RESUMO
In recent years, there has been rapid growth in demand for precision grade micro-electromechanical system microphones (MEMSMs). While new applications are increasingly demanding in terms of MEMSM performance, research and development of the technology is thus far keeping pace. To calibrate all these new MEMSMs, a reliable and cost-efficient method for bulk calibration of MEMSMs is urgently required. This paper describes a pilot version of such a method, carried out at the United Kingdom's National Physical Laboratory. The method was intended for use by MEMSM manufacturers themselves for bulk calibration. It may also be usable by manufacturers of devices that incorporate multiple MEMSMs for bulk calibration of these MEMSMs, provided that they are detachable from the devices' circuit-boards. The uncertainty of the method has been compared to that of the coupler comparison calibration which is a technique for acoustic pressure calibration of precision microphones. It is concluded that the pilot method is promising as the basis of a method for the bulk calibration of precision MEMSMs.
RESUMO
A new equation is proposed for the calculation of sound speed in seawater as a function of temperature, salinity, depth, and latitude in all oceans and open seas, including the Baltic and the Black Sea. The proposed equation agrees to better than +/-0.2 m/s with two reference complex equations, each fitting the best available data corresponding to existing waters of different salinities. The only exceptions are isolated hot brine spots that may be found at the bottom of some seas. The equation is of polynomial form, with 14 terms and coefficients of between one and three significant figures. This is a substantial reduction in complexity compared to the more complex equations using pressure that need to be calculated according to depth and location. The equation uses the 1990 universal temperature scale (an elementary transformation is given for data based on the 1968 temperature scale). It is hoped that the equation will be useful to those who need to calculate sound speed in applications of marine acoustics.