Feedback stupid idea

[Cuki79]

Hi Luthier friends,

Since I've been experimenting with Impulse response convolution, I am pretty sure one can "rebuild" the low-end of a guitar pickup signal by this method. My LR Baggs Lyric does not have any low-end... and the convolved sound has plenty. In this case, the pickup signal basically trigger a digital air resonance and a digital fundamental mode.

As a result, I ask if it would be possible to design a guitar with one or many specific side soundholes to prevent feedback:
1) The first idea is to detune or kill the "air" resonance
2) The second idea is to enable sound pressure from the amplifier / P.A. / Monitor to cancel at both side of the guitar top.
Here is an illustration I made from a picture I borrowed from the internet (probably Breedlove)

The blue arrows represent air pressure due to the P.A. If the air flowing through the differents soundholes apply the same pressure on the top (inside the box), than the air pressure outside, they should cancel each other don't they?

At 100 Hz for example, the wavelength is 3.4m, so both side will experience the same pressure. Of course it won't work at 5KHz.

Do you think this idea would work?

The IR could be done on the guitar with the side soundhols closed, so the LIVE sound could match the unplugged one.

Cuki

[B. Howard]

If a single side port doesn't relieve enough sound pressure to even really reduce feedback I doubt cutting more will be beneficial.

[Ned Milburn]

If an acoustic guitar works as it should, it WILL feedback at times when amplified. Electric guitars don't have a vibrating diaphragm (soundboard), and they feedback.

[Cuki79]

Originally Posted by Ned Milburn

If an acoustic guitar works as it should, it WILL feedback at times when amplified. Electric guitars don't have a vibrating diaphragm (soundboard), and they feedback.

That's definitely a point. In the case of electric guitar, suposing the response of the pickup is flat, the oscillation related to the time-travel of the signal to make a whole loop gives a feedback frequency even without sounding board.

Does it mean that acoustic guitar don't actually feedback on their soundingboard resonances? I thought it was the case...

Maybe the only thing you can hope is to reach the case of electric guitar presumed "flat response" and only gain few dBs headroom before feedback.

As far as the number of holes. I though the soundhole created an "air resonance" corresponding of the mass of air inside the box and theelastic properties of this air mass (following the principle of the Bass reflex). So I thought, adding holes to open the structure eventually would kill that resonance. I mean if there is no box, there is no air resonance isn't it?

A nice test would be to make a guitar with no box (that is to say an electric guitar that is very light), would it feedback less than an acoustic with its box?

Again I aknowledge my question may be totally stupid...

[emmsone]

Hi Cuki
Eric Weigeshoff of Skytop Guitars (i don't think he's a member of this forum though) makes guitars with huge soundholes on the sides, usually forgoing a soundhole in the soundboard, as he's one of the few people i've seen doing it this way round, perhaps he might be a good person to contact about your idea.

David

[Cuki79]

Thanks

Just wrote an email to ask. However from the two previous posts from professional luthiers I don't think there is hope

Cuki

[B. Howard]

Originally Posted by emmsone

Hi Cuki
Eric Weigeshoff of Skytop Guitars (i don't think he's a member of this forum though) makes guitars with huge soundholes on the sides, usually forgoing a soundhole in the soundboard, as he's one of the few people i've seen doing it this way round, perhaps he might be a good person to contact about your idea.

David

Eric is a client of mine, I do his finish work. We have discussed his trend towards side hole only builds. Mainly it is about how the instrument sounds to the player. He still gets good projection and toned but I have never heard him mention anything about feedback resistance.I will be speaking to him a little later and will get his thoughts on the matter and pass them along since I do not believe he is a member here either.

[redir]

Intuitively it seems to make sense. I'm not sure in the way you describe though, that pressure entering both spots will cancel out. But a large sound hole on the side does seem like it would relieve the top when it starts to go wild into feedback. I could be wrong but feedback is caused when the amp sound activates the strings which then drive the top. That's why you still get feedback on a solid body guitar. Seems to me that the more holes you have in the sides and or top of the acoustic guitar the less the sides and back influence the top and therefor you might soften the feedback.

[RussellHawaii]

Aloha, I'm late to this discussion, sorry!
I just got my Skytop guitar a month ago from Eric Weigeshop, I've had it a month now. I ordered it sight unseen ( sound unheard!) because I'm fascinated by this idea.
I have noticed it is naturally feedback-resistant compared to other acoustics, even tho it is plenty loud acoustically and very responsive. The difference is striking.
I'm not surprised to find this, as having no sound hole is like having the ultimate "Feedback buster" sound hole stopper. But I am very surprised how much it is true.

An additional benefit is that the player is getting so much sound directly from the guitar, that noticeably less amplification is needed for the same subjective volume! These are two separate phenomena.

[Rudy4]

At least one maker I've seen the work of makes a large side port with a sliding cover. You adjust it to whatever you want.

A better solution is to find an acoustic that you like and then install a transducer that will work well and give you what you want.

We always think there should be some way to engineer our way out of poor sound, but spending countless hours and dollars to that end wouldn't be how I'd want to spend my time. Less time spent trying to patch together a fix means less time to play.