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. There are a number of microphones that will work and measure pressure variances. Common diameters for condenser microphones are .125”, 250”, 500” and 1.0”. The trick is to determine which microphone will offer the best solution for a required application.
When choosing the optimum microphone, the parameters to look at include the type of response field, dynamic response, frequency response, polarization type, sensitivity required, 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 Field Types
There are three common application fields for precision condenser microphones. The first and most common is the free-field type. The free-field microphone is most accurate when measuring sound pressure levels that radiate from a single direction and source, which is pointed directly (0o incidence angle) at the microphone diaphragm, and operated in an area that minimizes sound reflections. A freefield microphone is designed to measure the sound pressure at the diaphragm, as it would appear if the microphone were not present. When a 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 the resulting correction factors are applied to the actuator response during calibration. These microphones work best in open areas, where there is no hard or reflective surfaces. Anechoic chambers, or larger open areas are ideal for these Free Field microphones.
Figure 2. Free field
The second type is called a Pressure Field. A Pressure Field microphone is designed to measure the sound pressure that exists in front of the diaphragm. It is described to have 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 to wavelength. The microphone will include the measurement changes in the sound field caused by the presence of the microphone. The sound being measured is typically coming from a single source. Testing of pressure exerted on walls, exerted on airplane wings, or inside structures such as tubes, housings or cavities are examples of Pressure Type microphone applications.
Figure 3. Pressure Field
The third type is called a Random Incident Microphone. This is also referred to as a “Diffuse Field Type.” The Random Incident type of microphone is designed to be omni-directional and measure sound pressure coming from multiple directions, multiple sources and multiple reflections. The Random Incident type microphone will have typical correction curves for different angles of incidence. 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 will be taken into account, in order to come up with a net zero correction factor. When taking sound measurements in a church or in an area with hard, reflective walls, you would utilize this type of microphone.
Figure 4. Random Incident Field
The main criteria to describe sound, is based upon 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 very low and not easily managed, another scale was developed and is more commonly used, called the Decibel (dB). The decibel scale is logarithmic and more closely matches the response reactions of the human ear to the pressure fluctuations. Here are some examples of typical sound pressure levels to use as a reference:
|0 dB = 0.00002 Pa
||Threshold of Hearing
|60 dB = 0.02 Pa
|80 dB = .2 Pa
|94 dB = 1 Pa
|100 dB = 2 Pa
|120 dB = 20 Pa
|140 dB = 200 Pa
||Threshold of Pain
Manufacturers specify the maximum decibel level based on the design and physical characteristics of the microphone. The specified maximum dB level will refer to the point where the diaphragm will approach the backplate, or where Total Harmonic Distortion (THD) reaches a specified amount, typically 3% THD. The maximum decibel level that a microphone will output in a certain application is dependent upon the voltage supplied, and the particular microphones sensitivity. In order to 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. The amount of pressure can be calculated by using the following formula:
Where P = Pascals (Pa) & Voltage is the preamps output peak voltage.
Once the maximum pressure level that the microphone can sense at its peak voltage is determined, this can then be converted to decibels (dB), using the following logarithmic scale:
Where: P = Pressure in Pascals
Po = Reference Pascals (Constant = 0.00002 Pa)
The above formula will provide the maximum rating that a microphone (when combined with a specific preamplifier) can be 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 cartridge thermal noise specification provides the lowest measurable sound pressure level that can be detected above the electrical noise, inherent within the microphone.
The inherent noise level of a microphone and preamplifier combination, will be greatest at both the lower and upper capabilities of the microphone. Each microphone will have its own noise characteristics, and the diameter of the microphone will have a major impact on the frequencies and noise levels of the microphone. Below is a typical representation of the noise effect at different frequencies for a microphone when used in conjunction with a preamplifier.
Figure 5. Typical Noise Floor Data, 1/3 Octave Band Analysis
Proper selection requires that the pressure levels, that are to be tested, fall between the microphones low-end noise level, called cartridge thermal noise, and the maximum rated decibel level of the microphone. In general, the smaller the microphone diameter, the greater the high-end decibel level will be. The larger diameter microphones are recommended for low range decibel measurements, since the inherent noise or cartridge thermal noise specifications are typically lower.
