GRAS 40SA Probe Microphone

Typical applications and use

• Acoustic-impedance measurements
• Exhaust-system measurements
• Near-field measurements
• Measurements at high temperatures
• Pressure-distribution measurements in smallenclosures

Design

The GRAS 40SA is constructed with detachable stainless-steel probe tubes which guide the acoustic signal to a microphone inside the housing of the 40SA. After being sensed by the microphone, the acoustic signal is passed on to an impedance matching wave guide which eliminates unwanted internal reflections. The result is a smooth frequency-response ranging from 2 Hz up to 20 kHz. The internal microphone is connected to a low-noise preamplifier with a high dynamic range; ensuring a measurement range from approximately 40 dB to 166 dB re. 20 μPa. The data sheet shows the preamplifier’s connections via its LEMO plug.

The GRAS 40SA Probe Microphone is internally compensated to equalize the internal pressure of the microphone with the static pressure at the probe’s tip . The static pressure within the 40SA will therefore adjust itself to the static pressure existing at the probe’s tip; which it does with a time constant of approximately 0.1 s.

The 40SA can be used with various probe lengths and is delivered with four standard probe lengths, i.e.: 20 mm, 40 mm, 80 mm and 160 mm. Intermediate lengths can be made by cutting these standard lengths. Also, the stainless steel tubes can be bent to a radius as low as 5 mm without downgrading the system’s acoustics. A flexible probe tube is also provided for use in measurements where stiff stainless-steel tubes are not practical. This does, however, slightly downgrade the system’s acousticperformance.

The right angled design of the 40SA makes it particularly well suited for measurements on exhaust-gas systems and other machinery in general as well as for scanning vibrating surfaces such as loudspeaker diaphragms and cabinets.

The compact size, low weight and all-stainless-steel construction of the 40SA make it robust, easy to handle and easy to mount.

The 40SA is provided with a 3 m integrated cable with a 7-pin LEMO connector. Extensions cables are available.