This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
In the case of both sound and light we find that if two waves should fall upon one another so that their crests coincide, the intensity of the sound or light is increased (Fig. 61), while if they fall on each other so that the crest of one wave fills up the trough of the other, they interfere so as to destroy each other's effect (Fig. 62); and thus two sounds produce silence, or two waves of light darkness. This is shown in the case of sound by a tube (Fig. 63), which divides into two branches, and these again re-unite. The length of one branch may be altered at will, so that the sound travelling through one branch has further to go than the other. It may thus be retarded so far as to throw it half a wave-length behind the other, and silence is produced. If lengthened still further, so as to throw the one sound a whole wave-length behind the other, the crests again coincide, and the sound is again heard. Increasing the length still further, so that the one sound is thrown a wave-length and a half behind the other, they again interfere, and silence is again a second time produced. This may be repeated ad infinitum, silence occurring whenever the one sound falls behind the other by an odd number of half wave-lengths.
1 St. Bartholomew's Hospital Reports, 1876, p. 155.
Fig.63. - Diagram of apparatus for demonstrating the interference of waves of sound. A and B, branches of a tube; c, sliding piece by which the branch B can be lengthened or shortened at will; D, tuning-fork; e, the ear.
Fig. 64. - Diagram showing the beats or alternate increase and diminution of the wave-heights by the interaction of two systems of waves of different wave-lengths. At A, two systems, having a relation to each other of 3 to 1, are indicated separately by dotted and complete lines. At B the resultant of the interaction of the two systems is shown. With such a relation as that shown in the diagram, and with those of a vibrating rod generally, such as n, 3n, 5n, etc, the interference of the systems is not complete, and silence cannot be produced by the interference of sounds-(From Ganot's Physics.)
In the case just mentioned, the waves are of the same length, but if they are of different lengths, instead of constantly reinforcing and interfering with others, they may sometimes strengthen and sometimes weaken each other. The result is more or less rhythmical increase and diminution of action, or as it is termed 'beats.' This is shown in the accompanying diagram (Fig. 64).
Instances of rhythm occur in the body, which strongly remind us of this condition; for example, the different rhythms of the heart under various conditions.