# Resonant frequencies in organ pipes

I have a series of doubts regarding the principle of organ pipes. For a given length of closed organ pipe there are various modes of vibration for the standing waves.I dont know if this is silly but what decides the frequency of standing wave in the column,is it the frequency with which a person blows into pipe or are the frequencies or various harmonics dependent only on the length of column (the ones we calculate by drawing the number of waves in the column). What happens if the frequency of source cannot be matched by any of the possible variations of waves (harmonics), do we get low intensity of sound in such setups due to negative superpositions, how do we draw the standing wave inside the column then, will the closed end still be a node?
I was studying the resonance tube experiment to determine the speed of sound in air that lead me here. I understood till the part where loudness was maximum because the air in that particular length vibrated at the same frequency, the problem comes when we find the frequency at the second resonance length, why is that wave not in its first harmonic as depicted in any available picture of the experiment, does this imply that frequency of wave in column is determined by source?

• Blowing does NOT have a single frequency. – ggcg Dec 28 '19 at 15:22

## 1 Answer

There are several questions here.

First, the factors that determine the resonance frequency of a piece of pipe are 1) the speed of sound waves in the pipe, 2) the length of the pipe, and 3) the nature of the termination of the undriven end of the pipe. 1) and 2) tell you how long it takes a sound wave to travel from one end of the pipe to the other and 3) tells you the phase of the wave reflected off the undriven end in comparison with the phase of the incoming wave. Organ pipe acoustical physics is well-documented; any well-written source will tell you how to combine 1), 2), and 3) to get the fundamental and the overtone series.

Next, the issue of what overtones the pipe will contain. This depends on the frequency content of the signal that is driving the pipe. If the driving frequency is random noise that contains a broad range of frequencies, then the pipe will select out the frequencies that are multiples of the fundamental and resonate at them. If the driving signal contains no multiples of the fundamental, then there will be no resonances excited in the pipe.

I cannot comment on your last question because I do not know what was depicted in your experimental materials.