manipulating the tonal balance of a built archtop

[Brian Evans]

So I have my archtop project strung up and test playing, next week I will take it apart again and start finalizing it. I am very happy with the sound. I have some "play dates" scheduled with a friend to compare and contrast - he has a couple of archtops, a petite bouche, I have a nicely made solid top jumbo flat top, I have a 1946 Epiphone.

Here is the thing. At this point it is the loudest guitar I have access to by a long shot, it's the best balanced, it has a fully even mellow tone with a touch of shimmer but no twang, it sounds like the soundclips on the Benedetto site, but I would like a touch more bass response. It has a solid one piece bridge that weighs 30 grams. I've read bigger bridge, heavier bridge, smaller bridge, smaller feet, full contact. All to improve the same thing. Less recurve, more recurve, bigger F-holes, smaller F-holes, etc. I do have a small side port that does open up the sound noticeably, but not overwhelmingly. It dies if I block off the f-holes.

My instinct is to reduce the mass of the bridge, and maybe (if I make a new one) increase the size of the feet. It's not a full contact bridge but I might try that too (bridges aren't all that hard and pretty non-invasive). I have searched the current forum and the archives for clues, probably hence my confusion... Any ideas I should try?

Brian

[Bob Gramann]

In my thinking, increasing the mass of the bridge cuts some treble. You can try sticking Blutack or some other putty on it to see the effect of that.

[Barry Daniels]

Archtops are always more in the treble range compared to flattops. Just the nature of the beast.

[Randolph Rhett]

Archtops are always more in the treble range compared to flattops. Just the nature of the beast.

That's a good thing, IMO. Archtops have their own sound. I would call it more "mid-range" than treble, and usually have more projection and clarity than typical flat top guitars. As I tell the occasional potential client, they are not really sit around the campfire strumming big chords type instruments.

That is not to say they have to be tinny. A good archtop has rich tone, but generally more bell like and clear on single notes than your typical dreadnaught. I liken it to saxophone vs clarinet. Flat tops are more like a clarinet and an archtop is more like a saxophone. Both are woodwind instruments, and both can be played to sound mellow or brash. However they favor different sounds and have musical uses to which each is more often suited.

That said, using putty to experiment with the weight of the bridge is an excellent idea! Post soundcloud clips of before and after (assuming you have a reasonably good mic) and I'm sure people here would be happy to give you feedback.

[Alan Carruth]

It's possible to alter the bass tone quite a bit with the tailpiece. Both the length of it and the mass can have an effect, and even the length of the tailgut can make a difference.

Given the different ways that exist to attach a tailpiece, it's not possible in a short post to go into all the different ways it might vibrate. However, all of them probably can swing up and down, and that's a motion that can couple with the top effectively. Basically there are three things that determine the pitch of the resonance of the tailpiece swinging like that: the string tension, the mass of the tailpiece, and the location of the center of mass (or maybe the center of moment). All else equal, the higher the string tension the higher the tailpiece resonant pitch. The more massive it is the lower the pitch. The pitch will tend to be lowest when the center of mass of the tailpiece is close to halfway between the bridge and the bottom suspension point.

As the tailpiece swings up and down it's pulling on the top of the bridge. The shorter the back string length between the bridge and tailpiece the more strongly the tailpiece motion is coupled into the top.

If the tailpiece resonant pitch is close to either the 'main top' or 'main air' resonant pitches there will be coupling between them. In most cases the tailpiece resonance is closer to the 'air' pitch (often around C~131 Hz) than the 'top' pitch (nearer B ~ 247 Hz). Since the top moves a lot at either pitch matching whichever one is closer will affect the tone.

For increased bass response it's most helpful to get the tailpiece to couple with the 'air' pitch, since that's the one that gives the most reinforcement to the lower fundamentals. When you get the pitches to match pretty closely there is a 'split', which is analogous to what happens in a bass reflex system. That is, the feedback between the two elements causes there to be output from the system at two frequencies, above and below the pitches of the two elements independently. Basically, in terms of the sound of the instrument, the 'main air' peak is split into two peaks that are somewhat lower in amplitude than the uncoupled 'air' resonance would be. However, the area under the curve on the spectrum chart, the
total available horsepower' in a sense, is larger for the split peak. The output is also spread out over a wider range of notes, so you're less likely to have one 'hot' note with poor sustain. Instead the low range tends to be 'darker', or 'fuller'.

It's pretty easy to hear that 'swing' resonance of the tailpiece by simply tapping on it and listening carefully. If it's a bit on the high side as compared with the 'air' resonance, you can always mass load it with a bit of poster putty and get an idea of the outcome.

Back when I made my first archtop Classical guitar I took it in to show my friend Aaron Green, who was sharing shop space at the time with Steve Connors. Arron was out, and Steve sat down to play it. I had made it with a very light tailpiece, and taped on a hunk of lead to get the pitch to match that of the 'air' resonance. When Aaron came in he listened for a while, and asked what the piece of lead was there for. I just leaned over and removed it as Steve kept playing, and Aaron remarked:"Who turned off the speaker!?" It's not usually that dramatic.

I once heard Jim D'Aquisto say that he could change the tone of one of his guitars over a very wide range by changing the tailpiece. It's a relatively easy thing to do, often overlooked, and generally fully reversible if you don't like the outcome. What's to not like?

[Randolph Rhett]

The tailpiece? See, that's why I love this forum. Never even occurred to me!

Thanks Alan!

[Michael Jennings]

Allen…and All,

I have a 1946 Epiphone Broadway. Was the Idea behind its "Frequensator" tailpiece speaking to what you were explaining? It does have a very even response in the bass comapared to some other archtops I've played. [By even I mean it isn't less powerful than the midrange].

Still the base on an archtop is its own thing… focused? but completely unlike a flattop.

[Brian Evans]

I also have an early Epiphone, with the Frequensator tailpiece, and it's purpose eluded me until I started to really research the physics behind compensation. We all know that compensation is mostly down to the change in tension of the string due to stretching the string when fretting. While the strings tension is based on the scale length and pitch, the change in tension from fretting is based on the total string length from tailpiece to tuning machine post - the whole length gets stretched and so the added tension on a long string is less than on a short string - as you might have on a Fender tailpiece and a locking nut system, for example. So the longer bass strings on the Frequensator equipped Epiphone require somewhat less compensation at the bridge. Now, it's a neat idea but does it amount to a hill of beans? I have no idea, I just think it's kind of neat.