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Soundhole bevel effect on tone?
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Author:  James Ringelspaugh [ Sun Nov 18, 2007 4:17 am ]
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I was reading an old article by Jim Williams on Smallman's bracing designs. At some point he went off on a small tangent about how he shapes the underside of the soundhole. He used to just round it over, but an acousical engineer friend who designs speaker boxes said he should make it sharp at the edge, then bevel it back towards the rim on the inside... something like this:



The only explanation was that the engineer knew what he's about, and though he didn't understand a lot of the terms he used his friend convinced him.

Can anyone enlighten me on how this soundhole design would affect tone?
Author:  JohnAbercrombie [ Sun Nov 18, 2007 8:06 am ]
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Interesting idea.
It sure wouldn't look too good to my eyes.
BTW, I've heard some great-sounding guitars with just the opposite bevel (with a doubler around the soundhole), so the effect (if any) may be fairly subtle. Perhaps it's the sharper edge that's the thing?

Cheers
John
Author:  TRein [ Sun Nov 18, 2007 9:25 am ]
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I would like to meet the person who could hear the difference in the way the underside of a soundhole bevel is shaped. He or she could then tell me how many angels can fit on a pin head.
Author:  Billy T [ Sun Nov 18, 2007 10:54 am ]
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     I noticed on Charles Fox's Ergo there are hole in the braces right by the soundhole. These are viewable from the top easily. I was wondering about that too! Probably structural!

     I would have to hear the difference to believe it also!

Author:  Alan Carruth [ Sun Nov 18, 2007 2:49 pm ]
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Hoo boy.

Art Benade did a lot of research on wind instruments years ago. One thing he looked into was why old recorders sounded better than new ones. He finally figured out that the only difference he could see was that the new ones had sharp edges on the tone holes, and the old ones were rounded off by wear. He rounded off the hole edges on a new recorder, and it sounded like an old one. The recorder makers got mad at him: rounded over edges were a sign of sloppy workmanship, and they would never let that out of the shop. So they keep looking for the 'secret'.

Air flows in and out of the soundhole. In doing so it has to get around the corner, and that entails some loss. If you radius off the corner correctly the loss is reduced; the higher the velocity the smaller the radius you can use. For the flow velocities we're looking at I think the 'correct' radius would be on the order of a foot or so. So much for that.

It's possible that the eddy formed on a sharp edge like that might just act like that radius, though. Those speaker guys are working with stuff that's even less efficient that what we do, and they ned all the help they can get.
Author:  Hesh [ Sun Nov 18, 2007 2:56 pm ]
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[QUOTE=Alan Carruth]Hoo boy.



 [/QUOTE]

Al I have been waiting for you to weigh-in here and I got a big kick out of your answer.  That says it all to me.
Author:  Billy T [ Sun Nov 18, 2007 4:46 pm ]
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I got the greatest regards for your knowledge Al! It's technically reasonable!
    But!
I'd still have to hear it!
Author:  Kevin Gallagher [ Sun Nov 18, 2007 5:23 pm ]
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Martin was doing that exact bevel bak in the early 90s. They've since
stopped, but tye did it on many thousand of guitars back then.

I didn't buy the difference and it could not be heard when a typically
round edged soundhole was on one instrument and the bevel was on
another.

The much smaller diameter holes on a recorder and the velocity of the
breath forcing air through them is a far cry from the dameter of the
soudhole on a guitar and the velocity at which air travels through it.

This is about as much as a stretch as anyhting I've seen online as an
airflow promoter or restrictor. It's just not an issue.

It does weaken the edge of the soundhole significantly, though as well as
making it a prime wear target for stray pick strokes.

Regards,
Kevin Gallagher/Omega Guitars
Author:  Arnt Rian [ Sun Nov 18, 2007 11:07 pm ]
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I think all the Santa Cruz guitars that I have seen have had soundhole edges looks more or less like the illustration in the first post.
Author:  Alan Carruth [ Mon Nov 19, 2007 4:49 am ]
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Kevin Gallagher wrote:
"The much smaller diameter holes on a recorder and the velocity of the
breath forcing air through them is a far cry from the dameter of the
soudhole on a guitar and the velocity at which air travels through it.

This is about as much as a stretch as anyhting I've seen online as an
airflow promoter or restrictor. It's just not an issue. "

It has been a while since I studied any of Benade's writing on this, but my memory of it is that most of the flow through the recorder goes through the windway, not the tone holes, There is some flow, but not the sort of volume you seem to imply.

