GRAS 41AM Permanent Outdoor Microphone System for airport noise

Typical applications and use

• Permanent outdoor-noise monitoring
• Airport-noise monitoring
• IEC 60651 Type 1 noise measurements

Design

It complies with IEC 651 Type 1 and ANSI S1.41983 Type 1 requirements and can be used with any suitable electronic sound or vibration measurement system. It is PTB approved as part of an IEC 651 Type 1 system.

A 1⁄2-inch precision condenser microphone and thick-film preamplifier ensure maximum stability and performance. Both microphone and casing are made of stainless steel.

Precise in-situ calibration checks at 1000 Hz are enabled any time via a built-in electrostatic actuator and test oscillator.

The 41AM is fitted with anti-bird spikes, a windscreen, and a rain cap which do not impair its acoustic omni directional characteristics. The rain cap, which contains the electrostatic actuator, remains in position while calibrating the microphone with a pistonphone.

The large dynamic range of the 41AM is obtained via a twin circuit board with a DC converter for supplying 200 V for microphone polarization as well as a 120 V supply for the preamplifier. The sensitivity of the amplifier can be adjusted by 12 dB and the gain set to -20 dB, 0 dB or +20 dB (via a pair of circuit-board jumpers) to give optimum signal-to noise ratios on long cables. The gain is normally set to 0 dB when leaving the factory.

An A-weighting flter for increased dynamic capability is included in the 41AM. A circuit-board jumper is used to select either the A-weighting or linear frequency response. A-weighting or linear response can be specfied when ordering, otherwise Lin-weighting will be active on delivery.

The built-in calibration oscillator can be switched on or off by short-circuiting a pin to ground in the output socket of the 41AM. The electrostatic actuator is factory set to a calibration level of 90 dBre. 20 μ Pa at 1000 Hz. A calibration level of 94 dB can be specfied when ordering.

The electrostatic actuator and microphone assembly is electrically isolated from the casing of the 41AM; thus eliminating EMC and ground loops.

A silica-gel desiccator is located inside the lower half of the casing and a humidity-indicator window for checking the condition of a second desiccator islocated in the top half.

The internal venting of the 41AM is via a capillary tube which terminates under the rain cap, it also carries the calibration-signal connection to the electrostatic actuator. By removing the rain cap (left-hand thread), the connection to the actuator is accessible.

With the windscreen removed, the unit can be readily re-calibrated using a pistonphone (fitted with Adaptor RA0009); though rarely necessary since actuator calibration is normally done on a regular basis.

The Pole Adapter has a G 11⁄2-inch thread (ISO228/1). The Tripod Adapter can be screwed directly to the Pole Adapter. The output socket of the 41AM is located under the Pole Adapter and both plug and cable are well protected.

Among the available accessories is a Calibration Control Box AC0001. This has a lead which plugs directly into the output socket of the 41AM. The AC0001 has sockets for signal output, remote-control calibration and external power (12 –18 V DC, e.g. from a GRAS Mains/line Adapter AB0002/AB0003 *). It also has an on/off switch for local calibration control.

* AB0002 for 230 V AC; AB0003 for 120 V AC