Selecting and Specifying Measurement Microphones

From Free Field to Random Incidence

Microphones measure broadband sound pressure levels from a variety of sources. When the microphone signal is post processed, the frequencies can be correlated with the sound source, and if necessary, related back to the wavelength of the sound. Acoustical measurement of this sound, through the use of high precision condenser microphones, provides a better understanding of the nature of the sound. In some applications there are a number of microphones that can work and measure the sound pressure level. Common diameters for condenser microphones are 1/4” (6 mm), 1/2" (12 mm), 1” (25 mm). The key is to determine which microphone will offer the best solution for a required application. 

A device called a preamplifier houses the electronics that assist the mechanical microphone in working properly. With some microphone designs, a preamplifier will be housed in the same package as the microphone. In other designs, as is the case with the design which is compliant with the IEC 61094-4 (see page 14) working class standard of microphones, the preamplifier is a separate component. The preamplifier will have an impact on the microphone’s characteristics and may limit its use. For example, if you read a prepolarized microphone’s specification for temperature, it is rated at 120°C. Some people will misconstrue that and presume that you can perform an acoustic measurement test in environments to that 120 °C, only to find that in real life applications they can not, because the preamplifier is rated at 80 °C and thus the limiting factor. Experienced acoustical engineers check not only the microphone specifications, but also the preamplifier, or microphone and preamplifier system specifications.

A single microphone can be used to measure amplitude and frequency characteristics. The amplitude is sound pressure expressed in pascals (Pa) which can be converted to either psi or decibels, and the frequency is represented in hertz (Hz.) Multiple microphones containing excellent phase characteristics, when combined with the correct software, can be used to analyze the particle velocity and direction of sound making them excellent choices for noise source location.

When choosing the optimum microphone, the parameters to look at include the type of response field, polarization type, dynamic response, frequency response, and temperature range. There are also a variety of specialty type microphones for specific applications. In order to select and specify a microphone, the first criteria that needs to be looked at is the application and what the sound and environment represent

Microphones Response Types

There are three response types for precision condenser microphones, which are: Free Field, Pressure, and Random Incidence responses. The output of these three different styles will be similar at lower frequencies, but as the frequency becomes higher, it becomes critical to select the right response type in order to obtain the most accurate test results. For any microphone all three responses can be determined, however, the microphone is named for the response for which it has the flattest frequency response.

The most common is the free field type, shown in the figure to the left. The free field microphone is most accurate when measuring sound pressure levels that radiate from a single direction and source, which is pointed directly (0° incidence angle) at the microphone diaphragm, and operated in an area that minimizes sound reflections. A free field microphone is designed to measure the sound pressure at the diaphragm, as it would appear if the microphone were not present. When any microphone is placed in a sound field, diffraction effects will alter the sound pressure when the frequency is high enough so that the wavelengths are similar in size to the dimension of the microphone. The effect is accounted for in the design of the microphone and calibration to ensure the most accurate test data. Free field microphones work best in open areas, where there are no hard or reflective surfaces. Anechoic chambers or larger open areas are ideal for free field microphones.

The second response type is called a pressure microphone, shown in the figure to the left. A condenser style Pressure microphone is used similarly to a piezoelectric or standard pressure transducer in the sense that it is typically mounted in a duct, or cavity to measure acoustic pressure. The pressure response field microphones are commonly flush mounted to a wall or panel and measure the sound pressure that exists in front of the diaphragm, without the need for it to correct for its own presence. A pressure field is described to have the same magnitude and phase at any position in the field. A pressure microphone is commonly used in an enclosure which is small in size when compared to wavelength. Testing of sound pressure exerted on walls, exerted on airplane wings, or inside structures such as tubes, housings or cavities are examples of Pressure Type microphone applications.

The third type is called a Random Incidence Microphone, shown in the figure to the left. This is also referred to as a “Diffuse Field Type.” The Random Incidence microphone is designed to be omni-directional and measure sound pressure coming from multiple directions, multiple sources and multiple reflections. The random incidence microphone will compensate for its own presence in the field. An average of the net effect of all the calibrated incidence angles is taken into account, in order to output an accurate response in a diffuse field. This is accomplished within the design and calibration by the microphone manufacturer. When taking sound measurements in a reverb chamber, church or in an area with hard, reflective walls, you would utilize a Random Incidence microphone, to accurately measure the sound from multiple sources.

Microphone Size (Diameter)

The microphone size plays a part in the selection process. In general, large diameter microphones have higher sensitivity which are better for low frequency and low noise (ie: computers) measurements, while smaller diameter microphones are better suited for high frequency and high amplitude (ie: gun blast) applications.

Polarization Type

Test and measurement microphones can be broken down into two categories, externally polarized microphones and prepolarized microphones. For most applications either type will work well. The prepolarized are better suited for humid applications. They are recommended when changes of temperature may cause condensation on the internal components. This may short-out externally polarized microphones. Conversely, at high temperatures, between 120 – 150 °C, externally polarized microphones are a better choice, since the sensitivity is more stable in this temperature range, provided that you can isolate the preamplifier or have a preamplifier capable of this same maximum temperature rating.

An Externally Polarized microphone set-up requires the use of a separate 200V power source. The most common set-up consists of 7-conductor cabling with LEMO® connectors and in some cases cost effective 5 pin designs are used. Externally polarized microphones are the traditional design. There are more models available and they are still utilized for special applications or for compatibility reasons.

The modern prepolarized test and measurement microphone designs with an ICP® preamplifier are powered by a cost effective and easy-to-operate, 2-20 mA constant current supply. This can be supplied by an ICP® signal conditioner (or directly by a readout that has a 2-20 mA constant current power built-in.) This design, for example PCB’s 377 and 378 series, enables the owner to use standard coaxial cables with BNC or 10-32 connectors (in lieu of the multiconductor cabling with 7 Pin LEMO® connectors), for both current supply and signal to the readout device. The prepolarized design also saves set-up time, since it can be used simultaneously with ICP® accelerometers, force, strain or pressure sensors that have built-in electronics. This newer design has become very popular in recent years due to its time and cost savings and ease of use characteristics.