A— MATHEMATICAL AND PHYSICAL SCIENCES 35 



numerical values of loudness some 14 phons less than with the British 

 Standard zero. In Germany the phon is based on a zero of 0-0003 dyne 

 per sq. cm. which is equivalent to a 4 db. difference from the B.S. zero. 

 As, however, a different method of listening is employed, the slight 

 discrepancy between the two scales is not known exactly. The American 

 scale agrees with the British, except that in the States it is customary to 

 use the decibel not only for expressing intensity measurements, but also 

 for loudness levels, it being implied in the latter case that the decibel 

 figure quoted refers to the energy level (above the arbitrary zero) of the 

 standard tone when it matches the sound to be measured. As already 

 mentioned, the British Standards Institution recommends that for greater 

 clarity the decibel should be restricted to energy ratios ; and in its defini- 

 tion of the phon, the Institution kept open the way for eventual inter- 

 national agreement on the unit of equivalent loudness. 



Happily such agreement came about at an international conference held 

 in Paris last July, when it was unanimously agreed that the decibel and 

 the phon should be adopted respectively as international units of intensity 

 level and equivalent loudness, full agreement being secured on questions 

 of the reference tone (1000 cycles per second), the reference zero (0-0002 

 dyne per sq. cm.) and the technique of listening. All these matters are in 

 accord with the definitions in the acoustical glossary of the British 

 Standards Institution. 



The phon, which has already proved of great service in many classes 

 of noise measurement in this country, came in the nick of time to meet 

 the present demand for noise abatement : the ability to measure is of 

 course vital to such a movement. I may perhaps refer to the public 

 interest which was excited when the decibel and the phon were introduced 

 into everyday language. Mr. Punch made play with the decibel, anxiously 

 enquiring ' how many decibels it took to talk the hind leg off a donkey ' ; 

 while W. R. in the Observer was moved to welcome the phon in the 

 following terms : — 



' Hail ! newest unit, welcome to the host 



Of ergs and amperes, kilowatts and therms, 

 Best of the lot, you shall be valued most 

 Among these unintelligible terms. 



For you alone can make men realise. 



In figures plain, the awful din they make. 



So that at last some genius may devise 

 A means of curbing it, for Reason's sake.* 



The experimental realisation of the fundamental scale of phons is one 

 for the standardising laboratory, as will be appreciated from the subjoined 

 definition of the phon taken from the British Standard Glossary of 

 Acoustical Terms and Definitions, 1936 (No. 661). For example, the 

 specification of a free progressive wave postulates an acoustical environ- 

 ment corresponding to infinite space, which in practice can only be 

 conveniently simulated by placing the ' normal observer ' (in practice 



