GRAS 41AC-2 LEMO Outdoor Microphone with RemoteCheck for Community and Airport Noise

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The 41AC-2 is a small precision outdoor microphone according to IEC 61672-1 for monitoring of community noise and the noise of overhead aircraft.

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GRAS 41AC-2 LEMO Outdoor Microphone with RemoteCheck for Community and Airport Noise


The 41AC-2 is a small precision outdoor microphone according to IEC 61672-1 for monitoring of community noise and the noise of overhead aircraft.

Typical Applications and Use

With state of the art weather protection and extensive use of high-grade stainless steel, the 41AC-2 is ideal for long-term unattended monitoring of community noise and noise from overhead aircraft.

It can easily be configured for measurement of noise with 90 degrees of incidence, typically community noise, or – with the proper correction data – for measurement of noise with 0 degrees of incidence, typically overhead aircraft. A USB flash drive with the necessary 1/12 octave correction data is part of the delivery.

41AC-2 has built-in RemoteCheck technology that makes it possible to remotely check the measurement chain for changes.


The 41AC-2 has been designed to meet the need for a precise and reliable outdoor microphone that can be part of a permanent installation. The 41AC-2 is waterproof and fulfils the requirements of IP-55. It is easy to configure and calibrate and comes with a RemoteCheck function that makes it easy to monitor the measurement chain and detect errors.

It uses a 40AF-S2 1/2” Ext. Polarized Free-Field Microphone, High Sensitivity and a 26AJ 1/2’’ RemoteCheck Preamplifier with Integrated Connector.

Microphone and preamplifier are mounted inside a high-grade stainless steel housing. The top cone – also made of stainless steel – has been perfected for best acoustical response with the 40AF-S2 microphone. In addition to its acoustical function, the top cone acts as a bird spike.

The 40AF-S2 microphone has been modified for optimal response with respect to the sound field and the top cone’s internal cavity. The protection grid is a special design that offers added water protection.

The wind screen eliminates the effects of air flow and also serves as part of the 41AC-2’s water protection. It is glued to a detachable tube that ensures correct positioning. A locking system eliminates the risk of accidental dislocation. It is designed to withstand a wide range of weather conditions and temperatures for a very long period, i.e. easily a year or longer.

It comes with an adapter for pipe mounting. An adapter for tripod mounting is also part of the delivery.


The 41AC-2 uses an externally polarized microphone with a LEMO type preamplifier with RemoteCheck. It requires a cable with 7-pin LEMO connector and an input module that can provide a 200 V polarization voltage.

If you want to benefit from the built-in RemoteCheck technology, your analyzer must be able to provide a sinusoidal test signal of 5 V RMS or greater, applied to pin 1 of the LEMO connector.

System Verification

The built-in RemoteCheck functionality allows you to set up a monitoring routine that will detect changes to your measurement chain.

The RemoteCheck technology works by applying a precision AC signal to an RC network in the preamplifier. The test signal is attenuated by about 40 dB by this network. Therefore, a test signal with an amplitude of about 5 V RMS will result in a response signal corresponding to about 94 dB SPL.

RemoteCheck verification is based on comparison of historical data from frequent RemoteCheck measurements. Depending on the background noise level, the expected accuracy of the verification signal will be within 0.15 dB to 0.5 dB. If the background noise is about 60 dB SPL, the accuracy will be about 0.15 dB, if it is 70 dB SPL it will be about 0.5 dB.

To get the best possible accuracy, you must therefore monitor the background noise and select your RemoteCheck measurement values from samples obtained in situations with low levels of background noise.

As the RemoteCheck does not generate a signal proportional to the microphone’s sensitivity, it cannot replace a proper acoustical calibration. For proper sensitivity calibration, we recommend using a pistonphone like the G.RA.S. 42AP Intelligent Pistonphone.


Before leaving the factory, all GRAS products are calibrated in a controlled laboratory environment using traceable calibration equipment.
An individual test certificate stating the sensitivity and frequency response is included with each product.

Performance and Warranty

GRAS outdoor microphones are made of components from our proven standard portfolio and are all manufactured of high-quality material and branded parts that were chosen and processed to ensure life-long stability and robustness.

All parts are manufactured and assembled at the factory in Denmark by skilled and dedicated operators in a verified clean-room environment. The microphone diaphragm, body and unique protection grid are made of high-grade stainless steel and make the microphone set resistant to physical damage as well as corrosion caused by aggressive air or gasses.

This, together with the enforced gold-plated microphone terminal which guarantees a highly reliable connection, enables us to offer 5 years warranty against defective materials and workmanship. The windscreen comes with a 6-month warranty.

The warranty does not cover products that are damaged due to negligent use, an incorrect power supply, or an incorrect connection to the equipment.

Service and Repairs

All repairs are made at GRAS International Support Center located in Denmark. Our Support Center is equipped with the newest test equipment and staffed with dedicated and highly skilled engineers. Upon request, we make cost estimates based on fixed repair categories.

If a product covered by warranty is sent for service, it is repaired free of charge, unless the damage is the result of negligent use or other violations of the warranty. All repairs are delivered with a service report, as well as an updated calibration chart.