Guitar one frequency

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KreherGuitars
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Guitar one frequency

Post by KreherGuitars » Thu Feb 18, 2021 1:20 am

Hello all, i just boxed up my first guitar. I took a frequency reading of the top on the sides before i put the back on. That was clearly at 178hz. I took a couple fft readings of it boxed and they all look like my attachment. peaks at 12hz 47hz 115hz and 224 hz.What can be made of this? Not an image i was expecting to see
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guitar one just boxed.jpg

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kiwigeo
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Re: Guitar one frequency

Post by kiwigeo » Thu Feb 18, 2021 9:04 am

12Hz........is the neck on the instrument?
47Hz.......noise from power supply? Is the peak always there or only when you tap the top?
115Hz - Helmholtz? a bit high. Block sound hole and retest. if the peak disappears then its the Helmholtz
224Hz - is the back live? Tap the back to verify.

Perhaps you could take us through your tap testing process and detail the set up (equipment being used etc)
Martin

North
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Re: Guitar one frequency

Post by North » Thu Feb 18, 2021 5:28 pm

So you plot is made with both top and back glued? But not bridge or any other elements? Right?

KreherGuitars
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Re: Guitar one frequency

Post by KreherGuitars » Thu Feb 18, 2021 6:07 pm

Yes so, i is the top and back on the sides. Freshly out of the clamps and freshly flush trimmed. I am using Spectrum analyzer. And it is set up the way Trevor describes on his O’Brien video. I am using a Dayton audio umm6 mic. The neck is not on. The guitar is a OM size. It is a little less than 4” at the tail lock and 3.5 at the head. My tap testing process is also as describes in the O’Brien video. And I have taken very clear spectrums with it. Like I said, the top seemed to be at 178 when it was on the sides and in the mold before the back. I have heard that the top jumps up about 30hz after the back is on. So I was expecting something like a 208 frequency in there. Then I could bring that to where I want it with edge thinning and the bridge mass. But if you have seen anything like this with with frequencies that aren’t really immediately recognized, plz help.

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kiwigeo
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Re: Guitar one frequency

Post by kiwigeo » Thu Feb 18, 2021 6:50 pm

Ok so we need to nail down the 224Hz peak. As I noted earlier tap the back and see what frequency that gives you. Some Chladni testing will help you nail down exactly what is generating each peak. I'd ignore the peak at 12 and 47Hz.....I often get low F peaks when I tap test my instruments. Are you holding the instrument in playing position when you tap test or is it sitting on a bench?

KreherGuitars wrote:
Thu Feb 18, 2021 6:07 pm
Yes so, i is the top and back on the sides. Freshly out of the clamps and freshly flush trimmed. I am using Spectrum analyzer. And it is set up the way Trevor describes on his O’Brien video. I am using a Dayton audio umm6 mic. The neck is not on. The guitar is a OM size. It is a little less than 4” at the tail lock and 3.5 at the head. My tap testing process is also as describes in the O’Brien video. And I have taken very clear spectrums with it. Like I said, the top seemed to be at 178 when it was on the sides and in the mold before the back. I have heard that the top jumps up about 30hz after the back is on. So I was expecting something like a 208 frequency in there. Then I could bring that to where I want it with edge thinning and the bridge mass. But if you have seen anything like this with with frequencies that aren’t really immediately recognized, plz help.
Martin

KreherGuitars
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Re: Guitar one frequency

Post by KreherGuitars » Thu Feb 18, 2021 8:40 pm

I am holding it in playing position. I need to practice getting more even taps. Some are harder than others. Maybe a bouncy ball on a stick like seniõr Gore. I recently received all my items to generate some chladni patterns. Just waiting mom a digital amp. But in the meantime, I really like your soundhole blocking idea. That’s a project for today. And a good start.

Alain Lambert
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Re: Guitar one frequency

Post by Alain Lambert » Fri Feb 19, 2021 2:47 am

if your current frequency is 50Hz, then the 47 peak is probably some noise from the current. Make sure you do not place your microphone on the same table as your computer or use some foam to isolate it.

KreherGuitars
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Re: Guitar one frequency

Post by KreherGuitars » Fri Feb 19, 2021 6:15 am

So i got to my shop today and first thing i did was take another tap test. And behold, it must have been outside noise creating those frequencies. It looks alot more normal now. looks like both frequencies need to come down a bit tho. Would you reccomend bridge edge thinning? i will hopefully be able to generate some chladnis soon and that will be more telling of what is going on and what needs to happen. Very exciting for me
Also, i am sitting on a stool with the box in playing position. Laptop is to my left on my bench. Mic is on a stool 1 meter in front of me pointed at the guitar body. Mic is on a rubber mat. I capture a spectrum of ten taps around the bridge area. i just need to create soundproof room haha. and get nice even taps.
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Guitar1uncoupled.png
Guitar1boxed.jpg

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