Liutaio Mottola Stringed Instrument Design



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Cannone Acoustic Bass Guitar

When designing an acoustic bass guitar (ABG) for good upright-like tone, the bigger the body the better. It should come as no surprise that in order to support the low bass notes of a bass in the way an upright bass does, the body of an ABG would have to be about the same size as that of the upright. A body this big causes some problems in the area of playability for an instrument that is meant to be played sitting across your leg while you are sitting, or hanging from a strap if you are standing. Starting with the body size typical of large guitars and commercial ABGs, if you make the lower bout longer you risk unbalancing the instrument. Lengthening the upper bout reduces the portion of the neck which is playable and also puts that playable portion farther away from the player's body. Making the body wider makes it more difficult for the seated player to get an arm around the thing. Same goes for making it deeper. One way to add more capacity to the body is to put some more body in a place that won't interfere (much) with playability. Extending the upper bout on the bass side in the direction of the neck is one possibility. But the process is nowhere near as simple as it may at first appear.

Initially appeared: June 1, 2014
Last updated: Saturday, August 15, 2015

About the Instrument

The double bass is tuned an octave below the lowest four strings of the guitar. In order to support the lowest notes, the body of a bass has to be approximately four times the capacity as that of a guitar, all else being equal. The function of the body that primarily supports these low notes is called a Helmholtz resonator, and it consists of two parts - the mass of air enclosed by the body and also the mass of air in and around the soundhole. As mentioned, the more air enclosed by the body, the lower the natural resonant frequency of the resonator.

So it would make sense to simply add a protuberance onto the body that increased the amount of air it enclosed. You might not be able to quadruple the volume of air that way, but you can make it bigger and thus lower the resonant frequency. But unfortunately there is a complicating factor. The body only behaves like a Helmholtz resonator if its shape is not too long in any one direction and if the hole is (more or less) in the middle of a broad plate. Adding a tube-like protuberance onto the body at the upper bout does not so much increase the mass of enclosed air in the body as add what amounts to be an organ pipe to that body. The upshot is that the desired effect is not what you get. Instead you get a more complex coupled resonator that does not behave according to any simple equation.

It took a bit of prototyping to come up with an enclosure of this type that would produce a resonance near the low A of the bass, where the lowest resonance of the upright bass is. Prototyping included tuning of both the body extension and that of the main body, and placing the soundhole at the end of the extension gave the best results. It turned out that the construction was more accurately described by the equations for a type of speaker enclosure called a 4th order bandpass enclosure (but, because the coupling to the extension does not directly include the top of the body, not quite).

Anyway, the resulting enclosure has a nice resonance at the A string, right where the double bass has it. How does it sound? Fine, but frankly no better than that of a more conventional big-body ABG like the Tinozza. The big issue here (and with other attempts to lower the lowest resonant frequency such as the use of soundhole tubes) is that at the same time frequency is lowered so is radiation efficiency. In fact, radiation efficiency is lowered more than frequency is. So the instrument sounds quite conventional when played acoustically. Things are a bit different when the instrument is miced though. In this case the low resonance comes through very well, giving the instrument a much better double bass-like tone. The rest of the resonant peaks are a lot more guitar-like than double bass-like, so the effect is far from perfect.

Given these limitations and given that it takes specially sized materials and special construction techniques to build the instrument, I think a more conventional big-body ABG is a better bet. Still the Cannone was an interesting experiment, and the instrument is quite usable.

The word Cannone is Italian for cannon. This is a nod to the gun barrel-like body extension.