Sounding in the soundhole

[Roger Häggström]

I don't do much tapping of the wood, I use my thumbs to feel the stiffness of the top when shaving down the braces before gluing back the bottom. I do give the bridge a big thump with the back off to hear if the top has enough sustain after the chock.

With the box closed and the bottom glued I do something that might be a bit unusual. I use my voice as a soundgenerator and sweep the frequencies from below and beyond the helmholtz resonator frequency. The frequency where the guitar body resonates the most. I'm interested in how steep and wide the "gauss curve" around that frequency is. A very smooth and wide curve is great, the guitar will sound nice. If the curve is steep, narrow and defined the guitar will have more of a blues sound and maybe a wolf note too.

I might add that the whole process looks very silly indeed! :-)

Sometimes I do shave off the top or the ends of the ladder brace in front of the bridge if the "sounding" tells me that it's needed until my thumbs and my ears tells me that the stiffness of the top is OK.

After vibrating the guitar the "gauss curve" will always flatten out and be wider.

With the Y-axis as volume and the X-axis frequency; good above and bad below.

[Robbie Collins]

It makes sense. The bottom graph basically shows a parametric eq with a high q factor centered around the loudest resonant harmonic ... which would sound nasal. I'd be surprised if the actual frequency response was symmetric, aka, "normally distributed."  Hard to imagine that your voice produces enough amplitude to excite the top. Of course, loudness abides by the inverse square rule so maybe you're kissing it. ;-)

[Roger Häggström]

I hear and feel the response from the guitar body as I pass the resonance frequency. The voice is strong enough even without shouting. I just hum in the hole :-)