GRAS 47AC 1/2" CCP Infra-Sound Microphone Set

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The GRAS 47AC is a 1/2" CCP precision condenser microphone set for infra-sound measurements in open acoustic fields.

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GRAS 47AC 1/2" CCP Infra-Sound Microphone Set


The GRAS 47AC is a 1/2" CCP precision condenser microphone set for infra-sound measurements in open acoustic fields.

As a free-field microphone, 47AC is designed essentially to measure the sound pressure as it would appear if the microphone were not present, the sound field pointing towards the microphone.

At low frequencies, the disturbing effects of its presence in the sound field are minimal because the wavelengths are large compared to the size of the microphone.

At higher frequencies (>1 kHz), the effects of diffractions generally cause the measured sound pressure levels to increase with frequency. In a free-field microphone, the effects of diffraction are compensated for to provide a flat frequency response in a free-field for 0º incidence.


The 47AC is especially designed for infra-sound measurements and can measure down to 0.09 Hz. To make this possible, it is furnished with an integrated low frequency adapter with a specially designed pressure equalization system and a dedicated preamplifier.

To make it possible for the microphone to measure sound with very long wavelengths, the pressure equalization system has a long settling time. It is therefore important that the 47AC is allowed sufficient time to settle in conditions with varying ambient pressure. How long depends on the specific circumstances, but about half a minute after the ambient pressure has stabilized is sufficient in most cases.

The preamplifier has TEDS for automatic sensor identification and reading of calibration data.

Typical applications and use

The low-frequency property combined with its high sensitivity and robust design make 47AC the obvious choice for infra-sound measurements - a fast growing discipline following the need for monitoring and reducing low-frequency noise from, for example, power and production plants and wind turbines.

With a lower limit below 0.1 Hz 47AC is also well suited for realistic measurements of supersonic booms which requires a microphone with a high bandwidth and the ability to capture steep low-frequency pressure variations.


To benefit from 47AC’s low frequency capabilities, it is important that the analyzer input module, or sound level meter is designed to match 47AC. Many data acquisition/recording systems have a fixed or adjustable high-pass filter. In many cases this filter can have a cutoff frequency at 3 to 5 Hz,  if not higher. Therefore, check if your data acquisition/recording system or conditioning power supply has a high pass filter (either hardwired or user configurable) and that it is compatible with the frequencies you are interested in.

We recommend that you consider using a GRAS 12AL power CCP module. It has a high pass filter at 0.07 Hz when used with the 47AC infrasound microphone set.

System verification

For sensitivity calibration at 250 Hz, we recommend using a pistonphone like GRAS 42AP Intelligent Pistonphone. A check at 1 kHz can be performed using a GRAS 42AG Multifunction Sound Calibrator.

Low-frequency calibration requires the use of a special low-frequency calibrator. We therefore recommend that 47AC is send to GRAS for calibration.

Quality and warranty

All GRAS microphones are made of high-quality materials that will ensure life-long stability and robustness. The microphones are all assembled in verified clean-room environments by skilled and dedicated operators with many years of expertise in this field.

The microphone diaphragm, body, and improved protection grid are made of high-grade stainless steel, which makes the microphone resistant to physical damage, as well as corrosion caused by aggressive air or gasses.

This, combined with the reinforced gold-plated microphone terminal which guarantees a highly reliable connection, enables GRAS to offer 5 years warranty against defective materials and workmanship.


If you accidentally damage the diaphragm on a GRAS microphone, we can - in most cases - replace it at a very reasonable cost and with a short turn-around time. This not only protects your investment, but also pleases your quality assurance department because you don’t have to worry about new serial numbers, etc.


Before leaving the factory, all GRAS microphones are calibrated in a controlled laboratory environment using traceable calibration equipment.

Depending on the use, measurement environment, and internal quality control programs, we recommend recalibrating the microphone at least once a year.

The 47AC comes with two calibration charts, a standard calibration certificate stating its sensitivity and frequency response above 250 Hz, and a chart documenting its frequency response below this frequency.