STME58
1170 posts
Feb 03, 2015
9:12 AM
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I am curious about comb slot design. I have not seen slots optimized for different keyed harps. I can understand that this would be a stocking nightmare that most manufactures would not want to put up with unless it really made a big difference in the instrument.
If the harmonica acts as a Helmholtz resonator with the oral cavity as the body and the slot as the neck, It would be possible to design the sots so that the oral cavity could remain the same size for all of the blow notes and have perfect resonance on each note. I suspect most players learn to adjust so well that this may not actually be an advantage.
Has anyone done or know of any study done in this area.
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harpwrench
982 posts
Feb 03, 2015
9:43 AM
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Sort of. The resonant size of the chamber for hole 10 on a hi G is too large for the blow note. On those harps I modify the comb to make the chamber smaller, so the note is playable.
Last Edited by harpwrench on Feb 03, 2015 10:08 AM
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arzajac
1579 posts
Feb 03, 2015
9:46 AM
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I've tweaked the slots of my combs in many different ways. I've focused on two endpoints: One being tone and the other the response of the instrument - in other words, how much work it takes to make the notes play.
I tested the hypotheses that a thicker comb creates better tone as well as that a thinner comb provides better response. I've sought out the the opinion of a few players and fellow harp techs on this. About half of those who tried normal-versus-thick-versus-thin (using the same reed pates) found no discernible difference. The other half felt that the hypotheses were accurate.
I have tweaked the size of the chambers and found that too narrow chambers negatively affects the tone but does not improve response. Wider-than-standard chambers don't seem to improve tone. I also worry about decreasing the surface area in contact with the reed plate to provide an airtight seal. So beyond standard width, I haven't found benefit in making them larger.
I tried creating a big resonance space at the and of the chambers but found that folks did not universally think it improved the instrument.
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Custom overblow harps. Harmonica service and repair.
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nacoran
8243 posts
Feb 03, 2015
10:17 AM
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Arzajac, how about a front drilled hole? I've wondered if you could make a round reed chamber where the chamber was wider in the middle but got back close to reed slot sized by the time it tapered back to the top. Might allow you to push the parameters a bit. Maybe not practical for mass production, but it might be a useful investigative tool.
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Nate
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harpwrench
983 posts
Feb 03, 2015
10:27 AM
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Also Hohner MBs GMs and SP20's have always had shorter reeds on Db and higher keys. Short slot reed plates will work on a long slot comb, as is done on a GM or SP20. But after 100+ years they continue to make 2 different combs for the MB, one with shorter slots. I imagine that's been studied more than once or they wouldn't still do it. Or maybe sticking to that plan was just part of the deal when extra terrestrials provided the blueprints for the perfect harmonica back in the 19th century.
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STME58
1171 posts
Feb 03, 2015
10:58 AM
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I am always impressed by how much knowledge is on this forum and how willing people are to share.
Nate, The Seydel 1847 Nobel I have has the geometry you describe. It is a great harp but I am not sure how much is attributable to the hole geometry.
I placed the Helmholtz formula in a spreadsheet to see what it would predict for slot length. I am not sure the Helmholtz formula applies to a slot with a reed over it, but it is a place to start.This assumes a 6mm x6mm slot opening. IN the last column I adjusted the slot length to maintain resonance at the same cavity volume and saw it went from 20mm down to 5mm in one octave. So this approach would need some modification to be useful
Helmholtz Spreadsheet
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HarveyHarp
641 posts
Feb 03, 2015
2:03 PM
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I sometimes put Short Slot Special 20 plates on Short Slot Marine Band Combs. Its a little work, but it works great when done. I have A D that I did a while back. This is not to place to sell it, but I will list it on the For sale 2015 page now. It has slots cut to locate the Sp20 covers
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HarveyHarp
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arzajac
1581 posts
Feb 03, 2015
4:48 PM
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Nate, like this?
Edit - Looks like a chandelier!
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Custom overblow harps. Harmonica service and repair.
Last Edited by arzajac on Feb 03, 2015 4:49 PM
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Brendan Power
465 posts
Feb 03, 2015
5:15 PM
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As STME58 stated, the comb chambers act as Helmholz resonators, which means that each has its own resonant frequency. When you get up to the high notes,that can interact with the reed pitch in a bad way.
Rick Epping is very knowledgeable about all this stuff. He made a type of pan flute from a diatonic harmonica comb, covering both sides and then blowing across each chamber to get its tone note - as pan flute players do.
