- #1
Freixas
- 298
- 39
In this post, I will describe, as best I have been able to determine, how a Western free reed works. As it is unlikely I will run into a physicist here who specializes in reeds, I will just ask people to speculate on the behavior of a reed given what you know about fluid dynamics or the mechanics of springs.
As Western free reeds are used in a variety of musical instruments, they have been widely studied. My problems are in finding the research, getting access to it, and understanding it. I am not a physicist. I am looking for a lay person's description of the reed's behavior, but one grounded in physics. Instruments which use the reed include the harmonica, accordion, concertina, melodica and even the organ.
It's called a free reed because it moves freely through an opening. It's called a Western free reed because it sits above the opening—Eastern free reeds sit within the reed plate.
Let me start with an introduction to a Western free reed:
The entire assembly is called the reed, although the tongue is also called the reed. The tongue is the flexible piece of metal that vibrates and produces sound. It is attached to the reed plate with a rivet, screw or weld. Near its base, the tongue lies just above the hole in the reed plate. Part way down its length, the tongue bends upward so that the tip lies about as high over the hole as tongue's thickness—this is called the gap. The whole thing sits on an exhaust port. When we want a reed to vibrate, we open the exhaust so that air can flow past the tongue, through the reed plate hole and out the exhaust.
The hole in the reed plate is almost the same size as the tongue. There is an extra 0.02–0.03 mm on the sides and 0.1 mm at the tip.
I am going to focus on one instrument, the melodica. Reeds in this instrument generally sit in a large air chamber into which the performer blows. When a key is pressed, an exhaust port is opened and the airflow begins. Here's what I think happens (with questions):
In #4, I'm guessing that the flow past the tip is somewhat aimed toward the final exhaust (which is toward the right) and actually serves to lower static pressure above the tongue and begin raising the tongue. I haven't studied springs at all, but I'm pretty certain that the further the metal bends, the greater the force to return it to its starting position.
#5 seems a little iffy if we accept the arguments made for #3. Air is still flowing in and still wants to exit. It should still want to push the tongue down. The static pressure below the reed, however, should be higher than in #3, so it won't assist this movement. But the main force driving the reed upwards would seem to be the spring.
These are the places that I'd love to hear what people think. Now, let me add some bonus questions:
This is a variant of #4 in which the exhaust walls (at least, near the reed plate) have been narrowed. I think with this design, the reed might function poorly since a region of low static pressure might be created below the tongue tip and would resist the upward movement. I would think #4 would be the better design.
In this alternate, we seal the reed in an airtight box except for the opening to the main air chamber. Speeds in the main air chamber are low and pressure is high. By channeling the air through a narrow tube, we increase the dynamics pressure and lower the static pressure in this reed box. Whereas before, the static pressure was acting on all parts of the tongue equally, we can now focus dynamics pressure right on the tip. Since the tongue also acts as a lever, this may make it easier to push the tongue down. Conversely, it might make it hard to get it to move up.
This might be a difficult question to answer. I can model airflow with SimScale, but only with a static geometry. I could build a reed box and try it out, of course, but whether the reed sounded or not, it wouldn't help me understand what was happening.
There's no penalty for guessing on any of this. Hopefully, some of you will make higher quality guesses than I'm making.
By the way, the Western free reed makes a sound only when air flows in one direction. If you study the six steps again, but imagine the flow in the opposite direction, it should be clear why—there will never be a point at which the air is blocked (or mostly blocked). The reed will probably bend upwards a bit, but won't vibrate.
As Western free reeds are used in a variety of musical instruments, they have been widely studied. My problems are in finding the research, getting access to it, and understanding it. I am not a physicist. I am looking for a lay person's description of the reed's behavior, but one grounded in physics. Instruments which use the reed include the harmonica, accordion, concertina, melodica and even the organ.
It's called a free reed because it moves freely through an opening. It's called a Western free reed because it sits above the opening—Eastern free reeds sit within the reed plate.
Let me start with an introduction to a Western free reed:
The hole in the reed plate is almost the same size as the tongue. There is an extra 0.02–0.03 mm on the sides and 0.1 mm at the tip.
I am going to focus on one instrument, the melodica. Reeds in this instrument generally sit in a large air chamber into which the performer blows. When a key is pressed, an exhaust port is opened and the airflow begins. Here's what I think happens (with questions):
- No air is flowing. The static pressure on either side of the tongue is equal (presumably its the same as outside pressure).
- The exhaust port is opened. Since the air chamber is large, air speeds (dynamic pressure) in it are low and static pressure is high. Air flows through the gap. This drops the static pressure below the tongue and the tongue is forced down into the gap. I've estimated that with a volumetric flow if 0.5 L/s, the air speed is about 41 m/s.
- This is where it gets interesting. When the reed moves into the hole, many sources say the airflow is blocked. In fact, there is still a small space between the tongue and the reed plate walls. It is very small, about 1.552-6 m2. Air flowing through this opening would need a speed of around 322 m/s to follow the continuity formula. To the extent that the flow is blocked, the static pressure below the reed rises to match the external pressure.
- The tongue moves below the plate. This allows air to vent. The spring metal has a force that wants to move it back to its resting position, so it begins to rise.
- We've blocked the flow again. The theory is that on the upswing, the static pressure below the tongue is not as low as it was at #2, so there is little pressure difference blocking the upward movement.
- The spring metal now wants to move downward and, as the gap closes, the static pressure below the tongue drops. The cycle repeats.
In #4, I'm guessing that the flow past the tip is somewhat aimed toward the final exhaust (which is toward the right) and actually serves to lower static pressure above the tongue and begin raising the tongue. I haven't studied springs at all, but I'm pretty certain that the further the metal bends, the greater the force to return it to its starting position.
#5 seems a little iffy if we accept the arguments made for #3. Air is still flowing in and still wants to exit. It should still want to push the tongue down. The static pressure below the reed, however, should be higher than in #3, so it won't assist this movement. But the main force driving the reed upwards would seem to be the spring.
These are the places that I'd love to hear what people think. Now, let me add some bonus questions:
This might be a difficult question to answer. I can model airflow with SimScale, but only with a static geometry. I could build a reed box and try it out, of course, but whether the reed sounded or not, it wouldn't help me understand what was happening.
There's no penalty for guessing on any of this. Hopefully, some of you will make higher quality guesses than I'm making.
By the way, the Western free reed makes a sound only when air flows in one direction. If you study the six steps again, but imagine the flow in the opposite direction, it should be clear why—there will never be a point at which the air is blocked (or mostly blocked). The reed will probably bend upwards a bit, but won't vibrate.
