Confused on this frequecy response

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Jim watts
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Confused on this frequecy response

Post by Jim watts » Thu Oct 09, 2014 11:04 am

I just completed a guitar the other day and took some impulse response data of it using VA.
The first graph shows the main air, top and back response ( I think). The second graph shows the top with the sound hole block by a foam plug. I blocked the sound hole because I was wondering what the 174 Hz response was. I was originally thinking it was my back which confused me also being lower than the top, but I could envision it happening. But it appears to be my top. My confusion comes in with the back appearing to have a more powerful response than the top, when I'm tapping the top in the bridge area.
Am I interpreting this right?
The bracing pattern is an X with a light lattice in the lower bout.
BTW: The guitar has good volume and good clear sound.
Thanks for any ideas on what's going on.
Jim
Ryans  guitar finished.jpg
top main sound  blocked guitar finished.jpg

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kiwigeo
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Re: Confused on this frequecy response

Post by kiwigeo » Thu Oct 09, 2014 4:32 pm

Interesting..I would have expected the main top peak at around 190 - 200Hz.

Do you have a spectrum recorded while tapping the back?

I tap top and back and then plot both spectrums on same Excel chart
Martin

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Trevor Gore
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Re: Confused on this frequecy response

Post by Trevor Gore » Thu Oct 09, 2014 4:46 pm

Hi Jim,

I think I know what's going on, but I think you can figure it out, too!

Have a look at Page 2-22 and see if it gives you any ideas, then let's discuss further.

Jim watts
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Re: Confused on this frequecy response

Post by Jim watts » Fri Oct 10, 2014 2:05 am

Thanks for the hint Trevor, I"ll have a look there a little later today.

Jim watts
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Re: Confused on this frequecy response

Post by Jim watts » Thu Oct 16, 2014 1:53 pm

Ok, so it appears that I made my back a lower main frequency than my top.
The frequency responses below lead me to this conclusion.
The fist graph shows the back with the sound hole plugged as was suggested by Martin, and the uncoupled back is setting at 183.3.
I then took Trevors advice ( I think) and added 31 grams of mass to the middle of the back and looked at the coupled response curve. The back dropped down to 165 HZ while the top moved down to 193.5 Hz (not much).
I would have guessed that the back would of gone up in frequency with the box closed, not dropped.
Also my conclusion could be totally wrong! Waiting for smarter people to come along :?
I think this works ok for me in this case as I don't believe the back is particularly active.
Soo.... if this happens to you I wouldn't panic, the guitar sounds really good and the customer is thrilled.
Interesting stuff!
Here are the graphs.
ryans back sound hole plugged tapped on back.jpg
ryans guitar 31 grams of mass added to the back tap on front at bridge.jpg

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Trevor Gore
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Re: Confused on this frequecy response

Post by Trevor Gore » Thu Oct 16, 2014 3:05 pm

Hi Jim,

I won't be able to get to this to answer it in detail for a few days as I have a modal tuning course coming up and I'm fighting computers and projectors at the moment! (Trying to get them all to work together - what was wrong with Windows XP and Office 2003 anyway?????)

A couple of questions though: You have concluded that the back is pitched lower than the top, but you have reported the uncoupled top at 175Hz and the uncoupled back at 183.3Hz. Is that right?

Have you done any Chladni testing?

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kiwigeo
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Re: Confused on this frequecy response

Post by kiwigeo » Thu Oct 16, 2014 9:02 pm

Trevor Gore wrote:
Have you done any Chladni testing?
As Trevor suggests do some Chladni testing on both top and back....this should help you nail down the back peak.

If its any consolation.....on my last two builds I didn't manage to get the back going live as planned but the guitars still sounded good.
Martin

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Trevor Gore
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Re: Confused on this frequecy response

Post by Trevor Gore » Fri Oct 17, 2014 12:09 am

OK, I've got W7, Office 2013 and the projector sorted, I think. (Or at least until it screws up again!)

Now back to this guitar of yours, Jim...

The first thing to remember is that when we talk about "top" and "back" resonances we are really meaning the T(1,1)x modes as shown in Fig 2.4-1 on P 2-32, as below:
Modes.jpg
One might expect the T(1,1)3 to be the taller peak (relative to the T(1,1)2) due to the larger top surface area involved and the fact that it is in phase with the "exhaust" from the sound hole. Whether it is or not depends on the mobility of the top and how active the back is. So it's worth having a look at what happens when the 4 DOF model is fired up (P 2-37 and the relevant appendix with all the maths).

