G.R.A.S. 40AM 1/2" Prepolarized Free-Field Microphone, Wide Frequency

Freq range: 3.15 Hz to 31.5 kHz
Dyn range: 20 dB(A) to 163 dB
Sensitivity: 14.5 mV/Pa


G.R.A.S. 40AM is a high-precision electret condenser microphone made according to IEC 61094-4 requirements. The electret technology for microphones was introduced in the beginning of the 1960’s and has been continuously improved over the years. At G.R.A.S., we have developed and refined our own techniques. This guarantees that our prepolarized microphones are as stable and robust as our traditional, externally polarized versions in daily use scenarios.

The 40AM’s design is based on the original reference ½” externally polarized pressure microphone, but is optimized for free-field measurements. This results in a doubling of the upper 3 dB frequency limit. All other parameters remain unchanged, making 40AM the perfect free-field complement to the G.R.A.S. 40AO ½” Prepolarized Pressure Microphone. 

40AM is individually factory-calibrated and delivered with a calibration chart stating its specific open-circuit sensitivity, pressure, and corrected free-field frequency responses. 

Typical applications and use

The compact design and low sensitivity make 40AM very robust and stable. This, combined with its wide frequency range, makes it an all-round free-field microphone suitable for both field-work and laboratory measurements. 


The 40AM 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. 42AB Sound Level 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 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.


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 5 to 16 k
Frequency range (±2 dB) Hz 3.15 to 31.5 k
Frequency range (±3 dB) Hz 3.15 to 40 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 152
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 149
Open-circuit sensitivity @ 250 Hz (±2 dB) mV/Pa 14.5
Open-circuit sensitivity @ 250 Hz (±2 dB) dB re 1V/Pa -37
Resonance frequency kHz 32
Microphone cartridge capacitance, typ. pF 17
Microphone venting Rear
IEC 61094-4 Compliance WS2F
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 6.5 / 0.229

Typical frequency response. Upper curve shows free-fi eld response for 0°, lower curve shows pressure response.

Free-field corrections for different angles of incidence