Paul Farmer and Dana Johnson thread on reed (harmonica and concertina) embossing. I'd thought I'd share Dana's findings with fellow tinkerers here on Harp-L : Regarding the chamfering of the reed plates, On concertinas that use "concertina reeds" and not accordion reeds, the chamfer is anywhere from 5-15 degrees from perpendicular to the reed plate surface, and serves to make a tapering hole. the size of the hole where it exits the backside ( where the flap valves are) depends on the thickness of the plate, and at those tapers, you should still be able to use the existing valves effectively. A number of different factors influence reed maximum volume and responsiveness, and there is a progressive if not scaled difference in responsiveness between high notes and low that you will never overcome. That is partly due to the mass of the reeds themselves, and the inertia they need to overcome to get moving. The chamfer of tapering relief does allow a reed to begin to speak at a lower pressure, since it can release the pressure effectively as the slot it is passing through becomes wider at low pressures. If the slot is not overly tapered, the reed can still operate nearly as well at high pressures, because the higher velocity higher pressure air trying to escape around the edge of the reed actually clogs the gap with itself. There is a limit to the amount of air that can pass through an opening at any pressure, and doubling the pressure does not double the amount of air that can pass. The larger reeds need a somewhat thicker plate, because they move farther at the tip than a short reed under the same pressure, and require the extra thickness to develop enough volume. Responsiveness may be somewhat improved by the chamfering operation, (though Steve Dickenson of Wheatstone Concertinas says it should be on the sides ideally and not at the end of the slot . I did some experimenting here, and he seems to be right.) but you will likely have to sacrifice some volume to get it. Switching to a stiffer reed will get the volume back, but at the expense of responsiveness. The one place I have seen where you can make some difference, is in the materials the reed is attached to, the materials and construction of the reed chambers and such. Reeds depend on a lot of feedback to keep oscillating, and surroundings that soak up vibration or damp it at all, like soft chambers, lightweight reed plates, or weak contact between reed plates and the surfaces they are connected to, drain energy away from the reed. In a concertina, a loose reed shoe may still sound, but will be quiet, not because it can't transmit its vibration, (which should be going into the air stream anyway) but because the shoe can wiggle as the reed moves, and bleeds off the reed energy. I am not too familiar with harmonica construction, but if there are gaskets that are not tight in the area of the low reeds, or if the partitions between reeds could be made from harder denser material, the reeds may retain more energy if these areas are dealt with. Unfortunately, trying to tinker an existing harmonica like that may not be very feasible. I have beveled the tip of weighted reeds, since they are otherwise disproportionately thick at the tips, but this was done on the top side of the reed, otherwise I would leave it alone, because it will alter the pitches significantly among other things Rich directed me to a web site in scandinavia where someone had developed a method of making a circular pocket in the reed plate at the reed tip purporting to increase the volume of the high reeds. Relieving the end of the window the reed moves through may slightly improve responsiveness, but will reduce volume in the end too. I think scooping out on the top won't accomplish more than slightly increasing how high the reeds tips are set off the surface of the reed plate, which would be easier to do, (though a reed set to low will be responsive, but choke off at high pressures, and one set high will allow a lot of high pressure punch, but not be so responsive.) Everything is a trade off! Rich also mentioned once that some company drilled a small hole from the inner face of the reed window at the tip that came out on the upper surface as a way to help balky reeds to start. It would do this by providing a stream of air below the reed tip that would pull it into the window at the beginning, as blowing a stream of air over a piece of paper will cause it to flap up to the stream of air. This may be practical on larger reeds of an accordion, but unless you are very good with a tiny drill, it would be incredibly hard to do successfully on a harmonica. Every instrument has its limitations, but if you were to outright replace some parts like the partition materials, or carefully adjust the flap valve thickness, you might be able to improve an instrument that was designed mostly for ease of production. I wouldn't look at the chamfer as an answer to your responsiveness problems, but if you have some old instruments to experiment on, it won't hurt to try. remember though, start with just a little first, and after 15 degrees, it will cease to have much effect. In a concertina, it is done to allow them to be played at pianissimo levels, and responsiveness is part of the overall design of the reed pan, chamber, valves and reed profile. Sincerely, Dana Johnson The chamfering at the sides of the reed slot are done so that the slot will be narrowest (unaltered) on the side of the plate that the reed is attached to and then widen out on the opposite side of the plate. Regarding the chamfering at the top of the reed slot that the end of the reed swings through there seems to be 2 schools of thought. One, as discussed by Dana, is to angle the slot on the same side as the reed sits which will enable the reed gap to be made smaller which is very handy for when it comes to gapping for overblows. The other approach is to chamfer the front of the slot from the underside. The rationale behind this is that the narrower the passage that the front of the reed has to pass through the less energy that is required for the reed to make its trajectory. Here is an excerpt from a post sent to a squeezebox discussion group (since it is taken from a public forum I am assuming that there is no problem reprinting it here): I think that you will find that a parallel-sided vent (as we call it, rather than slot) will work well, but not with the same characteristics that are essential for good (much less top quality) reed response. Concertina reeds shoes are punched and swaged in order to get tapered vents. This is required for ultra-quick and low pressure starting of the reed. Additionally, some concertina reeds have a tip scoop (a design that a recent accordion reed designer/technician has developed as well, though he uses a drill to undercut the tip). The mechanics of a reed starting is mainly dependent upon the air column (and a few other dimensional and stiffness parameters). Imagine a vent which is not only parallel, but which's tip is curved back towards the reed's root (in an arc), and of a thickness equal to the length of the reed. A reed in such a shoe will never sound at all because the reed blocks the air (can not dump it in order to start vibrating) no matter now hard it presses on the reed. On the other extreme is a shoe that is paper thin. Even the slightest amount of air (volume, pressure) will start the reed vibrating as it needs to travel very little distance before it dumps its air load and is available to return (vibrating) back. Tapering the vent accomplishes that while giving dimensional beef to the shoe for manufacturing and installation purposes as well as mass so that the energy produced goes mainly towards enabling the reed to vibrate rather over wasting some of it in vibrating the shoe. Parallel-sided reedplate vents (such as accordions have) require a higher amount of pressure to start the reeds because the first available dump is when the tip of the reed either clears the reedplate or (for thicker plates) when the reed arcs back far enough to dump air within the vent at the tip (tangential voiding). The reason why accordion reeds are simply punched out (with parallel sides) is mainly for cost savings in manufacture and assembly facility. Before the days of waxed installation (requiring a thicker plate for adequate side securance) accordion reedplates were much thinner - and reeds more responsive (of comparative qualities)! Even today, new instruments with thinner reedplates (some bandoneons, some bayans, Hohner BA bass reeds, helikon bass reeds, etc.) are discernably more responsive than typical PA reeds. Tapered vents are "not necessary for sound quality and playability" but is one solution to enabling a reed to speak as quickly and with as little pressure as possible. |
Created on ... gennaio 13, 2002