Is the Air Resonance Frequency of a guitar of any significance to you as a player?

[M19]

Originally Posted by Bob Womack

Speaking of which, what is the airspeed velocity of a laden African swallow?

You need to log into that other AGF.








Avian Global Forum.

[mcduffnw]

Originally Posted by TRose

..."I’m just on the steep part of the learning curve when it comes to guitars and enjoying lots of rabbit holes."

Hi TRose

Be more than a wee bit wary of the "rabbit holes"...

Many a good and fine man has gone down said rabbit holes "in search of"...and gotten lost in the Warren, and never found their way back out...and their pure and basic love and enjoyment of playing the guitar and the wonderful music that they can make with it has suffered irrevocably.


duff
Be A Player...Not A Polisher

[cdkrugjr]

It's very important to the fundamental design of the guitar that we not create a powerful resonator at a string pitch. Otherwise that string will be Much louder than any other, what's informally called a "wolf note."

A bow imparts Much more energy to a string than does even your most heavy-handed bluegrass buddy. Bowed players talk about this a TON.

Tap on the back of the guitar with your thumb. You'll hear a tone Usually somewhere between the 6th and 5th strings, and if you poke around on the fretboard you'll probably find that it's between two notes when you're tuned Standard.

That's intentional.

[mcduffnw]

Originally Posted by Michael Watts

it matters to me to know what the top, sides and back have been tuned to and what frequencies the instrument is most sensitive to.

Ahhh but Michael...

You are in the 1% of guitarists who has the innate God given and blessed talent, and hard earned and won skills, to make such knowledge actually useful to your playing.

But, for the vast majority of we "Weekend Hackers"...love and miss you Bud Collins RIP...ruminating on such eclectic information can leave us concerned, "dazed and confused"...needlessly.

Besides, if you were not told the ARF specs by a builder of a guitar, You most certainly and easily could figure it out on your own, with your hands, your ears, your head and your heart {;-)


duff
Be A Player...Not A Polisher

[musicman1951]

First time I've read about ARF. It may matter a great deal to me, but certainly not as a number. I have no interest/trust in such a number when selecting instruments.

I play them and listen. They work for me or they don't. I find science very interesting and am grateful for any knowledge gained along the way, but I'll trust my ears for selecting instruments.

[TRose]

Thanks to all for your thoughts and the information.

Perhaps the spec could be useful even to a weekend hack, couch player if they were buying a guitar on line. But, only actually playing the instrument will always be the gold standard to liking/ bonding with the guitar. Specs only help us narrow down our options.

As a kid I’ve blown on soda pop bottles to make “music” but I had never heard it called the Helmholz frequency.

It was this exhibition at the MET that prompted me to connect some dots and then enter the rabbit hole:

https://www.metmuseum.org/exhibition...ssel-orchestra

I solemnly swear to never allow rabbit holes and like distractions to limit my love for and appreciation of the guitar. I promise to continue practicing daily. I’m a hack. But I’ll stick with it.

Thanks,
Tom

[redir]

It's important on responsive guitars particularly as was mentioned in the Kent description becasue you can get some bad wolf notes on the main air pitch. The body shape and size and the stiffness of the top/back/sides is what determines the main air pitch, I would disagree for the most part on wood species but it might play a small role. Dreds can be as low as E while smaller guitars tend to be around G. Classical guitars are often around A.

I built one dred that is spot on E and I liked it so much I left it that way. Many luthiers today use some sort of method to tune the main air pitch. Ideally you would either want the main air pitch to be a note that is not common to many of the more popular scales like say an Db or something. But even better then that would be on a pitch that is some cents off a less common note. That way you never activate the main air resonance when playing in any key.

So if you built a guitar that resonated right on A you could change that pitch in a few ways. You can add mass to the top near the bridge, shave the back braces in the lower bout, add mass to the sides, or make the soundhole larger.

A flemenco guitarist might love a guitar that resonates on A so it's not necessarily a bad thing either. Hawaiian slack key players used to find the main air pitch and then tune the guitar to that.

