Infra-Sound Microphones

Normal measurement microphones have a very wide frequency range and cover most practical applications. There are however special situations where special microphones are required. One such application is the measurement of infra-sound. Normal measurement microphones have a low-frequency cut-off at about 3 Hz, but in some applications it is required to measure down to close to 0 Hz. Examples of such applications are the measurement of sonic booms and explosions (both chemical and nuclear), and noise generated by machinery such as diesel engines, wind turbines and specially designed mechanical transducers.

To achieve a very low low-frequency cut-off of down to 0.09 Hz, a special microphone combined with a special preamplifier and a low-frequency adapter is used. To account for pressure variations close to 0 Hz, a special ambient pressure equalization system with a very long settling time is used. For this reason, it is necessary to allow the microphone system sufficient time to settle if it has been subjected to fluctuations in ambient pressure, for example from changes in altitude, wind or a closing door. Microphone, preamplifier and low-frequency adapter are assembled and calibrated as a complete unit and should not be separated by the end-user.

A standard sensitivity calibration at 250 Hz can be performed using a Pistonphone. A low-frequency calibration requires special equipment and must be performed at our calibration laboratory.

  1. 40AZ 1/2" Free-field Mic, Low Frequency
    Freq range: 0.5 Hz to 20 kHz
    Dyn range: 14 dB(A) to 148 dB
    Sensitivity: 50 mV/Pa

    G.R.A.S. 40AZ 1/2" Prepolarized Free-field Microphone, Low Frequency

    The 40AZ is a prepolarized free-field microphone with a large dynamic range and a wide frequency response. Its externally polarized equivalent is G.R.A.S. 40AN.

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  2. 40AN 1/2" Free-Field Mic, low frequency
    Freq range: 0.5 Hz to 20 kHz
    Dyn range: 14 dBA to 149 dB
    Sensitivity: 50 mV/Pa

    G.R.A.S. 40AN 1/2" Ext. Polarized Free-field Microphone, Low Frequency

    The 40AN is an IEC 61094 WS2F classified ½” externally polarized free-field, low-frequency microphone with rear-venting. The prepolarized equivalent is G.R.A.S. 40AZ.

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  3. 46AN 1/2'' Free-field Set
    Freq range (±3 dB): 0.5 Hz to 20 kHz
    Dyn range: 17 dB(A) to 149 dB
    Sensitivity: 50 mV/Pa
    Use: For low frequency acoustic diagnostics

    G.R.A.S. 46AN 1/2'' LEMO Free-field Standard Microphone Set, Low Frequency

    The G.R.A.S. 46AN is a 1/2" Traditional Power (LEMO) free-field microphone set for measuring medium sound pressure levels at low frequencies. It is terminated with a 7-pin LEMO connector and is TEDS compatible. The equivalent Constant Current Power (CCP) type is G.R.A.S. 46AZ.

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  4. 46AZ 1/2'' Free-field Set
    Freq range (±3 dB): 0.5 Hz to 20 kHz
    Dyn range: 17 dB(A) to 138 dB
    Sensitivity: 50 mV/Pa
    Use: For low frequency acoustic diagnostics

    G.R.A.S. 46AZ 1/2'' CCP Free-field Standard Microphone Set, Low Frequency

    The G.R.A.S. 46AZ is a 1/2" Constant Current Power (CCP) free-field microphone set for measuring medium sound pressure levels at low frequencies. It is terminated with a BNC connector and is TEDS compatible. The equivalent traditional 200 V (LEMO) type is G.R.A.S. 46AN.

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  5. 47AC 1/2" CCP Infra-Sound Mic Set
    Freq range: 0.09 Hz to 20 kHz
    Dyn range: 20 dB(A) to 148 dB
    Sensitivity: 8 mV/Pa

    G.R.A.S. 47AC 1/2" CCP Infra-Sound Microphone Set

    The G.R.A.S. 47AC is a 1/2" CCP free-field microphone set optimized for infra-sound measurements down to 0.09 Hz.

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