He then compared the comb notes to reed pitches and noted the effect. Especially in the high range, he found that if the reed pitch is higher than the resonance frequency of the chamber, its pitch will be flattened to a greater or lesser degree.
I can vouch for that. I'm currently working on my Twin-Harmonica System harmonicas, which have a mouthpiece in front of two harps. Your mouth is further from the reeds, so the chamber is longer with a deeper resonant frequency. As a result, I have found that on high pitched harps the high notes will sometimes not sound at all!
That's how significant Helmholz resonance can be. The fix is to reduce the chamber volume to raise its resonant frequency, especially for very high reeds.
For the basic range I don't think it has a lot of effect to change the stock comb chamber size or shape. However Richard Sleigh feels that bigger chambers in the lower range sound better, which is interesting. Richard knows his onions, that's for sure!
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florida-trader
627 posts
Feb 03, 2015
6:22 PM
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I can’t say that I have done a lot of scientific testing but I have learned some things from feedback I have received from customers. I used to make laser cut acrylic combs. They were cut from sheets of acrylic that ranged in thickness from .220” to .240”. Initially, I was concerned that guys would not like thinner combs. I put a full disclosure statement on my website and eBay listings so people would be aware that the combs were a little thinner than the stock combs. I never had one complaint about the combs being too thin. The feedback I received was exactly the opposite. They liked the acrylic combs BECAUSE they were thinner. They told me that they felt the compression was better and the harps were more responsive.
Nate – I think you are talking about a design like this. This would be a front view of the comb and reed plates. The cross section of the chambers are round when viewed from the front.
The top and bottom of the slots are just wide enough for the reed slot on the reed plates. Am I right?
Interesting theory.
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Tom Halchak
www.BlueMoonHarmonicas.com
Last Edited by florida-trader on Feb 03, 2015 6:23 PM
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arzajac
1582 posts
Feb 03, 2015
7:28 PM
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Tom, that makes more sense... I misread Nate's description...
And I agree with you about slightly-thin combs. That has been my feedback as well.
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Custom overblow harps. Harmonica service and repair.
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harpwrench
984 posts
Feb 03, 2015
8:09 PM
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JF was making brass combs like that 20 years ago, but stopped. Possible advantage with wider sealing surfaces, rather than resonance or other theories.
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nacoran
8246 posts
Feb 03, 2015
8:48 PM
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Tom, yep, that's what I was thinking. A way to sort of maximize the size of the reed slot. Of course, I have no idea if that's optimal or not. If it turns out smaller slots are better there are other spots in a reed chamber that a reed wouldn't swing through, like where the sprues are on Sp20s, or a slope on the back edge of the reed slot where the blow reed doesn't reach as it swings through it's arc.
(I haven't tried a discrete reed comb, but I heard some complaints that it just seemed a little fat in the mouth. I've wondered if a careful line following the arc of the reed as it swung might be able to make it thinner- sort of a carefully shaped sprue. Probably a pain to CNC but fairly easy for a 3D printer.)
Brendan, it's funny how touchy resonance is. I almost posted an audio link the other day. I was trying to make a didgeridoo sound the by singing a note into the harp as a played a drone note, but as a side effect, I noticed as I modulated closer and farther from the note of the harp with my voice I started getting all sorts of interference sounds. I know that's the principle in a tremolo harp. At certain pitches I could actually get the harp reed to choke just by changing my vocal pitch (or at least I think just by changing my vocal pitch- I was trying to keep my mouth in the same shape, just changing what was going on in my throat. I guess I should figure out a way cancel out that possibility in a test.)
Also from my crazy lab of things I have no idea what to do with- I've got these brass shims I play with. I got them because I thought their giant (several inches long) size might make them useful to demonstrate techniques on, but playing with them on the edge of my desk I discovered that stacking them lowered the pitch. Now I want to try a low tuned harp with stacked reeds. (I've got some theories on reed stress relief but again, mostly crazy theories.) I should stop hijacking this thread though. I'll have to make a crazy post about some of my invention ideas in my own thread soon.
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Nate
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HarveyHarp
644 posts
Feb 03, 2015
10:47 PM
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Back in the early 2000s, I made some crude round hole combs for a guy in Daytona Beach to fit Lee Oscars. He was an endorser, so that is why the Lee Oscar. I had some left over Harp Maple from a racquetball court. I had to make the holes pretty big to get the same amount of air flowing as the Stock comb, so they were pretty thick. He said that they were the fastest harps he ever played, but he was scared that Lee Oscar would get upset. Go figure. I still have them, and they play great.
