What are the Characteristics of Measurement Microphones?

Now that we have discussed some general features of measurement microphones, we can look in more detail at the characteristics of these transducers. Characteristics describe the performance of a microphone and are summarized in the specification tables that appear on technical data sheets and microphone calibration charts.

CONTENTS

1. Characteristics — directivity and size
2. Open-circuit sensitivity
3. Frequency response
4. Dynamic range

To the untrained eye, a microphone calibration chart contains a lot of technical information filling a small space. But this information is invaluable if the best microphone is to be selected for a measurement task or if a microphone's potential is to be fully utilized. What follows here is a guide to the various microphone characteristics that are found in specification tables.

The three most important characteristics to be considered in selecting a microphone are the sensitivity of the microphone (and expected sound level to be measured), the frequency range of interest, and the type of sound field in which the measurement will be made.

 

Characteristics — directivity and size

DID YOU KNOW?

One of the early Brüel & Kjær field calibrators was known as a Falling Ball Calibrator. It used hundreds of small ball bearings, similar to those used in ballpoint pens. In the device, the ball bearings would fall from a cavity, through an orifice, and impact a metal plate. The resulting noise, broadband in nature, could be used as a calibration source. Since the exact level produced varied from unit to unit, each would have a unique and documented sound pressure level (SPL).

 

Open-circuit sensitivity

Factory calibration

Field Calibration

Loaded sensitivity and correction factor, K0

Frequency response

Actuator-response curve

Free-field and diffuse-field response corrections

Frequency-response range

Free-field-, pressure-field and diffuse-field response microphones

Low-frequency cut-off

High-frequency cut-off

Dynamic range

Lower limit of dynamic range

Upper limit of dynamic range

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