G.R.A.S. 40AO 1/2" Prepolarized Pressure Microphone

Freq range: 3.15 Hz to 20 kHz
Dyn range: 20 dB(A) to 163 dB
Sensitivity: 12.5 mV/Pa
  • 40AO 1/2

Typical applications and use

The broad working range and reliability has made 40AO the preferred prepolarized microphone for coupler and ear simulator setups and is the natural supplement or replacement of existing solutions. The 40AO is especially suited for audiometer calibration when using a sound level meter / analyzer without a 200 V polarization feature.

It is used for production line testing in coupler setups for test of hearing aids, earphones, headphones, and headsets.

The KEMAR Manikin can also be configured with 40AO for wide-band binaural sound quality recordings.

40AO can be used in

  • G.R.A.S. IEC 60318-1 Ear Simulator RA0039
  • G.R.A.S. IEC 60318-5 2cc Coupler RA0038 (incl. protection grid)
  • G.R.A.S. KEMAR Manikin 45BA

40AO is included in

  • G.R.A.S. IEC 60318-4 (60711) Ear Simulator RA0045-S1
  • G.R.A.S. IEC 60318-1 & -2 Ear Simulator System Type 43AA-S2

Other coupler systems can be configured with the 40AO and a CCP preamplifier on request.

The 40AO is a very versatile microphone when connected to the RA0067 Transmitter Adapter for ½" Microphones. The 40AO becomes a high-impedance sound source when the RA0067 takes a calibration signal directly from the signal generator and makes the microphone behave like an electrostatic loudspeaker. In a coupler, this gives a frequency response that is as good as when the 40AO behaves like a microphone.


The 40AO requires a standardized ½" or ¼" CCP preamplifier and an input module that supports this technology with a BNC, SMB, or Microdot connector.

System verification

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

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


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/calib.

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 5 to 12.5 k
Frequency range (±2 dB) Hz 3.15 to 20 k
Dynamic range lower limit (microphone thermal noise) dB(A) 20
Dynamic range lower limit with G.R.A.S. preamplifier dB(A) 25
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
Open-circuit sensitivity @ 250 Hz (±3 dB) mV/Pa 12.5
Open-circuit sensitivity @ 250 Hz (±3 dB) dB re 1V/Pa -38
Resonance frequency kHz 32
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 -0.007
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 6.5 / 0.229

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.