You must choose the microphone that is best for the type of field in which you will operate it. The three types of measurement microphone are free field, pressure field, and random incidence. These microphones operate similarly at lower frequencies but differently at higher frequencies.
A free-field microphone measures the sound pressure from a single source directly at the microphone diaphragm. It measures sound pressure as it existed before the microphone was introduced into the sound field. These microphones work best in open areas free of hard or reflective surfaces. Anechoic chambers or larger open areas are ideal for free-field microphones.
Figure 3. Free-Field Microphone
A pressure-field microphone is designed to measure the sound pressure in front of the diaphragm. It has the same magnitude and phase at any position in the field. It is usually found in an enclosure, or cavity, which is small when compared with wavelength. Pressure-field microphone application examples include testing the pressure exerted on walls, on airplane wings, or inside structures such as tubes, housings, or cavities.
Figure 4. Pressure-Field Microphone
In many situations, the sound is not traveling from a single source. Random-incidence or diffuse-field microphones respond uniformly to sounds arriving simultaneously from all angles. You use this type of microphone when taking sound measurements in a church or an area with hard, reflective walls. However, for most microphones, the pressure and random-incidence responses are similar, so pressure-field microphones are often used for random-incidence measurements.
Figure 5. Random-Incidence Microphone
The main criterion for describing sound is based on the amplitude of the sound pressure fluctuations. The lowest amplitude that a healthy human ear can detect is 20 millionths of a pascal (20 μPa). Since the pressure numbers represented by pascals are generally low and not easily managed, another, more commonly used scale, the decibel (dB) scale, was developed. This logarithmic scale more closely matches the response reactions of the human ear to the pressure fluctuations.
Manufacturers specify the maximum decibel level based on the design and physical characteristics of the microphone. The specified maximum dB level refers to the point where the diaphragm approaches the backplate, or where total harmonic distortion (THD) reaches a specified amount, typically 3 percent THD. The maximum decibel level that a microphone outputs in a certain application depends on the voltage supplied and that particular microphone’s sensitivity. Before you can calculate the maximum output for a microphone using a specific preamplifier and its corresponding peak voltage, you first need to calculate the pressure in pascals that the microphone can accept. You can calculate the amount of pressure using the following formula:
Where P = pascals (Pa) and voltage is the preamplifier output peak voltage.
After determining the maximum pressure level that the microphone can sense at its peak voltage, you can convert this amount to decibels (dB) using the following logarithmic scale:
Where P = pressure in pascals
Po = reference pascals (constant = 0.00002 Pa)
This formula provides the maximum rating that a microphone, when combined with a specific preamplifier, is capable of measuring. For the low-end noise level, or minimum amount of pressure required, you need to review the cartridge thermal noise (CTN) rating of the microphone. The CTN specification provides the lowest measurable sound pressure level that can be detected above the electrical noise inherent within the microphone. Figure 6 shows the typical representation of the noise level at different frequencies for a microphone when used in conjunction with a preamplifier.
Figure 6. The inherent noise level is greatest at upper and lower capabilities of the microphone.
When selecting a microphone, you must confirm that the pressure levels you are testing fall between the microphone’s CTN and the maximum-rated decibel level of the microphone. In general, the smaller the microphone diameter, the greater the high-end decibel level is. The larger diameter microphones typically have lower CTN, so they are recommended for low-range decibel measurements.
After you consider the type of microphone field response and dynamic range you need, review the microphone’s specification sheet to find the usable frequency range (Hz). Smaller diameter microphones usually have a higher upper frequency level capability. Conversely, larger diameter microphones are more sensitive and better suited to detect lower frequencies.
Manufacturers place a typical tolerance of ±2 dB on the frequency specifications. When comparing microphones, make sure you check the frequency range and the tolerance associated with that specific frequency range. If an application is not critical, you can improve the usable frequency range for that microphone if you are willing to increase your allowable decibel tolerance. You can check with the manufacturer or look at the calibration sheet for a particular microphone to determine the actual usable frequency range for specific decibel tolerances.
Traditional externally polarized and modern prepolarized microphones work well for most applications, but they do have some differences. Externally polarized microphones are recommended for high temperatures (120 °C to 150 °C) because the sensitivity level is more consistent in this range. Prepolarized microphones tend to be more consistent in humid conditions. Sudden changes in temperature that result in condensation on internal components may short out externally polarized microphones.
Because externally polarized microphones require a separate 200 V power source, you are limited to 7- conductor cabling with LEMO connectors in this setup. The newer, prepolarized microphones have become more popular because they are powered by an easy-to-use, 2–20 mA constant current supply. With this design, you can use standard coaxial cables with BNC or 10-32 connectors for both current supply and signal to the readout device.
Microphone sensitivity decreases as the temperature approaches the maximum specifications of the microphone. You must be aware of not only the operating temperature but also the storage temperature of the microphone. Operating and/or storing a microphone in extreme conditions can adversely affect it and increase its calibration needs. In many cases, the required preamplifier can be the limiting factor for operating temperature range. Although most microphones can operate to 120 °C without any loss of sensitivity, the preamplifiers required for these microphones typically operate in the 60 °C to 80 °C range.