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HarveyHarp
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boris_plotnikov
1036 posts
Feb 04, 2015
6:45 AM
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I didn't make a scientific research, but I tried thicker and thinner combs and noted that thinner combs have more attack, more high end and feels like it's a bit more airtight, while thicker combs sounds more fat (more low mid frequencies) and feels a bit more airy and less responsive for me. I prefer thinner combs much more than regular stock, but blue players can like thicker because of tone.
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STME58
1173 posts
Feb 04, 2015
9:00 AM
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Brendan, thank you for the pitch pipe example. That changes my thinking on the subject. I was thinking of a Helmholtz resonator with the volume of air in the mouth acting as the "spring" and the air in the reed slot acting as the "mass". Another way of looking at is is the lot itself acting as a closed pipe resonator. The natural frequency in this mode is determined by the length of the pipe. In this mode the resonant length (1/4 of the wavelength) for A2 (110Hz) is 77mm and for A5(880Hz)it is just under 10mm. It is pretty clear that we are in the range of hitting closed pipe resonance of the reed slot on the high end of the harp. Both the mathematics and the empirical example of the comb pitch pipe indicate this.
Last Edited by STME58 on Feb 04, 2015 9:02 AM
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isaacullah
2925 posts
Feb 04, 2015
6:00 PM
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I'm a little late to the party here, but I do know that the Late Great Chris Michalek made some of his combs so that they were thicker on the low end of the harp, and thinner on the high end. I believe that he did this for his higher-end harmonnicas only (but I can't remember for sure). None of the combs I have that he made are like this, so it's probably not something he did a lot. I remember he said it was a bit of pain to get the combs like that because he had to do it by hand on his shop belt sander. Anyway, the logic was along the lines of the discussion here, that the lower reeds benefited from having a larger chamber, and vice versa. I don't know if he was thinking specifically about resonance (or the connection of the resonance of a particular comb chamber and note pitch), but I do know he felt resonance of the body was a key factor in harmonica tone.
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Last Edited by isaacullah on Feb 04, 2015 6:01 PM
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nacoran
8248 posts
Feb 04, 2015
7:02 PM
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Isaac, that's an interesting idea.
I've got a few harps where the back of the comb is thicker than the front- 10 mm vs. 6.5 mm I think. The only modern comb I've seen like this is on tremolos.
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Nate
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Milsson
183 posts
Feb 04, 2015
11:00 PM
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What's going to happen if you have a cold and your tonsils are swollen? Or if you got an exceptional big tongue? The few mm^2 you'll get from the comb will not give a difference. Shure the ultra high harps were you play "in front of your teeth" I could have a difference but in the normal range I can't believe it will have any impact, at all, on your sound.
The cost of making combs will be much higher and that's money that could be put in reed work.
If I thought it would have a difference I would construct a comb,like a GM, that potrude in the back. Normal thiknes in the front and thiker in the back. Then I would continue the slot with drilled holes inside the part that protrude in the back.
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STME58
1176 posts
Feb 05, 2015
1:15 PM
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I can understand the opinion expressed by @Milsson that if you have a small precise chamber in the harp and a big sloppy variable camber in you mouth, how could a small change in the little chamber make any difference at all? It is the nature of resonance that small changes make a big difference. In the case of Helmholtz resonance, the critical factor is the ratio of volume between the cavity and the neck. It would be very easy to overcome a change in the slot volume by adjusting your oral cavity volume to keep the resonance the same. I expect an experienced player would do this subconsciously to get the tone they want. Hence a change in slot volume would not be perceptible to the player. Does that mean it is not important? I don't know.
In the case of closed pipe resonance, the critical factor is the length of the pipe and there is nothing you can do with mouth shape to correct for it. My calculations show that on the low end of the harp the length of slot to resonate at the pitch of the reed is many times longer than the harp. on the high end it is quite possible to get a channel length that will resonate near the pitch of the reed. Brendan reports problems with resonance on high reeds and this matches what the closed pipe resonance math predicts. Brendan stated that reducing the volume of the chamber eliminated the problem. I am curious if the volume was reduced by shortening the slot. Narrowing the slot would effect Helmholtz but not closed pipe. Shortening the slot would effect both. Of course there may be other types of resonance going on, and they all interact.
