Old build, dead guitar

[Kevin Manuele]

Several years ago I built a dreadnought from public plans, using fairly good guitar wood, with build advice from Cumpiano's book. I was happy with build quality, but the sound was dead. Later I decided to start doing some more building, purchased Gore and Gilet's books, and did some measurements on that older guitar.
Attached is spectrum plot of coupled front and back (using Audacity).
Noting:
the out of phase peaks at about 205 Hz.
the 'peak' at about 165 is fairly broad, constant amplitude from 165 to 172 Hz.

I wonder if the members here can comment on the results, and maybe suggest why the guitar sounds as it does. I don't intend to fix this guitar, but I might experiment with suggestions from the group.

Thanks in advance
Kevin

[jeffhigh]

The guitar may just be overbuilt with a top which is too heavy and stiff.
You can't really tell just from a tap spectrum unfortunately, and tapping the back is fairly irrelevant.
You would need to do the deflection testing and uncoupled testing tap testing.

[Dave M]

As Jeff said. The bridge rotation test in the book is very useful. It has pinpointed a classical that was overbuilt for me recently.


Dave

[kiwigeo]

A "dead" sounding guitar could be due to something as simple as a poorly seating saddle. Worth checking....it was the reason my 30 year old Yairi classical went "dead".

[Kevin Manuele]

Thanks to all.

Will do deflection test, and uncoupled taps

Kevin

[Kevin Manuele]

Thought I had posted this a few days ago, but trying again:

The uncoupled top frequency is 153 Hz. With the deflection readings , mobility comes out to about 14.5e-3, which seems pretty good.

But the bridge rotation is barely 1 degree (0.97). Makes me wonder about relationship between bridge rotational stiffness and mobility ?

I note that the modal mobility equation reduces to omega/K, or 2*pi*f/K. Simplifies the calcs a bit.

Comments / questions appreciated.

thanks

Kevin

[Mark McLean]

It could be that the centre of the soundboard is pretty stiff (overbraced, or very heavy bridge plate) which results in the low figure on the bridge rotation test - and maybe is impeding the acoustics. On the other hand, if the periphery is mobile (e.g. thinner edges, untucked brace ends) it allows a reasonable amount of deflection, but it is only flexing at the edges and the central portion is a stiff mass? I think the bridge rotation test is telling you something important about the performance of your soundboard and bracing. Shaving some bracing through the soundhole might get it a bit more lively.

[Kevin Manuele]

That makes a lot of sense.

I'll check inside and maybe do some tweaking on the X's around the bridge.

Thanks !!

Kevin