Diffraction – wave spreading around an edge

The wavelength of water waves may be several metres. If the wavelength is of a similar size to a gap in a harbour wall, then the wave will diffract  or spread out into circular ripples. This is quite a good aerial image which shows the effect clearly.  If the wavelength  is much smaller than the size of the gap, then only a little bit of diffraction will occur at the edge of the wave where the wave falls over on itself to form circular ripples and most of the wave passes straight through without significant direction change. We can see clearly that the depth of the water is pretty constant, if it weren’t the wavelength of the waves would change, and notice there’s no change in wavelength, frequency or speed.

Here are two images, one showing a narrow gap with circular diffraction, the other a wide gap with only small circular bits at each side.

These diagrams are important, so learn them.

Here’s a slightly more complicated diffraction image, showing multiple diffractions around different edges in sea waves.

 It’s worth noticing that the waves actually pass through each other, their amplitudes adding up in space and time.

I just got back from the mountains where TV signals sometimes are a bit wobbly, because the waves can’t diffract well enough over the mountains to be received clearly in the valley. Work out the wavelength for yourself. The TV signal is a radio wave travelling at 300 million m/s and might have a frequency of  600MHz. Got it? Wavelength = speed/frequency = half a metre. Mountains are a lot bigger than this so there isn’t much diffraction around them. We have to use much lower frequencies or bounce the wave off a satellite.

The idea of diffraction is important in loudspeaker design. Speakers that produce low frequency bass notes are called “woofers”. They need to move a lot of air, so need to be quite large. Diameters of 30 cm or more are common.

However, a typical high treble note has a frequency of 5000 Hz or so, which corresponds to a wavelength of  almost 7cm – sound travelling at 330 m/s in air. This is much less than the diameter we need for a woofer, so if we try to use the woofer to generate a high frequency note, the sound wave will beam straight ahead without significant diffraction, just like water waves passing through a wide gap. So, you won’t hear those notes unless you are right in front of the speaker.

It’s clear, then that the speakers which  produce high frequency notes, called  “tweeters”, must have a much smaller diameter than required for the woofer.

 In the picture, the woofer is above and the tweeter is below. A simple electric circuit inside the enclosure directs low frequencies to the woofer and high frequencies to the tweeter.