462 ENERGY MAY BE PROPAGATED BY VIBRATIONS flers do not build up back pressures and should be of value in silencing airplane exhausts. Reverberations Are Produced by the Reflection of Sound Waves. In the open country one can often hear several echoes produced by a loud sound, as it is reflected back and forth between mountains or hills; a dynamite blast in the mountains often produces such reverberations. Reverberations are not desired in broadcasting studios and auditoriums. Depth-sounding devices used by many ships depend upon the measurement of the time required for sound waves to travel to the ocean bottom and back to a receiving microphone. Airplanes use similar devices to determine their distance from the ground. Sound reflection is also used in geophysical prospecting. Vibrations produced by explosions in the ground are recorded by seismometers placed at various positions. Sound waves travel faster through salt domes and other possible oil-bearing formations than they travel through solid rock. The presence of approaching airplanes can be detected by the use of large concave mirrors, five feet or more in diameter, which converge the sound waves on a sensitive sound-detector. The Reflection of Sound Waves Causes Acoustical Problems. Megaphones, trumpets, and horns of various kinds act as reflectors, increasing the intensity of the sound in one direction, just as the reflector in a flashlight concentrates the light in one place. Sounds are more intense in a room or hall, because they are reflected back by the walls. Sometimes, however, the sound has to travel so far that the reflected waves are not received at the same time that the original waves arrive. These echoes are one of the sources of poor acoustics in rooms. This difficulty can be remedied in part by hanging draperies against the walls or by lining the walls with materials which absorb rather than reflect sound. People's clothes absorb sound; the echoes characteristic of a large empty hall are not so noticeable when it is filled with people. The scientific basis for acoustics in rooms and halls has been worked out. In order to obtain proper acoustical properties, the size, shape, and the materials of construction must be considered. After a hall is once built, consideration of the furnishings and the location of the audience and the sound source are important. It is needless to say that only highly trained specialists can be depended upon to solve these problems satisfactorily. Many large auditoriums that have been
ACOUSTICS 463 designed without consideration of the principles of acoustics have proved to be very unsatisfactory. Intensity of Sound Is a Major Problem of Acoustics. In the open the intensity of sound falls off so rapidly that only speakers with very powerful voices can be heard by a crowd of people. The modern public-address system which amplifies speech by use of a radio amplification system discussed in Unit VII, Section 7, has revolutionized public speaking both in the open and indoors. It is important that the speaking units of public-address systems be properly designed and located. Good reflecting surfaces augment the intensity of sound. The Shape and Size of a Room Help to Determine Its Acoustics. The old method was to design a building to fit some period of architecture and then apply corrective acoustical tricks if needed. Modern architects who plan buildings with acoustics in mind find that acoustical requirements often produce new, interesting, and pleasing shapes and contours. Acoustics should play an important part in modern functional architecture. Large concave reflectors may be so placed in auditoriums that sound waves are reflected to the audience. The difficulty with such reflectors is that they cannot reflect the sound waves equally to all of the audience. In large auditoriums, sound waves are best reflected from large flat surfaces placed above, below, and behind both sides of the speaker. Curved surfaces are avoided because they would concentrate the sound too much in certain spots. Ceiling domes, curved ceilings, and large reflective areas at the back of a room should be avoided. Extraneous Noise Should Be Eliminated in Offices and Public Meeting Places. An auditorium should be located in a quiet place if possible, and it should be constructed of sound-proof, or sound-deadening, materials. Outside noises which may enter the, building through small cracks around windows and doors may be kept out by means of weather stripping. The elimination of windows altogether will simplify the problem. Double sets of doors such as one finds in motion picture theatres helps to keep outside noises from coming in the doorway. Ventilating ducts should have sound-absorbing materials placed in them, or they should be constructed of sound-deadening materials so that they will not transmit the sound of the blowers or carry sounds from one room to another. Upholstered seats and carpeted floors help to prevent unnecessary noise.