332 



SPEECH, MUSIC AND HEARING 



where a change or fluxation in the frequency may occur as, for example, 

 phonographs and sound motion pictures. The minimum perceptible 

 change in frequency as a function of the sensation level is shown in 

 Fig. 13.12. It will be seen that the ear is most sensitive to frequency 

 changes at the higher frequencies. 



FREQUENCY IN CYCLES PER SECOND 



Fig. 13.12. The variation of A///" with frequency for various sensation levels. A/ is the 

 change in frequency. Sensation level is level above threshold. (After Shower and 

 Beddulph.) 



The minimum time required for a pure tone to excite the ear in order 

 to be sensed as a pure tone is of interest in sound reproduction from the 

 standpoint of the transient response and hangover. It appears that the 

 time is independent of the frequency and is about one twentieth of a 

 second. 



The minimum perceptible change in intensity ^^ which the ear can 

 detect is of interest in certain types of sound reproduction where the level 

 may change as, for example, fluxations in the voltage gain of an amplifier. 

 The minimum perceptible change in intensity level of pure tones as a 

 function of the frequency for various sensation levels is shown in Fig. 13.13. 

 These characteristics show that the ear is most sensitive to intensity level 

 changes at the higher sensation levels. 



13.17. Timbre (Tone Quality). — The three physical characteristics 

 which describe a tone are loudness, pitch and timbre. Loudness and pitch 

 have been defined in preceding sections. Timbre is that characteristic of a 

 tone which depends upon its harmonic structure as modified by absolute 

 pitch and total intensity. The harmonic structure is expressed in the 

 number, intensity, distribution and phase relations of its components. 

 Timbre, then, may be said to be the instantaneous cross section of the 

 tone quality. It ranges from a pure tone through an infinite number of 



26 Fletcher, " Speech and Hearing," D. Van Nostrand Co., New York. 



