Lipped comb & Hemholtz Resonance

Anything apart from the two mainstream default harmonicas (Solo-tuned fully-valved chromatic, and un-valved Richter 10-hole diatonic). Alternate tunings, different construction, new functionality, interesting old designs, wishful-thinking... whatever!
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harpdog123
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Lipped comb & Hemholtz Resonance

Post by harpdog123 » Sun Apr 16, 2017 10:24 pm

I just saw John Cook machine a lipped comb with a mouthpiece that was further away from the comb than a standard diatonic. I know that increased distance can cause problems with the pitch of the reeds and their performance and I was wondering if this problem could be fixed by narrowing the width of the reed slot.

David Pearce

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winslowyerxa
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Re: Lipped comb & Hemholtz Resonance

Post by winslowyerxa » Tue Apr 18, 2017 5:42 pm

By "slot," do you mean the chamber in the comb (as opposed to slot in the reedplate)?

My experience with long, narrow chambers is that it makes the pitch problem worse. Both the Suzuki SCT-128 tremolo chromatic and the Hohner Harmonetta place one reed behind another in a long, narrow chamber, and both suffer from pitch depression as a result, and largely on the higher pitches. In the case of the XB-40, the higher keys (C and regular D, which was discontinued in favor of Low D), the highest pitched reeds suffered pitch depression from the chambers being too large. I think that the problem, where it occurs, is one of total volume creating a resonant frequency lower than that of the reed

In the case of a mouthpiece on a diatonic, I doubt that a few added millimeters in length would be a problem. For awhile I was making Discrete Combs for Lee Oskar harps, which require a lipped front to keep the covers from sliding off the front (same as most lipped diatonics). My machinist and I came up with a glued-on front mouthpiece that added maybe 3mm to the total length. Didn't seem to affect pitch. And I widened, rather than narrowed, chambers on the DC models to counter a different problem: the reduced vertical chamber size made reeds reluctant to respond to all but the lightest attacks, so widening the chambers helped.

harpdog123
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Re: Lipped comb & Hemholtz Resonance

Post by harpdog123 » Wed Apr 19, 2017 12:18 am

Yes I did mean the width of the reed chamber, rather than the reed slot.

Thanks for the info!
David Pearce

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Brendan
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Re: Lipped comb & Hemholtz Resonance

Post by Brendan » Fri Apr 21, 2017 7:58 am

I think there is a bit of confusion here between narrowness and length of chambers and total volume.

Helmholtz Resonance Coupling is the phenomenon where the resonant frequency of the combined chamber of mouth and comb slot interacts with the pitch of the reed. If the Helmholtz resonance is lower than the reed pitch it causes the reed to play flat, or even stop playing altogether.

It's a vexed issue for me, as I'm creating some harmonicas with longer travel between mouth and reeds, such as those in my TwinHarmonica System. As Winslow said, it's the high reeds where the problem becomes apparent - the high octave of a standard C chromatic, for example.

Where there seems to be some confusion in the discussion is conflating chamber length and narrowness with chamber volume. It is the total chamber volume that matters. The way to reduce the effect of HRC is to minimise the total volume, which could be done by narrowing the comb chamber.

Narrowness of comb chambers per se is therefore not a bad thing, as seems to be inferred. It can actually be a good thing.

Of course in the Harmonetta no amount of narrowing will overcome the extreme length of the chambers, which is why pitch issues still occur.

HRC is like gravity: a natural law from which there appears to be no escape! It affects accordions too, and despite many years of knowledge about this problem there is no solution beyond reducing chamber volume to the absolute minimum however you can, and when even that fails simply avoid making harmonicas with longer travel to the reeds in high pitches.

In my TwinHarmonica System double chromatics I recommend G as the highest key, and for C use C tenor range. As Winslow noted, the Hohner XB-40 had this problem in keys above C, and D was the highest key made I believe. Even stock chroms like the CX-12 suffer from it in the very highest holes, which is why it's good to fill in the upper chambers with ramps.

You can mitigate it yourself by adopting a smaller mouth embouchure, which Rick Epping recommended in his notes for the XB-40. That can have a striking effect of raising the pitch of flat reeds, but it's awkward having to do that and tone is affected.

David, I guess you are asking in respect of your Harmonicaster? Seeing the design, this was something that I warned you could come up against, going on my experience. Sticking with lower-key harps is probably your only option with the current design.

My friend Zombor Kovacs was convinced there might be a counterintuitive 'trick' that could be used to unlock the chamber/reed coupling effect - for example, by diverting the air into a side chamber, or to funnel air to the reed from the rear. We tried testing a couple of these ideas but the results were inconclusive, and we eventually I gave up. He remained sure there could be a way, but I haven't heard anything more about it.

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triona
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Re: Lipped comb & Hemholtz Resonance

Post by triona » Fri Apr 21, 2017 9:02 am

Could these effects be the reason for the conical design of the reed slots by yonberg too?
http://en.yonberg-harmonica.com/content/8-technology


And here another question for better understanding:
Brendan wrote:
Fri Apr 21, 2017 7:58 am
Helmholtz Resonance Coupling is the phenomenon where the resonant frequency of the combined chamber of mouth and comb slot interacts with the pitch of the reed. If the Helmholtz resonance is lower than the reed pitch it causes the reed to play flat, or even stop playing altogether.
Is here to find one of the reasons, why bending and overblows are working better or worse with some reeds / channels / harmonicas than with others?

Another reason would be the presence or absence of valves, which is not the question here. Are there still more factors of influence here?


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triona
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harpdog123
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Re: Lipped comb & Hemholtz Resonance

Post by harpdog123 » Sat Apr 22, 2017 2:35 pm

Hi Brendan, I was asking because I want to try a double-decker diatonic design that uses two stacked diatonics. I thought that narrowing the reed chamber might offset the extra distance added by the mouthpiece.

Best Regards!
David Pearce

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winslowyerxa
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Re: Lipped comb & Hemholtz Resonance

Post by winslowyerxa » Tue Apr 25, 2017 11:12 pm

triona wrote:
Fri Apr 21, 2017 9:02 am
Could these effects be the reason for the conical design of the reed slots by yonberg too?
http://en.yonberg-harmonica.com/content/8-technology
Is here to find one of the reasons, why bending and overblows are working better or worse with some reeds / channels / harmonicas than with others?

Another reason would be the presence or absence of valves, which is not the question here. Are there still more factors of influence here?
Valves will have an effect. When i was working on a Harmonetta, I noticed that the upper part of the pitch range wasn't valved, and I tried adding valves, which made the pitch depression problem much worse; I ended up removing the valves. Why this happens I don't know. Perhaps the leakage afforded by the absence of valves lessens the coupling of the reed to the chamber resonance, but that's purely a guess.

Bending seems to work fine in most holes of diatonics unless the pitch is so high or low (as on the lowest-pitched keys) that the reed's pitch is simply outside the range that human oral cavity resonance can match. Overbends seem to be affect most by reed design and adjustment, slot tolerances and overall airtightness. I'm not sure anyone has experimented with changing chamber volume to affect overbending capabilities.

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