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Dirty-South Blues Harp forum: wail on! > Overblows – What’s going on?
Overblows – What’s going on?
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Grey Owl
565 posts
Feb 18, 2016
7:14 AM
What a fascinating instrument the harmonica is, there’s nothing else quite like it. Not designed to play any sort of bends and yet here they all are as if by magic. I don’t really understand the physics involved in producing conventional draw bends let alone the Overblows.

I’m wondering what forces are at play in producing an overblow. We know that a conventional draw bend involves a cooperating vibrating opposing blow reed. With an overblow the only role the opposing blow reed fulfils is that it stops vibrating (chokes) so that all the forces are focused on the single reed.

The only difference I can see is that on a draw note the air is drawn from the rivet end towards the free end whereas the overblow flows the opposite direction sort of pushing the air against the free end as it makes its way to the rivet end.

Here is a stroboscopic video (as seen on the forum before) showing an ultra slow motion capture of a reed undergoing an overblow and normal draw note. I can’t see much difference here.



Also what do you think allows an overblow to achieve multiple rises in pitch (a draw bend can only manage a maximum of 3 half step bends in line with the 3 half step difference between the blow and draw notes on hole 3.

Any ideas?


PS Here are the first two videos in the stroboscopic series featuring blow and draw and draw bends.



Last Edited by Grey Owl on Feb 18, 2016 7:15 AM
WinslowYerxa
1078 posts
Feb 18, 2016
9:39 AM
The bending limits of a reed have nothing to do with whether it's blow or draw, or whether the reed is opening (overblow) or closing (standard vibration, with the reed moving into the slot before springing back).

What limit the bending range of a a reed are:

1) reed gapping

2) player skill

3) whether the reed is isolated or in tandem with another reed.

1) To vibrate and sound a note, a reed needs to chop through the slot and then emerge. Bending range and gapping interact in the following ways:

a) When a reed bends down in pitch, it pulls closer and closer to the reedplate. At a certain point, it will no longer emerge from the slot and the reed will stall. To increase potential bending range of a closing reed, you can gap the reed higher so that it can travel farther before stalling.

b) When a reed opens, it pushes away from the plate. As you bend the note up, the reed moves father away from the slot until it no longer passes through the slot to chop the airflow. Lowering the reed gap can help increase the bending range of a reed in its opening behavior.

2)

3) When reeds are isolated and do not interact, then bending is limited only by the above considerations. An overblow is a partial realization of this, although the need to keep the blow reed quiet (and the air leakage through the blow slot) still limit bending range: block the blow slot with your finger and note how the overblow gets easier to control and may gain in bending range.

But when reeds interact in a dual reed system. the bending range of the higher-pitched reed is limited not by the reeds' innate capabilities but of the difference in pitch between the two reeds. For instance, the maximum three-semitone bending range found in Draw 3 of a standard diatonic is a function of the difference in pith between the draw reed (B on a C-harp) and the blow reed (G on a C-harp). Bends will include Bb, A, and Ab.

However, if you tuned Blow 3 down a semitone, or tuned Draw 3 up a semitone, you'd get an extra semitone of bend. Lee Oskar has a tuning (also used by Carlos del Junco) where he raises Draw 1 by two semitones and lowers Blow 1 by two semitones (like E draw and Bb blow on a C-harp). Result: five semitones of draw bend (Eb, D, Db, C, B).

===========
Winslow

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arzajac
1752 posts
Feb 18, 2016
10:33 AM
I posted a (boring) video about this a while ago:

http://harp.andrewzajac.ca/OverblowVideo
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Grey Owl
566 posts
Feb 18, 2016
10:57 AM
Thanks Andrew, I'll check that out.


Winslow

Thanks for your observations.

Just to say I was aware of the constraints of the draw bends in relationship to the note values of the blow and draw reed as stated my OP.

I didn’t know Carlos retuned draw and blow 1 as one of his tuning options. Interesting.

‘b) When a reed opens, it pushes away from the plate. As you bend the note up, the reed moves father away from the slot until it no longer passes through the slot to chop the airflow. Lowering the reed gap can help increase the bending range of a reed in its opening behavior.’



If you are referring to the overblow reed here then the slow motion evidence on the first video would seem to suggest otherwise as you can see the reed dipping in and out of the slot. Are you suggesting the higher you raise the overblow the less it will disappear into the slot and possibly travel further away from the reed plate?

Thanks

Ps As a point of interest I just took a single reed plate and blew through the back of the slot with an overblow mouth shaping and raised the note on hole 6 in pitch by five semitones. I’m going to tighten the gap and see if the pitch can be raised higher still.
arzajac
1753 posts
Feb 18, 2016
11:32 AM
"As a point of interest I just took a single reed plate and blew through the back of the slot with an overblow mouth shaping and raised the note on hole 6 in pitch by five semitones. I’m going to tighten the gap and see if the pitch can be raised higher still."

I suggest that in a "real-life" situation with two reeds in the channel, the hard part is keeping the other reed still. That will limit the upper limit of the pitch you can attain rather than how high the individual reed can go.

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WinslowYerxa
1079 posts
Feb 18, 2016
11:56 AM
Let me clarify.

The point at which an opening reed no longer chops through the slot is the point at which it stops sounding a note. You can definitely observe the reed traveling away from the plate as you bend an overblow up in pitch:

1. Take a reedplate and stand in front of a mirror in good light.

2. With the reeds facing away from you, exhale into the slot for the longest reed and get it to sound its lowest opening pitch (slightly less than a semitone higher than its closing pitch).

3. Bend the note up, and watch the reed. You'll see two things:

a) the reed's amplitude of vibration will get smaller as the pitch goes up, as it's getting farther and farther from its most efficient vibrating pitch.

b) its center of up-and-down vibration pushes farther and farther away from the reedplate

Note: By increasing breath flow, you can increase the amplitude of the reed's swing, thus helping to extend its bending range by inducing it to swing farther and thus still chop thru the slot.
===========
Winslow

Check out my blog and other goodies at winslowyerxa.com
Harmonica For Dummies, Second Edition with tons of new stuff
Join us in 2016 for SPAH on the San Antonio River Walk!

Last Edited by WinslowYerxa on Feb 18, 2016 3:23 PM
Grey Owl
567 posts
Feb 18, 2016
12:51 PM
Andrew 'I suggest that in a "real-life" situation with two reeds in the channel, the hard part is keeping the other reed still. That will limit the upper limit of the pitch you can attain rather than how high the individual reed can go.'

Yes, without a doubt. Without customisation I would have to set the blow reed so tight to increase the upper limit of the OB that it would be barely playable as a blow note. I like your explanation regarding how resonance affects the bends btw.

Thanks for the clarifcation Winslow, I will check that out and maybe video it so I can analyse it easier.
nacoran
8948 posts
Feb 18, 2016
2:40 PM
The Banson harmonica worked by stopping one reed with slide didn't it? (With a risk of snagging though.) There is a page on Turboharp saying they've prototyped a more economical version but aren't moving forward with it for the moment.


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