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This question refers to the idea that I've heard that the tap tone for the (guitar) back and soundboard should be different notes-eg a semitone apart or some other amount.

Does this apply to free-hanging braced plates, or assembled guitars, or both?

Will some terrible result (wolf notes or similar) ensue if the 'tap tone' of both plates is the same?

I've got a couple of 75% completed guitars hanging in the shop and one seems to produce the same sound whether I thump the soundboard or the back. The bridge has not been installed, so in theory this should change the vibration characteristic of the soundboard, though I'm not clear on whether this will raise or lower the tap tone.

Thanks

John

the bridge should lower the tone (note/freq) of the top. I think theoretically
the top and back need to be different notes although I just go for a nice tone
on either. I bet you'll be just fine.

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John Mayes
http://www.mayesluthier.com

Thanks, John.

It may be hard to tell the difference after you have joined the box. Mostly try and check this out before glueing the plates to the rims. After, there are other factors such as chamber resonance that tie the two together making it difficult to hear exactly what the plates are doing.

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Tickle your guitar daily, and it'll tickle you back.

The 'correct' relationship will depend somewhat on what stage you're at. It's also fairly easy (alas) to miss the top and back tap tones entirely, so let's start there.

When you tap on the top or back, assuming the back is at least somewhat active in the low range, you're going to be activating the 'bass reflex couple' between the top, back, and air. This produces two prominant tones, the 'main air' and 'main top' pitches, at least. The air pitch will be the lower one, usually by about an octave.

The 'normal' situation for a complex sound that starts all at once is that there's a single object making it that has a fundamental tone and a bunch of overtones, that may or may not be harmonically related. Your ear and brain tend to lump all of the energy in that sound together, and assign it to the lowest pitch. The low tone thus 'masks' the higher overtones, and it's hard to pick them out.

The way to isolate them is to plug the soundhole as much as you can without actually touching the vibrating part of the top, in the lower bout, and tap on the top and back that way. Blocking the hole changes the pitches, of course, but at least you can get an idea.

I like to end up with the 'main back' tap tone about a semitone higher than the 'main top' tap. This puts them close enough together so that they can move a lot of air and give a strong 'bass reflex' action, but far enough apart to avoid 'wolf' notes usually. This is where I want things after they've all settled in.

Normally, the pitch of the 'main top' tap tone will drop a bit in the first month's playing in, and then will more or less level off. Often on my guitars this drop is about a half a semitone, so if they're a half semitone apart when I first string it up, I figure that's OK. This can sometimes give a slight 'wolf' near the 'main air' pitch, down around G on the low E string; a bit of 'chuff' on the attack. It goes away as the top plays down.

Gluing on the bridge generally tends to drop the 'main top' tap tone a bit as well. This is a variable thing: I usually plan on about another half a semitone, but you can't always predict it closely. The bridge adds both mass and stiffnes to the top, and it all depends on who's winning. With that in mind, then, I'm normally pretty satisfied if the 'main top' and 'main back' tap tones are at the same pitch before I glue on the bridge.

One little kicker; the 'main back' tap tone can actually rise a bit when you put the strings on, since the back is being stretched like a drum head by the string tension. The extent of the change will depend on how strong the back is relative to the load it's taking, and that's something I haven't the math to predict.

When you rout for the binding that loosens up the edge of the top a bit, and drops the 'main top' tap tone. It may or may not drop the 'main back' tone, but usually doesn't, iirc. Gluing on the binding gets some of that stiffness back, but not all of it. So, if the 'main top' tap tone is just barely higher than, or equal to, the 'main back' tone when you first glue the two plates on, you're in the ball park. I'll note that just clamping the plates in place isn't the same as gluing them: the degree of the deviation depends on how well they fit, it seems to me.

Finally, I'll say that this is what I do, and it makes guitars that sound like mine. If you want a different sound, maybe you need to use different tap tones. There are lots of systems that work well.     

Thanks for the thoughtful responses, guys!

