Deflection testing is a mechanical tuning process that is taken from techniques used by the violin masters. Unlike Chladni testing or tap tuning which deal with the frequencies found in a plate, deflection testing instead measures the mechanical properties of the plate. The stiffness of a violin plate can be analyzed by twisting the plate with your hands and removing wood until it twists just the “right” amount. This method of deflection tuning can require years of practice and has limited accuracy. The method of deflection tuning we used involved putting a known load on a certain point and then measuring how far the plate would deflect. This method was developed in the 1970's by Roger Siminoff.
While we were designing our deflection tuner we wanted to not only be able to measure the deflection of the plate but also to be able to carve the braces at the same time without removing the top from the tester. This made our design more practical for a production setting however the design could be simplified if it was going to be used for test purposes only. In the pictures the back is already glued on but the idea is to deflection test while the top is on the rim or possibly even free. We built our tester with a metal frame for rigidity but a tester could probably be made out of wood as long as care was taken to ensure that it could still hold a plate without bowing and distorting the reading.
Pressure was applied where the bridge would eventually fall and the two readings were taken at 1 3/8 " off to each side. Originally we designed the tester with the dial indicators underneath it but we later added pivoting wings to allow us to get our readings without bending over. Each wing consists of a steel rod pivoting directly at its center with a carriage head bolt on one end in contact with the top and a dial indicator on the other end. A rubber band keeps the bolt pressed up against the top. The ring around the top was cut out of a 15' radiused disc so that the tester accomodates the radius of the guitar.
The idea behind deflection tuning a guitar top is to try to repeat the deflection (stiffness) of a high quality guitar on future instruments. Measuring and attempting to repeat the exact amount of pressure and deflection applied by the strings of a high quality flat-top guitar would be pointless because the pressure is applied horizontally and not vertically to the bridge. When guitar strings are brought up to tension they can easily pull the area behind the bridge up 20 thousands of an inch. Attemting to repeat the deflection applied by strings could make sense however if one wanted to deflection tune a mandolin, violin, or any other instrument where the pressure is applied vertically to the bridge.
Unfortunately, we were unable to iron out all the little kinks in our deflection tuner until testing was already completed on most of the guitars. Only the jumbo guitars have complete Chladni data.