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All things constant does a larger soundhole typically mean added bass to a guitar? I've read this many times and am curious if any of you agree/disagree with this theory. If in fact it does add bass does it also detract from the treble or mids? I ask this because I have a guitar that is relatively bright, it is full, clear and loud but on the bright side...would increasing the size another 1/8" help or hurt the bass. The soundhole is 4" now, the guitar in question has a 20 1/2" long body, 15 3/4" at lower bout, 11" at the upper bout, approx. 3 3/4" deep at neck, and 4 5/8" at tail. The thing sounds amazing now so I hate to screw with it unless it seems reasonable that the change will in fact alter that which I think it will.


Greg

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Gwaltney Guitars

Allen Carruth will probably be our best in put here but just using common deductive insight I would think that more than anything a larger than normal sound hole will cause the air pressure in the chamber to decrease therfore cuting down on the resopnce of the top.

If you wistle a low note then a high note, notice that it takes less air pressure to sound the low note than a high note. The air has to move faster to produce a high tone. The lower the pressure the slower the air mass moves. Acually it really a mater of pressure drop. The greater the pressure drop the faster the air moves coming out of the chamber. The smaller the hole, the greater the pressure drop. That is to say the difference in the pressure inside the chamber when compared to that outside the chamber. So I would say that it does not add to the bass but detracts from the trebel.MichaelP38713.7058217593

To increase bass response you make the port (or soundhole in this case) SMALLER. Seeing as this may not be possible, unless you bind the inner edge, what may be needed is to loosen the bracing at the top edges by shaving them thinner to increase the flex and hence bass response in the top. Alan is the master of all this stuff, he might suggest shaving some back braces instead, because the back contributes more to bass than highs. Wait to hear what he has to see before you go at it.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

This is fascinating talk, I do look forward to Alans thoughts.

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Gwaltney Guitars

I took the question as being: "If you add a larger hole, will that increase the base responce?" my answer was "no" it just decreased the pressure drop across the sound hole ther by decreasing the higher feq. I do not believe it increases the bass responce of the top at all. but mainly the the transplacement of the air into and out of the chamber.

I agree that loosing the diaphram is the best way to add bass respnce. I did not mention this, thinking that the real questions was how a large sound hole affected the bass/trebel.

One thing I did not mention is with the decreased pressure differential between the chamber and atmosphere, is the loss of rebound energy which will affect bass, trebel and sustain of each. This also only pertains to the affect of the air movement, not to the tonal responce of the top inparted from the string energy. There is a lot that goes into the sound produced by a guitar. I concentrated only on the the basic affect of the air movment.
MichaelP38713.7514351852

Greg:
Lay your guitar on it's back. Tap on the bridge like a tom-tom. Take a piece of paper and gradually slide it over the sound hole and listen while tapping the bridge. Keep tapping and slowly open the sound hole. You are chainnging the size of the soundhole with the paper. What did you hear?

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tim...
http://www.mcknightguitars.com

There's a trade-off involved here (surprise!). With a bigger soundhole the
box has a higher resonant frequency, but also a higher volume output at
that frequency. The converse for a smaller hole. So there isn't a clear
answer to the question what will get "more" bass. You would need to
specify
what bass frequency you want more of. The air resonance of the box will
give some support to frequencies close to its strongest resonance (the
"Helmholz" resonance). So usually a resonance around G on the low E
string
is a good compromise. If you tune it lower with a smaller hole you can
get more volume on lower bass tones, but you will lose some volume
higher up in the bass.

You can also lower the resonant frequency by moving the hole closer to
the upper end of the box. If I remember correctly (Al can help me out
here, I expect) this will not result in a volume drop, but it will narrow the
band of support for the bass. No free lunch on this.Howard Klepper38713.886400463

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

So what is 'more bass', he says with a Zen sort of look..... :)

Most of the sound output of the guitar comes from the 'bass reflex couple' between the 'main top' resonance and the 'Helmholtz' or 'main air' resonance. The top and the air work together to produce two peaks in the low to mid range, usually around G on the low E string, and near the open G pitch about an octave higher.

The 'actual' main air pitch is set by the size and location of the soundhole in relation to the mass of the air in the box. A larger hole _raises_ the 'actual' air resonance frequency. It also allows more air to flow in and out of the box, so the pressure change in the box is greater.

The 'actual' main top resonance pitch is set by the size and stiffness of the top. Making the top loser will drop that pitch.

The pitches you get when you put the thing together are the result of those 'actual' values as modified by the way they are coupled together. One result of the coupling is to push the two frequencies apart. Thus, on a rigid guitar shaped box the 'actual' main air resonance might be around 125 Hz, and on a backless box with a flange around the edge to simulate the back stiffness, the 'actual' top resonance would be likely to be around 180 Hz. The coupling between them gives the resultant frequencies, at more like 100 and 200 Hz.

