Sound Hole Question, Dumb, Maybe?

[sjm1580]

For the past couple of years I have gotten into building stereo speakers. Basically there are two types, sealed and ported. Taking the ported types as an example (which are vented to the atmosphere) there are complex calculations to optimize the size of the port to maximize the drivers to a low end frequency. Too large of a vent and the speaker will have no detail in the lower frequencies and too small a vent and the drivers won't be able to adequately define the low end frequencies.

My question, is this somehow taken into account by luthiers in sizing a sound hole? Larger sound hole, for say a deadnaught and smaller for a parlor size?

Just curious?

[The Bard Rocks]

I suspect it is taken into account less often than we'd like to think. Soundholes seem to be mostly of a similar size. Soundports, on the other hand, vary widely, and I don't understand why - other than an aesthetic choice or happenstance. I have not read of many studies on the relationships between size and placement that have been conclusive - too many variables, too few guitars made to experiment on. And not enough of the public who cares, just us and a few others.

[mercy]

sound hole sized is defined what those in the past have found works. In general a smaller sized helps the bass and a larger sized emphasizes the treble. 4" inside diameter is the "standard" size of a dred sound hole. 3-7/8" for an OM and SJ. 3-1/2 - 3-5/8" for a OO or parlor model. Ya see how that works?

[brandall10]

Besides the bass/treble relationship, a soundhole also defines how well the instrument focuses (smaller) or disperses (wider) the sound. For a small instrument, it's a balance between being quiet or boxy.

Also I gather the bass/treble response is more governed by the size of the hole relative to the size of the top, everything else equal. So a smaller body guitar should have a smaller hole to maintain relative tonal balance. But of course different guitar shapes/depths, whether there is a cutaway, or bridge placement (ie. 12 vs. 14 fret) change the tonal response considerably.

[sjm1580]

Originally Posted by mercy

sound hole sized is defined what those in the past have found works. In general a smaller sized helps the bass and a larger sized emphasizes the treble. 4" inside diameter is the "standard" size of a dred sound hole. 3-7/8" for an OM and SJ. 3-1/2 - 3-5/8" for a OO or parlor model. Ya see how that works?

So if I understand what you are saying, no matter the interior volume of say an OM ( which I guess vary quite a bit) they all have a 3-7/8" sound hole? Interesting!

[ljguitar]

Originally Posted by sjm1580

For the past couple of years I have gotten into building stereo speakers. Basically there are two types, sealed and ported. Taking the ported types as an example (which are vented to the atmosphere) there are complex calculations to optimize the size of the port to maximize the drivers to a low end frequency. Too large of a vent and the speaker will have no detail in the lower frequencies and too small a vent and the drivers won't be able to adequately define the low end frequencies.

My question, is this somehow taken into account by luthiers in sizing a sound hole? Larger sound hole, for say a deadnaught and smaller for a parlor size?

Just curious?

HI sjm

If Alan Curruth wades into this discussion, listen to him. He's done a lot of study of how the Helmholtz frequency relates to sound hole and side ports. So has Tim McKnight (both of these men have built a ton-o-great-guitars). Alan built a test guitar for side ports…called the corker.



Alan and Tim have done experiments on how changing the size of the sound hole changes the bass frequency and how the side ports affect overall volume of guitars.

[lowrider]

So this brings up another dumb question; does the sound come off of the top or out of the hole?

I think it comes off of the top and the whole just lets air move in and out so the top can move. If I'm right, small changes to the size of the hole shouldn't make much difference.

[lovgren]

Bob Taylor claims that it's very easy to make minor adjustments to a design to accommodate a standard 4" soundhole, so that's just what almost everyone does.

[mirwa]

I never stick with a one size fits all sound hole, on custom builds, i ask the customer what it is they are looking for (sound wise), the sound hole size is just one of the parts to the cake recipe that i use.

Steve

[Alan Carruth]

Some good questions: volumes have been written about some of this stuff.

In the low range a guitar is a 'bass reflex cabinet'. Unlike your loudspeakers, though, the 'air' resonance at the hole and the 'speaker-type' top resonance (the lower bout) are not tuned to the same pitch. In isolation they'd be about a musical fourth or fifth apart, with the 'air' resonance lower. Once the guitar is assembled they couple and push each other apart in pitch, usually ending up about an octave apart, generally roughly a G~98 Hz for the 'air' pitch and close to the open G string pitch at ~196 Hz. Exactly where they end up has a bearing on the timbre of the guitar, so it's one of the things that a maker tries to get under control by varying the size of the sound hole.

The structure of the guitar is a lot more complicated than speaker enclosures, so the acoustic response is too. Speakers are made with rigid walls as much as possible, and high losses in the cabinet, to spread out the resonances. Ideally the only resonances involved are the speaker and 'air'. The object is to have a 'flat' response over a wide frequency band.

On a guitar there are lots of moving parts, which couple promiscuously at all sorts of pitches. The response is anything but 'flat', which makes the sound 'interesting'.

The lowest pitches the guitar makes have a wave length in air of about 14 feet. The box is too small to make that sound efficiently, and it's radiated as if from a 'point source', in all directions equally. The player hears the fundamentals of the low notes as well as the audience.