Alan-
Again, thank you for your typically erudite and understandable explanation- I really appreciate your help.
I make a point of reading your posts and I'm very seldom disappointed- you should really collect those thoughts together in one place: 'Collected Wisdom, etc...' A book would be possibly worth your trouble- put me first in line to buy it.
An alternative would be to make copies available for download at your website. Till that happens, I'll just keep on printing them out as I read 'em!

Thanks


John

Alan, I, too, always greatly appreciate your thorough and extremely informative posts. I have an observation and a couple questions.

The observation is that I don't think I've ever played a guitar that didn't have a note, usually around G2 (3rd fret, low E string), that sounds somewhat "tubby", lacking the clarity and sustain of other notes. (The notes at least a half-step above and below are usually somewhat affected as well.) It's my understanding that this is due to the main air resonance. I bring it up now because you said above, "... so if [the main top pitch and the main back pitch are] a half semitone apart when I first string it up, I figure that's OK. This can sometimes give a slight 'wolf' near the 'main air' pitch, down around G on the low E string; a bit of 'chuff' on the attack. It goes away as the top plays down." It's that last sentence about it going away that really caught my attention. Are you saying that on your guitars, once the top has "played down", the relationship between the main top resonance, the main back resonance, and the main air resonance results in a guitar that does not have this tubby G note? Doesn't the main air resonance still give you that tubbiness regardless? Or are you talking about a separate issue - a "wolfiness" from the top and back that is distinct from the "tubbiness" from the air?

BTW, I've read with great interest your recent posts about what I'll call the "marimba bar resonance" of the whole length of the guitar, and how that can broaden the peak associated with the main air resonance if it's tuned right.

Second question: I confess I've never really listened to the tap tone of the top (main top pitch) with the strings off compared to with the guitar strung up and tuned. I'll have to do that next time I change a set of strings. What do you find generally happens to the main top pitch when the top is put under string tension compared to when the strings are off?

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Todd Rose asked:
"Doesn't the main air resonance still give you that tubbiness regardless? Or are you talking about a separate issue - a "wolfiness" from the top and back that is distinct from the "tubbiness" from the air? "

This is complicated. Remember that the 'main air' resonance is actually part of the bass reflex couple, and involves both top and air motion. If you look at the guitar, you'll find that there's a lot of top motion at the 'main air' pitch. If that's strong enough, and it happens right at a pitch that you're playing, it actually reacts back on the string. In some sense, the string wants to vibrate with two different lengths, depending on whether it's moving horizontally (with respect to the plane of the top) or vertically. This is one thing that can cause the 'chuff'. Another can be the interaction between the top and back when they are very close in pitch: you hear a difference frequency.

The lack of sustain of the 'tubby' note has more to do with how strong the air mode is, and how quickly it extracts energy from the string. You can alter that, for example, either by adding weight to the bridge or by restricting the soundhole. Adding weight keeps the energy from leaking out of the strings so fast, and restricting the soundhole size both changes the air mode pitch and reduces the strength.

"What do you find generally happens to the main top pitch when the top is put under string tension compared to when the strings are off? "

On flat tops; nothing. On archtops the pitches of the main top modes drop.

Todd,
On a side note about wolf tones, I have been aware of them since I’ve been building acoustics. They’ve always bugged me. I finally gave up trying to control them when Alan told me that some of the best guitars have strong wolf tones. One of my resent builds has a fairly extreme wolf at G sharp but it also is the loudest flat top I’ve made yet. Go figure.
Thanks again Alan!

Wade

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Wade
Nashua, NH
http://www.wadefx.com

Al, thanks for your reply. I'm not certain I'm understanding what you mean by "chuff". If you could use other words to describe that particular effect, it may help me understand your last post. Otherwise, I'm pretty sure I'm following you. Thanks.

