Microphones

Broad range of standard measurement microphones and preamplifiers for precise and reliable measurement, and recording of acoustic signals

G.R.A.S. has concentrated its efforts and expertise on the development and production of front-end acoustic products. This includes in principle all products necessary for the precise and reliable measurement, and recording of acoustic signals, from the transducer to the input of the A/D converter. The main line of instrumentation includes a broad range of standard measurement microphones and preamplifiers, all designed and manufactured in accordance with international standards. These are complemented by a wide range of more specialized transducers and accessories for more specific applications such as sound intensity microphones, artificial ears, ear and mouth simulators, telephone-testing equipment, HATS, piston phones and calibrators etc.


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MICROPHONE SELECTION
Measurement microphones are available in many types covering various frequency ranges, dynamic ranges and application situations. The following guide is provided to help in selecting the right microphone for a given application. Selecting a microphone involves a number of choices, which can be summarized as: • Externally-polarized vs. prepolarized •Free-field, Pressure-field or Random-incidence • Dynamic range • Frequency range
Surface microphones for general purpose measurements on planar and curved surfaces. Wide useful frequency range reaching up to 20kHz and a large dynamic range topping at around 136 dB.

Surface Microphones

Array Microphones

Array microphones
for situations where concurrent measurements are required at several points in an array. For example in the analyses of: • Sound fields • Sound power • Transients Also of particular value when measurement opportunities are few and far between either because of costs or repeatability. Close manufacturing tolerances together with the advantages of the TEDS chip, provide these array microphones with a high degree of interchangeability; a major advantage when used in multiples forming arrays and matrices. All have a coaxial SMB output connector. See also Array Modules PR0001, PR0001-1 and PR0002.

Probe Microphones

Probe microphones
for measurements in difficult or inaccessible situations, for example at high temperatures or in conditions of airflow. Its right-angled design makes it particularly well suited for measurements In exhaust systems and machinery in general, as well as for scanning surfaces such as loudspeakers and cabinets. The small size, low weight and all stainless steel design of the probe’s tip make it robust, durable, easy to handle and simple to mount.

Window Microphones

Window microphones for architectural acoustics.
The output from a condenser microphone is a very high-impedance signal and is therefore very sensitive to the capacitive loads of cables. This makes it necessary to introduce a driver with high input impedance and low output impedance. Such a driver is called a preamplifier. The frequency range of a preamplifier is determined by its electronic circuit and is typically more than 200kHz at the high end and 1 – 10Hz at the lower end. The lower end is determined by the input impedance of the preamplifier and the capacitance of the microphone. High microphone capacitance gives a low cut off frequency. The dynamic range of a preamplifier is defined as the range between the highest level the preamplifier can handle without distortion, and the lowest level it can measure. The highest level is related to the preamplifiers supply voltage, whereas the lowest level is related to the electrical noise generated by the preamplifier itself. There are today two different preamplifier principles in the world of acoustics. One is the traditional type for externally-polarized microphones often referred to as the “LEMO” type because of its 7-pin connector and has become an industry standard. It is voltage driven and can handle high voltage signals of up to 50Vpeak.
1-2 inch Preamplifier - General-purpose preamplifier 7-pin LEMO connector
1-2 inch Preamplifier - General-purpose preamplifier 7-pin LEMO connector
1-2 inch Preamplifier - General-purpose preamplifier 7-pin LEMO connector
1/2 inch Preamplifier – General-purpose preamplifier 7-pin LEMO connector
1/4 inch Preamplifier – General-purpose preamplifier 7-pin LEMO connector
1/4 inch Preamplifier – General-purpose preamplifier 7-pin LEMO connector & 14” to 12” adapter
The other principle uses a Constant Current Power (CCP) supply and was introduced around 1996 to the world of high-precision acoustics. Before that, the quality of CCP preamplifiers was not as good as the voltage driven LEMO types, but that is not the case today. A CCP preamplifier uses a Constant Current Power supply, which must lie between 2mA and 20mA (nominally 4mA), to produce a constant nominal voltage level of 12 Volt DC (referred to as the bias voltage). The output signal from the microphone superimposes fluctuations around this DC level. The great advantage of CCP preamplifiers is that they use a two wire system where the signal is superimposed on the wire through which the current is kept constant. This means that simple co-axial cables can be used instead of the more complex 7-core cables used with the voltage driven LEMO types. This is traded off by accepting a lower upper limit in dynamic range (due to the lower driving voltage of a constant-current source) which limits the maximum output signal to approximately 8Vpeak, and the necessity of having to use repolarized microphones. The range of available prepolarized microphones is still not as wide as for externally-polarized microphones, although G.R.A.S. was the first in the world to introduce 1/4” prepolarized microphones, and have just released a 1/8” prepolarized microphone.
1-2 inch CCP Preamplifier -with gain and filters
1-2 inch CCP Preamplifier -with gain and filters
1/2 inch CCP Preamplifier with gain and filters
1/4 inch CCP Preamplifier
G.R.A.S. microphone preamplifiers are all small robust units optimised for acoustical measurements with condenser microphones. They are all compatible with measurement microphones as defined in the international standard IEC 61094 “Measurement Microphones, Part 4: Specifications for working standard microphones”. All G.R.A.S. preamplifiers are built around a small, thick-film precision amplifier with very high input impedance. The casings are made of stainless steel for maximum strength and durability with minimal sensitivity to vibration and microphonics. They will work within their specifications up to a temperature of 70°C (158°F). Special versions for use at temperatures up to 120°C (248°F) are available on request. The only effect of elevated temperature is a slight increase in inherent noise level. This will change the lower limit of the dynamic range of the microphone/preamplifier combination, thus limiting the ability to measure very low sound pressure levels. CCP (Constant Current Power) is the same as IEPE (Integrated Electronic Piezo-Electric) and CCLD (Constant. Current Line Drive) and is compatible with many other constant current driven products.