As you go up in pitch the 'top' and 'hole' radiation starts to get out of phase. Higher order resonant modes kick in, with more and more small areas vibrating out of phase with each other (the 'telephone speaker catastrophe'). What you hear is a sort of sum of everything after a lot of it cancels out. The sound becomes more and more directional, coming off the top and out of the hole going toward the audience. The player hears this mostly through room reflections; if the room is big, dead, or noisy, the player might not hear much at all. This tends more to be the case with higher quality guitars.

'The Corker' was the first of three sound port 'test mules' I've made. The idea was to figure out what ports do, and what the best size and location would be.

Any time you punch another hole in the box it's like enlarging the sound hole: the 'air' resonance rises in pitch, and becomes a bit stronger. The further from the 'main' hole the port is, the larger the effect for a given size port. Usually this effect doesn't make a noticeable difference in the sound of the guitar out in front, but it can.

There are various internal 'air' resonances that are not normally 'heard' by the usual sound hole. Adding a port someplace else allows them to be heard. Many of these are normally 'wasters': and power that they get from the strings is dissipated in the box with no particular effect on the sound, and they show up as 'dips' in a power spectrum. A port can turn one of these dips into a (usually small) peak in the output, usually in a higher frequency range.
Your ears are set up to notice small changes at the threshold of perception, so those things can be picked up, even though they don't involve much power. Thus a port can make a noticeable difference in the timbre of the guitar, even when there's not really much change in the overall output or spectral shape.

Finally a port you can look into as you play will tend to shoot some of that internal high frequency sound out toward the player, making up for the lack of reflections in a 'dead' or noisy room. That's the utility of a port; it's a monitor for the player. The folks who tend to find these useful are ones who play a lot of restaurant gigs, and people with age-related high frequency haring loss.

Even a small port in the right place can be a useful monitor. The smaller it is the less it alters the 'normal' sound of the guitar, but it does have to be big enough to be useful in the context.

You would not believe how much work went into all of this.

[sjm1580]

Originally Posted by Alan Carruth

Some good questions: volumes have been written about some of this stuff.

In the low range a guitar is a 'bass reflex cabinet'. Unlike your loudspeakers, though, the 'air' resonance at the hole and the 'speaker-type' top resonance (the lower bout) are not tuned to the same pitch. In isolation they'd be about a musical fourth or fifth apart, with the 'air' resonance lower. Once the guitar is assembled they couple and push each other apart in pitch, usually ending up about an octave apart, generally roughly a G~98 Hz for the 'air' pitch and close to the open G string pitch at ~196 Hz. Exactly where they end up has a bearing on the timbre of the guitar, so it's one of the things that a maker tries to get under control by varying the size of the sound hole.

The structure of the guitar is a lot more complicated than speaker enclosures, so the acoustic response is too. Speakers are made with rigid walls as much as possible, and high losses in the cabinet, to spread out the resonances. Ideally the only resonances involved are the speaker and 'air'. The object is to have a 'flat' response over a wide frequency band.

On a guitar there are lots of moving parts, which couple promiscuously at all sorts of pitches. The response is anything but 'flat', which makes the sound 'interesting'.

The lowest pitches the guitar makes have a wave length in air of about 14 feet. The box is too small to make that sound efficiently, and it's radiated as if from a 'point source', in all directions equally. The player hears the fundamentals of the low notes as well as the audience.

As you go up in pitch the 'top' and 'hole' radiation starts to get out of phase. Higher order resonant modes kick in, with more and more small areas vibrating out of phase with each other (the 'telephone speaker catastrophe'). What you hear is a sort of sum of everything after a lot of it cancels out. The sound becomes more and more directional, coming off the top and out of the hole going toward the audience. The player hears this mostly through room reflections; if the room is big, dead, or noisy, the player might not hear much at all. This tends more to be the case with higher quality guitars.

'The Corker' was the first of three sound port 'test mules' I've made. The idea was to figure out what ports do, and what the best size and location would be.

Any time you punch another hole in the box it's like enlarging the sound hole: the 'air' resonance rises in pitch, and becomes a bit stronger. The further from the 'main' hole the port is, the larger the effect for a given size port. Usually this effect doesn't make a noticeable difference in the sound of the guitar out in front, but it can.

There are various internal 'air' resonances that are not normally 'heard' by the usual sound hole. Adding a port someplace else allows them to be heard. Many of these are normally 'wasters': and power that they get from the strings is dissipated in the box with no particular effect on the sound, and they show up as 'dips' in a power spectrum. A port can turn one of these dips into a (usually small) peak in the output, usually in a higher frequency range.
Your ears are set up to notice small changes at the threshold of perception, so those things can be picked up, even though they don't involve much power. Thus a port can make a noticeable difference in the timbre of the guitar, even when there's not really much change in the overall output or spectral shape.

Finally a port you can look into as you play will tend to shoot some of that internal high frequency sound out toward the player, making up for the lack of reflections in a 'dead' or noisy room. That's the utility of a port; it's a monitor for the player. The folks who tend to find these useful are ones who play a lot of restaurant gigs, and people with age-related high frequency haring loss.

Even a small port in the right place can be a useful monitor. The smaller it is the less it alters the 'normal' sound of the guitar, but it does have to be big enough to be useful in the context.

You would not believe how much work went into all of this.

Thank you for such a detailed report, greatly appreciated and fascinating!

[John Arnold]

Martin soundholes graduate in size depending on the body size, while Gibson has used a 4" hole on most of their models.
The graduation increment is 1/8"
D and 0000 = 4" (except for large soundhole dreads)
000 = 3 7/8"
00 = 3 3/4"
0 = 3 5/8"