A-WEIGHTED FILTER

The human ear has a frequency range from about 20 Hz to

20 kHz. Within this range the sensitivity of the human ear is

not equal for each frequency. In order to approach a natural

hearing response, weighting filters are introduced. One of

these filters is the A-weighted filter. The A-weighted filter is

commonly used in signal-to-noise ratio measurements,

where sound is compared to device noise. The filter im-

proves the correlation of the measured data to the signal-to-

noise ratio perceived by the human ear.

SENSITIVITY

Sensitivity is a measure for the transfer from the applied

acoustic signal to the output of the microphone. Conven-

tional JFET microphones and microphones with built-in gain

have a sensitivity that is expressed in dB(V/Pa), where 0dB

= 1V/Pa. A certain pressure on the electret of the micro-

phone gives a certain voltage at the output of the micro-

phone. Since the LMV1024 microphone has a digital output,

the sensitivity will be stated in dB(Full Scale/Pascal) or

dB(FS/Pa) as opposed to conventional microphones. This

section compares the various microphone types and their

sensitivity. Examples are given to calculate the resulting

output for a given sound pressure.

Sound Pressure Level

The volume of sound applied to a microphone is usually

stated as a sound pressure in dB SPL. This unity of dB SPL

refers to the threshold of hearing of the human ear. The

sound pressure in decibels is defined by:

SPL = 20 log (P

Where,

SPL is the Sound Pressure in dB SPL

P

P

In order to calculate the resulting output voltage of the

electret element for a given sound pressure in dB SPL, the

absolute sound pressure P

absolute sound pressure in decibels referred to 1 Pa instead

of 20 µPa.

The absolute sound pressure P

P

P

P

JFET Microphone

Translation from the absolute sound pressure level to a

voltage can be done when the electrets sensitivity is known.

A typical electret element has a sensitivity of −44 dB(V/Pa).

This is also the typical sensitivity number for the JFET mi-

crophone, since a JFET usually has a gain of about 1x (0

dB). A block diagram of a microphone with a JFET is given in

Figure 5.

Example: Busy traffic has a Sound Pressure of 70 dB SPL.

Microphone Output = SPL +C+S

Where,

SPL is the Sound Pressure in dB SPL

C is the dB SPL to dBPa conversion (−94 dB)

S is the Sensitivity in dB(V/Pa)

Microphone Output = 70 – 94 – 44 = −68 dBV

This is equivalent to 1.13 mV

The analog output signal is so low that it can easily be

distorted by interference from outside the microphone. Addi-

tional gain is desirable to make the signal less sensitive to

interference.

20133440

FIGURE 4. A-weighted Filter

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