harpdude61
1078 posts
Oct 03, 2011
7:00 AM
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I asked a question awhile back about what is happening with reed(s) when bending an overbend. I think we sorta figured it out. This one is a little different.
This question probably has a simple answer. I find how the harmonica works fascinating.
I think we all agree that when bending a note, the two reeds in the hole act together to create the bent note. On a C harp, one blow is C and one draw is D, so we draw bend down to get Db. ...But why?
We have two reed pitches here with their own identity, yet combining them makes a different pitch.
Now I go to my piano and hit the notes C and D at the same time and I do not get Db. I get a partial chord that has quite a bit of tension.
It just "seems" that if both reeds were sounding, that two seperate pitches would result.
Maybe the answer will better help me understand the answer to my first question.
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MrVerylongusername
1968 posts
Oct 03, 2011
7:33 AM
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When you hit a piano string, you hear the string's vibrations. Hit two keys and you'll hear two strings' vibrations (and harmonics caused by the two notes interacting)
When you blow a harmonica reed, you hear the air flow being rapidly cut and changed by the vibrating reed, not the reed itself.
When two reeds vibrate the pitch you hear is the combined effect of both reeds acting on the single stream of air - hence one note. One stream of air, one note
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gene
932 posts
Oct 03, 2011
7:42 AM
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I get what you're saying, but just to be difficult:
You do not hear the piano string (or anything else). The only way you could hear the piano string is if it were attached to your ear drum. You hear the pulses that the vibration of the string causes in the air. Same as the reeds.
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MrVerylongusername
1969 posts
Oct 03, 2011
7:47 AM
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No.
the string passes its vibrations to the non moving air around it (via sounding boards and resonators) causing a wave of vibrations that we called sound.
The sound of the harmonica is the vibration of a moving stream of air, the vibrations being caused by the chopping of that air stream.
They are very different.
Last Edited by on Oct 03, 2011 7:48 AM
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gene
933 posts
Oct 03, 2011
8:28 AM
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OK!
Hmmm....This lends credence to the view that comb material makes no difference in the sound of a harp...The comb is not a sounding board.
Last Edited by on Oct 03, 2011 8:34 AM
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MrVerylongusername
1970 posts
Oct 03, 2011
8:53 AM
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Exactly! Any vibration energy passed to comb is minuscule.
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harpdude61
1081 posts
Oct 03, 2011
9:01 AM
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Interesting stuff y'all, but I'm sure someone will debate the comb/tone thing.
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Todd Parrott
722 posts
Oct 03, 2011
9:05 AM
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They probably will, but I hope they do it in a separate thread.
Interesting stuff indeed! This would also seem to help reinforce the idea that the coverplates have more to do with shaping the "vibration of a moving stream of air" than the comb.
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harpdude61
1083 posts
Oct 03, 2011
9:38 AM
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Good point Todd....Air capacity of the coverplate vs. openess of coverplate..etc.....Does opening the coverplates like some of you do add volume AND change the color of the tone?
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groyster1
1464 posts
Oct 03, 2011
9:44 AM
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no doubt at all that it increases the volume but the tone,not so sure
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gene
934 posts
Oct 03, 2011
10:46 AM
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"...the idea that the coverplates have more to do with shaping the "vibration of a moving stream of air" than the comb...."
You're probably thinking the same way I am, but I don't like your wording. I don't believe the coverplates *shape the moving stream of air*, but rather enhance the resonance of the soundwaves that have been produced...In other words, the coverplates serve as a sounding board.
Between the *s, I know that's not what you said, but some might interpret it that way. That's why I didn't like the wording.
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Todd Parrott
723 posts
Oct 03, 2011
11:10 AM
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@gene - No problem... I understand what you are saying. Yes, we are describing the same thing - poor wording on my part.
@harpdude - Good question. I'm still experimenting, but I've found that the openness of the Golden Melody plates on the green-combed harp I did recently seems to change the tone a tad, maybe volume a little bit too, as I also did the same on another D Golden Melody with a dymondwood comb and experienced some of the same results. The biggest difference I hear though are when side vents are added, which I have grown not to care for on higher keys.
The best way to determine this stuff is to do an A/B comparison with 2 harps on the same comb, same tuning, same key, etc., and use a good condenser mic to record and compare the results.
