Roverharp
3 posts
Dec 13, 2010
8:51 PM
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Recent discussion about GM combs has me revisting the question of the overall volume of each hole. Guys have said that thicker or thinner custom combs have improved response and increased loudness. So, what's the corelation? Are the thicker combs the better ones? Or is a hole with a resonant frequency closer to the reed's pitch better such that you'd want thinner combs for higher keyed harps and thicker combs for the low keys?
Like cover shape I think chamber size contributes to tone as well. Put MB covers on a SP20 and there is still a slight tonal difference. I don't really believe comb material affects tone so I attribute the difference to either chamber size or the reeds being positioned further from the front of the harp than the sandwich style construction allows for.
Paul
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nacoran
3392 posts
Dec 13, 2010
9:59 PM
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I think bigger chambers should make louder harps, along with wider open backs on harps.
The converse is that a larger chamber means you have to change the pressure of a larger area to get the reed to start swinging. From my limited experience I think you're right that the reeds being farther foreword may make them more responsive.
Of course there are really two directions you can make the chamber bigger. You can make it taller or you can make it wider. I think the way a harmonica works taller would allow for a bigger increase in the amplitude of the sound. If you offset a thicker comb with tighter chamber tolerances you might get something, but I'm just guessing from a series of barely related snippets of information I've read around the internet on topics from harmonicas to sounds.
I'm not sure how much torsional swinging there is in the chamber. You wouldn't want the chamber rubbing the reeds and I have no idea how much room you need for the odd turbulence of OB's and whatnot. I've wondered what would happen if you made a comb to reed plate tolerances, maybe even screw mounting reeds to a metal comb and doing away with reed plates all together. I have no idea what it would do, aside from probably being really hard to work with and being really airtight.
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Nate
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hvyj
916 posts
Dec 14, 2010
4:04 AM
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Narrower combs have a quicker or more lively response. Thicker combs have more depth of tone.
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walterharp
504 posts
Dec 14, 2010
6:50 AM
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seems like the response time of the chamber size to air pressure would be negligible relative to the volume of your mouth, throat and lungs. Even if you just use your mouth as a bellows to rapidly make small notes like Popper, the volume of your mouth is so much larger than that of the chamber, it would only be an effect of a less than 1%
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nacoran
3397 posts
Dec 14, 2010
9:06 AM
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Walter, the thing is, how much of that air in your mouth/lungs do you get into the harp in that first moment? I'd think a tighter chamber might let you play with less air.
Although, I wonder also... here is an experiment for a customizer... Lee Oskars have notably wider holes than most harps. Without taking one apart I'm not sure if it has wider chambers. It is noted for not being great at overblows, and they say that has something to do with torsional problems in the vibrating. My one custom comb seems to have my narrowest chambers, although it's for a different brand harp than any of my other ones, so that may be nothing.
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Nate
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walterharp
505 posts
Dec 14, 2010
2:50 PM
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The way I think about it, is that to start the air flowing and the reed sounding, for example, you need to increase or decrease the pressure of the whole system. For example think about a good draw note. You need to expand your chest cavity (increase volume of lungs), that low pressure wave will propagate almost instantaneously through your trachea, larynx, pharynx, and mouth to the harmonica. It does not make sense that the pressure wave would take substantially longer to propagate through that little tiny chamber than the rest of the system. The time delay must be in the largest of the chambers because the most air needs to move to change the pressure. Thus the reed chamber is really small relative to all those others.. does that make sense?
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Roverharp
4 posts
Dec 14, 2010
3:50 PM
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@walterharp - What you say is logical but isn't borne out on practice. Winslow Yerxa has stated that he made his Discrete Comb as small as he could. Smaller versions impaired response. The XB-40 also has some interesting engineering in the top couple holes to reduce cubic volume. (There's also 'Tate Ramps' although these might deal more specifically with aerodynamics than chamber volume). So, something is going on.
The Discrete Comb example also shows that reducing chamber size doesn't automatically improve response.
On the other hand I actually once calculated the resonant frequency of a SP20 comb to see if it more closely matched any particular key harp. The results for those small chambers were so high that I couldn't determine anything good or bad.
Paul
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nacoran
3403 posts
Dec 14, 2010
4:54 PM
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I think maybe the size of the lungs/throat/mouth may be less relevant because we have more control over what is going on inside. We spend our whole lives learning to control breath and tongue and lips. Once it leaves the mouth we lose a lot of that control. Maybe we can get some back by sealing pressure in with a tight harp and a tight cup. That said, maybe a larger reed chamber would give you a more even response. I don't know. It's all just a thought experiment until someone tests it and lets us know the results. :)
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Nate
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walterharp
506 posts
Dec 14, 2010
5:37 PM
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@mrvery.....thanks, interesting...those articles only speak about chamber size with regard to resonant frequency, with the primary influence seeming to be length. they do not speak about the volume of the chamber, with respect to pressure generation to the reed... i read this (pun intended) to suggest the main effect on sound of the reed chamber is related to the resonance, not how quickly pressure is propagated in the chamber
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MrVerylongusername
1410 posts
Dec 14, 2010
8:47 PM
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Yes I noticed that too. I think building pressure up is not the issue. The reed does not start to move because of a build up of pressure, it starts to move because of localised pressure reductions created by separation vortices at the reed tip. Having read the papers on Turbodog's site it would seem that laminar airflow is the key factor. If I've read those papers correctly it appears that the initial reed movement is into the chamber - somewhat counter-intuitive.
I do however wonder if chamber height makes a difference, considering that the biggest difference between harps and squeezeboxes is that harmonicas are a two reed system acting together
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hvyj
922 posts
Dec 15, 2010
5:39 AM
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Some knowledgeable people compare the effect of thick combs to that of double reedplates. I don't know--I'm just passing along something I've heard.
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MP
1135 posts
Dec 15, 2010
9:48 AM
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i put tall wide slot combs on two harps and my impressions are consistent with MrVs suppositions.
localized air pressure is slightly reduced affecting -not so much how quickly the reed initially responds, but general overall response, ie; how fast it increases in volume. it's like one of those volume pots that go from soft to loud very gradually, and then jump out at you real loud.
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MP
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Buddha
2721 posts
Dec 15, 2010
10:02 AM
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i've experimented with everything. the best results I have had were with conical chambers but they are a PITA to make so I don't do it.
Without saying more, my buddha harps have the optimum chambers for tone, response and volume. I used to make different size chambers for different notes, and holes but got too many complaints from end users who simply thought my combs were uneven.
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"Music in the soul can be heard by the universe." - Lao Tzu
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walterharp
507 posts
Dec 15, 2010
1:28 PM
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to go that route you might want to studiously avoid chamber sizes that do resonate.. why, because only one note of the several that can be made with each chamber would resonate... making the sound uneven
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