GRAS 42AC Pistonphone, high pressure, Class 1

Sound pressure level: 134 dB
Frequency: 250 Hz
ANSI: S1.40
IEC: 60942

GRAS 42AC Pistonphone, high pressure, Class 1


Typical applications and use

  • Reference sound source
  • Precision microphone calibrations
  • Microphone comparisons
  • Precision hydrophone calibrations


With a microphone placed in the coupler of the pistonphone, the calibration level is:

  • 134 dB 1 re. 20 μPa
  • 125.4 dBA re. 20 μPa (with A-weighting applied)

At a static ambient pressure of 101.3 kPa, no further correction factors need be applied.

The 42AC is an extremely stable laboratory standard sound source which can also be used for field calibrations – it retains its high accuracy even under hostile environmental conditions. The 42AC complies with all the requirements of IEC Standard 942 (1988) Sound Calibrators Class 1 when corrected with barometer ZC0003K (included). The 42AC is delivered with a ½" coupler (RA0048) for calibrating ½" microphones directly. The 42AC-S2 is a version of the 42AC that is delivered with a 1" coupler (RA0023) for calibrating 1" microphones directly.

The pistonphone works on the principle of four reciprocating pistons actuated by a precision machined cam with a sinusoidal profile. The rotation speed of the cam is controlled to within 0.5% via a tachometer signal in a feed-back loop.

The 42AC has a dual-colour LED above the ON/OFF switch to indicate both battery condition and stable operation. When the pistonphone is operating properly, the LED shows green, indicating that the speed of the cam is correctly locked to give 250 Hz. If it shows red while the pistonphone is switched on, the speed is incorrect; most likely because of low batteries.

The operating procedure is straight forward, simply fit the microphone into the coupler of the pistonphone and switch on. The pistonphone will now produce a constant sound pressure level on the diaphragm of the microphone.

The Pistonphone 42AC is compatible with GRAS ½", ¼", and ⅛" microphones and all other microphones having the same standard diameters. Adapters are included for calibrating ¼" and ⅛" microphones. Where applicable, the coupler RA0023 is also available for calibrating 1" microphones.

Each pistonphone is factory calibrated with an accuracy of ±0.14 dB re. 20 μPa and is supplied with an individual calibration certificate stating the exact value and test condition. The exact value is adjusted to be 134 dB within ±0.05 dB under reference conditions .Since the output level of a pistonphone depends on the static ambient pressure, the 42AC is delivered with a barometer which shows directly on a printed scale what must be added or subtracted to the output level of the pistonphone. The barometric correction at a given altitude very seldom varies by more than ± 0.2 dB.

Adapters for the GRAS 41AL Environmental Microphone and Outdoor Microphone Systems 41AM and 41CN are available for use with Pistonphone 42AC fitted with a 1" microphone coupler RA0023. A two-port high-pressure calibration coupler for ½" microphones (RA0042) is available for making comparison calibrations with a reference microphone. This can also be used for measuring the P-I (Pressure-Intensity) index of intensity systems at 250 Hz.

Octopus couplers are also available for simultaneously calibrating up to 8 microphones.


Sound pressure level
134 re. 20 μPa
ANSI standard
IEC standard
Temperature range, operation
°C / °F
-10 to 55 / 14 to 131
Battery type
4 x AA alkaline (IEC LR 6)
g / oz
325 / 11.464

Calibration Accuracy at reference conditions:
1/2" microphone: ±0.14 dB
1" microphone:  ±0.2 dB

GRAS Sound & Vibration reserves the right to change specifications and accessories without notice.

Ordering info



Coupler for ½" microphones. Applies only for 42AC


Adapter for ¼" microphones


Adapter for ⅛" microphones




Four LR6-AA alkaline batteries



Adapter for ¼" microphones


Adapter for Outdoor Microphone System 41AM


Adapter for Outdoor Microphone System  41CN


Adapter for Environmental Microphone 41AL


Pistonphone Coupler for 1/2" microphones


Digital precision barometer

GRAS Sound & Vibration reserves the right to change specifications and accessories without notice.

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