# What does the sound coming from an open column look like?

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1. Mar 4, 2015

### fiddleback

I believe I understand what happens inside an air column that is open at both ends when you cause a disruption such as blowing into it. But I am not sure what happens in the space between the air column and your ear. Please let me explain what my current understanding is and tell me if I'm wrong.

The ends of the open air column are observed to be pressure nodes and the air around the column has a characteristic speed at which sound travels. When the column is disrupted, the air in the column moves such that the pressure is a superposition of the standing waves that fit onto the column's end nodes (multiples of half-wavelengths). Additionally, each of the component standing waves is equivalent to two traveling waves moving in opposite directions, which can be used to determine the frequency at which each of the standing waves oscillates. Now my guess is that the sound coming out of each end of the column is a diffracted version of the superposition of the traveling waves moving in each direction through the column. Is this correct?

Last edited: Mar 4, 2015
2. Mar 4, 2015

### Simon Bridge

The resonating column interacts with the outside air at the open end (if we idealize that the column walls are rigid).
At the open end, the displacement of air molecules is a maximum - so you have air pushing in and out of the hole, pushing on the air molecules in the "outside" air.
Thus the sound from that end spreads out mostly spherically as a travelling wave.

The inside and outside are coupled systems - what you usually do with a musical wind instrument is make a vibration near it, which gets amplified by the harmonics of the instrument. The whole instrument vibrates the air around it so what someone some distance away hears is the tones and not the initial vibration. The actual wave about an instrument can be quite complicated but the phenomena is not usually called diffraction.

3. Mar 5, 2015

### fiddleback

I was thinking in terms of pressure, but thinking in terms of displacement is more intuitive in this case. I see now I'm going to need to pick up another text. But thinking in terms of displacement gives a better sense of how vibrations get pushed towards the listener even if it doesn't give a sense of the waveform that's being sent. Thank you.

4. Mar 5, 2015

### Simon Bridge

You could imagine a flexible membrane over the "open" end of the rigid pipe - then it's going to look like how a speaker works right?
The waveform will be spherical. The amplitude will vary in pace with the end displacement.

5. Mar 5, 2015

### fiddleback

And now it is perfectly clear. Thank you for that last remark especially. Cheers.