Microphone Parameters Explained

Selecting the Right Microphone

When selecting a measurement microphone, it is important to understand the measurement requirements and the demands they impose on the microphone. This is necessary because, although measurement microphones are precision instruments that are optimised for particular measurement tasks, they still offer a wide operational range. In fact, such is the versatility of Brüel & Kjær microphones, that you might be tempted into a "that one will do" philosophy when selecting a microphone, simply because a microphone comes within the required general performance parameters. However, if you have a good understanding of the measurement requirement, it is possible to choose the best microphone for the measurement task in hand.

Type of Sound Field

A good way of narrowing down the choice of microphone is to consider the type of sound field in which the measurements are being made. For measurements made away from reflecting surfaces or in acoustically well-damped indoor environments,for example, when making outdoor measurements with a sound level meter or in an office providing a lot of natural acoustic damping, a free-field microphone is clearly best. But for measurements made in small closed couplers or close to hard, reflective surfaces, a pressure-field microphone is more appropriate. An example of this could be, for example, a set of pressure-sensing microphones positioned at different points across an aircraft wing. A complete picture of the pressure variations across the wing surface can then be established. For measurements in enclosed areas where reverberation is likely, pressure-field microphones adapted for random-incidence measurements are best. This is because the random-incidence response of a pressure-field microphone is much "flatter" or constant across the frequency range, than that of a free-field response microphone.

Dynamic Range

The lower limit of the dynamic range is dictated by the inherent noise of the microphone and preamplifier combination. The upper limit of the dynamic range is dictated by the maximum sound pressure level (3% total harmonic distortion). Due to the very wide dynamic range of the microphone, it is normally either the lower or the upper limit of the dynamic range that is of interest.

Frequency Response

Although particular types of microphone are optimised for particular purposes, they still have a wide operational frequency range. The frequency range is very interdependent with other parameters and the frequency response should, therefore, be considered in relation to other selection requirements such as the type of sound field and the dynamic range.

Polarization

There are two different types of microphone construction, one that employs an external voltage supply to polarize the backplate to the diaphragm air gap (externally polarized), and one where the polarization charge is stored in an electret layer on the backplate of the microphone (prepolarized). Generally there are only small differences between the specifications for externally polarized and prepolarized microphones, but these differences make them suitable for different purposes. Prepolarized microphones are used for portable sound level meters and with DeltaTron preamplifiers. Prepolarized microphones also offer slightly better performance in very humid environments. Externally polarized microphones are generally more useful for general field and laboratory use and for high-temperature measurements. And for special measurements, externally polarized microphones offer a broader range to choose from.

Standards

Abbreviations used for Standards in Tables
	IEC 61094 - 4		IEC 61672		ANSI
A	IEC 61094 – 4 WS1F	I	IEC 61672 Class 1	K	ANSI S1.4 Type 1
B	IEC 61094 – 4 WS2F	J	IEC 61672 Class 2	L	ANSI S1.4 Type 2
C	IEC 61094 – 4 WS3F			M	ANSI S1.12 Type M
D	IEC 61094 – 4 WS1P
E	IEC 61094 – 4 WS2P
F	IEC 61094 – 4 WS3P
G	IEC 61094 – 1 LS1P
H	IEC 61094 – 1 LS3P