14 Sound and the Ear /I : 3 



if the frequency is lowered from, say, 1 ,000 cps to 500 cps, the listener 

 hears a decrease in frequency of one octave. (A frequency ratio of 2 

 is called an octave in music.) Below about 30 cps, the listener cannot 

 really distinguish tones or tell whether the frequency is being raised, 

 lowered, or held constant. If the frequency is lowered to, say, 1 cps, 

 the tone identified is not the applied sound frequency but rather some- 

 thing in the neighborhood of 1,000 cps. 



Likewise, at high frequencies a point is reached above which a person 

 can no longer distinguish tones. In addition, the threshold sound 

 pressure rises very sharply. This latter effect limits experiments at the 

 high frequency end of the spectrum. The exact frequency range in 

 which this sharp rise occurs varies widely from individual to individual. 

 For one graduate student who worked in the Pennsylvania State Uni- 

 versity Acoustics Laboratory, this sharp rise occurred around 25 kc. 

 He could tell that 23 kc was higher in pitch than 22 kc. The author's 

 ears failed to respond to reasonable sound pressure levels if these were 

 above 1 7 kc in frequency, whereas his wife did not hear above 6 kc. 



The highest frequency which normal humans hear varies by a factor 

 of more than three. 4 This may seem large, but it is small compared to 

 the variations in the pure tone thresholds. Variations from one 

 individual to another may be as high as 40 db within the normal range 

 of hearing. These numbers, when translated into actual acoustic 

 pressures, represent a pressure ratio of a hundredfold, truly an enormous 

 variation. 



In an ordinary room, the lowest sound pressure levels one can hear 

 are limited by the ambient noise. In a very quiet room, where all the 

 ambient noise is below the hearing threshold, the physiological noise 

 level is approximately at the threshold of hearing. This physiological 

 noise is due to a variety of causes: the pulse in the ear, the muscles 

 contracting, breathing, and any motion of the joints. Physiological 

 noise is effective only at those frequencies where the ear is most sensitive ; 

 that is, the range 1-4 kc. 



Most sounds come to the ear from the air. Some, such as a few of the 

 physiological noises, are transmitted by bone conduction. The entire 

 structure of the middle and inner ear discriminates strongly in favor of 

 airborne vibration as opposed to bone conduction. However, a suffi- 

 ciently strong signal can be conducted by the bone. The bone con- 

 duction threshold can be observed by blocking the ears 5 or by applying 

 a vibrator directly to the head. The sound pressure levels necessary 

 for hearing by bone conduction are about 40 db higher than by air 

 conduction, and the threshold curve is much flatter. 



4 Eight kc to higher than 25 kc. 



5 This may raise the threshold. 



