20. G.R.A.S. 47AC is a 1/2" CCP microphone for infra-sound measurement

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

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Introduction

The G.R.A.S. 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.

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Design

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.

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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.

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Freq range: 0.09 Hz to 20 kHz
Dyn range: 20 dB(A) to 148 dB
Sensitivity: 8 mV/Pa

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