Purnell Guitars

From the beginning of my guitar making career I have experimented with the use of a sound post in the guitar. My first attempts didn’t work but when I introduced a new way of strutting the back about five years ago, most of the issues I was having were resolved. The use of a raised longitudinal back bar, a method I copied from steel string makers, allowed the sound post to be easily positioned and limited the amount of dampening that had occurred in the past. I found that my lightweight Torres influenced guitars, with their quick response and warm rich tone, could have their character subtly manipulated with the addition of the post. All my guitars with the raised back bar can have a sound post fitted if the player wishes. In this page I will explain my thoughts to the effect it has and my method of installing one (see YouTube link for my video demonstration at the bottom of the page).

Similar to that in a violin my sound post is positioned between the raised back bar and the harmonic bar below the sound hole. It is simply a piece of strut wood (any ridged piece of wood will do) wedged in using one’s fingers through the sound hole and can be easily positioned in a matter of seconds, without having to remove the strings and with no tools needed.

The sound post is not an integral part of the instrument, like that of the violin family, but is an addition which the player can use to subtly influence the sound of the guitar. A bit like using an electric guitar effects pedal. It has three main effects when positioned. Firstly it raises the air resonance by around one tone. My guitars which are relatively lightly built, have an air resonance of F#. This goes up to G# with the post installed. Secondly I find it enhances the mid range frequencies of the guitar, cutting off some of the lowest bass and highest treble frequencies. Thirdly it seems to give the decay of the notes a more linear quality.

As far as I can tell there is no loss of volume. There is dampening as one would expect, but this effects certain frequencies as I have explained rather than volume.The sound post, positioned where it is on the harmonic bar, leaves the lower bout of the soundboard free to move. Resting on the raised back bar minimises the dampening of the back. The post is basically stiffening the guitar, hence the rise in air resonance, with the minimum, almost insignificant amount of extra weight added. The fact that the post connects the soundboard and back almost at their centre, which is important, gives rise to an interesting change to the notes decay and is the most fascinating consequence for me.

With the tone rise in air resonance and boost of the mid range frequencies you can drive the guitar harder using the sound post. There is a more focused quality to the strings and more separation playing chords. It seems that there is an increase in projection. I believe there is a slight increase in sustain too. These changes are what one might expect when increasing the stiffness of the plates. Perhaps the most interesting quality the post gives the guitar is the change in the notes decay. It feels like the joining of the front and back result in some sort of feed back where energy/vibration is retained a bit longer in the soundboard and not lost to the sides and back. Hence the linear quality to the sustain and a “bloom” to the notes. My lightweight guitars have a quick response and the post has the effect of slowing/dampening the initial attack of the notes. There is a small loss of character to the overall sound which is given over to, I find, a more even, focussed quality and with a bit more clarity. Although the effects are relatively subtle I personally leave my guitars sound post in all the time.

These are very much my own thoughts to the use of a sound post and its affect. I have yet to gain much feed back from players. It is often dismissed as a bad idea by other makers whose concerns relate to over dampening the guitar. One maker who I believed was thinking along similar lines to me was in fact Torres. His guitar La Leona was very lightly strutted and the soundboard was supported by its tornavos which was joined to the back with small supporting posts. In this case the guitar would probably collapse without the tornavos. Apparently this was Torres own guitar and his favourite sounding. This experimental guitar is attributed as having a unique sound. I think its sound quality wasn’t due to the effects of the tornavos, but more to do with Torres joining the two plates via this supporting structure. It was a method of supporting the soundboard with the minimum of extra weight and therefore dampening. This was a large part of Torres work and thinking I believe.

The reason Torres probably didn’t continue with La Leona’s design was due to the effects that changing humidity have on the instrument. High humidity resulted in the supporting posts to fall out as the guitar swelled. I have heard from makers copying La Leona that they don’t use the supports for the tornavos for this reason. This problem with high humidity is well known to instruments of the violin family where the post becomes loose as the instrument swells. This isn’t really a problem with my method. Firstly my guitars are made without the need for the post. It is just a means to alter the character of the instrument in a non invasive way. The post does become loose, and you can hear it buzz when it does, but this can easily be corrected by putting a slightly longer one in. Another way of combating a loose post is to fit one in summer or when the humidity is high. My own guitar has two posts, one for summer and one for winter. Because I only play at home, changes in humidity are less extreme and I very rarely have to refit the post. There is room for a bit of adjustment without changing it. A concert guitarist travelling around would have to make sure the post was secure before performing.

Please click on this link for a demonstration on how to install and position the sound post in one of my guitars. https://youtu.be/cq18P6ag2LQ