Table of contents

A symposium of the Acoustics Group of the Physical Society, held on 18th February 1949, surveyed recent advances in (a) the investigation of the fundamental structure of matter; (b) telecommunication and allied applications; (c) use of mechanical forces set up by intense waves.

In (a) derivation of elastic constants of matter was an important field especially as small samples such as single crystals could be used. Losses incurred in propagating waves were surveyed with the help of an electrical transmission line model and simple versions of this were established to represent relaxation phenomena based on Maxwell's hypothesis of shear elasticity as a time function and on Kneser's treatment of loss due to delay in a storage process. There was excellent agreement of the latter with recent results on acetic acid.

Available sources of ultrasonic power were surveyed and the importance of barium titanate as a powerful and strongly coupled piezoelectric transducer was emphasized.

An expression for the receiver/transmitter power ratio in telecommunications systems was examined for gaseous, liquid and solid media and, from available data, optimum frequencies for various ranges were deduced. These were found to be in accord with experience in echo-sounding, earth exploration and in propagation in metals.

Accounts were given of experience with flaw detectors and echo-sounding which showed that these were becoming important industrially and in navigation; work on blind aids was unpromising. Advances in timing and time delay devices were described.

There is a dearth of important industrial applications of the use of intense waves in spite of the interesting phenomena which have been demonstrated in the laboratory. The importance of the study of cavitation was pointed out. Results were discussed for killing bacteria, disintegrating proteins, emulsifying, soldering aluminium and refining the crystalline structure in solidification of light alloys. Stress was laid on the wideness of the frequency spectrum over which these phenomena occurred and on the difficulties and importance of maintaining temperature constant and of measuring intensities during investigations.

A new method for measuring the reactances in the circuit requires measurements of equivalent capacitance at several frequencies around that of resonance, preferably differing from the resonance frequency by from ½ to 5%; the accuracy is unaffected by capacitance in parallel with that of the specimen. The measurements can be made with a suitably screened substitution Schering bridge; it should be suitable for frequencies between about 50 and 600 kc/s, and the only unusual component needed for it is a variable condenser which can be read to 0.001 pF If there are no unwanted modes of vibration, the equivalent circuit can be measured to within 0.1%.

The piezoelectric coefficient d is sometimes calculable from the equivalent circuit and the dimensions of the crystal, but there are difficulties which limit the accuracy of d to about ½%. Results obtained include, for example, d₁₁ for quartz at 20° C.=6.9₅ × 10^-8 cm/E.S.U of potential.

The effects of various surface treatments on the plastic properties of zinc crystals are examined. Roscoe's observation of the hardening effect of a surface film of oxide is confirmed; electrolytically polished specimens have critical shear stresses of about 33 gm mm^-2 whereas oxidized specimens have values up to 65 gm.mm^-2. Similar effects are observed on the rate of creep under constant stress The Rehbinder effect - softening crystals by immersing them in paraffin and oleic acid - can be produced on oxidized specimens but not on others. Oxidized specimens do not respond instantly to immersion and the time of response is proportional to the viscosity of the liquid. It is suggested that the liquid penetrates the oxide film and reduces its ability to harden the metal; the action of the oxide film itself is not yet clear.

It is shown that sharp yield points can be produced in crystals of zinc containing nitrogen The gas is introduced in a preliminary melting operation, either by bubbling it through the metal or by allowing it to be absorbed from the atmosphere, using a flux to keep the surface clean. Strain ageing treatments are needed to develop the yield point clearly, but once it is developed it returns at the same level after each successive treatment at the same temperature Ageing experiments at room temperature show that overstrained zinc crystals soften by recovery before the yield point returns; zinc differs from iron in this respect. General conditions for producing yield points in metals of common crystal structures are briefly discussed.

A simple device is described which can maintain the stress on a creep specimen constant to within 9.8% during uniform extensions up to 100% The apparatus is suitable for slow rates of strain. Examples are given of creep curves obtained with this device from tests on lead and a lead-tin alloy extending to 1,000 and 400 hours respectively; good agreement with the Andrade creep equation is observed in both cases

A method is described by which the surface tension of surfaces occurring in the systems copper-hydrogen or argon and copper-lead-hydrogen or argon is determined. At temperatures round 850° C the surface tension between copper grains is 640 dyne/cm. and between copper and the equilibrium lead-copper liquid phase 340 dyne/cm. The surface tension of a copper-gas surface is the same whether the gas is hydrogen or argon, within the limits of precision of the experiment, and has the value 1,800 dyne/cm. When lead vapour is present in the gas atmosphere at a partial pressure of the order of 0.1 mm. Hg the surface tension of the copper-gas surface has the much lower value of 780 dyne/cm It is suggested that the reduction is due to the adsorption of lead at the copper-gas surface The failure of the liquid phase to exhibit a contact angle of zero against solid copper surfaces is thereby understood

Optical interference filters are composed of thin films deposited on a glass base by volatilization in a high vacuum. The narrowness of the transmission bands characteristic of these filters demands a high degree of accuracy in the regulation of the thickness of the films if maximum transmission is to occur at a specified wavelength. A method of observing the optical thickness of transparent films during deposition is described and it is concluded that its accuracy is sufficient, for instance, to enable the centre of a fourth-order transmission band to be placed within 20 A. of any wavelength in the visible spectrum.

Microphotometry of photographs of distant objects with small angular subtense has shown that inaccurate results are obtained if the apparent contrast is calculated on the assumption of uniform sky brightness. No evidence was found of any edge effect due to inward small angle scatter.