Acoustic Guitar Resonance Problem

[jmcelroy]

Hi all,

I'm recording my acoustic guitar (Guild GAD-30r) in my (fairly low-budget) home studio. One thing I noticed in the process is that the guitar has a significant resonance centered around A. I'm haven't figured out which octave, but it's relatively high. If if I play a chord and stop suddenly the resonance is quite pronounced and lingers for a substantial amount of time. This resonance comes from within the guitar itself, and is not an artifact of the room in which it is played.

While I appreciate that the guitar resonates so well, the fact that it is centered at an obvious pitch becomes quite annoying when I record. Does anyone have any useful tips for minimizing the effect of this resonance? In order to scope the term "useful" I would offer "get a different guitar" as one that's probably not so useful to me.

Thanks for reading!

--John

[mixsit]

It rings long enough or loud enough to come off as a flaw in the character of the instrument? As in more than a fraction of a second when you strike and mute the strings? Odd.
Higher than open A?

[jmcelroy]

Originally Posted by mixsit

It rings long enough or loud enough to come off as a flaw in the character of the instrument? As in more than a fraction of a second when you strike and mute the strings? Odd.
Higher than open A?

Yes. If I strike my strings with a pick while my left hand is completely muting them the resulting resonance is quite evident for 4 seconds or more. The peak of the resonance is either bimodal at G5 and A5 or there's one mode and it's broad enough to pick up both G and A. Perhaps my ears aren't good enough to distinguish between the two possibilities. If you would like I could try to get a sound clip up somewhere.

[mixsit]

If it was a single low pitch point, maybe a narrow eq notch, but four seconds? Wow.
Since it can't be a string, can you mute it by dampening different spots around on the body?

[jmcelroy]

Originally Posted by mixsit

If it was a single low pitch point, maybe a narrow eq notch, but four seconds? Wow.
Since it can't be a string, can you mute it by dampening different spots around on the body?

Yes!!! Yes I can!!! And it was so OBVIOUS! I was muting the strings below the nut, but the strings were still ringing above it. If I wedge a cloth underneath the strings up by the tuning pegs the problem is completely solved. I was thrown off by the fact that the guitar body was amplifying the resonance (hence the term "resonance chamber" ) and so I didn't realize where it was actually coming from.

Mixsit, you just totally saved my weekend. I would have have had a little storm cloud over my head all day before I thought of that. What a great and wonderfully level-headed suggestion!

[mixsit]

Trouble shooting: Define, isolate, elliminate.

Cool. That is indeed a fine thing to have happen.

Next?

[jmcelroy]

Originally Posted by mixsit

Trouble shooting: Define, isolate, elliminate.

Yes, yes. I troubleshoot for a living (computer engineer and research scientist) but it's sooooo much harder when your emotions get in the way, no? That's why surgeons don't operate on their loved ones! I lost my cool!!

Originally Posted by mixsit

Cool. That is indeed a fine thing to have happen.
D

Indeed. Thanks for taking the time to help out!

[DrewPeterson7]

Originally Posted by mixsit

Trouble shooting: Define, isolate, elliminate.

Bingo.

FWIW, a narrow notch centered at the right frequency (hint, boost first until you make it REALLY jump out, then flip it down to a cut) should have worked anyway, but it's obviously a less-than-ideal fix.

[ggunn]

Originally Posted by DrewPeterson7

Bingo.

FWIW, a narrow notch centered at the right frequency (hint, boost first until you make it REALLY jump out, then flip it down to a cut) should have worked anyway, but it's obviously a less-than-ideal fix.

That would have also notched what he was trying to record, which he probably wouldn't want to do. I encountered the same thing when I was recording a friend's 12 string. I made a mute out of a wine cork cut in half lengthwise, a strip of foam, and a bit of duct tape. I put it across the strings just north of the nut. Problem solved.

[mixsit]

Originally Posted by ggunn

That would have also notched what he was trying to record, which he probably wouldn't want to do. I encountered the same thing when I was recording a friend's 12 string. I made a mute out of a wine cork cut in half lengthwise, a strip of foam, and a bit of duct tape. I put it across the strings just north of the nut. Problem solved.

