Waves and Sound

demonstration | amplitude and swell | extra | links

Demonstration: Slinky waves

A slinky can be used to show transverse and longitudinal waves, and also standing waves.

Standing waves might be easier shown by using a longer piece of string, perhaps with a small weight on the end.

Amplitude and swell

To a physicist amplitude is the maximum displacement from a zero value (the size of the crest from zero).

However, in the big wide world physicists are in the minority, and how the size of a wave is described is very likely not with the amplitude. For example, wave swell in coastal weather forecasts.

Swells are described in terms of significant height. Significant height is defined as the average height, from trough to crest, of the highest third of the waves. This means that some swell or sea waves will be notably larger than the significant height. For example, if the forecast is for 4-metre swells, then the occasional 6-metre wave should be expected.

Emphasis added. And FWIW you're a nutter if you go surfing in a 4 metre swell.

Extra

Anyone notice the bouncing waves when we used the slinky? When a transverse wave (or pulse) hit the end of the slinky it bounced off the fixed end, but on the other side of the slinky. When a longitudinal wave (or pulse) bounced off the fixed end of the slinky it just headed back along the slinky.

A tsunami is a real risk for many people in low-lying areas around New Zealand. Make sure you have a plan for what to do in the event of a tsunami warning and make sure your civil defence kit is up to date with food, water, batteries, and all the rest. See the back page of the yellow pages for more information.

Being able to work out the wavelength of a particular radio frequency can be very useful. For example it allows one to work out how long to make an aerial for a citizen's band (CB) radio, which operates on the 27 MHz band. The simplest aerial is a ¼ wave aerial. That (relative) length gives the best signal – it's where the radio attached to the aerial will be at the maximum amplitude of the incoming signal. This means the aerial needs to be ¼ the wavelength, which for CB works out at about 2.78 metres. Aerials like this are called whip aerials and look quite impressive as they tend to swing around a bit.

The conductor in an aerial can be coiled up making the aerial length shorter overall while keeping the actual length of the conductor the same, or they can use capacitors to shorten things even more.

Because the frequency of UHF CB (ultra high frequency CB, also called personal radio service, or PRS) is much higher – about 476 MHz – wavelengths are much shorter, so a ¼ wave aerial is a bit under 16 cm.

Another example is chaff. Chaff is conducting material (such as wire or aluminium foil) used to confuse radar. When the chaff is half the wavelength of the radar system it's being used on, it's really reflective. However, when chaff is falling it can be at any angle, so because they are not likely to reflect the radar emissions straight back to their source a lot of chaff is used – as it falls through the sky some of it will be at just the right angle for long enough to reflect in the right direction.

Corner reflectors reflect radar very well, since they reflect straight back to their source, whatever angle it's at. It's a principal known as retroreflection. Examples are catseyes (on the road), car number plates, bicycle and car reflectors, etc.

A corner reflector is often used by sea kayakers to make themselves more visible – it's the round thing in the photo.

For best visibility they're normally mounted on a pole, so because of its extra height this person's aluminium-decorated hat also worked quite well in this gentleman's radar trials.

However, his trials also showed that the kayaks were not visible more than a mile away from a given radar platform. My thought – take a fog horn as well.

Links

Java-based wave tank. Sonic boom, two slit experiment (see Light) etc.

MetService rain radar.