# Organ pipe sound source

## Homework Statement

Organ pipe sound is driven by a compressed air jet flicking alternately in and out of the pipe in step with the fundamental frequency. My question is whether sound waves of the higher frequency harmonic series are emitted during the intervals when the jet is outside of the pipe and not contributing energy to the system, and if so, what drives them?

Thank you.

## The Attempt at a Solution

haruspex
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Gold Member
2020 Award
What determines the fundamental frequency? Think about why the jet should flick in and out at that frequency.

PeterO
Homework Helper

## Homework Statement

Organ pipe sound is driven by a compressed air jet flicking alternately in and out of the pipe in step with the fundamental frequency. My question is whether sound waves of the higher frequency harmonic series are emitted during the intervals when the jet is outside of the pipe and not contributing energy to the system, and if so, what drives them?

Thank you.

## The Attempt at a Solution

I think you are forgetting how the Organ pipe sounds. Remember resonance. The air can flick in and out at any rate at all, but you only get resonance with specific frequencies (fundamental and overtones), so you only hear specific frequencies sounding.

haruspex
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Gold Member
2020 Award
The air can flick in and out at any rate at all,
Not sure how you mean that. As I hinted, it flicks in and out at the rate it does for a reason. Understanding that reason is key to answering the question.

The question is the energy source to drive the harmonic frequencies. If the jet flicks in and out of the organ pipe, adding energy only while flicking into the pipe, and if the fundamental frequency of the pipe were 110 Hz, then during each second there are one hundred and ten 4.5 ms gaps during which energy is not being added to the pipe. But during those 4.5 ms gaps, harmonic sound waves are being generated. For example, to sustain the harmonic frequency of 1100 Hz requires an energy pulse inside the pipe every 0.9 ms. Where is that energy coming from during the aforementioned gaps?

haruspex
Homework Helper
Gold Member
2020 Award
The question is the energy source to drive the harmonic frequencies. If the jet flicks in and out of the organ pipe, adding energy only while flicking into the pipe, and if the fundamental frequency of the pipe were 110 Hz, then during each second there are one hundred and ten 4.5 ms gaps during which energy is not being added to the pipe. But during those 4.5 ms gaps, harmonic sound waves are being generated. For example, to sustain the harmonic frequency of 1100 Hz requires an energy pulse inside the pipe every 0.9 ms. Where is that energy coming from during the aforementioned gaps?
Please try to answer my question in post #2, even though you do not yet see the relevance.

The fundamental frequency imposed on the jet is determined by the length of the pipe and the speed of sound, that is the time that it takes for each compression/rarefaction event initiated by the jet to transit the length of the pipe.

haruspex