Low Frequency Calibration of Measurement Microphones

Abstract

The calibration of measurement microphones below 100 Hz is not very well covered by the present IEC standards. The uncertainty increases rapidly and for very low frequencies it goes toward infinity. This paper approaches this issue and presents a unique way to verifying and calibrating the low-frequency response of measurement microphones. Using a small isolated calibration volume and applying a constant force to a large piston inside this volume, you obtain a direct proportional relation between force and sound pressure, allowing calibration of measurement microphones down to 0.01 Hz. LOW FREQUENCY MEASUREMENTSModern technology has caused a more frequent occurrence of low frequency sound phenomena. Thunder, water-falls and wind are sources in nature, but human activity has added many more sources causing distress and annoyance among people sensitive to low frequency sound. Heavy machinery and traffic often produce sound of a continuous cyclic character while aircraft take-off, sonic boom, pile driving, and blasting produce single events of an often startling nature.The very long wave length associated with low frequency waves e.g. 170 m (500 feet) at 2 Hz, makes it very difficult to absorb the energy of the travelling wave. Very low frequencies may travel thousands of kilometers. Free field conditions do not exist and in dosed volumes isothermal to adiabatic conditions play an important role.The measurement of low frequency sound pressure levels calls for special instrumentation not described in the present standards for sound level meters. Sound level meters are only specified for use in the frequency range above 20 Hz. At 20 Hz the tolerance of the weighting is ±2.5 dB and proper standardized calibration procedures including the microphone are non-existing. The consequence is the jump in standardized hearing threshold curves based on discrepancies in measurements reported over many years. Fig.l In order to measure perceived sound pressure levels at the threshold of human hearing, one should cover the frequency range down to 2 Hz and a dynamic range from 120 dB at 2 Hz and up to the maximum level one is interested in. As seen from fig. 2, the equal loudness level contours cover a range of more than 140 dB for full coverage of measurements involving humans.The instrumentation used to measure the sound pressure in the infrasound range 2 -20 Hz must be able to measure correctly in this frequency range. A normal type 1 sound level meter is not well suited. The tolerance on the frequency weighting alone below 20 Hz is ±3 dB and at 16 Hz it is +5 -∞. This does not include the microphone in most calibration situations. The database on which the ISO 226-2003 is based may thus be rather vague at 20 Hz, which may explain the lack of continuity in the equal loudness contour curves shown in fig. 1. The data trace for the infrasound equal loudness contour level may be considered more realistic, as the experimenters may have been more aware of the instrumentation requirements.

Numerical Study on the Phase Sensitivity Variation in Low Frequency Primary Microphone Calibrations

“…Moreover, due to the free field requirement and the wind noise in the infrasound measurement, the use of the in situ method in the infrasound range is quite limited. When multiple microphones are connected to the loudspeaker or exciter-based calibrator, their phase consistency can be calibrated in the frequency 2 of 15 range of 0.01 to 100 Hz [12][13][14][15]. Y.C.…”

“…to the loudspeaker or exciter-based calibrator, their phase consistency can be calibrated in the frequency range of 0.01 to 100 Hz [12][13][14][15]. Y.C.…”

A Modeling and Feasibility Study of a Micro-Machined Microphone Based on a Field-Effect Transistor and an Electret for a Low-Frequency Microphone

“…As shown in Figure 10 , to generate acoustic pressure in the frequency region below 500 Hz, a small loud speaker-driven pressure chamber was designed and fabricated. The use of a closed chamber enables amplification of low-frequency acoustic pressure by the Helmholtz resonator principle, and the upper cut-off frequency can be tuned by changing the chamber volume [ 21 , 22 ]. Therefore, to make the chamber volume behave as an acoustic capacity element with a Helmholtz resonator, the longest dimension of the chamber must be shorter than 1/6 of the wavelength of the upper cut-off frequency of the chamber.…”

Primary calibration for airborne infrasound utilizing the vertical gradient of the ambient pressure

“…It is also unlike the laser pistonphone, where the positioning of the equalization vent inside or outside the sound field can be controlled individually, at least for back-vented microphones. The positioning of the vent in regard to the sound field greatly influences the sensitivity of the DUT at low frequencies, because the vent acts as an acoustical high-pass filter when subjected to the sound field [15]. For this reason, it is important to note whether the vent is subjected to the sound field and to calibrate a microphone the way it is intended to be used later.…”