When guitar makers refer to a “live back”, they mean that a soundbox has been created in such a way that as you play the back plate vibrates and adds to the sound being produced by the soundboard. A live back instrument has a more complex timbre.
A useful way to understand guitar acoustics is to think of the soundbox as a resonator with three connected parts, each of which has an effect on the other two. It’s an example of a complex system in which you can’t change one element without affecting all the others – this is known as coupling.
The three coupled resonators in an acoustic guitar soundbox are the top plate (or soundboard), the back and sides assembly, and the air contained in the box. How these three react together when you play the guitar determines the sound (timbre) of the instrument.
This, by the way, opens up the possibility of changing the overall sound by messing with any one of the three resonators – if you know what you’re doing. For example, you can vary the size of the sound hole to change the air resonance, which will in turn affect how the other two resonators respond.
You’ll notice I carefully used the words “back and sides assembly” rather than just the word “back”. For the three element model to work there’s no need for the back and sides to do anything other than vibrate up and down for them to play their part. The soundboard, we know, behaves in a more complicated way and is the most important element in creating a good guitar sound.
But what if the back plate developed ambitions of its own and didn’t want to limit itself to going up and down as a rigid block with the sides? What if it had been watching Mr Soundboard doing his smartypants modal tricks just across the way and wanted to get in on the action? Maybe they could make beautiful music together.
This needs a different design approach, of course. Traditionally guitar backs are reinforced by four crosswise spruce braces laid like the rungs in a ladder. Cleverly, we call this ladder bracing – but that’s just the kind of guys we guitar makers are.
So how does the traditional method work out for poor old under-appreciated Mr Back?
Here is the spectral signature of a a Martin 000-18 made in the early seventies. I recorded the tones produced first by tapping the soundboard at the bridge and then again by tapping the back plate underneath the bridge. Here’s the response of the back plate;
You can see that the back response has one main peak at 202Hz followed closely by another at 221Hz.
The rest of the signature doesn’t really mean much – remember that the loudness is being measured on the dB scale which is not exactly intuitive. The 202Hz peak drops away by over 10dB into the trough to its right, which means the sound level at the trough is less than one tenth of the peak.
If we take the -40dB level running through the centre of the graph, the 202Hz peak is about 1,000 times more intense. So anything less than -40dB isn’t very significant.
If the Martin has a live back, we’d expect the back to contribute to the overall tonal signature. In other words, some peaks in the back signature should imprint themselves onto the overall signature produced by tapping the top to simulate the impetus given by the strings when you play.
In the chart below the back signature is red and the overall top signature in blue.
There’s no really strong imprinting of the back signal onto the overall signature in the Martin. The peak at 100Hz is the resonance of the air body (the coupled Helmholtz resonance), so you would expect that to appear on both signals. The back plate peak at 202Hz is echoed rather reluctantly in the top. Better is the next peak at 221Hz which clearly reinforces the main top peak – that’s one for Mr Martin. At 258Hz there’s another increase in the overall response that matches a back peak as well.
And my live back score for the Martin 000-18 is……(drum roll)…..three and a half!
I have also done the same test with one of my earlier Jumbo models. Here’s the back signal:
The first thing you’ll notice is that this back signal is similar to but a little more complex than the Martin’s – there is quite a peak at 412Hz that isn’t there in the Martin. I can’t claim any credit for this at that stage in my building career. Both instruments have the same four rung ladder back brace system, so I don’t quite know how I managed it except by perhaps making the back plate a little thinner.
Anyway, how well does this imprint itself onto the overall tonal signature?
Well, I give this maybe five out of ten. There’s a reasonable imprinting visible at the main back response of 210Hz, a small one at 258Hz, another at 275Hz, not a bad one at 412Hz.
The overall tonal signature of the Jumbo is more complex and interesting than the Martin, and you can hear a difference between the two. How much of the difference is due to the live back effect is hard to tell.
This is all very fine, and I’m excited about the response of my latest guitar, a bamboo classical, that uses Greg Smallman-style lattice bracing on the top and a new experiment of mine for the back bracing. The initial tests for live back are very encouraging – I’m giving it seven out of ten so far, but it doesn’t have its neck on yet.
…is there any evidence that a live back guitar actually sounds better than any other? Is having a live back necessarily a good thing?
And anyway, don’t you have your body in contact with the back when you’re playing? Doesn’t that damp out any back vibration anyway?
I’ll tackle these questions in a later blog. At least now you know what “live back” means.