It is easy to show that there is appreciable flow through a guitar soundhole, at least at the 'Helmholtz' resonance. Any sort of a screen will drop the output of the A-0 mode appreciably, even if it does not restrict the area. For example, if you use a parchment rose the total area of all of the openings will be quite close to that of the unrestricted soundhole, but the strength of the A-0 mode will be cut by half or more, simply because of the added drag of the edges.
Author:  grumpy [ Mon Nov 19, 2007 5:02 am ]
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Another way to show the amount of air flow from  soundhole is to hold a match to it. Tapping the top should blow the match out, as will a good strum.
Author:  Don Williams [ Mon Nov 19, 2007 5:25 am ]
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[QUOTE=grumpy] Another way to show the amount of air flow from  soundhole is to hold a match to it. Tapping the top should blow the match out, as will a good strum.[/QUOTE]

If you fill it with butane or propane first, the effect is even more dramatic.
Author:  grumpy [ Mon Nov 19, 2007 6:58 am ]
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If you fill it with butane or propane first,

That's also a good way to remove the back(and/or the top, and/or the sides...)
Author:  JimWomack [ Mon Nov 19, 2007 6:59 am ]
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[QUOTE=Don Williams] [QUOTE=grumpy] Another way to show the amount of air flow from  soundhole is to hold a match to it. Tapping the top should blow the match out, as will a good strum.[/QUOTE]



If you fill it with butane or propane first, the effect is even more dramatic.[/QUOTE]


Wow! I'd pay to see that.
Author:  Howard Klepper [ Mon Nov 19, 2007 7:02 am ]
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Methane should work, too. And it's free.
Author:  grumpy [ Mon Nov 19, 2007 7:13 am ]
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And there's no lack of it around the OLF....
Author:  Kent Chasson [ Mon Nov 19, 2007 7:16 am ]
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Coming to YouTube soon, I'm sure.


That would a good way to get a visual A/B test of the edge effect too.  Might have to find a second person to do the B test but art and science would never progress without some sacrifice.
Author:  Kevin Gallagher [ Mon Nov 19, 2007 7:39 am ]
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    Alright then, The small diameter of the wind way of a recorder in
reayionship to the velocity of the air passing through it is a far cry from
the same relationship of the same things on a guitar with the soundhole
being the wind way.

    Anytime that Helmholts resonators come up in defending any principle
concerning guitars...there's a mess that follows and as many opinions as
there are people involved. Guitars are not accurate representation of
Helmholtz resonators and the technology being applied to them is
reaching as well...in my opinion.

I don't see the relevance of cross referencing recorders and guitars since
their function and operation are so completely different as is the mode of
creation of their respective tones.

   I've seen and played guitars from a few builders who have tried to vary
the edge of their soundholes and it just didn't make a difference in their
tone.

Regards,
Kevin Gallagher/Omega Guitars

   

Author:  Don Williams [ Mon Nov 19, 2007 10:53 pm ]
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Wow, Did Grumpy use an emoticon????????

There's a first!

Author:  grumpy [ Tue Nov 20, 2007 2:02 am ]
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Author:  K.O. [ Tue Nov 20, 2007 4:40 am ]
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Author:  Don Williams [ Tue Nov 20, 2007 4:54 am ]
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[QUOTE=grumpy]
[/QUOTE]

Yeah, but I can see you smilin' behind that.

Author:  Alan Carruth [ Tue Nov 20, 2007 5:29 am ]
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Kevin Gallagher wrote:
"I don't see the relevance of cross referencing recorders and guitars since
their function and operation are so completely different as is the mode of
creation of their respective tones. "

But sound is still sound.

What Benade found was that the sharp edged holes produced significant high pitched 'edge tones' because of the turbulence of the air trying to make it around the sharp corner. In the case of a wind instrument this would introduce an 'hiss' in the sustained tone, and reduce the efficiency. I don't think you'd hear as much of a hiss in guitar tone, but the efficiency issues are still there.

I should, of course, have used the 'A-0' terminology, rather than talking about a 'Helmholtz' mode. The guitar as we play it is certainly a far cry from a 'classic' Helmholtz resonator, but understanding how the Helmholtz mode works is a neseccary preliminary to knowing what the guitar does in the low range. Certainly, for example, enlarging the soundhole on a guitar raises the pitch of the A-0 mode, just as it would with a Helmholtz mode. The fact that there are more variables involved doesn't alter that.
Author:  Kevin Gallagher [ Tue Nov 20, 2007 1:32 pm ]
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Al,
   That makes sense to me. I can see the sharp edges of the recorder
creating high pitched wind tones as the breath of the player is forced
across what are very shallow cavities once a finger is placed over the tone
holes.

   It would be interesting to be able to document the differences created i
a guitar as that soundhole edge was modified. I know I can't read it here. I
wish Fred dickens was still around and had access to some of the gear
that we used stick our instruments in fron of down at Bell Labs in Jersey.

   He was not only a genius when it came to acoustics and the behavior of
air and chambers when they interacted to create tone, but was also very
innovative when it came to divising new ways to put their equipment to
use.

    I can't help but to think that Fred's exposing all of the egineers there to
guitars had a lasting effect on their use of it in other areas. He never said,
"You can't do that.", but just found a way to get things done. He's missed.

   Thanks for shaking me into seeing what you were saying.

Regards,
Kevin Gallagher/Omega Guitars
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