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.
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 number of errors have come to light in the above paper with regard to units of salinity. In each example of the error, the unit for salinity has been stated as "%" instead of "‰"; this erroneously represents the values as being in percent rather than in parts per thousand. The reader should note that the units for salinity throughout the paper are intended as parts per thousand. On p. 2776, line 15 of Sec. IIB, it should read "parts per thousand" rather than "percent."The postal code for the National Physical Laboratory should read TW11 0LW.
Sound is a small fraction of the pressure wave spectrum. In terms of wavelength, we can hear far more than we can see: while deep red light has waves about twice as long as those of deep violet, the lowest-pitched sound waves we can hear are about ten times longer than the highest. ‘Sound waves’ explains infrasound, the lower frequencies detectable by touch rather than hearing, and the audio range, describing the fundamental differences between sound power and loudness, and frequency and pitch. It also considers sound underwater. Sonar systems can be passive systems that simply detect sounds, while active sonar transmits sounds and detects those that are reflected.
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