Implications of back resonance lower than top

You can ask questions here about Trevor and Gerard's exciting new book on Luthiery.

Moderators: kiwigeo, Jeremy D

Post Reply
Craig Bumgarner
Blackwood
Posts: 181
Joined: Thu Apr 11, 2013 10:28 pm
Location: Drayden, MD, USA

Implications of back resonance lower than top

Post by Craig Bumgarner » Sat Mar 11, 2017 1:12 am

I keep running into Selmer style guitars with soft backs with low monopole peak resonance (T(1,1)3. So low, they are below the top monopole peak, T(1,1)2. I don't think it is intended other than it makes the guitars lighter and light weigh is revered though the reasoning is not scientific that I know of.

For instance, a guitar I looked at last night, by a well known builder in this style, had a (T(1,1)2 of 260 hz and a back of T(1,1)3. (2.4 semi-tones). I didn't like the sound much, thin and tinny sounding. Another I looked at by a respected Spanish builder of Selmer style guitars, a builder of multi-generation talent, had a top of 252 and back of 237, 1.1 semi-tone difference. Again a fairly thin sound.

Another general example are a whole bunch of cheap guitars built in Paris in the 50s and 60s that had backs laminated to roughly a spherical shape (~15'-20' radius) backs with no back braces. The backs are very flexible and have peaks well below the top, sometimes as much as 7 semi-tones! The soft backs result in neck rotation over time and the action becomes unplayably high. With this fixed, however, some of these guitars can sound very good in style. Very fundamental in tone and dry, but often very loud as the tops are also fairly flexible and SM of 25 or better can be seen on these guitars. Unlike the guitars in the paragraph above, they tend to have a lot of character, very throaty, soulful, responsive and loud.

So, I keep wondering about this. The active back, four semi-tone target described in G&G certainly works for me and some of the better guitars in this Selmer style, such as original Henri Selmer guitars from the 30s and 40s and at least some of the Bustato Gran Modeles and backs around 4 semi-tones higher than the tops, +/-. These guitars are at the top of the vintage heap.

Would anyone like to comment on the implications of back peaks below top peaks? Maybe from just a straight physics point of view The book talks about what happens when they are close to each other, split peaks, loss of clarity and separation and we have reviewed it here in the forum, but what is the implication of a back well below and separated from the top peak? Is a peak a peak whether it is below or above? I guess it depends on the amplitude of the peak as well. These soft backs have fairly high back peaks. I have not tested but my guess is the SM of these soft backs might be fairly high (light weight, high deflection).

Thoughts?
Craig Bumgarner

Bumgarner Guitar Blog

User avatar
Trevor Gore
Blackwood
Posts: 1306
Joined: Mon Jun 20, 2011 8:11 pm

Re: Implications of back resonance lower than top

Post by Trevor Gore » Sun Mar 12, 2017 11:59 pm

The first writer on the sonic effects of top and back coupling that I came across was Jurgen Meyer, papers published in the early 80's, full references in the book. He did a lot of detailed work with a listening panel relating sound to guitar structure, and one of the things that the listening panel rated poorly was guitars where the main top peak was "split", meaning that the T(1,1)2 was of similar amplitude and spaced close to the T(1,1)3. The "split" look appears when the two peaks are about a tone or less apart. Gerard always had a maxim "always make the back stiffer than the top". Of course, the question that is raised next is "how close is too close?" for the two peaks. I tap tested and listened to a lot of guitars until I learned to hear "split peaks" (which have a sound much as you've described above) and came up with the 4 semitone rule, which I've never had reason to change. Check out Fig. 2.3-11 for some experiments I did to drop the T(1,1)3 frequency below the T(1,1)2. By mass loading like I did in those tests, I also kicked down the back's mobility, of course. If the back is made less stiff, its mobility will go up and the "back" peak might end up higher in amplitude than the "top" peak.

Also check out the motions of the T(1,1)1 and T(1,1)3 modes in Fig. 2.4-1. Note the phase reversal of the sound hole air flow. As those two resonances are brought closer together, something has got to give, possibly a major attenuation of the air flow over a frequency band somewhere between the frequencies of the two resonances. That would go some way towards explaining the well-reported "thin" sound.

Anecdote time: I was at the Healdsburg Guitar Festival in 2009 looking over the guitars, which were uniformly bright, shiny and immaculately presented. I asked to play one maker's guitar and was sadly disappointed by the sound (which was a not infrequent occurrence at that event). I recognised the "flabby back" syndrome immediately and turned the guitar around and tapped the back with the end of a finger. The back was pitched so low you could just eyeball the vibrations and count them. I smiled at the guy and said "Beautiful finish!" as I handed the guitar back to him. He gave me a death stare, and I got the impression he'd heard that once too often at that show.

Craig Bumgarner
Blackwood
Posts: 181
Joined: Thu Apr 11, 2013 10:28 pm
Location: Drayden, MD, USA

Re: Implications of back resonance lower than top

Post by Craig Bumgarner » Tue Mar 14, 2017 1:09 am

Thanks for the insights Trevor, will read up on 2.3-11.
Craig Bumgarner

Bumgarner Guitar Blog

Post Reply

Who is online

Users browsing this forum: No registered users and 2 guests