Preamplifier with ultra low frequency cutoff for infrasonic condenser microphone

Rasmus Trock Kinnerup, Kresten Marbjerg, Per Rasmussen, Arnold Knott, Ole Cornelius Thomsen

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Measuring infrasonic sound sets high requirements on the instruments used. Typically the measurement chain consists of a microphone and a preamplifier. As the input resistance of the preamplifier forms a high pass filter with the capacitance of the microphone in the picofarad range, measuring ultra low frequencies becomes a challenge. The electric preamplifier presented in this paper together with a prepolarized condenser microphone form a measurement system. The developed preamplifier connects the microphone signal directly to the input of an operational amplifier with ultra high input impedance. The bias current for the preamplifier further complicates the signal amplification. A configuration of two diode-connected FETs provide
the input bias current. The resulting input impedance of nearly 1 TW yields a total lower limiting -3 dB cutoff frequency of 8 mHz and a dynamic range of 95 dB. Being able to measure down to ultra low frequencies in the infrasonic frequency range will aid actors in the debate on wind turbine noise. Sonic booms from supersonic flights include frequencies down to 10 mHz and the preamplifier proposed in this paper will aid scientists trying to modify the N-shaped shock wave at high level which prohibits flights in land zones.

Original languageEnglish
Title of host publicationProceedings of ASME 2012 Noise Control and Acoustics Division Conference at InterNoise
PublisherAmerican Society of Mechanical Engineers
Publication date2012
ISBN (Print)9780791845325
Publication statusPublished - 2012
Event41st International Congress and Exposition on Noise Control Engineering - Marriott Marquis, New York City, NY, United States
Duration: 19 Aug 201222 Aug 2012
Conference number: 41


Conference41st International Congress and Exposition on Noise Control Engineering
LocationMarriott Marquis
Country/TerritoryUnited States
CityNew York City, NY


  • Acoustic noise
  • Cutoff frequency
  • Electric impedance
  • Electric impedance measurement
  • High pass filters
  • Shock waves
  • Condenser microphone
  • Supersonic flights


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