Air Quality Monitors

Microphones

The microphone should be handled with great care, as it is particularly delicate and fragile.  In many cases, it can be removed form the meter and stored in a separate box for protection, although the act of transferring it could lead to damage.  It can be mounted directly onto the SLM or connected to it via an extension arm or cable.  The latter positions further reduce the risk that the instrument casing will distort the sound field and hence give a false reading.

The function of the microphone is to convert sound, ie., variations in sound pressure, into an electrical signal which can be represented on the decibel scale by a meter reading or a digital display.

Three types of microphone are commonly used for acoustic measurements:

Condenser microphone:  this is made up of a thin metal diaphragm positioned close to a rigid backplate’ between these two, a polarisation voltage is applied.  Fluctuating pressure, ie., noise, changes the distance between the diaphragm and the backplate which in turn changes the capacitance of the system.  This generates a voltage across a resistance which varies in proportion to the fluctuations in the sound pressure.

Electret microphone:  this is a variation of the condenser microphone.  It does not require an external polarisation voltage, the system being pre-polarised by the electret materials of the diaphragm that has permanent charges ‘frozen in’ to the material.  This type is of particular interest for use in potentially e4xplosive environments such oil refineries.

Piezoelectric or ceramic microphone:  changes in sound pressure create movements in a diaphragm which are transmitted as a force to a piezoelectric crystal.  The fluctuating charges on the crystal generate a change in potential across a resistor.  Like the electret, the piezoelectric microphone does not require an external polarisation voltage.

In all three cases, condenser, electret and piezoelectric, the signals are fed into an appropriate high-impedance pre-amplifier, often connected directly to the microphone, which is plugged into the SLM either directly or through an extension cable.

Relative to the other two types, piezoelectric microphones are cheaper and more robust and therefore generally suited to field use.  Condenser microphones have a higher sensitivity but they are also more expensive, less robust and are sensitive to humidity.  Other factors which might affect the microphone are temperature, vibration and magnetic fields.  It is a case of ‘horses for courses’, choosing the microphone which is best suited to the environment to be measured and which will give the required degree of accuracy. 

Overall, frequency response, dynamic range and sensitivity are generally the most important factors.  These are usually interrelated, sot he choice may well be dictated by the frequency range relevant to each particular case.  Most manufacturers provide individual calibration certificates for precision grade microphones.

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