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All things being equal (or as close as possible), what are the differeces between a box with wider sides, and a same size box (volume-wise) but with the top being wider and the sides being narrower? I'm working on developing my own design and was wondering what the sound results are between taller sides or a larger body, assuming the volume of the box is equal in both cases. Does anybody have any good information as to the differences?

Thanks!

John

What you're asking is 'what happens when I change the ratio of top area to body volume', if I understand you correctly. The answers to that are, as usual, a bit complicated if you dig deeply, but can be broken down a bit to aid in understanding.

First, a little physics. Most of the output of the guitar is produced by the 'bass reflex couple' between the top and the inside air. Changing things like the air volume alters the parameters of this couple, and can give some results you might not expect. The usual way the physics guys deal with this sort of thing is the isolate variables; changing one thing at a time to see what happens.

So, suppose you start with a 'standard' design and make the sides really deep. Fred Dickens did that once with a classical guitar, making it initially with 6" deep sides, iirc, but everything else the same as usual. It turned out the the pitch of the 'main air' resonance was just a little lower than 'normal'. You'd expect it to be much lower, with the extra body volume and relatively small hole. However, the depth of the box changed the way the couple worked: for a given motion of the top there was less pressure change in the body to resist rhe top motion, so the couple was weaker. What happens in these cases, and I've seen it too, is that the 'air' resonance pitch tends to stay the same, but the 'top' resonance pitch can drop.

Fred actually cut the sides down progressively to about 3" deep, or even less, and found that the 'air' resonant pitch only rose by about a semitone or a little more. He didn't comment on the timbre, but I'd expect it to be more 'forward' or 'cutting' as you made the body more shallow.

The reason for the change in timbre is that the 'bandwidth' of the resonances changes, becoming more narrow as the side height is reduced. This makes the output 'peakier', with more of a chance of having 'strong' and 'weak' notes or overtones. If there are only a few resonances in an octave this might make the sound 'uneven', but if there are a lot of peaks it just makes it 'interesting' or 'colorful'.

Obviously, as you make the sides deeper, the bandwidth of the resonances spreads out, and the peaks themselves also tend to be lower. The whole response is 'flatter'.

The opposite set of generalizations holds too: making the top bigger for a given side height is like making the sides shallower for a given outline. The bigger the top is the more 'bass balanced' the sound is likely to be, and it may actually be nmore difficult to make the guitar sound 'loud'.

As usual, you're loking for some sort of balance between 'attack' and 'sustain', 'bass' and 'treble', 'even' and interesting' timbre, and so on. The safe thing to do is stay somewhere in the range of instruments you like in the various design choices, but that still leaves you a heck of a lot of leeway.   

John,

Welp, I was composing my response when Alan posted his, so I wanted to read through what he had to say and see if my response coincided with his comments at all. I think they do, for the most part. I would just add to the following that the differences between the deeper and shallower guitars were slight -- but noticeable.

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I have built a few classicals with the same size body, or plantilla if you will, but varying depths. I find that the shallower guitars tend to lose some of the bass resonance, but they also tend to have better separation than the deeper guitars. The way I see it, this isn't all that surprising, since the added bass response from the deeper bodied guitars might tend to muddy up the mids and highs somewhat.

Hauser was known for building shallower guitars. I have a set of plans for a '43, drawn by Jeff Elliott, in which the depth at the tail is 90mm and at the heel is 86mm. Compare that to your typical Ramirez or Contreras in which the depth at the tail can be over 100mm and at the heel 96mm or more.

A friend of mine has a German-built "trammel guitar" -- 10-strings with 6 over the fingerboard and 4 floating. It uses a headstock extension for the added strings. This guitar looks more like one from the Viennese school, with a wider body than most classicals, but very shallow sides -- maybe 3" or so. He has it strung up with light guage steel strings. Not much bass response, but really punchy mids and decent highs. It sounds a lot like a Dobro in fact.

Best,

Michael
Michael McBroom38926.6713078704

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Thanks fellas, that's exactly the insight I was looking for. I think I understand what you're both saying. The hard part is finding that happy medium. In addition to tone and resonance, I was wondering what effect these changes may also have on volume. I agree they all must be closely related and changing one certainly changes the effects of others. I just haven't built enough instruments to learn the differences yet, but at least you guys gave me a starting point.

Thanks!

John

John,

Regarding volume, it seems once you get past a certain point in the shallowness of a guitar, volume will become affected. Have you ever played a "thinline" acoustic-electric? I own one -- an old Ibanez steel string, and I've played a number of others, including a Cordoba classical that had a depth of only 3" or so. None of these really thin guitars sounded very good to me unamplified. They were quiet and had very little bass response.

But within a more typical depth range, it doesn't seem that body depth has much of an effect on volume, if at all. To my ear (and also to my daughter's, who I frequently ask for comments regarding volume and qualitative judgements), the three Hauser-depth classicals I've built are every bit as loud as the others I've built with body depths that are more commonly found on Spanish built guitars.

Best,

Michael

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