G.R.A.S. 40HT 1/2'' Lownoise Microphone System for Confined Spaces

Freq range: 10 Hz to 16 kHz
Dyn range: 6.5 dB(A) to 113 dB
Sensitivity: 800 mV/Pa


G.R.A.S. 40HT can measure sound pressure levels below the threshold of human hearing; and is amply suitable for sound-power measurements on even very quiet products. Its very wide dynamic range permits measurements down to below – 2 dB re.20 µPa (in 1/3-octave bands) from 20 Hz to 20 kHz.

Typical applications and use

40 HT is for low sound pressure measurements and very low sound power measurements.


40HT comprises a special high-sensitive ½-inch Condenser Microphone G.R.A.S. 40AH, a ¼-inch High-impedance Preamplifier G.R.A.S. 26HG with adapter G.R.A.S. GR0010 for the ½-inch microphone, and special Gain and Filter Unit G.R.A.S. 26HT.

The Gain and Filter Unit and Microphone are an individually-matched combination. To complete the system, a special single or 10-channel power module (available from G.R.A.S.) is required, i.e. G.R.A.S. 12HF for single-channel measurements or G.R.A.S. 12HM for 1 to 10 channel measurements.

Each channel provides all voltages required for 40HT and polarizing its microphone.

The Gain and Filter Unit G.R.A.S. 26HT has a built-in overload indicator (which is repeated on the chosen power supply) and a frequency correction facility for both pressure-microphone operation as well as free-field microphone operation.


The G.R.A.S. 40AH microphone is an externally polarized microphone with a specially reduced inherent noise floor in order to achieve a high dynamic range and wide frequency range. Its diaphragm is specially tuned to yield high sensitivity coupled with low internal-noise generation.

Frequency response and noise floor

The chosen power supply has a two-position switchfor selecting which microphone operation to use, i.e.pressure or free-field. A typical free-field response for an angle of incidence of 0 ° is shown in the data sheet when the 40HT is switched to free-field operation. Also shown is a complete low-noise measuring system, a typical noise floor in 1/3-octave bands for both the linear and A-weighted cases.

Frequency range (±1 dB) Hz 12.5 to 10 k
Frequency range (±2 dB) Hz 10 to 16 k
Frequency range (±3 dB) Hz 6 to 20 k
Dynamic range lower limit (microphone thermal noise) dB(A) 6.5
Dynamic range upper limit dB 113
Set sensitivity @ 250 Hz (±2 dB) mV/Pa 800
Polarization voltage V 200 V / Traditional
Microphone venting Rear
Output impedance Ω 47
Temperature range, operation °C / °F –20 to 60 / -4 to 140
Temperature range, storage °C / °F -40 to 85 / -40 to 185
CE/RoHS compliant/WEEE registered Yes/Yes/Yes

Typical frequency response

Typical noise floor



High sensitive 1/2" microphone


Gain and filter unit


Lownoise 1/4" preamplifier with adapter for the 1/2" microphone

Optional items

G.R.A.S. 12HF

1-Channel Power Module for Lownoise Systems

G.R.A.S. 12HM

10-Channel Power Module for Lownoise Systems

G.R.A.S. AM0069

Windscreen for 1/2" Microphones

G.R.A.S. 42AA

Pistonphone, Class 1

G.R.A.S. RA0090

Pistonphone Coupler

G.R.A.S. AL0006


G.R.A.S. RA0096

Adjustable, high quality, stainless steel tripod adapter

G.R.A.S. AA0046

3 m LEMO 7-pin - LEMO 7-pin Cable for Low-noise measuring system

G.R.A.S. AA0047

10 m LEMO 7-pin - LEMO 7-pin Cable for Low-noise measuring system

G.R.A.S. AA0053-CL

Customized length LEMO 7-pin - LEMO 7-pin Cable for Low-noise measuring system