Right. And this is why/where 'eq falls. It's good for diminishing a tone (within a reasonable amount) -but not so much when the problem extends into the time domain.

[Richard Monroe]

OK, you now have a finger in the dike. If I were you, the next thing I would do is send a private message to "Light". a frequent poster on this board, who appears to be a badass luthier, and ask him what the correct permanent fix is. I'd be interested in his answer.-Richie

[jmcelroy]

Originally Posted by ggunn

I made a mute out of a wine cork cut in half lengthwise, a strip of foam, and a bit of duct tape. I put it across the strings just north of the nut. Problem solved.

Man, that's some Macgyver-style DIY stuff right there. I'm impressed. For right now I've just snuggled a micro-fiber guitar cloth under the strings and it seems to work well enough. Still, your approach is much cooler without a doubt.

[jmcelroy]

Originally Posted by Richard Monroe

OK, you now have a finger in the dike. If I were you, the next thing I would do is send a private message to "Light". a frequent poster on this board, who appears to be a badass luthier, and ask him what the correct permanent fix is. I'd be interested in his answer.-Richie

Consider it done. I'll point him over here so that he can leave a public response if he would like. Thanks for the tip!

[muttley600]

It sounds like what you have come up against in purely musical acoustic terms is the enclosure resonance occurring at a given frequency. Nearly every object has a frequency at which it is excited and naturally wants to resonate. In you case it sits around A440 and is the body of air inside the guitar. You don't want the natural resonance of the top or back at the same frequency. The body frequency has been described variously as the A, Ao, or C1 C2 c3 resonance. etc. That would be the most common cause but it may less commonly be the neck material having a fundamental resonant frequency at around 440. If so what I'm about to say does not apply.

Essentially it's the Helmholtz resonance and is distinct from the other acoustic resonances going on in the materials themselves and is directly related to the size and type of enclosure. It is quite easy to calculate where the Helmholtz resonance is in theory for any enclosure. Blowing over the top of a bottle to produce a note is what we are talking here.

However when you introduce other complications such as the shape of the enclosure, the size of the opening ( in this case soundhole size) and materials it is made of things get messy and you are unlucky if it falls bang on a common or fundamental frequency. It does happen.

The Helmholtz resonance also acts like any other harmonic series in that it has other related frequencies that are independent but relate to the fundamental frequency. These are the Ao or c1 ,c2, c3, modes etc. They are very important in speaker enclosures as well and some one with experience of those may chime in with some words on that one. They are also important to the projection and tone of any acoustic instrument and have been well researched and documented.

Enough of the physics. To rectify the problem you need to change one of the properties that is driving the Helmholtz resonance. They are the volume of the enclosure, the size of the opening or the mass and stiffness of the materials. The other thing you can do is tune down to take a440 out of the equation.

You can alter the size of the enclosure by placing an inflated balloon in the body or a sponge (yes it can work), you can alter the size of the soundhole (not practical) or you can alter the mass/stiffness of the body material (add mass or damp the vibration by adding a cloth under the strings such as you have done for example, but that can also take away much of the response of the instrument. In your case it may not be a problem this is just aheads up for others who may come across the same problem.

If the problem is mostly present when recording, try the balloon or sponge trick as the same thing will work if you have issues with live performance when miced or amplified. The only real way to deal permanently with it is to add or subtract mass from the top or back by shaving the braces.

Experiment with ways to damp or add mass to the body is the best way to deal with your problem in the short term. The permanent fix is to alter the frequency of the top or back which coincides with the body resonance.

[ggunn]

Originally Posted by muttley600

It sounds like what you have come up against in purely musical acoustic terms is the enclosure resonance occurring at a given frequency. Nearly every object has a frequency at which it is excited and naturally wants to resonate. In you case it sits around A440 and is the body of air inside the guitar. You don't want the natural resonance of the top or back at the same frequency. The body frequency has been described variously as the A, Ao, or C1 C2 c3 resonance. etc. That would be the most common cause but it may less commonly be the neck material having a fundamental resonant frequency at around 440. If so what I'm about to say does not apply.

