Interpreting Closed Box Tap Tests

[klooker]

I'm attempting a live back, X-braced, L-00 size guitar.

I just closed the box & did some tap tests.

From what I think I'm seeing, my coupled top response is high at 118 Hz. Is that the back around 263 Hz? Do my measurements look accurate?

Any help or insight would be appreciated.

Thanks,
Kevin Looker

[kiwigeo]

Welcome to the forum Kevin,

Trevor will chime in with expert advice....my ten cents worth below.

Check out this thread on parlour tap testing results: viewtopic.php?f=33&t=6073&p=77253&hilit ... ies#p77253

The peak at 118Hz....you can verify whether or not it's the main top resonance by re doing the tap test with the soundhole blocked. If the peak is still there then it's your main top resonance. If the peak is no longer there then its your Helmholtz peak.

The peak at 263Hz.....to a tap test on the back (mic on back) and see what sort of spectrum you get.

What's your set up for tap testing?

Cheers Martin

[Trevor Gore]

Hello Kevin,

I think you might have a problem with your tap testing set-up, so you're not getting meaningful data. Let us know what your set-up is and we can probably help you out. I'm guessing your signal levels are currently too high.

For a small body guitar around L-OO size (live back) you should be seeing something like this:

That's this guitar BTW.

[klooker]

Thanks for the replies.

I'm using a Shure PG57 into my Lenovo laptop.

The mic is mounted to the bench & I'm holding the guitar body (closed box only) about 1m away (maybe a little less). I have discovered that the fan on my computer generates about 90 Hz.

I did some more taps & got this.

Then I plugged the hole & tapped the top and back to try & measure the non-coupled responses of each.

I don't know if these are meaningful because my styrofoam plug doesn't fit perfectly snug. I put masking tape around the perimeter in an attempt to make a better seal.

I'm going to move my setup to a different room to see if the measurements change much.

Thanks,
Kevin Looker

[Trevor Gore]

Your new plots make a lot more sense, Kevin. What did you change?

It looks like you have the main air resonance T(1,1)1 at 115 Hz, with the T(1,1)2 at 258Hz and the T(1,1)3 lower at 247Hz. I'm assuming you haven't got the bridge on yet. It's always best to have the T(1,1)2 lower than the T(1,1)3, and that is likely to happen when the bridge goes on (which acts mainly as added mass on a X-braced guitar). If you do nothing else, you will have a fairly stiff, "middly" sounding guitar (a typical small body sound), which is fine if that is what you're after. If you want a fuller sound, you'll need a response that looks more like the one I posted. Thinning around the top edges will take the T(1,1) down (and also the T(1,1)2), but I doubt you'll get as low as 100Hz, as the sound hole is likely too large to achieve that. You may be able to get down to ~107Hz if you decide to head in that direction. It depends what sort of sound you're chasing.

[klooker]

The main thing I changed was holding the guitar body further from the mic.

Correct, the bridge isn't on yet.

I've already started thinning the perimeter of the top.

Would reducing the height of the braces have much impact? The lower legs of the X are not scalloped, just tapered and I would like to get the fullest sound out of the guitar.

Thanks.

[kiwigeo]

The main thing I changed was holding the guitar body further from the mic.

I was actually going to suggest holding the mic a bit closer. I use the same Shure mic but I usually have it closer to the guitar.

[klooker]

It may have something to do with having the gain set so high as I did.

I've just started using VA & like it compared to audacity. Being able to immediately see the results of your tap to see if you're getting consistent measurements as opposed to an anomaly is a great feature.

[Trevor Gore]

Would reducing the height of the braces have much impact? The lower legs of the X are not scalloped, just tapered and I would like to get the fullest sound out of the guitar.

Scalloping the lower legs of the X (and any lower face braces) will have a relatively small impact on the T(1,1)2 frequency but generally improves the bass response by "softening" the top, allowing greater excursion and so reducing the T(1,1)1. To reduce the T(1,1)2 frequency significantly, one usually has to attack the X area, but be sure you leave enough in there to support the static loads over time. If you have let the ends of the lower bout braces into the linings, the effect of all of these tweaks will be reduced. That's been my (somewhat limited) experience on X-braced guitars. I don't build them very often.

[johnparchem]

I did a Martin style 00 12 fret to the body a couple of years back I have a few spectrum graphs one of them is not that far from yours. Might give you some ideas as you go forward

Here is the spectrum of the guitar just after I closed it. You can see the top is around 242 Hz. The back is higher than the top

closedbox.jpg (92.32 KiB) Viewed 18733 times

I have a magic probe thickness meter. So I took the top down to the calculated target thickness that I got when I tap tested the plates using the books SS number, finished the guitar and put a bridge on. The Air 120 is around top is 216 the back is 275.
Here was the spectrum with the bridge and no strings. Sorry I do not have with stings but the top is around 208 with strings.

nostrings.jpg (59.76 KiB) Viewed 18733 times

Here is a recording of the sound I was happy with it.

youtu.be/

All three of my X braced 00 Ended up a bit above 200. My facate 00 was lower ending up with the air just above 106 and the top at 185

[klooker]

Thanks for the replies, John & Trevor.

More good information for me to think about as I try to carefully tweak the guitar.

I guess if this was simple, we'd get bored & take up a new hobby.

Kevin Looker

[kiwigeo]

I guess if this was simple, we'd get bored & take up a new hobby.

Kevin Looker

Precisely....

[Deems Davis]

Just finished boxing my 2nd SS falcate and got some numbers.

T(1,1)1 =103
T(1,1)2 = 199
T(1.1)3 = 219

Thinned the top to 2mm at the edges of lower bout while tap testing along the way and ended up with:

T(1,1)1 =100
T(1,1)2 = 196
T(1.1)3 = 224

I'm going to make up some weights and take some samples and see how that effects things. I've not adjusted the back brace other than the minimal scallop using Trevors's "tube and sandpaper" prior to gluing.

I am concerned that the spread between Top and Back isn't as great as I'd like it to be to achieve the desired target (I probably got the back too thin) . How much fiddling/adjusting do you guys do at this stage? Or do you wait for bindings, bridge,

[Trevor Gore]

You stand a reasonable chance of hitting ~100, 180, 224 with binding, finish, neck, bridge and a good dollop of side mass, which would be be pretty good numbers.

Carry on!