Understanding the fundamentals of microphones can help you choose the right equipment for your test environment. Learn more about the types of microphones, components, standards, maintenance, and applications.
Used with permission by PCB Piezotronics ( www.pcb.com)
Pressure variations, whether in air, water or other mediums, that the human ear can detect are considered sounds. Acoustics is the science or the study of sound. Sound can be generally pleasing to the ear, as in music, or undesirable, referred to as noise. The typical audible range of a healthy human ear is 20 to 20,000 Hz. A Sound Pressure Level (SPL) beyond the detectable frequencies of the human ear can also be very important to design engineers. Noise, Vibration and Harshness (NVH) is concerned with the study of vibration and audible sounds. Vibrations represent a rapid linear motion of a particle or of an elastic solid about an equilibrium position, or fluctuation of pressure level.
Harshness refers to the treatments of transient frequencies or shock. Usually treatments are employed to eliminate noise, but in some cases products are designed to magnify the sound and vibration at particular frequencies. The sound produced or received by a typical object, which may be above and below the frequencies that are detectable by the human ear, or amplitudes concerning its resonant frequencies, are important to designers, in order to characterize the items performance and longevity. Microphones can be used to make NVH, or sound, measurements.
When an object vibrates in the presence of air, the air molecules at the surface will begin to vibrate, which in turn vibrates the adjacent molecules next to them. This vibration will travel through the air as oscillating pressure at frequencies and amplitudes determined by the original sound source. The human eardrum transfers these pressure oscillations, or sound, into electrical signals that are interpreted by our brains as music, speech, noise, etc. Microphones are designed, like the human ear, to transform pressure oscillations into electrical signals, which can be recorded and analyzed to tell us information about the original source of vibration or the nature of the path the sound took from the source to the microphone. This is exhibited in testing of noise-reducing materials. Pressure from sound must be analyzed in the design stages to not only protect the materials around it, but also to protect the most precious and delicate mechanism designed to perceive it, the human ear. Like the human ear, microphones are designed to measure a very large range of amplitudes, typically measured in decibels (dB) and frequencies in hertz (Hz.)
To convert acoustical energy into electrical energy, microphones are used. There are a few different designs for microphones. The more common designs are Carbon Microphones, Externally Polarized Condenser Microphones, Prepolarized Electret Condenser Microphones, Magnetic Microphones, and Piezoelectric Microphones.
The carbon microphone design is a value-oriented design. This design is a very low quality acoustic transducer type. An enclosure is built. This enclosure houses lightly packed carbon granules. At opposite ends of the enclosure, electrical contacts are placed, which have a measured resistance. When the pressure from an acoustical signal is exerted on the microphone, it forces the granules closer together. This force presses the granules together, which decreases the resistance. This change in resistance is measured and output. A typical use of this item can be seen in early basic designs of a telephone handset.
A condenser microphone operates on a capacitive design. The cartridge from the condenser microphone utilizes basic transduction principles and will transform the sound pressure to capacitance variations, which are then converted to an electrical voltage. This is accomplished by taking a small thin diaphragm and stretching it a small distance away from a stationary metal plate, called a “backplate.” A voltage is applied to the backplate to form a capacitor. In the presence of oscillating pressure, the diaphragm will move which changes the gap between the diaphragm and the backplate. This produces an oscillating voltage from the capacitor, proportional to the original pressure oscillation.
The backplate voltage can be generated by two different methods. The first is an externally polarized microphone design where an external power supply is used. The power source on this traditional design is 200 volts. The second or newer design is called a prepolarized microphone design. This modern design utilizes an “electret” layer placed on the backplane, which contains charged particles that supply the polarization. This design, when coupled with an Integrated Circuit Piezoelectric (ICP®) circuit can provide great advantages. An inexpensive constant current supply can power the unit, instead of the more expensive externally polarized power supplies. Standard coaxial cables with BNC or 10-32 connectors can be used, instead of LEMO 7-pin connectors and cables. The coaxial cables can be driven long distances without degradation of the signal. The modern prepolarized designs are becoming increasingly popular for laboratory test and measurement, and field applications, due to their low cost and ease of use.