Essentially it's the Helmholtz resonance and is distinct from the other acoustic resonances going on in the materials themselves and is directly related to the size and type of enclosure. It is quite easy to calculate where the Helmholtz resonance is in theory for any enclosure. Blowing over the top of a bottle to produce a note is what we are talking here.

However when you introduce other complications such as the shape of the enclosure, the size of the opening ( in this case soundhole size) and materials it is made of things get messy and you are unlucky if it falls bang on a common or fundamental frequency. It does happen.

The Helmholtz resonance also acts like any other harmonic series in that it has other related frequencies that are independent but relate to the fundamental frequency. These are the Ao or c1 ,c2, c3, modes etc. They are very important in speaker enclosures as well and some one with experience of those may chime in with some words on that one. They are also important to the projection and tone of any acoustic instrument and have been well researched and documented.

Enough of the physics. To rectify the problem you need to change one of the properties that is driving the Helmholtz resonance. They are the volume of the enclosure, the size of the opening or the mass and stiffness of the materials. The other thing you can do is tune down to take a440 out of the equation.

You can alter the size of the enclosure by placing an inflated balloon in the body or a sponge (yes it can work), you can alter the size of the soundhole (not practical) or you can alter the mass/stiffness of the body material (add mass or damp the vibration by adding a cloth under the strings such as you have done for example, but that can also take away much of the response of the instrument. In your case it may not be a problem this is just aheads up for others who may come across the same problem.

If the problem is mostly present when recording, try the balloon or sponge trick as the same thing will work if you have issues with live performance when miced or amplified. The only real way to deal permanently with it is to add or subtract mass from the top or back by shaving the braces.

Experiment with ways to damp or add mass to the body is the best way to deal with your problem in the short term. The permanent fix is to alter the frequency of the top or back which coincides with the body resonance.

Very eloquent, but his problem was the ringing of the strings between the nut and the tuners.

[muttley600]

Originally Posted by ggunn

Very eloquent, but his problem was the ringing of the strings between the nut and the tuners.

I guess I fell for this bit

This resonance comes from within the guitar itself,

If it's its the strings behind the nut why has no one said get a new nut cut. Thread finished. Damn my speed reading.

[ggunn]

Originally Posted by muttley600

I guess I fell for this bit



If it's its the strings behind the nut why has no one said get a new nut cut. Thread finished. Damn my speed reading.

The OP first said that the ringing was coming from the body of the guitar, but later found he was mistaken. As we've discussed in other threads, since sound travels in solids much more efficiently than in air, sometimes rattles, ringing, etc. can appear to the ear to come from someplace far removed from the actual source. I can remember chasing a rattle around on my Strat for a while which sounded like it was coming from under the pickguard. It turned out to be that the knob at the end of my tremelo bar had come partly unscrewed. Luckily, I figured it out before I dismantled the guitar.

[muttley600]

Originally Posted by ggunn

The OP first said that the ringing was coming from the body of the guitar, but later found he was mistaken. As we've discussed in other threads, since sound travels in solids much more efficiently than in air, sometimes rattles, ringing, etc. can appear to the ear to come from someplace far removed from the actual source. I can remember chasing a rattle around on my Strat for a while which sounded like it was coming from under the pickguard. It turned out to be that the knob at the end of my tremelo bar had come partly unscrewed. Luckily, I figured it out before I dismantled the guitar.

Yeh thats the speed reading thing and my current stress levels. But the first thing you do with those elusive rattles is hit the note and grab everything you can reach with your other hand til you nail it. I have chased rattles all over guitars for hours before.

Sound doesn't travel through all solids better than air by the way, in many it travels very inefficiently. Its that mass/stiffness thing again.

