G.R.A.S. 40AZ 1/2" Prepolarized Free-field Microphone, Low Frequency

Freq range: 0.5 Hz to 20 kHz
Dyn range: 14 dB(A) to 148 dB
Sensitivity: 50 mV/Pa


The G.R.A.S. 40AZ 1/2" free-field microphone is a precision condenser microphone for low-frequency (including infra-sound) measurements in open acoustic fields.

As a free-field microphone, 40AZ is designed essentially to measure the sound pressure as it would appear if the microphone were not present, the sound field pointing towards the microphone.

At low frequencies, the disturbing effects of its presence in the sound field are minimal (large wavelengths compared to the size of the microphone).

At higher frequencies (>1 kHz), the effects of diffractions generally cause microphones to measure sound pressure levels increasing with frequency. In a free-field microphone, the effects of diffraction are compensated for to provide a flat frequency response in a free-field for 0º incidence.

The G.R.A.S. 26CG 1/4" high impedance preamplifier is available for use with 40AZ. The mounting thread (11.7 mm - 60 UNS-2) is compatible with microphone preamplifiers for WS2P/F microphones.

Typical applications and use

The low-frequency property combined with its high sensitivity and robust design make 40AZ the obvious choice for infra-sound measurements - a fast growing discipline following the need for monitoring and reducing low-frequency noise from, for example, power and production plants and the increasing number of wind turbines.


The 40AZ requires a standardized 1/2" or 1/4" CCP preamplifier and an input module that supports this technology with a BNC, SMB, or Microdot connector.

To benefit from 40AZ’s low frequency capabilities, it is important that the microphone preamplifier, analyzer input module, or sound level meter is designed to match 40AZ.

System verification

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

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


When leaving the factory, all G.R.A.S. 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: G.R.A.S. Traceable Calibration and G.R.A.S. Accredited Calibration.

G.R.A.S. 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 G.R.A.S. microphones are subjected to as an integral part of our quality assurance.

G.R.A.S. Accredited Calibration is performed by the G.R.A.S. 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/calibration.

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 reinforced 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.

Frequency range (±1 dB) Hz 1 to 10 k
Frequency range (±2 dB) Hz 0.5 to 20 k
Dynamic range lower limit (microphone thermal noise) dB(A) 14
Dynamic range lower limit with G.R.A.S. preamplifier dB(A) 17
Dynamic range upper limit dB 148
Dynamic range upper limit with G.R.A.S. preamplifier @ +28 V / ±14 V power supply dB 142
Dynamic range upper limit with G.R.A.S. preamplifier @ +120 V / ±60 V power supply dB 148
Dynamic range upper limit with G.R.A.S. CCP preamplifier dB 138
Open-circuit sensitivity @ 250 Hz (±2 dB) mV/Pa 50
Open-circuit sensitivity @ 250 Hz (±2 dB) dB re 1V/Pa -26
Resonance frequency kHz 14
Microphone cartridge capacitance, typ. pF 20
Microphone venting Rear
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 -0,008
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 62
CE/RoHS compliant/WEEE registered Yes / Yes, Yes
Weight g / oz 9 / 0,317

G.R.A.S. Sound & Vibration reserves the right to change specifications without notice.


Typical frequency response

Free-field corrections for different angles of incidence

G.R.A.S. CA0001

Traceable Calibration of Microphone

G.R.A.S. CA2001

Accredited Calibration of Microphone

G.R.A.S. Sound & Vibration reserves the right to change accessories without notice.