Transducer Electronic Data Sheet (TEDS)
IEEE P1451.4 is a standard for adding plug and play capabilities to analogue transducers. The underlying mechanism for plug and play identification is the standardization of a Transducer Electronic Data Sheet (TEDS).

A TEDS contains the critical information needed by an instrument or measurement system to identify, characterise, interface, and properly use the signal from an analogue sensor. The TEDS resides in embedded memory, typically an EEPROM, within the analogue transducer, as defined in the IEEE P1451.4 standard. IEEE P1451.4 defines the method of encoding TEDS information for a broad range of sensor types and applications. In order to cover such a broad range while also keeping memory usage to a minimum, the IEEE P1451.4 TEDS concept utilises templates that define the specific properties for different sensor types. Instrumentation and software to read TEDS information directly from the transducer is available, for example, from National Instruments, 01dB, OROS, etc.

1-2 inch TEDS Free-field Microphone1/2 inch TEDS Free-field Microphone
1-4 inch TEDS Pressure-field Microphone
1/4 inch TEDS Pressure-field Microphone

G.R.A.S. Sound & Vibration A/S is a member of the National Instruments Alliance Program and can offer a wide range of precision measurement instrumentation directly compatible, for example, with the NI 44XX series DSA devices. Each G.R.A.S. TEDS preamplifier and microphone combination is first sealed with a sticker. The TEDS is then programmed with the calibration information for the specific combination using IEEE P1451.4 TEDS template.

Each preamplifier and microphone described in the following is calibrated together as a combined unit and the information is stored in the built-in TEDS of the preamplifier.

Unprotected measurement microphones are sensitive to environmental factors such as wind, rain and snow. This shortcoming has been eliminated by specially designed units that protect the microphone and its diaphragm From the effects of outdoor use. Each has a Windscreen surmounted by four-pronged anti-bird spikes to prevent birds using it as a perch.

Perching birds and their excreta can seriously distort measurements or even overload the measurement equipment. Smaller birds have actually been known to nest on top of the earlier three-pronged anti-bird spikes. Hence, the introduction of the fourth, centre prongs.

Two types are available:

Outdoor Microphones for permanent outdoor use which are encased stainless steel units that can withstand all weathers year after year e.g. in airport noise monitoring systems.

Environmental Microphones are smaller units meant for temporary outdoor use in terms of days or weeks, such as in time limited consultant work.

Each type is available in the following two versions:

  • For airport noise monitoring, where the measurement direction points upwards (0º incidence).
  • For community noise- or traffic noise measurements, where the measurement direction is in the horizontal plane (90º incidence).

G.R.A.S. Sound & Vibration has more than 1500 of these units deployed all over the world, from the arctic cold in Norway to the humid jungles of Malaysia.

Outdoor Microphones, 0 & 90 deg IncidenceOutdoor Microphones, 0 & 90 deg incidence
Environmental MicrophonesEnvironmental Microphones

Normal measurement microphones as shown on have a very wide dynamic range and cover most practical applications. There are however special situations where special microphones are required. One such application is in the measurements of very low sound pressure levels. Normal measurement microphones have a noise floor around 10 dBA re. 20 µPa in 1/3 octave bands, while the human ear is able to detect levels down to around 0 dB. In fact the 0 dB level was originally defined as the threshold of the human hearing ability at 1kHz.

In some applications it is required to measure down to and below the threshold of the human ear. This is possible by using special high sensitive microphones combined with special low noise preamplifiers.

One of the applications of such microphones may be the measurement of the sound power of high-end personal computers. These are not only used in noisy office environments, but tend to move into living rooms, meeting rooms and hotel rooms. In some hotel rooms the traditional TV set has been replaced by a computer, delivering not only all TV channels, but also Pay channels, account status, wake up calls and other services. This requires the computer to be turned on all the time, and to avoid disturbances during sleep the noise level has to be below the threshold of hearing. In turn, this requires that component manufacturers of hard drives, fans, etc. also deliver very low noise devices.

In order to achieve the very low noise floor of the microphone and preamplifier, these have been specially matched and adjusted together. This further enables the microphone/preamplifier combination to be switched to be used for free-field measurements or for pressure-field measurements.

The special preamplifier and matching circuit require a higher supply current than can be obtained from traditional microphone Preamplifier supplies, the 40HH and 40HF must be used together with the 12HF low noise system power supply. To avoid damaging traditional microphone preamplifier supplies, the 7-pin LEMO on the low noise preamplifier is different from the 7-pin LEMO normally used for microphone preamplifers. The very high sensitivity of low noise microphones means that the sound pressure level used for calibration should be limited to 94 dB to avoid overloading. A special coupler, RA0090, for the 42AA or 42AP pistonphone is available to reduce the level from 114 dB to 94 dB.

1 inch Low Noise Microphone System1 inch Low Noise Microphone System
1-4 inch Low Noise Microphone System1/4 inch Low Noise Microphone System