A magnetic microphone is a dynamic microphone. The moving coil design is based on the principal of magnetic induction. This design can be simply achieved by attaching a coil of wire to a light diaphragm. Upon seeing the acoustical pressure, the coil will move. When the wire is subjected to the magnetic field, the movement of the coil in the magnetic field creates a voltage, which is proportional to the pressure exerted on it.
A Piezoelectric microphone uses a quartz or manmade ceramic crystal structure, which are similar to electrets in that they exhibit a permanent polarization and can be coupled with an ICP® design. Although these sensor type microphones have very low sensitivity levels, they are very durable and are able to measure very high amplitude (decibels) pressure ranges. Conversely, the floor noise level on this type of microphone is generally very high. This design is suitable for shock and blast pressure measurement applications.
The most popular test and measurement microphones are the capacitor condenser designs. The focus of the following information will be based on this design.
As mentioned in the previous section, condenser microphones are the most common design for test and measurement. The following figure shows a diagram of its components.
Figure 1: Microphone components
Prior to selecting an acoustical instrument, the consumer should determine whether there are certain standards that the product must comply with, for the application in which the product will be used. Whether for legal purposes, or for quality assurance programs, these standards will help determine the quality, accuracy and consistency of the products. Standards have been set for sound level meters, calibrators, microphones, or other related measurement components. There are numerous standards that correspond to the performance requirements, dimensions and characteristics of acoustical components. The most popular organizations for establishing these standards are the American National Standards Institute (ANSI) and the International Electrotechnical Commission (IEC.)
ANSI S1.4 is the American National Standards Institute’s standard for Sound Level Meters. It deals with the specifications for this product, and was originally written in 1983 and later amended to S1.4A in 1995, and is currently used today. ANSI S1.43 established in 1997 deals with setting a standard for compliance for Integrating-Average Sound Level Meters.
IEC 60942 was created with the specifications on sound calibrators in mind. There are three classes concerned with the standards of sound calibrators. The most stringent is the Laboratory Standard (LS). The LS class has the tightest tolerances and is designated for calibrators that will be used only in the laboratory. Class 1 instruments are considered for field usage applications, as is the class 2 standard. The difference of the class one verses the class 2 calibrator depends upon which class the associated sound level meter, that will be used in conjunction with the calibrated is classified as. A class one calibrator is intended to be used with a class one sound level meter. A class 2 calibrator is designed to be used with a class 2 sound level meter.
IEC 61094 deals with the condenser microphone. It establishes specifications on the mechanical dimensions and certain electroacoustic characteristics for working standard microphones, which are to be used as laboratory reference microphones. These condenser microphones require the highest accuracy, due to the fact that these units will determine the accuracy of other microphones in primary calibrations by the reciprocity method. IEC 61672 was instituted to replace 60651 and 60804. 61672 provide electroacoustical performance standards for Sound Level Meters. There are standards that range from the simplest integrated sound level meter that measures sound exposure levels, to sound level meters that measure time-average sound levels, to conventional sound level meters that measure exponential time-weighted sound levels. A sound level meter can have one, all or a combination of the previously mentioned measurements. There are two classes for the sound level meters. The standards are based around the same design characteristics, but differ in the tolerance limits and operational temperature specifications. To conform to class one or class two, a specified frequency response for sound incident on the microphone from either, one sound source and direction in an acoustic field, or from random directions. Class two tolerances are more liberal, compared to the class one standards.
