290 REPORTS ON THE STATE OF SCIENCE, ETC. 



(B.5.)^ is defined as a unit of equivalent loudness, a sound being said to 

 have a loudness of n phons when it is judged to be equally loud with a i ,000- 

 cycle reference tone at a level of n db above a reference pressure level of 

 0-0002 dynes per sq. cm. Thus, on the basis of phon values one can 

 arrange sounds in order of loudness, but it does not follow that a sound of 

 mn phons is m times as loud as one of n phons. On the other hand, loudness 

 values expressed in sones will presumably be true numerical magnitudes 

 which will conform to mathematical requirements such as that just stated. 

 Stevens also formulates an equation relating loudness to a power of the 

 number of just noticeable differences above threshold, and derives a method 

 of determining the subjective magnitude of a difference threshold. 



The claims of the various loudness scales described above, and those of 

 certain ' physical ' scales, were examined by Abbott (i), who concluded 

 that both were necessary, for different purposes. That of Fletcher and 

 Munson was recommended for psychological purposes, as using the best 

 available data for high and low levels, for which accurate information is 

 often more important than at intermediate levels. 



Bibliography. 



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2 I.e. British Standard, as stated in Publication No. 661 — 1936 of the British 

 Standards Institution. The quaUfication is necessary since the term phon has 

 already been used in Germany in the same sense, but with different defining 

 conditions. 