[TRose]

Originally Posted by redir

It's important on responsive guitars particularly as was mentioned in the Kent description becasue you can get some bad wolf notes on the main air pitch. The body shape and size and the stiffness of the top/back/sides is what determines the main air pitch, I would disagree for the most part on wood species but it might play a small role. Dreds can be as low as E while smaller guitars tend to be around G. Classical guitars are often around A.

I built one dred that is spot on E and I liked it so much I left it that way. Many luthiers today use some sort of method to tune the main air pitch. Ideally you would either want the main air pitch to be a note that is not common to many of the more popular scales like say an Db or something. But even better then that would be on a pitch that is some cents off a less common note. That way you never activate the main air resonance when playing in any key.

So if you built a guitar that resonated right on A you could change that pitch in a few ways. You can add mass to the top near the bridge, shave the back braces in the lower bout, add mass to the sides, or make the soundhole larger.

A flemenco guitarist might love a guitar that resonates on A so it's not necessarily a bad thing either. Hawaiian slack key players used to find the main air pitch and then tune the guitar to that.

Wow.
That’s really interesting.
Thanks.

[mercy]

I agree with Redir on all but one point. I question how many builders, and of course factories, tune their guitars. The jug tone is a factor of how the size and materials come out.

[printer2]

Originally Posted by mcduffnw

Ahhh but Michael...

You are in the 1% of guitarists who has the innate God given and blessed talent, and hard earned and won skills, to make such knowledge actually useful to your playing.

I thought the same thing but I would have moved the decimal place over a point or two.

Originally Posted by mercy

I agree with Redir on all but one point. I question how many builders, and of course factories, tune their guitars. The jug tone is a factor of how the size and materials come out.

There is a lot more to the story. I didn't want to get into it too much last night, just going to add a point or two today. For players like Michael knowing the resonance frequencies can be useful for the case of when they impede what he wants to do with the guitar. But he has a greater likelihood of running his fingers over a racehorse than the rest of us.

While every guitar will have a top and hole resonance, some will not have a back resonance, or two back resonances. Major resonances that is, also the back has two, one for the upper and one for the lower bout. The other part of the 'equation' is not just the frequencies but the 'Q' of the resonance, or the narrowness of the resonance and the height or strength of the resonance. The back resonance can be reduced by wood thickness and bracing, to give a top dominant sound field, and less variability in sound between having the back free to vibrate or up against one's belly. On the Q of the resonances, the greater the height the more efficient the guitar is at that frequency, the more likely it will 'drain' energy from the string producing sound. If on a note's frequency the note is louder but shorter in length. Not a problem for plywood guitars, more a problem on higher quality guitars. Plywood guitars (or ones built heavier, many factory guitars) have lower quality resonances, they are broader in nature and do not make themselves a problem. But for their mild manners you also get less output. In a factory this is a plus as the variability in the wood is not going to move the resonances on scale tones.

[Alan Carruth]

In it's low range the guitar acts like a 'bass reflex' speaker cabinet, of sorts. A good bass reflex system will have the 'Helmholtz' (rum jug) air resonance tuned to be at the same frequency as the resonance of the cone of the speaker in 'free' air. Either resonance by itself would produce an output peak in the sound at a certain pitch. When you put the two together the air moving in and out of the port in the speaker cabinet produces pressure changes that push on the speaker, and the speaker, of course, pumps air in and out of the hole. The two tings working together produce a system that puts out sound with two peak frequencies, above and below the pitches that the two elements were 'tuned' to originally. In a speaker cabinet the cone itself has very high losses, and they build a lot of padding into the box (which is otherwise rigid) to increase the losses there. This results in a setup where the two output peaks are low and broad. Ideally the output never varies more than 3 Db above or below a certain level, which to your ears is just barely noticeable as a change in 'loudness'. Thus the system has an effectively 'flat' response over a broad range of frequencies, and fall off only slowly as you go up from there. The response is 'flatter' than a simple speaker in a box (an 'infinite baffle'), and since it captures some energy off the back of the speaker it's also a bit more efficient, despite all the built-in damping.

