When two notes of nearly equal pitch are both sounded together a regular rise and fall occurs in the loudness of the tone heard. These alternations in loudness are called heats.
This may be demonstrated with organ pipes, or singing tubes may be used instead. Two glass or metal tubes about a metre long and 4 or 5 em in diametere clamped in a vertical position and have small gas flames inside them about a quarter of the way up from the bottom. For this purpose, suitable burners may be made from drawn-out glass tubing, as shown in Fig. 28.3. After adjusting the flame height and position for the best results, the tubes will give out a loud, continuous note. Usually, the tubes are not exactly the same length, so their frequencies are lightly different and strong beats are heard. By sliding a paper collar over the lower end of one tube its frequency may be altered as desired, with a corresponding change in the number of beats heard per second.
In the days when the majority of aircraft were powered by two or more piston type engines a throbbing sound could often be heard owing to beats between the engine notes.
Piano tuners sometimes utilize beats for tuning a piano string to the pitch of a standard tuning fork. If the pitch of the string is not equal to that of the fork, beats will be heard when they are sounded together. The tension of the string is now altered until the beats become slower and finally disappear, showing that the two notes are in unison.
Two tuning forks of equal frequency may also be used to produce beats, if the frequency of one of them is reduced slightly by loading its prongs with tiny pieces of wax or plasticize. Let us suppose that two forks A and B of frequency 256 Hz, are sounded together, but that B has had its frequency reduced to 252 Hz in the manner described. Consider an instant when both forks are exactly in step so that they are simultaneously producing compression‘s. Under these conditions they reinforce one another and produce a resultant sound of maximum loudness. After t s A will have made 3_ vibrations while B makes 31}, and at this stage A will be producing a expression a exactly the same moment that B is producing a rarefaction. The resultant disturbance of the air will now be a minimum. Again, after another t s A will have completed 64 vibrations and B 63. Once more the two forks are in step with one another and the resultant air disturbance is maximum. The loudness of the sound thus falls to a minimum and rises to a ma ‘- mum every t s or, in other words, there are 4 beats per second. Now 4 is equal t the difference in the two frequencies concerned, 256 and 252. In general, the number of beats per second given by two notes of nearly equal frequency II and j~ is given _ II – 12’ the resultant wave-form when two nearly equal waves combine – produce beats. The variation in the amplitude of the resultant wave indicates clear.; how the beats ‘occur.