First, the base-line model, which has a profile not dissimilar to your guitar, Jim (magenta line is measured, blue line is modeled for the first three peaks, one of my guitars):
Original settings.jpg
Original settings.jpg (41.47 KiB) Viewed 15510 times
My preferred profile is having the back a little stiffer, which results in a plot like this (blue line):
Stiffer Back.jpg
Stiffer Back.jpg (42.07 KiB) Viewed 15510 times
The three peaks are at 101, 189 and 226 Hz, which is a pretty good set of numbers (for a classical guitar).

If I revert to the base-line model and then make the top a little lighter (more mobility), I get a response curve pretty similar in shape to your guitar (blue line again):
Reduced top mass.jpg
Reduced top mass.jpg (40.86 KiB) Viewed 15510 times
The main peaks are at 101, 195 and 220 Hz, with the uncoupled plate frequencies (these come out of the 4-DOF model) of 190Hz for the top and 198Hz for the back (which also demonstrates the "repulsion" effect). So we can have the third peak higher than the second and still have the back pitched higher than the top. However, to my way of thinking, these are pitched too close together.

If, from here, we add a bit of mass to the back, we get a response pretty similar to what you saw with your guitar:
Added Back mass.jpg
Added Back mass.jpg (41.62 KiB) Viewed 15510 times
This has the uncoupled top at 189.6Hz and the uncoupled back at 186.9Hz (i.e. back lower than top). What I find with guitars with the the T(1,1)2 and T(1,1)3 too close is that the sound becomes somewhat un-focussed and has a sort of out-of-phase quality that one hears when one wires stereo speakers incorrectly, so I try to avoid that by spacing them a bit further apart (like the second response curve). Jurgen Meyer, in that extended set of listening tests that he conducted, also concluded that people didn't like guitars with "split main peaks" as he called it. Myer's test were with classical guitars, and it is on classical guitars that I've come across this type of thing most often, but I've also seen it on one or two steel string guitars and to me it has a similar effect. However, what I like and what you like may well be two very different things. So it's really just a matter of preference. But I think that's what's going on.

Jim watts
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Re: Confused on this frequecy response

Post by Jim watts » Fri Oct 17, 2014 1:18 pm

Trevor, thanks so much.
It appears to be a very high mobility top also. It deflects approx. .0075 inches at the bridge with a 2.5 lb load (I really need to accurately confirm the weight though), giving me something like 18 X 10 -3 s/kg using a main top frequency of 174 Hz, or 21 X10-3 s/kg at 198 Hz which seems extremely high.
I'm guessing the lattice in lower bout is responsible for some of this anyway.
Need to go through the books a few more times!


Jim watts
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Re: Confused on this frequecy response

Post by Jim watts » Wed Oct 29, 2014 3:34 pm

As I was trying organize some notes and photos of this guitar this evening I came across a photo of some Chladni test i did a while back on this guitar and remembered that Martin and Trevor asked if I had done any testing.
I thought I'd post the photo.
chladni on completed box ryan g.jpg

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Trevor Gore
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Re: Confused on this frequecy response

Post by Trevor Gore » Wed Oct 29, 2014 8:51 pm

That definitely looks like a T(1,1)3 response.

Jim watts
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Re: Confused on this frequecy response

Post by Jim watts » Thu Oct 30, 2014 2:39 pm

Thanks Trevor. That's another piece of the puzzle trying to figure this out. I don't think I could tell a T(1,1)2 Chladni pattern from a T(1,1)3 as they're both monopoles. What clues give it away?
Thanks,
Jim

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Trevor Gore
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Re: Confused on this frequecy response

Post by Trevor Gore » Thu Oct 30, 2014 2:58 pm

If you check here you can see that the T(1,1)2 is effectively a concentric dipole and the node line forms ~70mm in from the edges of the top. The top and back are in phase, so momentum equilibrium has to be achieved by the sides having to move. For the T(1,1)3, the back and top are out of phase, so momentum equilibrium can be achieved without the sides having to move, because the back movement "balances" the top movement, so the node line is further outboard, much as in your photograph. Also see Section 1.7.1..

Jim watts
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Re: Confused on this frequecy response

Post by Jim watts » Fri Oct 31, 2014 2:52 pm

Thanks Trevor,
I really appreciate you thoughtful answers and your references back the book section, it helps!
Jim

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