http://www.ck12.org Chapter 11. Vibrations and Sound
same frequency. We call this phenomenonsympathetic vibration.
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/109834FIGURE 11.14
https://www.youtube.com/watch?v=tnS0SYF4pYE
InFigure11.14, a set of pendulums are fixed to a horizontal bar that can be easily jostled. PendulumsAandE
have the same length. If one of them is set swinging, the horizontal bar will be forced into moving with a period
equal to that of the pendulum, which, in turn, will cause the other pendulum of the same length to begin swinging.
Any pendulum that is close in length to pendulumsAandE, for example, pendulumD, will also begin to swing.
PendulumDwill swing with smaller amplitude than pendulumsAandEsince its resonant frequency is not quite the
same as pendulumsAandE. Pendulums with lengths dramatically different from pendulumsAandEwill hardly
move at all. You can try a similar demonstration out yourself with the following simulation:
http://phet.colorado.edu/en/simulation/resonance
Resonance is a very common phenomenon, especially with sound. The length of any instrument is related to what
note it plays. If you blow into the top of a bottle, for example, the note will vary depending on the height of air in
the bottle. This plays an important role in human voice generation. The length of the human vocal tube is between
17 cm and 18 cm. The typical frequencies of human speech are in the range of 100 Hz to 5000 Hz.
By using the muscles in their throat, singers change the note they sing. A dramatic example of this is breaking glass
with the human voice. By singing at exactly the resonant frequency of a delicate wine glass, the glass will resonate
with the note and shatter.
http://www.youtube.com/watch?v=7YmuOD5X4L8
The resonance of sound is also a mechanical analogue to how a radio set receives a signal. TheFigure11.16 shows
one of the earliest radio designs, called a crystal radio because the element which detected the radio waves was a
crystalline mineral such as galena.
In modern times, the air is filled with all manner of radio waves. In order to listen to your favorite radio station, you
must tune your radio to resonate with only the frequency of the radio station. When you hear the tuning number of a
radio station, such as “101.3 FM”, that is the measure inmegahertz, 101 .3 MHz= 1. 013 × 108 Hz. The coiled wire
(called an inductor) and the capacitor inFigure11.16 act together to tune in a specific radio station.Effectively,
the capacitor and inductor act analogously to a pendulum of a specific length that will only respond to vibrations of