I : 3/ Sound and the Ear II 



L = .0 log (g (6) 



or, for a plane wave, 



L = 10 



log (J) (7) 



The latter is actually the original definition of a decibel. However, 

 T depends on temperature, the medium, and the wave shape, so that 

 the sound pressure level defined by Equation 5 is really a more con- 

 venient quantity. 



The use of a logarithmic unit is helpful in plotting graphs, and to 

 some extent loudness is proportional to the sound pressure level at fixed 

 frequency. The logarithmic unit makes it possible to compare two 

 sound pressure levels without knowing the absolute value of either. It 

 also makes it appear as if many acoustic measurements were more precise 

 than they actually are. The table on page 10 gives the sound 

 pressure level of several common sounds, as well as the sound pressure 

 amplitude p. 



In addition to decibels, persons working in psycho-acoustics have 

 used many exotic units such as phons 2 and sones. 3 The purpose of these 

 has been to bring the numbers measured into a closer correspondence 

 with the psychological sensation of loudness. These units all depend 

 on experiments on groups of people and are accordingly difficult to 

 interpret either in terms of any direct physical significance or even in 

 terms of their application to an individual. 



In this section, the physical quantities important in hearing have been 

 introduced, and their application to a study of hearing has been indi- 

 cated. The measurement of the typical values of these quantities, 

 significant in human hearing, has given rise to a variety of types of tests. 

 They are discussed in the following section. 



3. Hearing Tests 



There are various ways of studying hearing. Tests on humans which 

 do not involve any surgical techniques are discussed in this section. 

 Clinically, the most widely employed tests measure the threshold of 

 hearing. The observed thresholds are then compared with the normal 

 threshold. The simplest of these tests uses pure tones. However, the 



2 The loudness level of a sound measured in phons is defined as the sound 

 pressure level of a 1 kc pure tone which sounds equally loud to the average 

 observer. 



3 The loudness of a sound may be measured in sones. A loudness of 1 sone is 

 identical to a loudness level of 40 phons. The loudness of other sounds is measured 

 in sones by subjective comparison to a 1 sone loudness. Ideally, the loudness in 

 sones should be linearly related to the loudness level in phons. No such simple 

 relationship exists. 



