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Main Air Resonance (frequency) Test
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=22153		 Page 1 of 1
Author:  Chris aka Sniggly [ Mon Apr 27, 2009 3:54 pm ]
Post subject:  Main Air Resonance (frequency) Test
Some background first. This past Sunday I finished the assembly of the box on an OM 'ish guitar I am building and became curious as to what note the box produced. When tested...my previous 2 guitars both landed in the G# area plus or minus (still doing OM's).

Let me pause for a second. By test I mean I hold a chromatic tuner up to the soundhole and I tap (with my finger) the bridge area of the box. The tuner registers a note and I record what it says.

This particular box, which is both deeper at the tail and the head block, registered a perfect A on the tuner. It wasn't sharp and it wasn't flat. That wasn't designed in folks....I'm ain't THAT good. I'm not even sure it was a good thing.

I wanted to see what effect shaving braces AFTER assembly would have on the main air frequency of the box. So I did it. I shave and I tapped and though I could not discern a change in pitch...the sustain was getting much better. At some point I stopped and 'played' the box again for my tuner to hear.

Now the main air resonant frequency is 25 - 30 cent flat (with A as the reference).

I guess this is normal behavior but I was curious.

Chris
Author:  Sondre [ Mon Apr 27, 2009 6:21 pm ]
Post subject:  Re: Main Air Resonance (frequency) Test
When I use my tuner to measure the air frequency of a box, I often get inconsistent results. It may be my tuner that's the problem though... For me, it works best to simply hum inside the soundhole. You will notice it immediately when you hit the main frequency.
Author:  Chris aka Sniggly [ Mon Apr 27, 2009 6:30 pm ]
Post subject:  Re: Main Air Resonance (frequency) Test
I have found it that way too (voice in the soundhole).
Author:  Alan Carruth [ Mon Apr 27, 2009 7:38 pm ]
Post subject:  Re: Main Air Resonance (frequency) Test
It's interesting that making the box deeper _raised_ the pitch of the 'main air' mode. I'm assuming you used the same soundhole size and location that you had in the past? I can think of a reason for this, but there are a lot of other variables that can get in the way. At any rate, the drop in pitch when you shaved the braces is normal, and ties in with the thing I'm thinking of. ;)
Author:  ChuckB [ Mon Apr 27, 2009 9:44 pm ]
Post subject:  Re: Main Air Resonance (frequency) Test
Chris, I recently built an OM that I am thinking had a main air resonance at A. The reason I am thinking this is that when I played A anywhere on the fingerboard the result was a wolf note. After a lot of studying the archives and a lot of it from Alan Carruth (thanks Alan), I came to the conclusion that the main top and back resonance were A. And the best way, for the bracing on this guitar, to eliminate the matching resonance and hopefully the wolf note was to shave some from the back bracing (which I thought was a little stiff). And it worked! No more wolf note!.....So anyway my point is that you may have inadvertently eliminated a wolf note by experimenting and changing the main air resonce from A.

Chuck
Author:  Chris aka Sniggly [ Mon Apr 27, 2009 10:32 pm ]
Post subject:  Re: Main Air Resonance (frequency) Test
Chuck I appreciate the input...

If my understanding of resonant modes is accurate......the resonant mode of the back would almost always be lower than the resonant mode of the top (atleast when considered as separately). This assumes that the builder is using the typical woods for backs (mahoganies & rosewoods) and tops (spruces and cedars). This probably changes when the back is coupled to the top when you assemble the box. To what degree I have no idea.

The pitch of A is higher in frequency than G or G#. I have no data to back up what I'm about to say but I would think that you would NOT want your main air mode to be that high. I might not even be able to articulate the concept if a person brought me a guitar that had an A main air mode. I do assume however that my ears would tell me something's up with it. I noticed an elevated pitch on the tap of this current box. But I could not have told you it was an A before I tested it.

One other thing....if removing material from bracework lowers the main air resonance then there is a direct correlation to how loose you build a top and how low the main air mode is (within limits). There must be a sweet spot there somewhere. I can't imagine you there aren't limits on what you gain.

On my next build I am going to record the main air mode on a fully and completely braced top that HAS NOT BEEN carved and shaped yet.....then I'm going to carve and shape it....and see what the pitch is then. It should drop.

I'm just thinking out loud here.
Author:  Ken McKay [ Tue Apr 28, 2009 2:42 am ]
Post subject:  Re: Main Air Resonance (frequency) Test
Maybe you could set up a system to perform an impuse test using some computer software (freeware) like Audacity. Record the tap and analyze it using the analysis tool to show an FFT. The FFT will show all of the resonances in that impuse and you can usually pick out the top, back, air resonances and see the "damping" of each peak by eyeballing the width of each peak.

But then you have to decide what is good and what is bad if there is such a thing. pizza [:Y:] For this to be of any use you have to believe that it creates either consistency, or tonal copy of something that is known.
Author:  Alan Carruth [ Tue Apr 28, 2009 1:40 pm ]
Post subject:  Re: Main Air Resonance (frequency) Test
Normally the 'main back' tap tone is higher then the 'main top' tone, and often much higher.

In a complicated system wth as much coupling as the guitar has it's a little hard to say what the damping of any individual mode is on the basis of the peaks in the impulse spectrum. You can say something about the overall damping of the system from the peak to dip ratio but any one mode is more problematic.

An example would be the peaks around the 'main top' resonant pitch. Let's say that that the main top mode, as found with Chladni patterns, is around 200 Hz, and the 'main back' is something like a semitone higher, at around 215. In the impulse spectrum you are likely to see a peak at around 190, a _dip_ near 200, and another peak around 220. This is because of the way things change phase as you go through the resonant pitch, along with the differences in effectiveness of the two plates and the air coming through the soundhole because of things like phase cancelation. It is also often the case that the 220 peak will fall off fast, to something like a null at around 230-240, due, in part, to a second Helmholtz-type air resonance in the lower bout only (often near 250 Hz), AND the fact that the top 'cross dipole' mode is statrting to vamp energy too. And then there's side movement.....

Generally speaking, the peakier the response curve, the lower the overall damping, but that's about all you can say about that from a spectrum chart.
Author:  Jeremy Douglas [ Tue Apr 28, 2009 3:58 pm ]
Post subject:  Re: Main Air Resonance (frequency) Test
Don't forget that gluing the bridge on will change(decrease) the pitch of the top.
Author:  Sondre [ Tue Apr 28, 2009 4:59 pm ]
Post subject:  Re: Main Air Resonance (frequency) Test
Quote:
if removing material from bracework lowers the main air resonance then there is a direct correlation to how loose you build a top and how low the main air mode is (within limits)


Someone please correct me if I'm wrong, but I think I read somewhere that this is exactly the case. If I understood things correctly, the reason why it is difficult to calculate the Heimholz frequency of a guitar box is because the vibrations of the top, back and sides will make the frequency lower than expected. So a looser top would lead to a lower main air frequency.
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