The application of the NPL laser pistonphone to the international comparison of measurement microphones

Abstract

NPL has chosen to use the laser pistonphone as the basis for some of its measurements for international key comparison CCAUV.A-K2. The other laboratories taking part in the key comparison have all used the reciprocity technique for their low-frequency measurements, thus the use of a laser pistonphone allows the verification (or otherwise) of this technique at low frequencies. Since the use of this device for the calibration of microphones is not currently included in international standards, this paper describes this calibration method and gives a full account of the associated measurement uncertainty. While the NPL laser pistonphone has existed since the 1970s, its true value to the international community is only now being revealed.This paper also provides the necessary background information to support the key comparison data when they are eventually published.

A study on the pressure leakage correction of pistonphones at infrasonic frequencies

“…(13), and the solution of k mn in this equation stands for k Tmn or k Pmn , respectively. At t¼0, the gas is compressed adiabatically; the volume changes from V 1 to V 0 , the temperature changes from T 1 to T 0 , and the pressure changes from P 1 to P 0 .…”

“…In order to get the precise sound pressure in the chamber, extra corrections for effects of sound pressure leakage, heat conduction and wave motion are always made [7,9,12,13], which results that the final corrected values of sound pressure level (SPL) in the chamber of a pistonphone is…”

“…where p ref is the reference sound pressure, p ref ¼20 μPa; L LC , L HC and L WC are the values in dB of the pressure leakage correction [7,9,12,13], heat conduction correction [13,15,16] and wave motion correction [7,18], respectively, and each correction is independent [7,13]. A thin slit between the piston and its guide is usually designed to equalize the static pressure in and out of the chamber [7,13].…”

“…A thin slit between the piston and its guide is usually designed to equalize the static pressure in and out of the chamber [7,13]. In all references, the pressure leakage correction of pistonphones is done as the method proposed by NPL [7,9,12,13].…”

“…A thin slit between the piston and its guide is usually designed to equalize the static pressure in and out of the chamber [7,13]. In all references, the pressure leakage correction of pistonphones is done as the method proposed by NPL [7,9,12,13]. As stated in this report, by ignoring the heat conduction effects at low frequencies and wave motion effects at high frequencies, the acoustical impedance of pistonphones is expressible as the acoustical compliance of the chamber C, in parallel with the acoustical resistance of the leakage path R. Accordingly, the correction in dB for the sound pressure leakage effect is [7] L LC ¼ À10log 10 …”