Standing Waves in Open Pipes: How Do Sound Waves Reflect at the Open End?

[nokia8650]

Hi

How are standing waves set up in open pipes - my book says that the sound waves can be reflected from the open end of the pipe - how is this possible - surely the wave would just leave the pipe?

Thanks in advance

 

[tiny-tim]

How are standing waves set up in open pipes - my book says that the sound waves can be reflected from the open end of the pipe - how is this possible - surely the wave would just leave the pipe?

Hi nokia8650!

Sound is a traveling longitudinal pressure difference.

A sound wave in a pipe is a standing wave, and therefore cannot exist unless there is reflection at both ends.

First consider a single pulse (not a wave).

At a closed end, a pulse of high pressure is reflected as a pulse of high pressure.

At an open end, a pulse of high pressure is reflected as a pulse of low pressure (and vice versa) … a 180º "change in phase".

This is because when the high pressure pulse leaves the pipe, it spreads out spherically, rapidly becoming atmospheric pressure, and suction is created behind it.

In other words, there is a reduction in pressure behind it, and this low pressure will pulse back down the pipe.

It is this sudden reduction in pressure caused by the ability of the wave to spread out which makes the difference.

That's fairly clear for a single pulse, though not so clear for a wave, but the principle is the same.

For a lot of detail, including a rather good animation, see this Australian site: http://www.phys.unsw.edu.au/jw/flutes.v.clarinets.html#time

(Another way of looking at it is that the open end of the pipe is at high pressure, and the air beyond it is at ordinary pressure, so it behaves like a boundary between two fluids of different pressure, and so a reflection is to be expected. )

 

[nokia8650]

That makes sense, thanks a lot!