A Proposal For A Bi-Directional Concertina Reed
Posted 19 March 2015 - 08:01 AM
Posted 23 March 2015 - 07:35 AM
When the reed is in rest position, the tip is not exactly parallel with the frame, but slightly bend
upwards. The gap between the frame and the tip of the reed allows air to flow past the reed to P2.
This air flow will pull the tip of the reed downwards towards the frame, just like two pieces of paper
hanging parallel which, when you blow air between them, will move towards each other. The fact
that the gap decreases when the reed moves down towards the frame is important for the start of the
cycle. As the reed starts to move, tension energy is built up which replaces the suction of the airflow.
The suction of the airflow decreases as the gap gets smaller.
This is an excerpt from Wim Wakker description of free reed excitation http://www.concertin...rtina reeds.htm . I have read this only once and a long time ago, and have forgotten, that this explanation was included in Wim's article. I have just stumbled upon this today, led by a different thread on c.net, and thought it should be mentioned in this thread, that at least one profesional concertina maker thinks that gap airflow can work as I have tried do describe.
Posted 06 April 2015 - 08:33 AM
Reeds that are set too low will choke if the initial starting pressure is too high. What amounts to "too High" is dependant on the surroundings of the reed including the air volumes the reed is connected to. ( empirical, not theory ). Accordion reed bellows have much larger volumes than concertina bellows and I have found that without adjustments, the two reed types often object to a big change in their environment.
. When one of the tongues was valved (with a masking tape) the same reed was very prone to choking with sudden bellows movement]
Reeds are driven in more than one way. Generally the airflow through the acting reed damps out all but the first oscillatory mode ( and if the reed is too short for its width, its twisting mode ) the adjacent reed will vibrate sympathetically in any mode close to some multiple of the main reed's frequency. At higher pressures, I believe that these too are damped out to a great degree with no valve, with the inactive reed becoming nothing more than an obstacle to the air flow. With low pitched reeds which have quite a bit of mass, at low pressures, they can gain more energy from sympathy than they lose resisting the air stream, and drive some sound creation. Often if low pitched reed valves are of incorrect mass and stiffness, the energy from the sympathy reed is enough to kick the valve open slightly, allowing bypass air through. This causes a rough almost kazoo like sound that I generally call valve bobble. Increasing pressure holds the valve against this and the sympathetic vibration doesn't get high enough to overcome it. With no valve, you are effectively creating an opportunity for a free reed version of a wolf tone where the two reeds interact at differing frequencies creating an unstable state where reinforcement comes and goes. Some reeds are more susceptible to being pulled off pitch than others, so this effect may be very dependent on the reeds used.
Posted 06 April 2015 - 08:54 AM
This may be a valuable point. Since it makes so much difference to the operation of a traditional concertina reed, you may find that whatever design you create may require its own appropriate window design to make it work.
Do you understand the fundamental difference between traditionally made concertina reeds/frames and accordion reeds/frames?
Nope. But i ceased trying to describe the operation of the reed quite a ways back. Lately, I've been considering the pressure difference between top and bottom of the reed tongue initially -- before the tongue has begun to move with any significant speed.
Besides, since this thread started out as a design proposal, i can make the frame and tongue to have the shape that i want. ;-)
Edited by Dana Johnson, 06 April 2015 - 08:56 AM.
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