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well , I'm litle confused..

I lay guitar on table (without glued back) put microphone on soundhole and start to measure tap tone (with audicity and wavesurfer). Atached you will find printscrean of measure TAP1 with audicity and TAP2 with wavesurfer.

what confuse me is
1. in any case my main top resonance is around 180Hz(what does it mean) and
2. where you put microphone when measure tap tone becouse I find if I put microphone over top the results is much diferent

regards
Fric

ups when I try to upload pictures I can't do that . ???

So you do not tap as a loose plate? most of us tap the plate loose and tune for a nice defined musical note typically near a G for me anyway with good sustain and rich overtone. Then when the body is assembled completely we will do some fine tuning on the finger braces and tone bars via the sound hole to adjust the main resonate pitch. But if the body is not complete you are not measuring the main resonate pitch. you are measuring the only freq that the constrained top is resonating at.

pictures need to be less than 200kb and JPG format or posted as a URL directly from your web site.

Suspend the guitar in the air or just the top between you thumb and four finger...then tap.

Although called a sound hole, the sound does not really come from the hole ... it comes form the plate when set in to motion or vibration.

The sound hole allows for the air to move in and out of the box from the vibrating waves that are caused by the top when in resonance moving in and out / up /down thereby compressing and or sucking in . Of course this compression retraction to the air causes the air to become the carrier for the sound to our ears....but the sound comes from the plate in motion. You will get better mike pick up from the board not the hole. Actualy the sound comes from the strings vibrations which then pass thru the bridge setting the top into motion. The lagre surface of the top acts like an amplifyer for the sound waves.


blessings
the
Padma

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Audiences and dispensations on Thursdays ~ by appointment only.


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The lagre surface of the top acts like an amplifyer for the sound waves.

padama I tend to go with a lossy matching transformer model ...

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thanks Michael..
first thanks for info about picture it was under 200k but in GIF
second let me spec guitar: it’s classical with mounted top and neck , top is reinforced with lattice (balsa +CF) without bridge.
Next info may be useful for other who will use this bracing patern.
when I finish the top bracing I mean put CF grid then put balsa grid and again over the balsa cap of CF (demanding proces take 3 day) I leave the top 2 day to harden the epoxy, then I make first tap measure. I hang the top on my finger; tap on the bridge area and from the other side put mic to make record. the result is that top pitch is around 211Hz but I read David Schram said that epoxy need more day to harden so I measure next days and after 6th day resonant pitch stabilize around 235Hz,(thanks David it’s fact that epoxy nead more than 6 days to harden and to get real measure) than I sand and scrap the top and bracing and drop the frequency to 216Hz at that moment my top is 1.1mm around and 1.3mm around bridge area so only way to drop the freq. more down is to more scrap and sand CF and it’s tedious job so I decide to leave freq at that point (reason is I’m not proffesional builder and all my guitar is ended as present and I earn new experience that in next guitar I will put less CF and will see how she sound on this way).
Then I glue ttop and glue binding and purfling and start measure of tap mentioned in previous post.
Maybe I’m wrong but these facts confuse me becouse my back-rim is flat and table surfice also so I think that table act as massive back in my case and if you can se we can find main air on 96Hz but main top at around 180 confused me. (At that moment maybe I did wrong becouse I put mic over soundhole or I didn’t????????)
I will try as PADMA said to hang guitar and measure but in this case when side and neck are involved in game I don’t know what I will get.
so if anybody of you have suggestions it’s wellcome! and do anybody of you and how measure tap in this stage of building guitar?

at this moment I’m colecting parts to build amplifier for next step and it’s chladni and my question is according above story is do I get valid figure when the guitar is without back?

I see that ,in this case ,most of you leave back off so you can have better access to the brace for settingsbut some of you do settings through soundhole , in my case access through my 85mm soundhole is rather rather dificult.

regards fric
p.s. sorry for my english

I am using strobosoft, what i find is many times the most prominent reading I get at first is not the fundamental, but ( sometimes) the first partial , or another related note. ( sometimes )I need to vary the strenghth of my taps to find the fundamental . if the back is not glued on the guitar , I dont see how you are getting a " main air' resonance.of the two readings you are getting , I would interpret as the 96 hertz being the fundamental. Jody

Hi Jody,
I THINK that I get "main air " because , as I explain before , my back rim is flat and guitar lay on flat table so air leakage betwen guitar and table is minimal main is through soundhole. In that case table acts as hevy guitar back.

Fric

Fric, the air resonance is not an isolated absolute,it involves the other resonances of the guitar , it will change as the other resonances of the guitar are altered, if you glue on a thick back , the air resonance can change with the sanding of the back . like your back will definatly be more flexable than your table top( or at least i hope so !), and will therefore give a different air resonance mode . probable lower . jody

The tap tone pitches of the 'free' top plate have almost no relationship to those of the top once it's glued to the guitar. You can't predict one from the other without a lot of powerful software and much effort.