There are certain classifications that are commonly referenced. Types ranging from Type 0 through Type 2 dictate the tolerance and accuracy of the microphone system. These standards were created for Sound Level Meters, and get commonly referred to when mentioning components of the system, such as the microphone cartridge and the preamplifier. Type 0 refers to Laboratory reference. It is not required to satisfy the environmental requirements for field instruments, but must be is an extremely accurate type, since this is what other microphones will be judged against or tested to. Type 1 is for both laboratory use and for use in the field. The Type 1 standard is extremely accurate and durable. It is designed to take the challenges of the environment and still compile highly accurate and reliable acoustic measurements. This is a very popular type for research and design engineers. Type 2 microphones have standards that are more relaxed and easily achieved. The Type 2 is a general purpose type. These microphones do not have the high-frequency response, low cartridge thermal noise levels, or accuracy as the first two types, but offer a less expensive alternative for when the measurement accuracy is not critical.
Some manufacturers will designate their microphone, preamp or filter as a certain type or to meet a particular standard. In order to conform you must look at the complete system. The whole system is what must meet the standard, not just one component. PCB designs each individual component to exceed the specifications, so that when installed it meets the Type 1 specifications, even after the accumulation of tolerances of each component is taken into consideration. For more information on the above standards, please contact the above organizations. They can answer your questions and send you documentation on the specific standards that you may have an interest in.
After the proper selection of the microphone has been determined, the corresponding preamplifier, and ancillary equipment must be specified and installed. The necessary cabling, power supplies, signal conditioning and data acquisitions selections are to be made. Not all of these components are required for all test set-ups. Figure number 6 will show the typical set-up for an externally polarized system.
Figure 6: Externally polarized set-up
The diagram for the prepolarized system is shown in figure 7 below.
Figure 7: Prepolarized set-up
Components of the prepolarized design may be used with the externally polarized set-up. The prepolarized microphone can be used with a power supply that is designed for externally powered microphones and its associated preamp, provided that you set the supply voltage to zero.
Acoustical testing is performed for a variety of reasons, ranging from the design of new products, to monitoring products, to predictive maintenance functions, to personal protection. Some typical applications, for acoustical studies that require microphones may include:
Excessive sound pressure can cause damage to products or human hearing. Microphones are used to measure the pressure level exerted on a surface. Sound pressure can shake plaster off walls or cause damage to an airplane wing. Sound measurement is used in a variety of applications including: the study of door slams, clutch engagements, starter impact and sunroof noise. Analysis of engine noise in a cabin or car interior, or sound exhibited from consumer appliances are tested to extend the lifespan of the product and keep the external noise minimal, for the comfort of the user.
Increased sound levels, or changes in frequency can indicate that a product is not working to its capacity. Motors, gears, bearings, blades, or other industrial components can all experience changes in decibel level or frequency shift when not working properly. High precision microphones can be utilized to confirm that a product is experiencing a problem, or can be used to predict failure of a component
Universities, governments and independent companies have audio testing equipment to perform hearing tests and research projects. Microphones are used to test and calibrate the systems to ensure the accuracy of the test equipment.
Microphone tests can be performed and recorded for verification of pressure levels on products, and can be utilized in legal situations. Companies will use high precision microphone tests for proof of sound pressure levels during design. Microphones are used on sound level meters to ensure compliance with national standards for shop noise.
There are certain sound pressure levels that the human ear can be subjected to for specific amounts of time before ear damage can occur (dose). A few of these are industrial shop noise, airports, and automotive highway noise. Acoustic testing is performed so that a better understanding of the sound levels that are experienced in these surroundings is achieved, and the necessary adjustments can be made in order to provide greater personal protection. The automotive market will utilize high precision microphones for “Squeak and Rattle” tests in order to provide a quieter ride.
Acoustic holography, and pressure mapping are areas where microphone use has been increasing. Grids of microphones can be set-up to tell the difference in the sound pressure at different points around an engine or a car tire well. Calculations can be made per zone or spectrum. Some applications include Seismic activity monitoring, satellite tracking, and automotive and Industrial noise source identification. Microphones can be utilized to transform 2-dimensional complex sound pressure information, into 3-dimensional acoustic fields, using basic wave equations, to indicate surface intensity and radiation patterns.
