Voicing for more bass

[TEK]

Is there any "secrets" or suggestions on how to bring out more bass when building a acoustic? I have a few builds under my belt so far this year and have been pretty happy with the sound I am getting. The exception is the bass is not quite as strong as I would like. I am building mainly OM's braced after Martin plans and have been using Adi for braces. Is less bass caused from over stiff braces? If so, which braces should I work on.
I would appreciate any suggestion anyone could offer.
Travis

[Alan Carruth]

A lot depends on what you mean by 'more bass'. I know that sounds flip, but it's a real issue.

Bass response comes from the 'bass reflex' action of the box and the sound hole. There's a 'Helmholtz' type resonace of air flowing in and out of the hole, analogous to what you hear when you blow across the top of a wine bottle. There is also the 'main top' motion, where the lower bout moves like a loudspeaker, producing sound off the top directly and also pumping air through the hole. These two vibrations can't be isolated from each other on the guitar, and other things, like the back, also get in on the game. Usually the upshot is a pair of peaks in the low range, with the 'air' peak coming in around G on the low E string, and the 'top' peak in the spectrum about an octave higher, near the pitch of the open G string. Generally speaking the 'air' peak is lower than the 'top' peak, in part because there's some cancellation going on: the air motion through the hole and the sound coming off the top are out of phase at the 'air' pitch. At the 'top' pitch they're in phase and add.

The things you can work with, then, are:
1) the overall strength of the bass reflex couple,
2) the pitches of the 'air' and 'top' peaks, and
3) the relative strengths of the two peaks.

You make the couple stronger by:
a) making the hole bigger,
b) making the box shallower,
c) increasing the amount of air the top moves, or
d) getting some other player, like the back, into the act.

Making the hole bigger allows for easier air flow, but it also raises the pitch of the 'main air' resonance. Whether this will come across as 'more bass' depends on which effect wins. In general you have to make something a lot more powerful to get it to sound appreciably 'louder', so in this case the pitch change usually overbalances it, and the bass is not heard as stronger.

Making the box shallower means that a given amount of top motion gives you more pressure change in the box, and more air gets pumped through the hole. Surprisingly, it does not raise the pitch of the 'air' mode much, unless you go overboard. It does, however, make the air peak in the spectrum narrower, and increase the 'attack' of the note, and those can come across as 'brighter' sounding.

Scalloping braces is the usual way to get the top to move more air. It's actually more effective than thinning the top or braces around the edge. Again, this usually gives more 'punch' which seems to come across as 'treble', but it moves more air at both the 'air' and 'top' pitches.

Sometimes loosening up the back is the best way to increase the bass response. If the top and back tap tones are fairly close in pitch the two will work with the 'air' mode to pump more sound out. This is also structurally less risky than scalloping top braces.

Generally, lowering the pitches of the 'air' and 'top' peaks will come across as adding to the bass. Loosening up the top or back, and making the hole a little smaller will usually do that. OTOH, loosening up the top and back, opening up the hole and making the box shallower will make the reflex action stronger.

In the end it's all a matter of finding the balance you like. My general approach is to try a little bit of several things that will move it in the direction you want, rather than doing too much of one thing.

I'm sure others will chime in with opinions on wood and so on. Lotsa variables...

[TEK]

Alan,
Thank you for taking the time to give a very detailed answer to my question. Its guys like you that make this a great forum. Much appreciated!! Having just started building this year and learning the whole process on my own is challenging and it great to have guys like you to take time to help us newbies out.
Thanks again
Travis