There are two tubes that are identical except for their slightly different lengths. Both tubes have one open end and one closed end. A speaker connected to a variable frequency generator is placed in front of the tubes. The speaker is set to produce a note of very low frequency and then turned on. The frequency is then slowly increased to produce resonances in the tubes. Students observe that at first only one of the tubes resonates at a time. Later as the frequency gets very high, there are times when both tubes resonate.

Question

Explain why there are some high frequencies, but no low frequencies, at which both tubes resonate. Any help would be greatly appreciated :)

Answer 1

the tubes first resonate at their individual fundamental frequencies
... the longer tube will have a lower fundamental

the tubes will then resonate at the odd harmonics of their fundamentals

as the frequencies increase, some will be an odd multiple for both tubes