PHYSIOLOGICAL ACOUSTICS 287 



4. The sound may farther be described as "full," if the 

 fundamental tone be predominant, and as "empty" if it be 

 relatively feeble. This is exemplified in the difference of 

 quality between the sound of a piano-wire when struck with 

 a soft or a hard hammer, respectively ( 26, 38). 



5. When harmonics beyond the sixth or seventh are 

 present in considerable intensity, the sound is harsh and 

 rough, owing to the discords which these higher overtones 

 make with one another. If, however, the higher harmonics, 

 though present, are relatively weak, as in the case of the 

 stringed instruments of the orchestra, reed-pipes, and the 

 human voice, they are useful as giving character and expression 

 to the sound. Brass instruments, on the other hand, with 

 their long series of powerful overtones, are as a rule only 

 tolerable in combination with others, or for the sake of 

 particular effects. 



The analysis of the sounds of the human voice is naturally 

 a more difficult matter. In particular, the constitution of the 

 vowel sounds has been much debated, without any very definite 

 conclusion. The same vowel may be sung on a wide range of 

 notes, but preserves its peculiar character throughout ; and the 

 question arises, does this special quality depend solely on the 

 relative intensities of the various partial tones, or on the 

 predominance of one or more overtones of, or near to, a 

 particular pitch ? It will be remembered that the vibration of 

 the larynx is periodic, and that particular harmonics may be 

 reinforced by the resonance of the mouth-cavity, as in the case 

 of a reed-pipe ( 90). The balance of authority appears to 

 incline, though not very decisively, to the " fixed-pitch " theory, 

 which is the second of the two alternatives above stated. A 

 review of the subject down to the year 1896 will be found in 

 the concluding chapter of Lord Rayleigh's treatise. 



93. Interference of Pure Tones. Influence on the 

 Definition of Intervals. 



It has so far been assumed that the sensations due to 

 two coexistent simple-harmonic vibrations are produced quite 

 independently of one another. This appears to be in fact the 



