Theoretical study on the phase sensitivity of condenser type acoustic transducers at low frequencies

Abstract

The phase sensitivity of the condenser type acoustic transducers at low frequencies is crucial for locating large-scale natural and manmade activities, but is now commonly calibrated based on comparison methods. Although the primary method, which traces its sensitivity back to the international standard unit is few studied. Recently, the explicit sensitivity models of the condenser type acoustic transducers based on the laser-pistonphone technique are built, and can be used to study the phase responses of acoustic transducers at infrasonic frequencies. So that, in this paper, the phase sensitivities of acoustic transducers when its rear vent connected to the calibrating sound field or outside atmosphere are studied in detail. Secondly, time domain analysis of generated sound pressures by displacement excitation are derived to reveal the mechanism of phase variation. Calculations show two distinct sensitivities with 90° phase lead and −10° phase lag limits for vent in field and vent out field calibrations, which are dominated by the pressure leakage and heat conduction effects at infrasonic frequencies.

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].…”

“…Although the direct determination of gap impedance is enabled by geometrically measuring the gap dimension [13], precise measurement is more difficult for narrower gaps. Alternatively, NMIs and some research groups conventionally measure the decay time until the chamber pressure reaches 1/e of the initial pressure after a step pressure input [6,7,11,[17][18][19]. Assuming that the chamber interior is adiabatic, the decay time is equivalent to the product of the gap impedance and chamber impedance, i.e.…”

“…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.…”