G.R.A.S Sound & Vibration

Since its establishment in 1994, GRAS has been totally dedicated to the development and manufacture of high-quality measurement microphones and related acoustic equipment. It was founded by the Danish acoustics pioneer, Gunnar Rasmussen, who has been a major contributor to the world of sound and vibration with his unique ideas and designs. From the first reproducible 1” condenser measurement microphone that enabled quality measurements and instrumentation for acoustic calibration, Gunnar’s ingenuity and foresight led to the world’s most popular acoustic sensor: the ½” measurement microphone. The ¼” and 1/8” microphones followed with outstanding dynamic and high-frequency capability that brought higher definition and transparency into impulse noise diagnostics. Many variants have been made available for the years; all based on Gunnar’s original 1” pressure microphone design.

Measurement Microphones

The measurement microphone sets have been combined so they fulfil our users’ typical measurement needs. Independently of your measurement system and application you should be able to find a set that suits your needs. The microphone sets can be connected directly to all professional measurement systems and as indicated they are available for both CCP and 7-pin LEMO inputs.

If your measurement platform supports intelligent transducers according to IEEE 1451.4 (TEDS) you can simply plug in the microphones and they will identify themselves with their specific properties, types and calibration data. A feature especially appreciated by multi-channel users.

The CCP sets use high-quality coaxial cables, whereas the LEMO sets use a special, soft type of multicore shielded cable. If extended cables are to be used you should consider the possible influence on the upper frequency and dynamic ranges.

All microphone sets are delivered as a unit and are calibrated accordingly. The sets are delivered with calibration charts including sensitivity values and frequency response curves for the complete set. The sensitivity value can therefore be used directly in your system setup.

For frequent verification of the measurement chain a sound source will be required. GRAS supplies a 114 dB sound calibrator for this purpose. Depending on the use and your internal quality control requirements we recommend that the sets are recalibrated at least every second year. For a more accurate verification we recommend calibration with a pistonphone. GRAS supplies 114dB and 134dB pistonphones. Further, we recommend calibration of the sound sources against an extreme reliable reference. GRAS offers calibration services and recommends such calibration every second year.

Head & Torso Simulators

Audio design is becoming a vital part of the development of a lot of daily products like automobiles, computers, hearing aids, mobile phones etc. Either to improve on its basic function, or to optimize the clients' experience with the product.

KEMAR was initially invented in collaboration with the audiological industry for the use of hearing aid development, and is still the defacto standard for this industry – however since then the usage of KEMAR has spread into a multitude of other industries like: telecommunications, hearing protection test, automotive development etc.

The major difference between KEMAR and the standard head and torso simulators (HATS) on the market is that KEMAR is built on a large statistical research of the average human body – meaning the KEMAR HATS has the same acoustical properties as an average human, including distinct facial features.

Through the years GRAS has developed a range of accessories and special versions of KEMAR – allowing you to design a test setup that can allow you to research in your specific industry, whether this is for development of the next generation mobil phones or the next super car.

KEMAR meets the international standards that are specified by ISO, IEC as well as ANSI.

The KEMAR manikin is the recognised industry-standard for in-situ anthropomorphic testing in the fields of:

  • Telecommunications
  • Hearing conservation
  • Noise abatement
  • Sound recording and sound-quality evaluation.
Test Fixtures

Including products for ear- and headphone testing, hearing-protector testing and telephone testing GRAS' test fixtures are top of the line solutions to fulfill your every need.

Hemisphere Kits

The GRAS hemispheres are compliant with the ISO 3744, 3745 and 3746 (ANSI S12.54, S12.55, S12.56) standards and accommodate for 4, 10 and 20 positions. These are clearly marked to ease the mounting and maintain measurement repeatability.

Calibration Equipment

Calibration is an essential step in every precision measurement. It establishes the relationship between the sound pressure and the electrical output of the microphone.

Calibration can be performed at either 250 Hz or at 1 kHz. Both frequencies have their advantages and disadvantages.

250 Hz is in the frequency range where the frequency response of almost all microphones is flat and will therefore give the most accurate calibration.

If, however, the microphone is used in connection with measuring equipment with weighting filters, for example a sound level meter with A-weighting, the 250 Hz signal will be attenuated by the weighting network. In this case it might be advantageous to use a 1 kHz calibration frequency.

At 1 kHz, the weighting filters have 0 dB attenuation and will therefore not affect the calibration.

On the other hand, since the frequency response of the microphone is not flat, a small error could be introduced.