464 ENERGY MAY BE PROPAGATED BY VIBRATIONS One Can Learn an Important Lesson in Acoustics from the Bathroom. It is a well-known fact that people like to sing or whistle in a bathroom. Nearly everyone sounds like an artist (to himself) in a bathroom because the reverberations in the bathroom enable the singer to hear his own voice as it is reflected back to him with little loss of energy. The result of being unable to hear one's self sing results in tension and nervousness that decrease vocal efficiency. In the attempt to fill a hall with one's voice, one is apt to force it into too high a pitch. A modern device to aid artists utilizes public-address units directed toward the artist so that he can hear himself sing. A room which is well adapted to speech will be too dead for music, because the musician needs reverberation in order to blend one tone into another. Music halls should be so constructed that the wall back of the music source will act as a sounding board, while the wall behind the audience should absorb sounds. Large areas of wood nailed on furring strips of variable spacing permits them to vibrate in response to quite a range of low-frequency vibrations, thus amplifying the low frequencies. These wooden surfaces should be given a highly polished finish so as to reflect the high-frequency tones, which otherwise may be readily lost by absorption. Acoustics Should Be an Important Consideration in Building a Modem Dwelling. In building a modern home, many acoustical problems are presented. Noises from bathrooms, kitchens, and laundries should not be transmitted to other rooms in the house. The blare of a radio in one room should not be allowed to bother the occupant of another room. There should be a room where it is possible to play various musical instruments without bothering other occupants of the house or annoying the neighbors. Special types of acoustical boards may be used on the ceilings of kitchens or laundries to deaden sounds. Ordinary lath and plaster partitions often act as diaphragms or resonators to transmit sounds. Walls may be made sound-proof by building two or three thin partitions which are separated by absorbing materials such as cellulose building-boards. Walls constructed of hollow concrete blocks made from porous aggregates insulate against sound as well as heat. In general, the same types of material that are used for heat insulation also serve for sound-deadening. Porous material like hair felt, cellulose fibers, and textile materials reflect only a small amount of sound and absorb relatively large amounts. Solid plaster reflects over 97 per cent of sound waves.
ACOUSTICS 465 These diflferences are caused by the marked differences between the elasticity and density of air and solid materials. STUDY QUESTIONS 1. Discuss the factors which should be considered in providing proper acoustics for an auditorium. 2. Discuss the factors which should be taken into account in the design of a modern dwelling, and explain how each factor is controlled. 3. Would it be a good idea to put sound-deadening material on the ceiling of a music studio? 4. What is the advantage of putting sound-absorbing materials on the ceiling and walls of offices, banks, and restaurants. 5. Why are the acoustics better in a crowded auditorium than in an empty auditorium? 6. 7. Why do sounds travel better in an empty house than in a furnished house? Why should we support antinoise campaigns? 8. Why do younger people like " noise" parades which may annoy older people? 9. Why should a person be quiet in schoolrooms, public libraries, churches, theatres, and other public meeting places? 10. How should a radio broadcasting room be constructed from the point of view of acoustics? 11. Why are booths provided for telephones in public places? 12. Discuss the acoustics of the library in your school. Was the library designed so as to aid in the elimination of noise? 13. Discuss the acoustics of your classroom. What would you suggest to improve it as far as acoustics is concerned? 14. Discuss the design of the auditorium in your school from the point of view of its acoustics. 15. Why is the use of mufflers required by law? 16. Explain the method by which mufflers eliminate sound. 17. How does resonance increase the intensity of the sound waves produced by a generator? 18. What is meant by the terms, generator and resonator, when applied to musical instruments? 19. Can you use resonance to explain the "shimmy" or "drumming" of an automobile? 20. Can you use interference to explain the decrease in vibrations when one increases the speed of an automobile over a "washboard" road? 21. Differentiate between the two kinds of resonators. 22. What is meant by the terms, forced vibrations and sympathetic vibrations? 23. Why are crowds in grandstands cautioned not to stamp their feet in unison? 24. Explain what is meant by the term interference. 25. Explain how depth-sounding devices using sound waves operate. 26. Explain the use of sound waves in geophysical prospecting. 27. What are echoes, and how are they produced? 28. Try to explain the rumbling of thunder.