G.R.A.S. 40GI 1/2" Prepolarized Intensity Microphone Pair

Freq response: IEC 60651 Type 0 IEC
Dyn range: 27 dB(A) to 163 dB
Sensitivity: 12.5 mV/Pa


The 40GI microphone pair consists of two phase-matched free-field condenser microphones with extremely well-controlled phase characteristics for use in sound-intensity probes. Their standard mounting threads make them compatible with all the usual available makes of measurement-microphone preamplifiers.

As a microphone pair, the free-field frequency response of the 40GI complies with IEC 61043 1993 Class 1 and IEC 60651 Type 1 in the range from 20 Hz to 10 kHz.

Two 40GI microphones are included in 40GK microphone kit, which is used in the G.R.A.S. CCP Sound Intensity Probe Type 50GI.

40GI 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. 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 a sound-intensity probe such as the 50GI, 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 40GI requires a standardized ¼" CCP preamplifier and an input module that supports this technology. G.R.A.S. recommends using the G.R.A.S. 26AApreamplifier.

When used with 40GK in the 50GI Sound-Intensity Probe, the 40GI is combined with the 26CB ¼" CCP preamplifier.

System verification

For daily verification and check of your measurement setup, we recommend using a calibrator like G.R.A.S. 42AB Sound Calibrator.

For proper sensitivity calibration, we recommend using a pistonphone like G.R.A.S. 42AP Intelligent Pistonphone.

Quality and warranty

All G.R.A.S. 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 G.R.A.S. to offer 5 years warranty against defective materials and workmanship.


If you accidentally damage the diaphragm on a G.R.A.S. 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.


Before leaving the factory, all G.R.A.S. microphones are calibrated in a controlled laboratory environment using traceable calibration equipment.

Depending on the use, measurement environment, and internal quality control programs, we recommend recalibrating the microphone at least once a year.

Frequency range (±1 dB) Hz IEC 60651 Type 0 IEC
Dynamic range lower limit (microphone thermal noise) dB(A) 20
Dynamic range lower limit with G.R.A.S. preamplifier dB(A) 27
Dynamic range upper limit dB 163
Dynamic range upper limit with G.R.A.S. preamplifier @ +28 V / ±14 V power supply dB 153
Dynamic range upper limit with G.R.A.S. preamplifier @ +120 V / ±60 V power supply dB 163
Dynamic range upper limit with G.R.A.S. CCP preamplifier dB 150
Polarization voltage V 0 V / CCP
IEC 61094-4 Compliance WS2P
Temperature range, operation °C / °F -40 to 120 / -40 to 248
Temperature range, storage °C / °F -40 to 85 / -40 to 185
Temperature coefficient @250 Hz dB/°C / dB/°F -0.01 / -0.006
Static pressure coefficient @250 Hz dB/kPa -0.014
Humidity range non condensing % RH 0 to 100
Humidity coefficient @250 Hz dB/% RH -0.001
Influence of axial vibration @1 m/s² dB re 20 µPa 66
CE/RoHS compliant/WEEE registered Yes / Yes, Yes
Weight g / oz 7 / 0.247

Optional items

G.R.A.S. 26AA Set

1/4" Preamplifier with Integrated Cable for Intensity Probes

G.R.A.S. 42AB

Sound Calibrator, Class 1

G.R.A.S. 42AP

Intelligent Pistonphone, Class 0