I Need to control a problem "FREAK"

S

stoctony

New member
I finally got things together on my studio and have been playing through my amp, acoustic and electric, in the studio. I think I have most of the freqs covered. But for some reason, my D string when playing acoustic, does that thing that I know all guitarits hate. That humming thing that if you dont stop the string, it just keeps going and going and keeps getting louder. Now I may be off in left feild here but since none of my other strings do that, does this mean that my room is not absorbing that freq. I have read alot of Ethans stuff and have a very slight idea on room freqs and how to control them. So am I even close. If so, what freq is D and what thickness or what ever do I need to control it. Thanks in advance.

Tony
 
resonance feedback? or just feedback because the mic is pointed directly into the opening facing the back wall of the inside of the guitar, and there fore reflecting the wall in front of you. try a differnt angle with the mic, or take the monitors out of the same room as the guitar is being recorded in.
 
It's 146.84 hz.
http://www.techlib.com/reference/musical_note_frequencies.htm

BTW Tony, I don't know if you are using 703 panels in your room, but I've been telling people this for a long time, but no one seems to pay attention. I think all the talk about rigid fiberglass has people thinking it is a cure-all for homestudios. But here is the reality.

For those of you who think your fiberglass panels are absorbing everything in the room........

An OPEN "D" note(D string) is 146.84 hz.
It's wavelength is 7.69545083 FEET!! So for rigid fiberglass to have any absorption at that frequency, it would have to be at least 1/4 of the wavelength, or 1.9 feet thick. :eek: (depending on the thickness of your existing 703 and how many square feet, harmonics get absorbed which can throw off the richness of the timbre! THAT is what makes for a DEAD sound. If it were absorbing EVERYTHING, you would hear NOTHING but direct wavefronts. hence the usefullness of diffusers in conjunction with absorbers but that is whole 'nother discussion)

The next whole tone down is a "C". It's frequency is
130.8127847 hz. It wavelength is 8.64 feet. Now the fiberglass would have to be 2.2 feet thick.
The open E string has a frequency of 82.4068923 and a wavelength of a 13.1 feet. The panel would have to be a whopping 3.4 feet thick to absorb a simple open E note!! :p

Now take that the opposite direction. IF you are using 2" thick 703, set 1" off the wall, the first fundamental guitar note that it will have an affect on is your OPEN HIGH E string. That means everything between the high open "E" and the LOW open "E" is NOT being absorbed. That only leaves the notes from the high open "E" on up the fretboard!! That means there are ALMOST 2 octaves that are NOT being affected. Not very effective for usefull music frequency ranges.

Lets take it one step further. The low OPEN "E" on a BASS has a frequency of
41.20344615 hz and a wavelength of 27.42 feet! The panels would have to be 6.9 feet thick! So you can see, the usefullness for rigid fiberglass for low frequency is ZERO!

Here is one more bit of information. I see guitarists all the time who tune up a guitar starting at an OPEN A string to an ELECTRONIC TUNER. Because the meter or button says "440", they get the impression this string has a frequency of 440 cycles per second. Ha! Thats where you get into trouble.
In fact, the OPEN "A" is whats known as "A2" and has a frequency of 110 hz and a wavelength of 10.3 FEET . That means the fiberglass would have to be
2.6 FEET THICK!! to absorb down to an A2. This is where a lot of people get confused. Because the term "bass traps" have become synonomous with fiberglass panels,(don't confuse with ETHANS MEMBRANE TRAPS) people think it will absorb a guitars frequency range. I've got news for them.

BUT, if you use ACROSS CORNERS OF THE ROOM and at the wall/ceiling intersections, you can lower the absorpsion frequency and actually, because the distance to the boundary behind it varies, it becomes broadband. But remember, the TOTAL absorption in sabines is goverened by the SQUARE FOOTAGE of the fiberglass panels.

fitZ
:)
 
Easy way to test if it's a room mode, just move to a few different spots in the room and see what happens. One thing to consider is that the feedback frequency could easily be an overtone of the fundamental. I doubt this is a mode problem though, it's probably just a particularly resonant frequency of the guitar's soundboard. Try a different acoustic guitar, if the problem goes away it's the guitar.

Rick, fiberglass panels do have some absorption at 100Hz. You seem to be confusing maximized absorption and any absorption at all.

