cut off filter has been implemented. This filter

reduces the effect of wind and handling noise.

It’s also helpful to reduce the proximity effect in directional

microphones. This effect occurs when the sound source is

very close to the microphone. The lower frequencies are

amplified which gives a bass sound. This amplification can

cause an overload, which results in a distortion of the signal.

The LMV1012 is optimized to be used in audio band appli-

cations. By using the LMV1012, the gain response is flat

within the audio band and has linearity and temperature

stability Figure 5.

NOISE

Noise pick-up by a microphone in cell phones is a well-

known problem. A conventional JFET circuit is sensitive for

noise pick-up because of its high output impedance, which is

usually around 2.2 k

Ω.

RF noise is amongst other caused by non-linear behavior.

The non-linear behavior of the amplifier at high frequencies,

well above the usable bandwidth of the device, causes AM-

demodulation of high frequency signals. The AM modulation

contained in such signals folds back into the audio band,

thereby disturbing the intended microphone signal. The

GSM signal of a cell phone is such an AM-modulated signal.

The modulation frequency of 216 Hz and its harmonics can

be observed in the audio band. This kind of noise is called

bumblebee noise.

RF noise caused by a GSM signal can be reduced by

connecting two external capacitors to ground, see Figure 6.

One capacitor reduces the noise caused by the 900 MHz

carrier and the other reduces the noise caused by 1800/

1900 MHz.

20058711

FIGURE 4. dB SPL to dBV Conversion

20058712

FIGURE 5. LMV1012-15 Gain vs. Frequency Over

Temperature

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