Array microphones, are Free Field Type microphones, which are designed to offer a cost effective solution for multiple channel sound measurement. This makes Nearfield Acoustic Halography (NAH) measurements practical. Grids can be constructed to take 2D mapping measurements. The 130D20 and 130D21 have an integrated Microphone and Preamplifier. The 130 series utilizes the Prepolarized microphone design, and incorporate ICP® type circuitry, powered by a constant current signal conditioner. The 130 series provide an inexpensive alternative to the 377 series. The 130 series are accurate for frequency responses and great for trending, but are more sensitive to changes in temperature, and less accurate than the 377 series of high precision condenser microphones, when measuring dB.
Figure 8. Multiple channel array
Microphones are very stable over long periods of time, provided that they are handle properly. Components of the microphone are fragile and can get damaged by misuse. The diaphragm is made up of a very thin proprietary material that should be kept clean of dust, dirt, moisture and any type of imperfection (scratch, dent, etc.) The grid cap is designed to let through the true sound pressure level. The only function for the grid cap is to prevent items from coming in contact with the diaphragm. We do not recommend that you ever remove the grid cap, if possible. We do not recommend cleaning the microphone and if you take the precautionary measures to keep it clean and dry, it should not be necessary. In the event that you absolutely must take off the grid cap and clean the microphone diaphragm, we recommend using a soft cotton swab with a little alcohol. Make sure that the alcohol fully dries before supplying power to, and using the microphone. Do not touch the microphones diaphragm with your fingers or let it come in contact with any sharp or pointed object.
Accessories, like windscreens and desiccants, will help keep moisture off the microphone, and help maintain the specified sensitivity level. Nose Cones will help keep turbulence off the microphone diaphragm and still let the sound pressure level come in unobstructed.
Dust and dirt may not only in adversely affect the microphones performance, but also the preamplifier. If you should get dirt or dust on the backside of the microphone, or inside the connection area of the preamplifier, use a rubber bulb to blow clean, dry air into the unit in order to remove the dust.
Keep the microphone and preamplifier assembled while preparing for testing. Keep the maintenance caps on them and store them in their protective cases, when not in use. With proper maintenance, your microphone and preamplifier should provide stable and accurate results for years to come.
Due to the ease of use and the cost savings mentioned earlier, consumers have been dictating where research and design time be spent. Manufacturers have been coming up with a more diverse prepolarized microphone designs, so that end users have a larger selection to match that of the traditional microphones for different applications.
The rise in applications concerning Nearfield Acoustic Holography (NAH) has driven manufacturers to not only coming up with array microphone designs for large channel counts, but also the ancillary equipment to be enhanced. Array grids, both stationary and linear guided systems are becoming increasingly popular. This in turn has also promoted new designs in software systems designed for sound pressure mapping, large channel signal conditioning units, and newer readout and data acquisition designs that can handle the increased inputs and necessary data output.
Advanced designs of Sound Level Meters are very popular in both the industrial sector, and for health related measurements. End users want items that are both mobile and easy to use. These products come complete in one battery operated hand-held unit that does not require any assembly for daily usage. Sales of this product group continue to be strong, due to its convenient design. Future developments will be in the expanded software systems and designs made for specific target applications.
Transducer Electronic Data Sheets are becoming increasingly popular with the rise of large channel count tests, and for obtaining accurate results on a consistent basis. This system makes it easy to manage a number of microphones, by enabling the user to locate a specific microphone within a group of microphones. Data storage, calibration historical information, and physical characteristics of each individual microphone and preamplifier are easily retrieved, with the TEDS set-up. Software companies and data acquisition companies are placing a higher emphasis to manufacture designs to take advantage this technology, so that the end consumer can reap the rewards.
In general, consumers are looking for smaller packages that will accurately measure sound pressure levels and make their jobs easier and quicker in both the set-up process and data recording areas. They also want the costs to be minimized. The manufacturers will continue to look at designs that will offer the customer the best value for the application.