Top Frequency

[Dryfly]

While tuning up with my Polytune and muting the strings while it settled down I accidentally tapped the top and it registered as an A. I repeated the process on areas all around the top with the same result. Curious I tried the same exercise on my other guitar. Also registered as an A.
My question is whether this is a result of intentional design of is it simply nature and physics. It seems fascinating that it's an A at 440.

[zmf]

Originally Posted by Dryfly

My question is whether this is a result of intentional design of is it simply nature and physics.

Are you talking about guitars or the tuner?

[Dryfly]

Both guitar tops with the strings muted registered as an A.

[freaktone]

Very cool! Typical guitars have a resonant frequency of between F# and A evidently.

[mc1]

The guitar I picked up here does not have a strong tap tone, but seemed to register between C# and D, but not right on either. Since the tone didn't register strongly, I consider this guitars tap tone inconclusive.

[freaktone]

Do you guys think it's the tone of the top, or the resonant frequency of the guitar body? I'm not sure.

[Alan Carruth]

Most likely the resonance that your tuner was picking up was the Helmholtz-type 'main air' mode, which can come in around 110 Hz, although it's often lower. The 'main top' resonance, where the lower bout is moving like a loudspeaker, can also be around A, an octave higher, although, again, it's more likely to come in around G. There can be a top resonance up around 440 Hz, but it's normally not strong enough or sustained enough for a tuner to pick up, nor would it be driven strongly everywhere on the top.

The pitch and strength of these low range resonances has a lot to do with the 'character' of the guitar's sound, so it's not a coincidence that they're similar on two different guitars.

[Dryfly]

Originally Posted by Alan Carruth

Most likely the resonance that your tuner was picking up was the Helmholtz-type 'main air' mode, which can come in around 110 Hz, although it's often lower. The 'main top' resonance, where the lower bout is moving like a loudspeaker, can also be around A, an octave higher, although, again, it's more likely to come in around G. There can be a top resonance up around 440 Hz, but it's normally not strong enough or sustained enough for a tuner to pick up, nor would it be driven strongly everywhere on the top.

The pitch and strength of these low range resonances has a lot to do with the 'character' of the guitar's sound, so it's not a coincidence that they're similar on two different guitars.

So I'm still curious as to whether these frequencies occur naturally due to the physics of the body or do builders strive or make an effort to obtain them to influence the "character" of the instruments sound.
Or maybe they don't matter anyhow.

[Alan Carruth]

Dryfly asked:
"So I'm still curious as to whether these frequencies occur naturally due to the physics of the body or do builders strive or make an effort to obtain them to influence the "character" of the instruments sound."

Yes......

Any time you make a box with a hole in it there will be a 'Helmholtz' type air resonance. If the walls of the box are at all flexible they will also vibrate at some pitch or other, and the 'air' and 'wall' vibrations can 'couple'; influence each other in pitch and activity. In this respect the guitar, in it's lower frequency range (say, up to about the open G string pitch or even as high as B) acts a lot like a 'bass reflex' speaker cabinet.

With a speaker setup the design is worked out to produce a more or less 'flat' frequency response curve, so as not to distort the sound of the music being played through it any more than necessary. The box itself is made of stiff and heavy wood so that the vibrations of the box don't color the tone. The speaker itself is highly damped: it won't 'ring' if you tap it, and ther eis usually a lot of something like acoustic foam in the box to kill air resonances, again, to reduce any 'color' they might impart to the sound. The damping makes the efficiency of these is very low, but it doesn't much matter, since you can usually just crank up the amp a few notches.

Guitars are, if anything, designed to maximize 'tone color'. A guitar with the sort of 'flat' frequency response of a good loudspeaker would be boring. You could approximate that by putting extra light strings on a Les Paul, moving the pickup as far as possible from the strings to get as close to a 'linear' response as you can, and playing it at a very low setting through the 'flattest' amp you can find, with no effects. There's a reason they don't do that on recordings...

Guitars also are pretty close to the most efficient of instruments, in terms of turning the driving energy you put in into sound. There's not much power in a plucked string, so you have to make the most of it. One way they do this is to reduce the damping, which means that the various parts of the guitar can 'ring' when you tap them, and have a more or less defined pitch, as you have heard on your guitar. This inexorably produces 'tone color'.

So, on the one hand, anything you make that more or less resembles a guitar will share some of the same kinds of sound. On the other hand, a flat top guitar has a different character of sound from an arch top jazz guitar, or an Irish bouzouki, both of which are similar in size and tuning range. Things like the body size and shape, hole position and shape, and so on, all affect the timbre.

Over time various guitar designs have evolved to fill different functions. You would not normally use, say, a Flamenco guitar as a Bluegrass backup instrument, or vice versa. The various designs have been adapted to produce the desired sound. Makers will more or less stick to proven designs because that's the most reliable way to ensure that you'll get the sound you want.

In the end, then, there are some aspects of the sound that help define the tone of a guitar, as opposed, say, to a mandolin or banjo. These are the outcome of certain common design features, and tend to be 'strongly conserved'. It's probably not an accident that most guitars tend to have a similar body outline, for example': although the effect that has on the tone is subtle it is one of the things that defines the guitar, and you can't stray too far from that and still make a guitar. Much about string lengths and materials is determined by physics, and that sets limits on the range of variations: you can't break the laws of physics. From there you can shift things around within limits to produce the type of sound a particular style of music, or player, might want. Since much of that has already been worked out, there are 'standard' designs to produce the desired results, more or less.

[mercy]

Hum into your guitar's sound hole different pitches and when your guitar comes alive you will find the Hrtz of your guitar. Mine is C on the A string

[Alan Carruth]

C on the A string would be ~131 Hz, which is quite high for the 'main air' resonant pitch. That's about where I'd expect it on an arch top, or a small bodied flat top like a Martin 0 or 1.

You have to be careful when humming into the sound hole that you don't get your face too close. That partially blocks the hole and changes the pitch. Try to keep at least a sound hole diameter back, and preferably two.

[MartinGibsonFan]

Totally confused.

How can you register a C note on the A string

I'm assuming this ' humming ' is done with no strings being fretted

Totally confused here

I can understand an F# to A frequency with open strings

Since a G(six) is defined as a chord unfretted in standard tuning for all 6 strings.

J

[MartinGibsonFan]

I guess my recent post in this thread has crossed from another thread about humming into a guitar

The two topics are related, and I still don't understand how a C note can be observed on the A string without some fretting involved.

And if you are talking about the resonance of the TOP, I'm assuming that no fretting should be involved to determine top resonance?

Am I missing something here?

J

[redir]

If you play the 3rd fret on the A string you have a C

Luthiers tend to strive to get the resonant frequency of the guitar body just off of the main notes in at least the most common scales. I have a Dred that resonates on E and it sounds amazing for blues in E but a bit annoying when it's not wanted. My flamenco guitar is right on A which makes anything in A very strident.

Hawaiian slack key players were known to tune their instruments to take advantage of this phenomenon.

[MartinGibsonFan]

Originally Posted by redir

If you play the 3rd fret on the A string you have a C

Yes, I know that much

But still not sure of the top frequency, the top frequency would be the sum of all the strings being played in the open position.

Correct?

J