|
|
allows us to write the expression for y as (18.13) Graphs of the individual waves and the resultant wave are shown in Figure 18.22. From 1 ! f 2 )/2 and an amplitude given by the expression in the square brackets: (18.14) That is, the amplitude and therefore the intensity of the resultant sound vary in time. The broken blue line in Figure 18.22b is a graphical representation of Equation 1 # f 2 )/2. Note that a maximum in the amplitude of the resultant sound wave is detected whenever This means there are two maxima in each period of the resultant wave. Because the am- 1 # f 2 )/2, the number of beats per second, or the beat frequency f beat , is twice this value. That is, (18.15) For instance, if one tuning fork vibrates at 438 Hz and a second one vibrates at 442 Hz, the resultant sound wave of the combination has a frequency of 440 Hz (the f
beat " # f 1 # f 2 # cos 2&
! f
1 # f
2 2 " t " (
1 A
resultant " 2A cos 2&
! f 1 # f 2 2 " t y " $ 2A
cos 2&
! f 1 # f
2 2 " t % cos
2&
! f 1 ! f
2 2 " t S E C T I O N 18 . 7 • Beats: Interference in Time 565 y (a) (b) y t t Active Figure 18.22 Beats are formed by the combination of two waves of slightly dif- ferent frequencies. (a) The individual waves. (b) The combined wave has an amplitude (broken line) that oscillates in time. At the Active Figures link at http://www.pse6.com, you can choose the two frequencies and see the corresponding beats. Resultant of two waves of different frequencies but equal amplitude Beat frequency Quick Quiz 18.9 You are tuning a guitar by comparing the sound of the string with that of a standard tuning fork. You notice a beat frequency of 5 Hz when |