5.6 Vibrating air column
You learned in the previous section that for standing waves to be set up on a string there must be reflection. A travelling wave reaches the end of the string and is reflected. This results in a second travelling wave, which moves back up the string in the opposite direction to the first wave. The two travelling waves interact to produce a standing wave.
Standing waves are set up in an air column enclosed within a tube in a very similar way. Again there must be reflection. In this case, a travelling wave reaches the end of the air column and is reflected. This results in a second travelling wave, which moves back up the air column. The two travelling waves interact to produce a standing wave. But now the two travelling waves are sound waves. That is, they are longitudinal waves consisting of pressure compressions and rarefactions, rather than the transverse waves we saw on the string. Associated with the pressure changes are forward and backward movements of the air molecules in the direction of the tube axis.
Standing waves will be set up in an air column only at certain frequencies – the air column's resonance frequencies. These frequencies depend on the reflection conditions at the end of the air column.