Reed physics - frequency

[Little John]

To my layman's understanding, the sound of a reed is produced by the reed "chopping" the airflow as is passes through the reed frame (rather than being the oscillation of the reed itself). Since the reed passes through the frame twice (once in, once out) for each oscillation of the reed, does this mean that the generated frequency is twice that of the reed? For example, when we hear A4 at 440Hz is the reed actually oscillating at 220Hz?

 

I ask this partly because occasionally, playing very quietly, I can hear a subharmonic an octave below the fundamental.

[alex_holden]

I don't believe so. When you pluck a reed with your finger, it makes a sound at approximately the same pitch as when you draw air through it.

[Little John]

23 minutes ago, alex_holden said:

When you pluck a reed with your finger, it makes a sound at approximately the same pitch as when you draw air through it.

 

But maybe, even then, you're hearing principally the effect of the air being "chopped".

[alex_holden]

21 minutes ago, Little John said:

But maybe, even then, you're hearing principally the effect of the air being "chopped".

 

It's the same pitch (though very quiet) if you take the reed out of the frame and hold it at the clamp point with a pair of pliers, then pluck it in the open air like a kalimba.

[Mikefule]

If you twang a reed, it oscillates, sending a small pressure wave in each direction.  Think of waggling a canoe paddle slowly in a calm pond.  The waves will radiate but with the strongest force emanating from the flat side.  If it goes right left right left right left, it pushes the wave one way on every right and the other way on every left.

 

If you play a concertina reed, it waggles at the same rate.  It opens and closes the hole in the reed plate.  It goes open close open close open close.  The sound pulses are on the "open" which is the same frequency as EITHER the right OR the left in the example above.

 

So, simply, if a reed makes a C when you play it, it will make the same C if you twang it.

 

The difference in volume is interesting.  Imagine gently waving your hand in a large still pond.  It will make small waves caused by your hand pushing the water.  Now imagine waving

your hand across the jet of water from a hose.  The jet of water will nw have pulses in it a the same frequency, but with more power.  The frequency comes from the waving of your hand, but the volume comes from the powerful jet of water that you are interrupting and releasing.  Apply this to the concertina reed.  "Twanging it" is waving your hand in the pond; playing it is interrupting and releasing the powerful flow of air created by the bellows.

[David Barnert]

On 10/3/2020 at 6:02 AM, Mikefule said:

If you play a concertina reed, it waggles at the same rate.  It opens and closes the hole in the reed plate.  It goes open close open close open close.  The sound pulses are on the "open" which is the same frequency as EITHER the right OR the left in the example above.

 

But if the reed passes through the hole in the plate, spending equivalent time both above and below it, won’t there be two opens and two closes for each cycle of vibration of the reed?

[alex_holden]

1 hour ago, David Barnert said:

 

But if the reed passes through the hole in the plate, spending equivalent time both above and below it, won’t there be two opens and two closes for each cycle of vibration of the reed?

 

The two halves of the cycle aren't identical, and it seems that the brain is surprisingly good at perceiving the fundamental pitch of a repeating waveform even if it has a bunch of harmonics mixed in.

[Richard Mellish]

24 minutes ago, alex_holden said:

 

The two halves of the cycle aren't identical, and it seems that the brain is surprisingly good at perceiving the fundamental pitch of a repeating waveform even if it has a bunch of harmonics mixed in.

Indeed. For part of the cycle much of the length of the reed is completely clear of the frame and the air can flow freely. For part of the cycle the reed is down inside the frame and regardless of the exact profile of the frame the air flow is significantly impeded. So we should expect the sound to have a substantial component at the fundamental frequency. Having observed the frequency spectra of a few reeds I have been surprised at the levels of very many harmonics.

[Mikefule]

6 hours ago, David Barnert said:

 

But if the reed passes through the hole in the plate, spending equivalent time both above and below it, won’t there be two opens and two closes for each cycle of vibration of the reed?

It doesn't pass through the hole.  It is offset towards the direction from which the air comes. The air flow between the edge of the reed and the edge of the hole results in low pressure which draws the reed into the hole blocking it.  The air flow then stops and the reed springs back opening the hole.  The reed does not pass through the hole.  It opens and closes the hole.

[David Barnert]

19 minutes ago, Mikefule said:

It doesn't pass through the hole.  It is offset towards the direction from which the air comes. The air flow between the edge of the reed and the edge of the hole results in low pressure which draws the reed into the hole blocking it.  The air flow then stops and the reed springs back opening the hole.  The reed does not pass through the hole.  It opens and closes the hole.