The objective on the guitar is not to have a 'flat' response, but to make it more effective at producing sound in the low range. The top itself, seen as a speaker cone, is not big enough work well in the range of the lowest notes, and, at any rate, needs to be stiffer than you'd want it to make those low tones for structural reasons. A top mounted on a rim that has no back, but with the back edge stiffened up, will have a 'main' resonance at around 165 Hz, say E or so on the D string. A 'rigid' guitar shaped box with the hole open will have it's Helmholtz air resonance at around C or so on the low A string. When they work together on a guitar in playable shape the two output peaks of the combined 'bass reflex couple' go to around G on the low E for the 'air', and the open G pitch for the 'top'. The top and air working together can pump enough air through the sound hole to make a usable amount of noise.
There's a lot of leeway in where those end up, of course.

As has been said, when either of these resonances ends up too close to a played note you can get a 'wolf'. The top moves so much at both pitches that it can suck the energy out of the string in a hurry, and turn it into sound, so you get a note that's twice as powerful for half as long. You don't notice the extra power so much' your ears aren't set up that way, but you sure notice the lack of sustain.

This is exactly the same mechanism that causes the famous 'wolf tone' on 'cellos, but in that case the steady input of energy from the bow causes the whole system to go haywire, and produce a variety of 'growling' or 'warbling' tones. It sometimes does happen that the 'top' resonant pitch, say around the open G string, can 'flutter' or 'warble' a bit, but you often don't notice it.

Generally speaking, the more 'responsive' a guitar is, the more likely it is that one or another of it's resonances will become a problem, and be labeled as a 'wolf'. Lightly built guitars may need to be 'tweaked' a bit to move resonances off the pitches of played notes to avoid this. It's also possible to shift some of these peaks a bit to adjust the overall sound.

The basic timbre of the guitar is closely tied to the location of these 'air' and 'top' resonances. In general, the lower the 'air' pitch in particular the more 'bass balanced' and 'full' the sound of the guitar, while placing it high in pitch tends to give what I think of as a more 'forward' or 'open' sound. As has been said, there are lot of things in the construction of the guitar that influence the pitches and activity of those two resonances, so there are several ways to make these adjustments. Often, for example, shaving back braces can be a very effective way to lower the 'air' pitch without weakening the top.

[TRose]

Thank you all for the information!

[AgentKooper]

This is all pretty fascinating, and it alters to some degree what my idea of a guitar is. How does a factory operation ensure that production guitars are properly "tuned" to the extent they avoid having the same air resonance pitch as a string and causing the problems described above?

Or can they not ensure it, and does that explain why some guitars are perceived as duds?

[TBman]

Originally Posted by Bob Womack

Speaking of which, what is the airspeed velocity of a laden African swallow?

What? African or European?

[redir]

Originally Posted by AgentKooper

This is all pretty fascinating, and it alters to some degree what my idea of a guitar is. How does a factory operation ensure that production guitars are properly "tuned" to the extent they avoid having the same air resonance pitch as a string and causing the problems described above?

Or can they not ensure it, and does that explain why some guitars are perceived as duds?

They don't and generally it's not a problem in factory guitars, even good quality ones. The irony there being that you might have more problems from a very talented small shop luthier guitar where the guitar maker strives to build the most responsive of guitars and probably succeeded in doing so. But it can be fixed too.

I use deflection and acoustic testing to try and build responsive guitars typically with live backs. It's almost like I'm trying to build a guitar that is susceptible to wolf notes. I always err on the side of too stiff, that way I can fix the problem if it arises and so far it never really has. I mentioned before that I built a dred that resonates on E and it is definitelya wolf but I like it. I also have a flamenco guitar that is right on A and I like that too. When you play the E or A respectively on each guitar it's strident.

Factories don't have the luxury of specializing each guitar for a specific client but what they do have is thousands of iterations of an assembly line product, and often times decades of production, which helps them narrow down their process to make something really good for a more reasonable price.