Reducing feedback

[Jeff Chumley]

I have a request from a customer. My first custom order. The guitar he wants is essentially a byrdland style guitar but he doesn't want a free floating bridge. His initial request was to put a block under the bridge and install a tunomatic bridge on posts a la Les Paul. Now he is thinking of a traditional looking arch top bridge with a tunomatic top but he wants the bridge foot fixed and he still wants something tying the top and back together to reduce feedback.

The guitar will have a floating neck pickup and a bridge pickup mounted in the top and will be played almost exclusively as an electric.

So I have two questions.

1. WIll a big sound post help with the feedback? How about a block big enough to support a tuneomatic on posts? Other thoughts?

2. How about gluing the bridge foot in place like a flattop bridge and using a tunomatic as the top piece? IS gluing an archtop bridge in place done ever?

I'm nervous about building a guitar with a feature that may cause it to sound less than its best.

Thanks

[Rodger Knox]

1> A block between the top and back will reduce feedback, probably to an acceptable level. It doesn't need to be a large block, but it should probably cover the bridge footprint. It will also kill most of the acoustic power.

2> A TOM style should work fine glued in place, I believe one of the reasons for a floating bridge was to adjust intonation.

[Alan Carruth]

Anything you do to reduce feedback will make it sound 'less than it's best' in some respect. It's a reciprocity issue: if the room can hear the guitar, then the guitar can hear the room, and it will feed back if there's enough gain in the loop. Anything you can do to reduce the acoustic sound output will reduce feedback, and that's about the only way to do it.

Naturally feedback happens most strongly at resonances. Usually the 'main air' resonance is the one that feeds back the most, with the 'main top' resonance close behind. These are the two lowest pitch resonances of the guitar that can produce sound, and involve the largest areas that can interact with the room. If you can get those under control, the rest is relatively easy (depending on how high your customer wants to crank the amp).

The easy way to eliminate the 'main air' resonance entirely is to block the sound holes, or simply not make them in the first place. Gretch just paints them onto the 'County Gentleman', iirc, and I've seen a number of 'acoustic-electric flat tops that leave them out as well. The acoustic sound can be remarkable good without them: a bit 'thin' in the low register, perhaps, but not unsatisfying. Because of the way the air and top resonances work together, this should also reduce feedback at the 'main top' pitch.

To reduce feedback of the top, mass and stiffness are the keys. Your top is already a lot stiffer than most flat tops ever dream of being, so you're ahead of the game there. OTOH, most flat top players don't regularly have to compete with a sax player. Making the arch a little taller than you usually do, and the top a little thicker, can add a surprising amount of stiffness. That TOM bridge top also will add a lot of weight. You could also go whole hog and use maple for the top: I did that on one of mine, and it worked well until UPS destroyed the thing.

Smaller is also better in terms of feedback control. If you usually make a 17" box, try 16" or even 15".

[Jeff Chumley]

Thanks,

Sorry I haven't responded. I forgot to check the "notify me" box so I didn't see this until now. Your advice all makes very good sense dovetails nicely with what I've been thinking. It is very nice to get confirmation from experienced builders. I plan to go a bit thicker with the top and also tie the top and back together with a block under the bridge. I think at least some of the PRS arch tops work like this. I don't expect much acoustic sound but this guitar is more about the look and the electric sound.

Jeff

[Bill Raymond]

Instead of gluing a standard archtop bridge base in place, why not pin it with 2 small screws. There is a tutorial on the gretschpages.com forum, and I believe there's also some discussion on the chetboard about this.

[Jeff Chumley]

Thanks. I think that is what I will do. I figure a couple of screws inboard from the height adjustment posts countersunk so they are not easily visible.