[Richard Monroe]

Yo Muttley! I am reminded of an old story. A bunch of guys were in a chartered helicopter over LA, and their GPS navigation system went out. They needed to find LA international airport, and had no clue. The pilot said, "don't worry, I have an idea." He hovered next to a skyscraper and saw some guys working on computers. He opened a window, and indicated to the workers to do the same. When they did, he yelled out, "Where the hell are we?" One of the guys yelled back, "You're 387.2 feet in the air in a Sikorsky S-76 helicopter, hovering over the West Coast of The United States of America, city of Los Angeles!" The pilot yelled back, "Thanks, that's all I needed to know!" Within minutes, the helicopter set down at it's assigned pad at LA-X. As they disembarked, one of the passengers asked the pilot,"All that guy told you was that you were in a helicopter over LA. How the hell did you find the airport?" The pilot replied, "They gave me an answer that was entirely true, contained a bunch of detailed information I didn't need, and was completely useless. So I knew they could only work for- Microsoft tech support. They're at 333 South Grand Ave. in East LA. The rest was easy."

Do you work for Microsoft?-Richie

[muttley600]

Originally Posted by Richard Monroe

Yo Muttley! I am reminded of an old story. A bunch of guys were in a chartered helicopter over LA, and their GPS navigation system went out. They needed to find LA international airport, and had no clue. The pilot said, "don't worry, I have an idea." He hovered next to a skyscraper and saw some guys working on computers. He opened a window, and indicated to the workers to do the same. When they did, he yelled out, "Where the hell are we?" One of the guys yelled back, "You're 387.2 feet in the air in a Sikorsky S-76 helicopter, hovering over the West Coast of The United States of America, city of Los Angeles!" The pilot yelled back, "Thanks, that's all I needed to know!" Within minutes, the helicopter set down at it's assigned pad at LA-X. As they disembarked, one of the passengers asked the pilot,"All that guy told you was that you were in a helicopter over LA. How the hell did you find the airport?" The pilot replied, "They gave me an answer that was entirely true, contained a bunch of detailed information I didn't need, and was completely useless. So I knew they could only work for- Microsoft tech support. They're at 333 South Grand Ave. in East LA. The rest was easy."

Do you work for Microsoft?-Richie

Cute thing is those guys are all earning more than the helicopter pilot but they left that bit out.

[ggunn]

Originally Posted by muttley600

Yeh thats the speed reading thing and my current stress levels. But the first thing you do with those elusive rattles is hit the note and grab everything you can reach with your other hand til you nail it. I have chased rattles all over guitars for hours before.

Sound doesn't travel through all solids better than air by the way, in many it travels very inefficiently. Its that mass/stiffness thing again.

Inefficiently by relative standards, I guess, but sound velocity is proportional to mass. Water isn't very stiff, but sound travels faster/better in water than in air.

[Richard Monroe]

Well, Ggun, water may not be that stiff, but 32 feet of sea water or 33 feet of fresh water is equal to one atmosphere of pressure, which is 26 miles or so of air. That makes water many, many, more times dense than air.-Richie

[jmcelroy]

Originally Posted by muttley600

It sounds like what you have come up against in purely musical acoustic terms...

That, Muttley, was a beat down response if I've ever seen one. One day somebody is going to stumble on this thread and be really grateful to you!

[muttley600]

Originally Posted by ggunn

Inefficiently by relative standards, I guess, but sound velocity is proportional to mass. Water isn't very stiff, but sound travels faster/better in water than in air.

It's proportional to mass and stiffness. Take putty for example not very good at transmitting sound at all but very good and stopping it. With woods the speed of sound varies a lot from type to type and piece to piece. Many solids have high mass and low stiffness many barely transmit sound at all. The speed of sound in gasses is also different depending on several factors relating to mass stiffness, pressure etc, but you know this,

[Light]

Personally, I've got no problem with sticking a little piece of felt behind the nut. Not, perhaps, the most elegant solution, but it works, and save you a bit of money. We do that fairly frequently with some archtops (particularly old Epiphones with the Frequencator tailpiece. A new nut would probably also do the trick, but if it is working right otherwise, why?


Light

"Cowards can never be moral."
M.K. Gandhi