PROBLEMS 359 



VII-1. The velocity of sound is 340 meters per second at 16° C. What is its 

 velocity at 50° C? 



VII-2. A sounding tuning fork has a frequency of 100 cycles per second. Cal- 

 culate the wavelength of the pressure disturbance in air at (a) 20° C; (b) —10° C. 



VII-3. Calculate the wavelengths of the lowest and highest frequencies that the 

 average human ear can experience as sound. 



VII-4. What is the velocity of sound in water if on an increase of 1 atmosphere 

 pressure the water is compressed 4.78 X 10 -5 of its volume. 



Ans. 1456 meters/sec. 



VII-5. Express the ratio of a kilowatt to a microwatt as a simple number and 

 then express this ratio in decibels difference of power level. 



VII-6. If 10~ 16 watt/cm 2 is used as the reference level for expressing intensity 

 levels, what is the intensity level of sound having the following intensities: (a) 10 -3 

 watt/cm 2 ; (b) 1 microwatt/cm 2 ; (c) 2 X 10~ 10 microwatt/cm 2 ? 



Ans. (a) 130 db; (b) 100 db; (c) 3 db. 



VII-7. Explain why the quality of sound emitted by a radio is changed by turn- 

 ing the volume-control dial? 



VII-8. Using the normal standard loudness-level contours find the loudness 

 level of each of the following tones : 



Ans. 80 db. 

 Ans. 71 db. 



(1) A 1000-cycle tone having an intensity level of 80 db. 



(2) A 10,000-cycle tone having an intensity level of 80 db. 



(3) A 100-cycle tone having an intensity level of 80 db. 



Ans. 75 db. 



(4) A 100-cycle tone having an intensity of 10 -2 microwatt/cm 2 . 



Ans. 75 db. 



(5) A 100-cycle tone having an intensity of 10 -6 microwatt/cm 2 . 



Ans. 40 db. 



VII-9. You are listening to a source of pure 1000-cycle sound waves, identified 

 as having an intensity level of 50 db at the observer's ear. (1) What loudness level 

 will be observed when two such sources function simultaneously? (2) What is the 

 experienced loudness of one source? (3) What is the experienced loudness of the 

 two simultaneously sounding sources? 



Ans. (1) 53 db; (2) 2000; (3) 2600. 



VII-10. Construct a deafness audiogram with hearing loss in decibel units plotted 

 as a function of the logarithm of the frequency. Use the following data. At fre- 

 quencies 128, 256, 512, 1024, 2048, 4096, 8192 the right ear's loss was -5, 0, -5, 

 -10, -10, +10, -2; the left ear's loss was +20, +35, +35, +35, +20, +30, 

 +35 db. Is the right or the left ear defective? 



VII-1 1. For the average of five cases where the tympanic membrane was re- 

 moved, an air audiogram gave the following hearing losses at the frequencies cited in 

 the previous problem. Hearing loss +40, +50, +45, +65, +50, +45 db. Draw 

 conclusions as to hearing loss of an ear functioning without tympanic membrane. 



VIII-1. What is the magnifying power of a reading glass whose focal length is 

 25 cm, if it is held 15 cm from a book with the eye 50 cm from the lens? 



Ans. 1.7 X. 



VIII-2. Show that the magnifying power of the eyepiece of a compound micro- 

 scope can be represented by M.P. = 25/fi and that the magnification produced by 