Wade, thanks for your input as well. I'm reminded of a presentation by Mike Doolin at the last GAL convention in which he was discussing intonation problems with a classical guitar made by a well known and highly regarded builder. Apparently it was an excellent guitar, extremely responsive and powerful. Along with this came various notes whose pitches were pulled noticeably off course (these are my own words for describing the phenomenon) by string resonances in the instrument.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Todd,
The wolf notes I have experienced seem to have an initial bark, no pun intended, then the usual sustain gets zapped out of the note like there is some kind of cancellation going on. That notes harmonics seem to sustain ok but the dominant pitch is stifeld. Maybe that's the "Chuff" Alan is talking about?
If you "sing" a sweeping note into your sound hole, stop at the note that shakes the guitar box. That note, when played, will likely be a wolf.
I guess that's just the way it is..
I'd love to hear what others have done about it, if anything?



Wade

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Wade
Nashua, NH
http://www.wadefx.com

I think I read somewhere that you should try to put the resonance in between the "normal" notes. So, make the body air resonant between G to G#, rather than making it G or G#.

Todd, I think all guitars have that "wolf" note. It's more pronounced on some than others, and it's not always the same note but every guitar I've played the last few years has had it to some degree. I think it's a real negative.

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David White, Toronto

"All my favourite singers can't sing."

[QUOTE=Wade S.]
I'd love to hear what others have done about it, if anything?
Wade
[/QUOTE]

Hey, Wade- I found a thread on this in the archives here! (Guess who started it?)
Also, there is some stuff in the MIMF library as well.
It seems to be an ongoing problem for folks with good ears.
I guess I'm lucky- I don't know if I would recognize a wolf note if I had one. I can barely tell when it's time to tune up again!

Cheers

John

[QUOTE=Todd Rose] I'm reminded of a presentation by Mike Doolin at the last GAL convention in which he was discussing intonation problems with a classical guitar made by a well known and highly regarded builder. Apparently it was an excellent guitar, extremely responsive and powerful. Along with this came various notes whose pitches were pulled noticeably off course (these are my own words for describing the phenomenon) by string resonances in the instrument. [/QUOTE]

That was supposed to say "...STRONG resonances in the instrument."

"String resonances" would certainly confuse the issue, now, wouldn't it?

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

John,
I do seem to “hear” many of the nuances and quirks that exist in a typical acoustic build. It would make sense that most don’t seem to hear these things, especially since hardly anyone responds when ever I have asked about the topic. Maybe most folks are not bothered by it.

John microsmerf, I had heard “tuning” your box in between notes could help too.
The guitars that seem to have less an issue have “spread out” the wolf over a few notes rather than hitting on one particular note. The difficulty is predicting what your build is going to settle into in terms of the relationship between the main air, top and back resonances once it is finished and broken in. With all the variables that happen throughout a build, it is tough to tame the wolf. I think it is a good thing to be aware of though, one of the many things that make this craft so interesting!
Thanks for letting this topic drift a bit. Although, it is all related!
Wade

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Wade
Nashua, NH
http://www.wadefx.com

I think it's all just a matter of balance. Any responsive guitar will be likely to have a wolf, so you have to balance the desire for responsiveness with the desire for an even tone.

In general, the more there is going on in the guitar the less noticable little problems are. Lack of sustain in the fundamental of a note won't hurt as much if there are lots of higher harmonics with good sustain to cover up for it. The timbre of the note changes, but it's still there.

Adding some weight to the bridge might help, by slowing down the 'leakage' of energy from the string to the soundboard. you get less attack and peak volume, but a more even and less 'bright' sound.

Yup, strong resonances in the top can alter string pitches. They can also alter the pitches of only selected overtones, which can give you a whole 'nother sort of 'wolf'. On one guitar I built, a strong resonance around A=880 interacted with the B string to produce a buzz on the D note, third fret, that sounded just like fret buzz. The trick was, it stayed at that pitch, so shifting the tuning of the string altered the frett effected. It took almostr four hours to figure out what the cause was, and then only about a half hour to fix it, by shaving the upper ends of the tone bars just a little.