G.R.A.S. 40BF 1/4" Ext. Polarized Free-field Microphone

Freq range: 4 Hz to 100 kHz
Dyn range: 30 dB(A) to 172 dB
Sensitivity: 4 mV/Pa


It’s a low sensitivity microphone that is ideal for high-level and high-frequency measurements. It can measure high sound pressure levels of up to 166 dB due to its low sensitivity. Its frequency range reaches up to 100 kHz due to its small size. 

40BF 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 40BF is typically used for high frequency and high-level measurements, and acoustic-transient measurements. 

40BF is included in G.R.A.S. 46BF ¼” Free-Field Set.


The 40BF requires a standardized ½” or ¼” LEMO preamplifier and an input module that supports this technology with a 7-pin LEMO connector and polarization and supply voltages for microphone and preamplifier.

System verification

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

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/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 10 to 40 k
Frequency range (±2 dB) Hz 4 to 100 k
Dynamic range lower limit (microphone thermal noise) dB(A) 30
Dynamic range lower limit with G.R.A.S. preamplifier dB(A) 35
Dynamic range upper limit dB 172
Dynamic range upper limit with G.R.A.S. preamplifier @ +28 V / ±14 V power supply dB 163
Dynamic range upper limit with G.R.A.S. preamplifier @ +120 V / ±60 V power supply dB 172
Open-circuit sensitivity @ 250 Hz (±3 dB) mV/Pa 4
Open-circuit sensitivity @ 250 Hz (±3 dB) dB re 1V/Pa -48
Resonance frequency kHz 100
Microphone cartridge capacitance, typ. pF 7
Microphone venting Rear
IEC 61094-4 Compliance WS3F
Temperature range, operation °C / °F -40 to 150 / -40 to 302
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 -0.02
Humidity range non condensing % RH 0 to 100
Humidity coefficient @250 Hz dB/% RH -0.0013
Influence of axial vibration @1 m/s² dB re 20 µPa 60
CE/RoHS compliant/WEEE registered Yes / Yes, Yes
Weight g / oz 1.75 / 0.062

Typical frequency response (without protection grid). Upper curve shows free-feld response for 0°, lower curve shows pressure response.

Free-field corrections for different angles of incidence

Optional items

G.R.A.S. RA0086

Transmitter adapter for ¼” microphones

G.R.A.S. CA0001

Traceable Calibration of Microphone

G.R.A.S. CA2001

Accredited Calibration of Microphone