458 ENERGY MAY BE PROPAGATED BY VIBRATIONS The Ear Is Remarkably Sensitive. The human ear can detect sharp sounds that occur at intervals as close as a twentieth of a second. It can discriminate between tones or musical notes whose frequency differs by less than 1 per cent. It can analyze and sort out the components of a mixture of sounds, concentrating on one at a time. At least a part of this action, of course, must The other part of it may be due to the fact that we occur in the brain. have two ears, which enable us to determine the direction from which a sound comes. A sound coming from the left will reach the left ear before it reaches the right ear, and vice versa. Some deaf persons may be made to hear by vibrations which are transmitted through the bones of the skull, which cause the liquid in the inner car to vibrate, provided that the deafness is not due to an abnormal condition of the inner ear. One can determine whether it is the inner or outer portions of the ear that are not functioning properly by use of the tuning fork. If a tuning fork is heard better when it is placed next to the skull than when placed near the ear, the outer portions of the ear are abnormal. Certain hearing-devices amplify compressional waves and change them into vibrations which are transmitted through the bones near the ear, rather than through the outer ear. A very common type of deafness In factories, busy streets, airplanes, and even classrooms is the deafening due to masking. Under such noisy conditions one has to talk as loudly as possible in order to be heard, and it is no wonder that many students in noisy classrooms do not hear what the teacher is saying — they are "noise" deaf. STUDY QUESTIONS 1. Define pitch, intensity, and quality of sounds. 2. How could you prove that sound does not travel in a vacuum? 3. What is the effect of temperature on the velocity of sound in air? Work out an explanation of this effect. 4. Why does sound not travel through a vacuum? 5. What is the velocity of sound in air? 6. What is a harmonic series of overtones? 7. Suppose that one sees the steam of a whistle one minute before he hears the sound; how far away is the whistle? 8. What is meant by the frequency of sound waves? 9. Compare sound waves with electromagnetic waves as to the medium in which they travel and as to their rate of motion and sources. 10. In how many different ways may sounds be produced? 11. What is sound? 12. How may sound waves be analyzed? 13. Why do deaf people often place cupped hands behind their eat-s?
SOUND PRODUCED BY VIBRATIONS IN MATTER 459 14. What is the effect on the pitch of the sound produced by a sound motionpicture machine when the film is speeded up? 15. How fast would an airplane have to travel in order to catch up with the sound waves produced by its motor? 16. Suggest several reasons why sounds carry better on a foggy night than on a hot, clear, windy day. 17. In what respect is it correct to speak of supersonic waves as soundless sound waves? 18. Discuss some of the applications of supersonic frequencies. 19. What is meant by "noise deafness"? 20. Describe the human ear and explain why it is possible to hear and recognize several sounds simultaneously. 21. What is the Doppler effect? Illustrate with some examples. 22. W'hat is the frequency range to which the human ear is sensitive? 23. Discuss the principle of the phonograph. 24. What is meant by the statement that a speech has been broadcast by electrical transcription? What are the advantages which electrical transcription makes possible to radio broadcasting? 25. Suggest some possible uses of electrical transcription in the school. 26. What is meant by saying that sound waves are compressional waves? 27. Mention some of the consequ-ences of the relatively small velocity of sound in air. 28. What other information besides the time required for a sound to travel from a ship to the ocean bottom and back again would be required to determine the depth of water by depth-sounding devices? 29. In what respect would an airplane position-finder based on the reflection of electromagnetic waves be superior to one based on the reflection of sound waves? 30. Compare living in the country with living in a city from the point of view of sound. 31. What are some of the modern trends in the elimination of noise in cities? 32. Discuss social intelligence as applied to the use of a radio. 33. Discuss the factors which control the intensity of sound. 34. Differentiate between the intensity and loudness of sound. 35. Would ultrasonic sound waves be more or less intense than audible sound waves of the same amplitude? Why, or why not?