Once the type of microphone field response, and dynamic range has been taken into consideration, the frequency range (Hz) of interest, for the test requirement should be reviewed. Upon inspecting the microphones specification sheet you will find the usable frequency range of the specific microphone. Smaller diameter microphones will usually have a higher upper frequency level capability. Conversely, larger diameter microphones will be able to detect lower frequencies, generally better.
Manufacturers will place a typical tolerance of +/- 2 dB on the frequency specifications. When comparing microphones make sure that 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 individual calibration sheet for a particular microphone in order to determine the actual usable frequency range for specific different decibel tolerances.
As explained previously, test and measurement microphones can be broken down into two categories, traditional Externally Polarized microphones and modern Prepolarized microphones. For most applications either type will work well. The prepolarized tend to be more consistent in 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 – 150o C, externally polarized microphones are a better choice, since the sensitivity level is more consistent in this temperature range.
An Externally Polarized microphone set-up requires the use of a separate 200V power source. 7- conductor cabling with LEMO connectors is required in this set-up. 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 microphone designs are powered by a cost effective and easy-to-operate, 2- 20 mA constant current supply. This can be done with a PCB signal conditioner (or directly by a readout that has a 2-20 mA constant current power built-in.) This design enables the owner to use standard coaxial cables with BNC or 10-32 connectors (in lieu of the 7 Pin conductor cabling with LEMO connectors), for both current supply and signal to the readout device. The prepolarized design also saves set-up time, since it is interchangeable with vibration accelerometers 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.
Temperature will have an effect on the microphones performance. Sensitivity levels can be directly affected by extreme environmental conditions. As the temperature approaches the maximum specifications of the microphone, its sensitivity specification will decrease. The owner will need to be aware of not only the operating temperature, but also the storage temperature of the microphones. If operated and/or stored in extreme conditions, the microphone can be adversely affected and also will also require to be calibrated more often.
When temperature becomes a concern, a probe microphone offers an alternative solution. The probe microphone was designed for sound pressure measurements in harsh environments. It combines a microphone with a probe extension tube. This enables the user to get very close to sound sources. The probe tip will send the acoustic signal to the microphone inside the probe housing. By placing some of the critical components in the separate housing, this microphone type can be used in extremely high temperature applications, or where access to the sound source is too small for a typical condenser microphone.
Applications that require a microphone to be fully submersible provide their own challenges. Hydrophones were designed to detect underwater sound pressure signals. Industrial and scientific underwater testing, monitoring and measurements are accomplished with this corrosion resistant design. Different models are available for different sensitivities, frequencies decibel levels and operating depths.
Sound Level Meters are designed by manufacturers to provide a fast and convenient way to obtain a sound pressure level reading. This design contains all the components necessary to take a sound pressure reading. This small handheld unit includes the microphone, preamplifier, power source, software and display. This is an excellent choice for taking a dB measurement in an industrial setting, for community noise assessment, noise exposure measurements, artillery fire measurements, and many other applications. The Sound Level Meter can be provided with a number of options, including A Weighting, real time analyzers, and software options.
When measurements involving the magnitude and direction of the sound needs to be captured, an intensity probe is an excellent choice. By taking two phase matched microphones and placing a spacer between them, a user can not only tell the pressure level, but also the speed and direction of the propagating sound waves. Different sized spacers are available for measuring the particle velocity at different frequencies. The higher frequencies typically require a smaller spacer. Larger spacers are suitable for lower frequencies and for situations where reverberation is present.
For Near Field Acoustic Holography (NAH) applications where three dimensional field values are to be studied, an Array microphone set-up is recommended. By taking a number of array microphones and spacing them out in a predetermined pattern, and combining them with the appropriate software, spatial transformation of a complex sound pressure field is projected to effectively map the acoustic energy flow. Array microphones are an excellent choice for large channel count acoustic testing. Transducer Electronic Data Sheet (TEDS) are a recommended option for arrays, since they enable the user to quickly and easily identify a particular microphone. These TEDS chips and software enable the user to store information on the microphones model, serial number, calibration date, along with the specifications of the microphones sensitivity, capacitance, impedance, etc…. that can be downloaded and help ensure accurate test results.
Outdoor microphones have been developed to be able to withstand the rigorous environmental exposure that these microphones will be subjected to. Airport noise, or highway traffic noise has become increasing popular spots for test and measurements, to provide safety for humans. The Environmental microphones and Outdoor microphones provide different levels of protection for the internal components, while maintaining their high-accuracy specifications.