Special microphones

Special microphones are often required for applications where there are particular requirements regarding the methods of measurements and configurations.

GRAS special microphones are categorized as:

Array microphones

Array microphones are designed to be mounted on large or small arrays, for example for the analysis of sound fields, sound power and transients. Such systems are typically used for measuring and locating noise sources, and here the phase match is important to get good accuracy in the measurements. An important characteristic of array microphones are precisely that the microphones are phase-matched.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.

Flush-mount microphones

Our line of flush-mount sensors combines the high precision and reliability of GRAS measurement microphones with the need for very low installation height to fit the sensors into very confined spaces and narrow structures, e.g. in acoustic antennas and beams.

With an installation height of less than 10 mm and thin coax wiring, the flush-mount series can be integrated into literally any design without sacrificing aero-dynamic properties. They have integrated preamplifier with plug & measure functionality, wide linear frequency range and wide dynamic range.

Infra-sound Microphones

Normal measurement microphones have a very wide frequency range and cover most practical applications. There are however special situations where special microphones are required. One such application is the measurement of infra-sound. Normal measurement microphones have a low-frequency cut-off at about 3 Hz, but in some applications it is required to measure down to close to 0 Hz. Examples of such applications are the measurement of sonic booms and explosions (both chemical and nuclear), and noise generated by machinery such as diesel engines, wind turbines and specially designed mechanical transducers.

To achieve a very low low-frequency cut-off of down to 0.09 Hz, a special microphone combined with a special preamplifier and a low-frequency adapter is used. To account for pressure variations close to 0 Hz, a special ambient pressure equalization system with a very long settling time is used. For this reason, it is necessary to allow the microphone system sufficient time to settle if it has been subjected to fluctuations in ambient pressure, for example from changes in altitude, wind or a closing door. Microphone, preamplifier and low-frequency adapter are assembled and calibrated as a complete unit and should not be separated by the end-user.

A standard sensitivity calibration at 250 Hz can be performed using a Pistonphone. A low-frequency calibration requires special equipment and must be performed at our calibration laboratory.

Intensity Microphones

Intensity Probes

The GRAS intensity probes comprise two closely spaced, face-to-face microphones and a set of 1/4-inch preamplifiers as well as a control handle. The 1/2-inch and 1/4-inch intensity microphone pairs have been carefully manufactured and selected to have minimum phase difference.

Lownoise measuring systems

Normal measurement microphones 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 1 kHz.

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 measurements.

The special preamplifier and matching circuit require a higher supply current than can be obtained from traditional microphone preamplifier supplies, the Type 40HH and Type 40HF must be used together with the Type 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 pistonphones Type 42AA or Type 42AP is available to reduce the level from 114 dB to 94 dB.

Outdoor microphones

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. GRAS offers two types;

• Permanent Outdoor Microphone Systems that are designed to be part of permanent noise surveillance systems, i.e. for airport and community noise surveillance.They can withstand all weather year after year. They are equipped with four-pronged anti-bird spikes on top of the windscreen. This prevents birds from perching on top of the windscreen when the microphone unit is placed outdoors for long-term unattended noise surveillance. Birds perching on top of the windscreen can seriously disturb measurements or even overload the measurement equipment. Furthermore, bird excreta can distort the acoustic properties of the windscreen so that it no longer performs according to original specifications. Some smaller birds have actually been known to nest on top of the earlier three-pronged anti-bird spikes. Hence, the introduction of a fourth, centre prong.
• Outdoor Microphones designed for unattended outdoor use for very long periods. They have state of the art weather protection. Extensive use of high-grade stainless steel enables them to withstand a wide range of weather conditions and temperatures for a very long period, i.e. easily a year or longer. The protection grid is a special design that offers added water protection. The wind screen eliminates the effects of air flow and also serves as part of the water protection. In addition to its acoustical function, the top cone acts as a bird spike.

Both types are available with 0° incidence for airport noise monitoring and 90° incidence for community and traffic noise monitoring. All GRAS versions comply with the relevant international standards for noise monitoring.

Plate microphone kits

Probe microphones

Probe microphones are used for measurements in difficult or inaccessible situations, for example at high temperatures or conditions of airflow. The 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.

Surface microphones

The GRAS surface microphones are specifically designed for in-situ boundary layer testing, where non-intrusive mounting is essential. This eliminates the need to drill holes at the mounting location. Their low-profile design minimizes height, and they can be installed with a surrounding fairing to reduce self-generated turbulence. Equipped with an integrated preamplifier, they offer true plug-and-play functionality.

The 40PS-1 model features a cost-effective microphone capsule and is 2.8 mm thick. The 40LA and 40LS models use high-precision microphone capsules, with a reduced thickness of 2.5 mm. The 48LA and 48LX Ultra-Thin-Precision (UTP) microphones leverage cutting-edge manufacturing techniques, enabling a high-quality condenser microphone to fit into an ultra-slim 1 mm housing, while ensuring the solution remains cost-effective.

GRAS surface microphones are engineered for quick, easy mounting and rapid in-situ verification. They are ideally suited for boundary layer measurements, aeroacoustic testing, and applications involving small cavity measurements

Turbulence Screen Microphones