The assembled box is a 'bass reflex cabinet'. Air moves in and out of the soundhole to produce a pitch, as it does when you blow across the mouth of a wine bottle. As the air goes in, the pressure in the box rises, and pushes on the top, and as it flows out, it pulls the top inward. By the same token, the vibration of the string puts a force on the top that pushes it in and out, like the cone of a loudspeaker, and this top motion 'pumps' air through the soundhole. The top and the air have to work together, and in doing so they alter each other's pitch. That's why you can't use the equation in the acoustics book to predict the 'Helmholtz' air mode pitch accurately on a guitar: the 'main air' pitch that you actually see will be lower than the 'Helmholtz' pitch by an amount that reflects how strongly the top and air couple. A mic either at the hole or over the top will pick up both pitches, but it will 'hear' one or the other better. 95 Hz for the 'main air' pitch, and 180 Hz for the 'main top', is in the normal range, so you're on track.

Alan Carruth:
"The tap tone pitches of the 'free' top plate have almost no relationship to those of the top once it's glued to the guitar. You can't predict one from the other without a lot of powerful software and much effort."

Therefore, is there a reason to record the tone pitches of the free top plate?


Kind regards,



Edzard

Alan wrote : ......A mic either at the hole or over the top will pick up both pitches, but it will 'hear' one or the other better.

corect.... after testing I find that whereever you put mic on guitar, resonant pitch will be recorded . Only difference is in db scale
Thank's Alan

Edzard asked:
"Therefore, is there a reason to record the tone pitches of the free top plate?"

As usual, in posting a brief reply, I left out some important details. There is no simple _general_ way to predict assembled mode pitches from free plate modes, but two tops of the same wood, braced the same way, of the same shape, with the same modes, will make up into guitars that have the same assembled modes if the backs are also matched. In other words, it's a good quality control method, if nothing else.

Thanks for your time and answer(s). It's appreciated!


Kind regards,


Edzard

I'd like to add that predicting the exact pitches of tap tones may not be all that necessary anyway. Most of the people who've done experiments on this say that 'small' changes in these pitches don't effect our perception of the tone of the guitar much, with a couple of exceptions.

One thing is that notes that are very close to a strong resonant pitch will sound different. Shifting the 'main air' resonant mode from F to G will alter the sounds of those two notes, at least. The overall timbre of the guitar might not be effected much, though.

The other thing is that there are lots of resonant modes of the different parts of the structure, and the air in the box, and these interact in complicated ways. Altering the pitch of one or another of them could change a whole web of relationships, and that could change the general impression of the sound in some ways.

All of this is pretty fuzzy in many respects. It's _very_ difficult to make two guitars that are alike, and sound it, and the thought of making two that are alike except for one thing is close to absurd. (Ive' made 'almost matched pairs', and learned things from them, but you really have to keep in mind that the margin of error in thse tests is huge. There's always a chance that what you 'learned' isn't really true) To 'prove' that that one thing had a certain effect you'd need to do that sort of matched pair experiment some number of times, and probably a large number of times. The only approaches we have, then, are:
1) make an instrument that can be easily and quickly modified in some way, like my 'corker' guitar. This can allow for the testing of a number of different configurations, but, of course, there are only so many plausible tests of this kind that can be done. There's no way, for example, to easily and quickly swap out a back so that you can A/B the sound in a few seconds.
2) use a computer model that allows for easy changes of almost anything. One such model was produced by Howard Wright back in '96, and it was very productive. Of course, a computer model is not a guitar: limitations of computing power and model sophistication limit the aplicablilty of the results. These things are improving, of course, but they're out of the reach of us luthiers for the most part for now.
3) build lots of guitars, with the best possible quality control, and look for common elements. At present the biggest QC problem is with wood: any species of wood varies a lot, and the folks who are set up to make lots of guitars with otherwise good QC, like Martin and Taylor, can't afford the time to test wood before building guitars from it. Some enterprising researcher could learn a lot if they could convince one of the big factories to make a run of guitars with 'known' wood, and submit them for testing before they went out to be sold. The factory would probably reap some benefits as well.

The big problem with all of these: who's going to pay to get them done? Most of the computer modeling is done by students who are learning how to use the tools on their way to engineering jobs. Big guitar factories are more interested in applied than basic research: finding a decent substitute for mahogany before it goes on the CITES list is more important than 'Quantifying the exact tonal consequences of altered T(0,0) /T(1,0) mode spacing on the perception of bass-range 'punch' in spruce/rosewood Dreadnoughts'. They already have a good enough feel for that for their purposes. That leaves a bunch of us 'left-brain' luthiers putzing around and wasting time when we should be building stuff. No wonder progress is slow.