At the end of the day I guess it really doesn't matter a whole lot, because the differences are subtle and what sounds best is subjective, but it sure is a lot of fun experimenting. You said, "I find how the harmonica works fascinating." I would agree!
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nacoran
4700 posts
Oct 03, 2011
11:27 AM
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I wrote a big long post trying to explain, but in the process it turned into a comb thing. The action of our breath force on the tiny reeds is violent. The action of the reeds on the pitch of the sound we produce is violent. It's two force magnifiers fighting it out. We are, in our own little way, putting our finger on the guitar string to change the pitch, but we are doing it in a round about way. By changing the pressure at a certain frequency we tell one reed to shut up and the other reed to bend at a different rate than usual.
If you think of a small closet with a swing door on two opposite walls and a fan vent blowing or sucking air out we are able to change how fast one door or the other blows open, and then come up with strange combinations that get the doors to do all sorts of strange little things that effects how quickly they swing, all with the little fan.
What does a guitar player do? He just puts his finger on a string to make the vibrating part longer or shorter and plucks.
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MrVerylongusername
1972 posts
Oct 03, 2011
11:29 AM
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Hmmm... can't help thinking any coverplate vibrations would be effectively eliminated by the damping effect of the players grip.
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gene
936 posts
Oct 03, 2011
11:52 AM
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Good point, Mrveryveryextremelylongusernameorforumhandle. The vibrations might very well be dampened, but side vents and open backs will let more sound out unobstructed.
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WinslowYerxa
70 posts
Oct 03, 2011
1:20 PM
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Getting back to the original question of why two reeds produce a pitch that neither one is tuned to. For instance, blow C and Draw D producing a Db (or C#) when they're in the same hole and you play a draw bend.
The draw bend action that you perform tunes your mouth to a Db. Robert Johnston demonstrated this experimentally with a blowing machine and syringe as reported in his Acoustic Australia paper:
http://mis.ucd.ie/Members/RJohnst/oldharp.pdf
If you had just a draw reed by itself, it would lower in pitch to that note, which is within the range of notes that the reed can produce. The reed vibrates most efficiently at its tuned pitch, and the isolated reed will have less volume and a slightly duller tone quality at the bent pitch.
But with the blow reed in the picture something interesting happens.
Normally a reed is moved by the breath into its slot, like a door closing into its frame, before it springs back. This is called closing reed action and is how reeds are intended to vibrate in harmonicas, accordions, etc.
But a reed can also open - move away from the slot before springing back. When it does this, it sounds a pitch nearly a semitone higher than the closing pitch. This note can be bent UP in pitch just as a closing reed can be bent down in pitch.
When you play the Db bend, you're hearing both reeds. The blow reed is opening at a Db, and the draw reed is closing at that same pitch.
If you take the covers off a harmonica, play a bent Draw 4, then alternately block either the blow reed or the draw red with your finger, you'll find that both notes are sounding the bent note.
With the wider bends, such as Draw 3, at the bottom of the bend it will be mostly the opening reed sounding, and at the top of the bend mostly the closing reed.
Overblowing in Hole 4 is a matter of persuading the draw reed to open, sounding an Eb instead of a D, then bending it up slightly to put it in tune.
But there's also the inconvenience of the blow reed wanting to respond to the blow breath, and reeds are much more likely to close than open.
So how do you get the more efficient user of the exhaled breath (the blow reed) to be quiet and get the less efficient user (the draw reed in opening mode) to sound?
Your oral cavity tuning needs to be much more precise, and you have to work the air pressure right. It also helps to adjust the profiles of the reeds to facilitate overbending.
Last Edited by on Oct 03, 2011 2:01 PM
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harpdude61
1085 posts
Oct 03, 2011
1:37 PM
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Winslow..Great answer..I would like to hear your take on what happens when bending up an overblow or overdraw.
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WinslowYerxa
72 posts
Oct 03, 2011
2:02 PM
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Bending an overbend up is just a matter of continuing to tune your mouth to a new pitch higher than the one the reed is already sounding. If the note is within the range that the reed can produce, and if you can continue to keep the reed vibrating in opening mode (and the other reed quiet) then you can bend the pitch up.
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