A laser pistonphone designed for absolute calibration of infrasound sensors from 10 mHz up to 20 Hz

Abstract

There has been an increased demand for traceable calibrations at infrasonic frequencies in support of geophysical monitoring applications, an example being the Comprehensive nuclear Test Ban Treaty Organization, which provides a global international coverage for nuclear testing ban, and requires for the International Monitoring System. In this paper, a new laser pistonphone design is presented with the objective of establishing primary standards for sound pressure at very low frequencies down to 10~mHz. The piston is a modified accessorized loudspeaker driver whose diameter is equal to the diameter of the front pistonphone cavity. The volume velocity of the piston is measured through a laser interferometer and the current version was designed to have an upper frequency limit of 20 Hz, to overlap with the closed coupler reciprocity method of calibration. Particular attention has been given to the sealing to avoid the pressure leakage loss. The dimensions of the front cavity were designed to allow the calibration of a large variety of sensors, including microphones, barometers, manometers and microbarometers. Examples of calibrations for several sensors are presented and also an uncertainty budget for the Brüel & Kjaer type 4160 laboratory standard microphones, commonly used for primary calibrations. Finally, the metrological performance of the laser pistonphone is demonstrated by comparing the calibration results with those obtained with alternative methods.

Performance comparison of measurement microphones and microbarometers for sound pressure measurements near wind power plants

“…Since the uncertainty of the calibration of the microphones or the microbarometers used represents a significant contribution to the total uncertainty of a potential practical on-site measurement, a couple of complementing calibration techniques are currently in development. [27][28][29] These methods aim to cover the frequency range from 0.1 Hz to 20 Hz and will significantly improve the availability of traceable calibration standards and services for airborne infrasound.…”

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

“…The calibration was conducted in the frequency range from 0.1 Hz to 10 Hz in third-octave band steps. The same microphone set was calibrated at the Laboratoire national de métrologie et d'essais (LNE) in France in a laser pistonphone [5] in the frequency range from 0.01 Hz to 20 Hz. In the overlapping frequency range, these calibrations can be compared to mutually validate the two calibration methods.…”

“…In combination with the known geometry of the chamber, the alternating pressure in the chamber can be calculated from the piston displacement and is used as a reference. To date, multiple laser pistonphones have been realized as calibration facilities for infrasound sensors [3][4][5][6]. Apart from laser interferometers, the displacement of the piston can also be measured by other means such as a linear variable differential transformer [7].…”

Investigation of sound pressure leakage effect for primary calibration down to 10−2 Hz using a laser pistonphone system

“…Alternative calibration methods have been developed by NMIs and various universities [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Among them, the laser pistonphone method originally proposed by the National Physical Laboratory in the United Kingdom has gained prominence [6].…”

“…This discrepancy was pointed out by He et al [23], and it remains a large uncertainty source. Recently, gapless pressure-generating ports, such as a membrane-sealing structure between the piston and chamber and a sealed loudspeaker have been proposed [17][18][19]21]. Because such pressure-generating ports are not rigid, this type of laser pistonphone requires additional care when calculating sound pressure.…”