GRAS 42AA Pistonphone, Class 1

Typical applications and use

• Reference calibration source
• Precision microphone calibrations
• Microphone comparisons
• P-I index measurement at 250 Hz

Design

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

• 114 dB 1 re. 20 μPa
• 105.4 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 42AA 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 42AA complies with all the requirements of IEC Standard 942 (1988) Sound Calibrators Class 1, when corrected with barometer ZC0002K as well as Class 0, when corrected with barometer GRAS RA0168. The 42AA includes ZC0002K.

The pistonphone works on the principle of two 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 42AA 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 is green, indicating that the speed of the cam is correctly locked to give 250 Hz. If it is red, while the pistonphone is switched on, the speed is incorrect and 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 42AA is compatible with GRAS ½”, ¼”, and ⅛” microphones and all other microphones having the same standard diameters. Adapters are included for calibrating ¼” and⅛” microphones. Where applicable, the coupler RA0023 is also available for calibrating 1″ microphones.

Each pistonphone is factory calibrated with an accuracy of ±0.09 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 114 dB within ±0.05 dB under reference conditions. Since the output level of a pistonphone depends on the static ambient pressure, the 42AA 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.

For use as a Class 0 calibrator, a precision barometer (not included) with an accuracy of ± 1 hPa or better should be used. 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 42AA fitted with a 1″ microphone coupler RA0023.

A two-port calibration coupler for ½” microphones (RA0024) 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.