 

Are you saying that the tongue of the reed travels between being flush with the hole and being bent away from it without ever dipping into the hole? Wouldn’t the first puff of air push it in the direction of the hole?

 

Alex’s and Richard’s responses to my earlier post make sense, but I can’t make sense of this.

[alex_holden]

10 hours ago, Mikefule said:

It doesn't pass through the hole.  It is offset towards the direction from which the air comes. The air flow between the edge of the reed and the edge of the hole results in low pressure which draws the reed into the hole blocking it.  The air flow then stops and the reed springs back opening the hole.  The reed does not pass through the hole.  It opens and closes the hole.

 

Sorry Mike but I don't agree. I'm certain that the reed tongue passes below the vent opening and allows some air through in the lower part of the cycle. I suspect the opening is considerably more restricted than in the upper part of the cycle, but I don't have a good way to measure the difference.

[Richard Mellish]

If anyone really wants to know in more detail what's going on it mightn't be too hard to set up an experiment. A reed in its frame on a tuning rig. Bellows underneath (or a fixed chamber) connected to an air pump with a controllable throttle in between, so that the rate of air flow past the reed can be adjusted. Attach something like a fine wire to the tip of the reed so it's position can be adjusted. Measure the pressure difference across the reed as a function of flow rate and reed position. Then you might need some computer modelling.

[alex_holden]

9 minutes ago, Richard Mellish said:

If anyone really wants to know in more detail what's going on it mightn't be too hard to set up an experiment. A reed in its frame on a tuning rig. Bellows underneath (or a fixed chamber) connected to an air pump with a controllable throttle in between, so that the rate of air flow past the reed can be adjusted. Attach something like a fine wire to the tip of the reed so it's position can be adjusted. Measure the pressure difference across the reed as a function of flow rate and reed position. Then you might need some computer modelling.

 

I have done a very unscientific version of this, visually observing how fast the tuning bellows fall as I slowly push the tongue down towards and then through the frame opening. There is quite a narrow region where the vent is almost completely blocked, before it passes below the restriction and starts to allow air through again.

[David Barnert]

31 minutes ago, Richard Mellish said:

If anyone really wants to know in more detail what's going on it mightn't be too hard to set up an experiment. A reed in its frame on a tuning rig. Bellows underneath (or a fixed chamber) connected to an air pump with a controllable throttle in between, so that the rate of air flow past the reed can be adjusted. Attach something like a fine wire to the tip of the reed so it's position can be adjusted. Measure the pressure difference across the reed as a function of flow rate and reed position. Then you might need some computer modelling.

 

If this is set up the way most tuning rigs are (ie., reed shoe mounted in dovetail joint on the upper surface of the rig table), then you might want to put the air source (bellows, pump, what-have-you) above the reed to simulate the working of an actual concertina, where the air flows from the side of the reed assembly with the tongue.

 

16 minutes ago, alex_holden said:

I have done a very unscientific version of this, visually observing how fast the tuning bellows fall...

 

Unless, of course, you use the bellows as a suction device, as Alex does, here (I wrote the first half of this post before Alex’s appeared).

[Richard Mellish]

Yes, I assumed a pump sucking from below, to allow access to the reed as for tuning.

 

[Dana Johnson]

Reeds at their quietest enter the window until they reach a depth where the increased clearance allows enough air to pass by the reed to drop the forces causing the motion.  Reed momentum carries it to a point where the return spring force is greater than the remaining air flow forces.  Reeds like concertina reeds with a back draft angle in the window allow the reed to dump air on the sides of the reed as well as at the tip and can operate at lower pressure than reeds with vertical window walls, where the increasing gap at the tip is the only avenue for the air to pass by the reed requiring the reed to swing farther to begin cycling.  In concertinas, reeds commonly swing through the window and down into the reed pan port below at moderate volume.  A strobe makes watching the reed easy.  At higher volume, they swing quite far, and the swing is roughly equal in both directions from the clamping point. It is true that these reeds are “set” to have a small gap under the reed tip at rest ( about the tip thickness )they don’t start with the reed completely shutting off the air unless they have a resonator Chinese Sheng style. But they can and do swing through the full window depth of normal reeds.

Dana

[Chris Ghent]

A rough and ready confirmation of reed travel beyond the bottom of the frame; blow through a hand held reed and put your finger behind it and you will be able to feel the reed hitting your finger. 

[alex_holden]

I once restored a vintage Lachenal anglo where one of the reed pan wind slots had been routed off-centre, such that the reed tongue hit it and made a horrible buzzing sound when you tried to play the note loudly. I think it must have left the factory like that!