GRAS 40GK 1/2" Prepolarized Intensity Microphone Kit

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The 40GK is an IEC 61043-compliant ½" prepolarized sound-intensity microphone pair. Read about the externally polarized equivalent [40AK].

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GRAS 40GK 1/2" Prepolarized Intensity Microphone Kit


The 40GK is an IEC 61043-compliant ½" prepolarized sound-intensity microphone pair. Read about the externally polarized equivalent [40AK].

The microphones in the kitt are two 40GI microphones – two phase-matched free-field measurement microphones with extremely well-controlled phase characteristics for use in sound-intensity probes. This microphone pair has a free-field frequency response that complies with IEC 61043 1993 Class 1 and IEC 60651 Type 1 in the range from 20 Hz to 10 kHz.

The 40GK kit also includes solid spacers for 12 mm, 25 mm, 50 mm, and 100 mm intensity-probe configurations, as well as adapters for the 26CB ¼" CCP preamplifier set. The 40GK is included in the GRAS 50GI CCP Sound Intensity Probe.

40GK is individually factory-calibrated and delivered with a calibration chart, stating its specific open-circuit sensitivity and pressure frequency response.

Typical applications and use

The 40GI is designed for sound-intensity and sound-power measurements, as well as sound-source localization, which is why it is included in the 50GI probe in the 40GK kit. Each 40GI microphone pair is phase matched according to the phase specifications for a Class 1 Sound Intensity Probe in accordance with international IEC 61043, Electroacoustics - Instruments for the Measurement of Sound Intensity - Measurements with Pairs of Pressure Sensing Microphones, 1993.

The quality of a sound intensity probe microphone is its ability to measure the real part of a complex sound intensity in highly reactive sound fields as well as in sound fields exposed to high levels of background noise. This ability depends largely on the phase responses of the microphone pair. The individual phase matching of each 40GI ensures that any differences in phase responses are extremely small.

The 40GI is also used for ranking of sound sources. When mounted in the 50GI sound-intensity probe, the microphones can pinpoint the area generating the most noise, which may then require additional measurements, depending on the measurement procedures and processes used. The sound-intensity microphones reveal the weak and strong points when identifying noise sources. Sound-source ranking is useful as long as the background noise doesn't exceed the noise source.


The 40GK includes straight and right-angled adapters for ¼" CCP preamplifiers, which requires an input module that supports this technology with a BNC or Microdot connector.

In the 50GI Sound-Intensity Probe, the 40GK pair is combined with the 26CB ¼" CCP preamplifier set.

System verification

For daily verification and check of your measurement setup, we recommend using a calibrator like GRAS Sound Level Calibrator 42AB.

For proper sensitivity calibration, we recommend using a pistonphone like GRAS Intelligent Pistonphone 42AP.


When leaving the factory, all GRAS microphones have been calibrated in a controlled laboratory environment using traceable calibration equipment. Depending on the use, measurement environment and internal quality control programs we recommend that the microphone is recalibrated at least once a year.

We offer two kinds of calibration as an optional after-sales service: GRAS Traceable Calibration and GRAS Accredited Calibration.

GRAS Traceable Calibration is a traceable calibration performed by trained personnel under controlled conditions according to established procedures and standards. This is identical to the rigorous calibration that all GRAS microphones are subjected to as an integral part of our quality assurance.

GRAS Accredited Calibration is performed by the GRAS Accredited Calibration Laboratory that has been accredited in accordance with ISO 17025 by DANAK, the Danish Accreditation Fund.

If you want a new microphone set delivered with an accredited calibration in stead of the default factory calibration, specify this when ordering.

Learn more at gras/calib.

Quality and warranty

All GRAS microphones are made of high-quality materials that will ensure life-long stability and robustness. The microphones are all assembled in verified clean-room environments by skilled and dedicated operators with many years of expertise in this field.

The microphone diaphragm, body, and improved protection grid are made of high-grade stainless steel, which makes the microphone resistant to physical damage, as well as corrosion caused by aggressive air or gasses.

This, combined with the enforced gold-plated microphone terminal which guarantees a highly reliable connection, enables GRAS to offer 5 years warranty against defective materials and workmanship.


If you accidentally damage the diaphragm on a GRAS microphone, we can—in most cases—replace it at a very reasonable cost and with a short turn-around time. This not only protects your investment, but also pleases your quality assurance department because you don't have to worry about new serial numbers, etc.