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STME58
1177 posts
Feb 05, 2015
1:21 PM
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Nate, Here is a photo of a stock 1847 Nobel comb. Is this the profile you were talking about?
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WoozleEffect
55 posts
Oct 16, 2019
7:45 PM
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Ok, I take my last question back in favour of resurecting this thread.
So, we've got Sjoeberg custom golden melody combs with fancy chamber coutouring, and the Yonberg style chambers.
Have people come up with any new theories on why these shapes are beneficial?
I want to adjust the chambers a bit on some combs I'm milling, and would greatly appreciate the groups feedbacks 4 years or so after the original post in this thread.
What's new in this space, scientifically or anecdotally?
Cheers!
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Rob Laferrière
Russell, MB, Canada
www.woozleeffect.com
Last Edited by WoozleEffect on Oct 17, 2019 10:28 AM
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nacoran
10180 posts
Oct 17, 2019
3:16 PM
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You are talking about some pretty advanced machining, but more than that, computer modeling that would show you how they affect airflow. There are only a couple guys on the forum with access/knowledge that could do more than guess on stuff like that.
I mean, I can comment and guess... I've got a Kongsheng Solist and they have very thin tines/wide holes. It's a very loud harp. Correlation is not causation though. With combs I've noticed there are lots of things you can do that make them more comfortable or airtight (flat sanding combs and reed plates, rounding tines, rounding the ends of combs, but very little is being done with slot shape as far as I can tell aside from width, although Kongsheng has just introduced round holes (not an idea original to them... Hering has had a harp with round holes for quite some time and someone from BlueX was complaining about Kongsheng stealing the BlueX comb design- despite it not being patented and the fact that lots of people had had the idea before- it was sort of a case cost of machining coming down enough for other people to do it.
I'm always a little dubious when people make claims I can't test myself unless they can really demonstrate the science. I've seen more than a couple people go off into pseudo science with design ideas.
I'm not a machinist. I've had ideas I wanted to try out. I've got an idea for a chord harp that conceptually I know is sound but the matter of getting moving parts to work together is killing me. I've wondered if you maybe couldn't make harps a bit more airtight by angling the back edge so the tip of the swinging reed stays closer to the tip of the blow reed throughout it's swing, and wondered if you could expand on that to make a version of Winslow Yerxa's discreet comb much thinner by angling the divider so that it takes advantage of the arc of swing of both reeds (although that would leave one hole thinner than the other unless you made a weird mouthpiece).
The Kongsheng Babyfat, oddly, doesn't trigger my brother's tinnitus. I wonder if maybe what triggers him on other harps is the sympathetic vibrations of higher reeds... I blow the 4 blow and the 10 blow vibrates... Babyfat doesn't have a 10 blow, so no vibration. Or it's just their reeds.
I've thought about dividing the chambers between the covers a little bit... originally I had the idea for a bigger harp- I was having a problem cupping a 365 and thought dividing them in half might make that easier, but you could test whether that would block sympathetic vibrations in other reeds that way too.
Sjoeberg seemed to veer off into some unprovable stuff, or at least wasn't presenting it very scientifically when he started talking about his resonance holes. I think the Yonberg design is a manufacturing design, not an acoustic design. Their new reed shape is basically what you would come up with, as far as I can tell, if you didn't have a stamping machine to make reeds. There are issues with inside curfs that they seem to be dodging, although the wider base might, maybe, help with reed stability on OBs. (I have some ideas on that too, but again, just untested theories and get into really complex modifications of reeds in ways that are way beyond my mechanical skills!)
So, scientifically, not a ton, except make your harps airtight and comfortable to hold and play. Cover shape and material, something I have messed around with modifying, seems to have more effect than comb on tone, but the fine tuning of reeds beyond basic gapping is out of my league physically.
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Nate
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SuperBee
6213 posts
Oct 18, 2019
3:42 AM
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There are a few people making some changes to slot dimensions I think. I know AZajac has made some shaped chambers for higher keys in the Hohner combs.
I have a couple of Brendan Power “powercomb” in my 270s. They have slots ramped in a way which takes advantage of the capabilities of 3D printing which I guess would be more costly in other materials.
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Philosofy
927 posts
Oct 19, 2019
9:19 AM
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Years ago Joe Filisko made some brass combs that had conical holes. I played one that a friend had, it was good, but I couldn't tell any difference (other than it was a Filisko.) I asked Joe about it, and he seemed to say it was just an idea, they were a pain to make, and he didn't see or hear much of a difference, so it wasn't worth the trouble.
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