Increasing the soundhole size, by raising the air mode pitch, would tend to jack the final 'main air' resonance up a little. However, the extra air flow also increases the coupling strength, which tends to knock the pitch back down. It can happen that the two cancel out, so that in the end you get a resultant 'main air' pitch that has not changed at all, but the _top_ pitch can go up owing to the stronger coupling.

The end result could, indeed, be 'more bass', since there is more power at that main air frequency. However, it could also be 'more treble' owing to the rise in the 'main top' pitch. The only way to find out....

There are certainly other effects from changing the soundhole size, and those might be more elusive, if not _illusive_. I'm really curious about the 'upper corner frequency' of the soundhole, which is solely a function of it's size, but have yet to do the experiments needed to track that down.

If I may make a suggestion... Try changing the frequency of the 'main back' resonance. The back can couple with the air motion in the same way the top does, and if the 'main back' tap tone is close to the pitch of the 'main top' resonance you can actually drop the pitch of the 'main air' resonance and increase the flow at the same time. One big benefit with this is that you can try it out without making any permanent changes.

Get some poster putty, such as 'BluTac' or 'FunTac', and stick a lump of it on the back in the spot that gives you the most response at the lowest pitch, with the guitar face down on the padded bench. Try to drop the 'main back' tap tone to around a semitone above the 'main top' tap tone. Play and listen to the guitar with and without the putty load on it, and see which gives you the best tone. If you like the sound with the added weight, then you can shave back braces to get the tap tone down to the correct pitch. If not, no harm done.     

Having read many of Alans posts before, something told me that his suggestion would be to maybe shave the back braces !!!! Nice explanation Alan. I think I might be catching on !!! I like the putty idea - non intrusive.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Well, I got part of that post yesterday right! As soon as I put it up I thought I'd better check and make sure it was all correct. When I went to look up any experimental results the stuff I had was not much help. I had never actually done that particular experiment before, so...

I did four tests today on my 'corker' guitar. It has a rather large soundhole, and I've made restrictors for it, using foam core board and card stock, of various sizes. Thus I was able to try some things with a 'normal' soundhole size, plus one that had about 33% more area and one that had 17% less. I measured the response in the low range, from about 70 to 250 Hz, in three ways: an accellerometer on the top just behind the center of the bridge, a microphone inside the guitar down near the tailblock, and a microphone out in front, about two feet from the top in between the bridge and soundhole. The driver for all of these tests was the same: a 'stinger' clipped to the bridge over the 4th string, and driven by a computer generated sweep signal. I was also able to play and listen to it with different soundhole sizes, as I had a student in.

As I suspected, the larger the hole is the higher the sound pressure inside the box at the 'main air' frequency. Both the inside sound measurement and the accellerometer on the top gave stronger signals with the hole more open. That's the part I got right. However, the 'main air' pitch always goes up when the hole is made bigger, while the 'main top' pitch remains the same, at least for this range of hole sizes on this guitar. Going from the smallest hole, 71mm dia., to the largest, 89mm, raised the 'main air' pitch from 89.3Hz to 98.2, a couple of semitones, and the internal sound level jumped by 68% when the hole was enlarged. The top, however, moved with less amplitude at the 'main top' pitch when the hole was enlarged, with the accelerometer giving a signal that was 7% lower and the internal sound level being 19% lower at that frequency.

Out in front the measured sound level at the 'main top' frequency was only 4% lower with the smallest hole that it was with the largest. The 'main air' level dropped off by more than 50% with the smallest hole, though, so overall the sound level was much less, and the balance shifted strongly toward the mid-range top frequency peak as the hole was made smaller.

Not much of this came through in the percieved sound. I could hear very little difference when my student was playing between the largest and smallest hole diameter. When I played it myself the tone with the more open hole was preferable, although it was hard to pin down why. When I played it and the student watched he thought it sounded more 'distant' with the smaller hole, but when we did a blind test, with his back to me, his perception reversed. So much for that.

In the end, there simply was not much musically useable change made by enlarging the soundhole, at least on this guitar. Despite it's looks it's not a bad little box, with decent balance and tone. Perhaps a guitar that was more lacking in bass response would benefit more from enlarging the soundhole some. Unless the change in area was large I suspect there would not be a lot of difference percieved in the overall timbre, but the balance and clarity might both be enhanced. Again, there's really only one way to find out....     

Nice work Alan, your knowledge is impressive, I appreciate you sharing it with us all.

Thanks,
Greg

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Gwaltney Guitars