ON INSTRUMENTAL AIDS FOR DEAFNESS 261 



pitch of the lowest tone to which it resounds depends on its 

 total length, and to some extent on the diameter of its trumpeted 

 end, but it will be generally found that the fundamental tone 

 of the instrument is considerably higher than that employed 

 in speech. Apart from the natural raising and lowering of 

 pitch during speech, a man's voice employs notes of about 

 e' = 160 vibrations per second as a mean, and the resonator for 

 this would require to be about 42 inches long, though a shorter 

 instrument would suffice for listening to a woman's voice. 

 Moreover, as shown by Helmholtz and others, for the production 

 of a vowel quality of tone, several partial tones are necessary, 

 and the lowest of these is for most vowels much below the 

 fundamental tone of the instruments used in deafness. Let one 

 take, for example, one of the large shells of the type formerly 

 used, on account of its pearl-like colours, for decoration, and 



Fig. 4. 



hold it to the ear. It will respond to a very limited series of 

 tones, the lowest of which will probably be about b' = 490 or 

 c " — 5 2 3> an d the next an octave higher. These may be easily 

 found by striking the notes of a pianoforte while holding the 

 shell to the ear. 



The fundamental defect therefore of Class C of instruments 

 is that they reinforce only a few notes, and these of too high a 

 pitch, and consequently the quality of the sound is materially 

 altered ; vowels — on which the character of speech almost solely 

 depends — being entirely altered in timbre. 



Class D, see fig. 4, usually called resonators, are generally of 

 the shape of a small bell, with one narrow, bent, and tapering 

 tube leading from near the mouth of the bell to the external 

 meatus of the ear. The inefficiency of these is not reduced by 

 the addition of a more or less ornamental grille blocking up 

 the mouth of the bell, although it is sometimes claimed that 

 the grille has a " useful and necessary acoustical effect." 



