![]() At first you hold your finger steady, and the ball bounces up and down with a small amount of damping. Most of us have played with toys where an object bobs up and down on an elastic band, something like the paddle ball suspended from a finger in Figure 14.18. The phenomenon of driving a system with a frequency equal to its natural frequency is called resonance, and a system being driven at its natural frequency is said to resonate. The natural frequency is the frequency at which a system would oscillate if there were no driving and no damping force. ![]() Over time the energy dissipates, and the amplitude gradually reduces to zero- this is called damping. This is a good example of the fact that objects-in this case, piano strings-can be forced to oscillate but oscillate best at their natural frequency.Ī driving force (such as your voice in the example) puts energy into a system at a certain frequency, which is not necessarily the same as the natural frequency of the system. It will sing the same note back at you-the strings that have the same frequencies as your voice, are resonating in response to the forces from the sound waves that you sent to them. Sit in front of a piano sometime and sing a loud brief note at it while pushing down on the sustain pedal. The swish of the tyre and wind-noise contains a lot of high frequency energy, and you should find that this does not diffract around the corner as effectively as the rumble of engine.Before the start of this section, it would be useful to review the properties of sound waves and how they are related to each other, standing waves, superposition and interference of waves. You can experiment with this by listening to traffic noise from a busy road from around the corner of a building (not in a direct line-of-sight to the traffic), and then moving to a location a similar distance from the road but in direct view of the passing cars. However with a short barrier (the same length as the wavelength) diffraction is very effective and there is almost no zone of silence behind it.įrom this, we can reach the conclusion that with sound waves, it is the low frequencies (which have long wavelengths) which diffract around corners. Our simulation shows that with a ‘long’ barrier, there’s a lot of reflection of incident energy back towards the source, but although there is some diffraction or bending of the wave around the barrier, this still leaves a zone of silence behind it. The obstacle in the right animation has the same width as the wavelength of the sound.īy examining the three animations, decide which of these statements is correct in the following quiz. Ripple tanks with large, medium and small objects (left to right) obstructing a wave. ![]() ![]() The key to understanding diffraction is understanding how the relative size of the object and the wavelength influence what goes on. Have a look at this a simulation of three ripple tanks, each containing an object of different width, which obstructs the propagation of a wave. Diffraction can be clearly demonstrated using water waves in a ripple tank. ![]() The amount of diffraction (spreading or bending of the wave) depends on the wavelength and the size of the object. Waves can spread in a rather unusual way when they reach the edge of an object – this is called diffraction. What is the reason for this? Do light and sound share any properties that might cause this effect? Diffraction Around An Object Have you ever wondered why you can hear someone who is round the corner of a building, long before you see them? It appears that sound can travel round corners and light cannot. ![]()
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