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The measurement of steady high pressures in a fluid system with the highest accuracy demands the use of pressure balances (free piston gauges) of accurately known effective areas. This requires a precise knowledge of the way in which the effective areas of the piston-cylinder assemblies concerned vary due to the elastic distortion caused by the applied pressure.Two methods which have been directed to the solution of this problem are described. The first depends on a principle of similarity as applied to the deformations of two assemblies of the same general dimensions but constructed of materials having substantially different elastic moduli. The second method makes use of measurements of the flow characteristics of the pressure transmitting fluid using two pistons having a known difference of diameter.The distortion factors are shown to be representable as linear functions of the pressure, so that the effective area a t pressure P is connected with that at zero pressure by expressions of the form B P 7 A , (1 -?.P) where I may be termed the distortion coefficient.The final accuracy of the measured distortion coefficients 15 about 2%, which corresponds to an uncertainty in effective area of about 5 1 part in lo5 a t 1000 bars increasing in proportion to the pressure a t higher pressures. Some aspects of the practical calibration of pressure balances, carried out by direct balancing against assemblies calibrated by the methods described, are considered.

During recent years a program of work has been in progress at the National Physical Laboratory with the objective of providing improved data on several aspects of subjective acoustics. In connection with standards for audiometry, measurements have been made of the threshold of hearing for pure tones by earphone listening, and these have since been extended to the case of listening in free field. Latterly a redetermination of the equal-loudness relations for pure tones has been completed aimed at resolving discrepancies between former determinations and providing an improved basis for the establishment of a standard set of contours. These results apply to a large team of otologically normal observers, and cover the range from 25–15 000 cps and up to 130 db in sound pressure level. The results of this investigation enable the equivalent loudness of any pure tone to be expressed by simple formulas with coefficients varying smoothly with frequency. Considerable attention has also been devoted to the determination of the loudness scale, i.e., the function relating loudness level in phons to the magnitude of the loudness sensation expressed on the sone scale. An assessment of the experimental evidence has led to the formulation of a simple relation which appears adequate for practical purposes in noise measurement. Investigations are continuing on the determination of loudness levels of complex sounds from their objective spectra.

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