452 ENERGY MAY BE PROPAGATED BY VIBRATIONS The walls of a megaphone prevent the sound waves from spreading out as soon as they would spread from the mouth. Sound Waves May Be Refracted. Lake Inasmuch as sound travels at different speeds in layers of air of different temperature and relative humidity, sound waves may be bent as they pass from one medium to another, as shown in Fig. 218, to the extent that the convex waves Warm Air /rfFX/n^ bccome concavc in nature Fig. 218. Sound waves are refracted as they pass from cold air through warm air. and thus focus the sound at a given point. The sound thus appears to travel farther under these conditions. The sound of a guitar played on a boat on a lake on a hot summer night will travel long distances with remarkable clearness because of the refraction of the sound waves by the warm air over the cooler water. Sound Waves May Be Analyzed by Methods Which Use Compressional Waves to Produce Corresponding Wavelike Patterns in Beams of Light. Curves showing the nature of sound waves can be obtained by an instrument called the "phonodeik." The principle here employed is that the vibrations in the air move a sensitive diaphragm, w^hich is attached to a mirror reflecting a beam of light to a motor-driven revolving mirror, which, in turn, throws the beam of light on a screen in the form of a long wave. It is important to keep in mind that the waves in a beam of light do not resemble compressional waves in the atmosphere but rather serve as a diagrammatic representation of sound waves. Sound waves may also be directly recorded by the phonautograph, in which a stylus attached to the diaphragm traces a path on a smoked paper carried on a rotating cylinder. The telephone diaphragm and the microphone generate oscillating currents from sound waves, which may be received by a cathode-ray type of oscillograph. This oscillograph is of great value in telephone and radio research and testing. The phonograph, invented by Thomas A. Edison in 1877, recorded the movements of a stylus controlled by a diaphragm, by indentation in a sheet of tinfoil supported over a spiral groove on a metal cylinder. Such Later machines made the records on wax cylinders or disks.
SOUND PRODUCED BY VIBRATIONS IN MATTER 453 records can be greatly amplified by a special apparatus designed for the purpose of giving curves similar to those produced by the phonodeik. The phonograph not only records sound but also reproduces the sound thus recorded, because the diaphragm is caused to vibrate as the needle runs over the sound track. Thomas Edison's phonograph was followed by a series of improved machines under such trade names as the "graphophone," the "gramophone," the "electrola," and the "victrola." Today a very useful application of the phonograph is the dictaphone. Phonograph records are first made in wax, dusted with graphite, and electroplated to make the master disks. These master disks are used to make faithful impressions on plastic materials of various compositions in the hydraulic press. The loudness of a sound produced by a record depends on the depth of the track, while the pitch depends upon the relative frequency of the elevations and depressions of the vertical cut record. If the records are played too slowly the pitch is cut down. In the case of lateral-cut records, the loudness depends upon the swings of the track away from the center line. Modern instruments use electrical transcription which employs the vacuum tube (as described in Unit VII). Some sound motion pictures use a sound track produced on film by a device in which sound energy is transformed so as to vary the intensity of light falling on the film. Light passing through this sound track produces variations in the light received by the photoelectric cell, which changes the light into electrical impulses. These variations in electrical current are then amplified, as in the radio, by vacuum tubes, to a point where they can operate electromagnets powerful enough to cause rather large diaphragms to vibrate and produce the usual sound of the loud-speaker. Overtones Are Vibrations of Greater Frequency than the Fundamental Vibration. The time required for one complete vibration is called a period, while the number of complete vibrations (cycles) per second is called the frequency of a vibrating system. A string is capable of vibrating as a whole, thus producing the fundamental tone, or it may also vibrate in segments, giving not only the fundamental vibration but a series of higher tones. These higher tones, produced by vibrations of greater frequency, are called overtones. When the set of vibrations has frequencies in the ratios of the natural numbers, 1 : 2; 3 : 4; etc., it is called a harmonic series. The overtones of many vibrating bodies are not harmonic. For example, the overtones in the xylophone are in the ratios 1 : 2.756 : 5.404, etc.