Your statement here:

So for rigid fiberglass to have any absorption at that frequency, it would have to be at least 1/4 of the wavelength, or 1.9 feet thick.
Click to expand...

conflicts with published manufacturer data, and with Ethan's FAQ here:

http://www.ethanwiner.com/acoustics.html#rigid fiberglass

Ethan Winer said:
In practice, you don't necessarily have to measure wavelengths and calculate air gaps, and the first few inches of space yield the most benefit . . . And even though the velocity is indeed highest at 1/4 wavelength, there's still plenty at 1/8th of the wavelength too . . . Please note that the angle at which sound waves strike a fiberglass panel can make the panel and its air gap appear thicker than they really are.
Click to expand...
 
Rick, fiberglass panels do have some absorption at 100Hz. You seem to be confusing maximized absorption and any absorption at all.
Click to expand...
True, however, here is a quote from Ethans site.


"....703 rigid fiberglass that is two inches thick has an absorption coefficient of 0.17 at 125 Hz...."

That IS less than .2 of the maximum coeffecient per square foot. That MEANS that for a frequency of less than 125 hz it drops off considerably. 125 hz is around the frequency of a B2, which is the lowest B note on a fretboard. So what this says, it would take approximately 5 square ft of 2" panel to get maximum absorption of 1 sabine @125 hz. Not a very good coeffiecent if you ask me. And yea, I've ALSO been telling people not to put frames around the edges of thier panels, as tests have also confirmed the square footage of this edge on a panels perimeter, adds considerably to the overall absorption at angles of incidence greater or lesser than 90 degrees. Which soundwaves and reflections are seldom perpendicular to the face of the panel. So yea, it will absorb a lower frequency to an EXTENT. But certainly not anywhere near the 1/4 wavelength maximum. But let me qualify some things. I am only trying to clarify some things in regards to GENERAL conceptions that may be out there.
I AM BY NO MEANS A QUALIFIED ACOUSTICIAN, HAVE NEVER MADE ANY TESTS, DO NOT CONSIDER MYSELF EDUCATED IN THIS REGARD, AND HEREBY STATE THAT MY OPINIONS ARE EXACTLY THAT. Only opinions. If anyone cares to correct my opinions with tested fact, I'll be the first one to scream I"M AN IDIOT! :D HOWEVER, the obvious facts still remain. Wavelengths, thickness, frequencys are still FACT, and given the 1/4 wavelength absorption MAXIMUM is what I believe to be tested fact, I rest my case. That does NOT mean there are other values of absorption at lower frequencys. ONLY that it drops off in coeffiecient like a lead balloon the lower the frequency. AND, from all the posts I have read here, there seems to be a ggeneral conception that rigid fiberglass is a magic formula for making ones studio into a perfect acoustical environment. Thats all. Maybe I'm wrong, but thats what I've percieved after being a member here for 3 and a half years. My only point, was to clarify EXACTLY WHERE and WHAT the frequencys on a guitar are, and how they relate to absorption of 703. Nothing else.
BTW
there's still plenty at 1/8th of the wavelength too
Click to expand...
So what the fuck does that mean other than it will INDEED absorb HIGHER frequencys. Well that seems obvious to me.

fitZ :( :confused: maybe
 
RICK FITZPATRICK said:
An OPEN "D" note(D string) is 146.84 hz.
It's wavelength is 7.69545083 FEET!! So for rigid fiberglass to have any absorption at that frequency, it would have to be at least 1/4 of the wavelength, or 1.9 feet thick
Click to expand...



Now damn it! Someone needs to clarify this because I've been under the impression that the 1.9 feet is not how thick but the distance it should be from the wall.
 
HangDawg said:
Now damn it! Someone needs to clarify this because I've been under the impression that the 1.9 feet is not how thick but the distance it should be from the wall.
Click to expand...
That was my understanding too.
The 1/4 wave length is the distance from the wall (or other reflective surface) that you would want to position your treatment for effective absorbtion of that given frequency.
But, I'm sure its not that easy.
 
Michael Jones said:
That was my understanding too.
The 1/4 wave length is the distance from the wall (or other reflective surface) that you would want to position your treatment for effective absorbtion of that given frequency.
But, I'm sure its not that easy.
Click to expand...


Exactly, nowhere have I ever heard that you need 1.9 feet thick of absobtion material to control low frequencies.

But I do agree that 1" of 703 no matter where you put it will have much effect on 140hz
 
Guys,

The ideal air gap is equal to the panel thickness. So for a rigid fiberglass panel that's four inches thick, you'll extend its absorption down one octave by spacing it four inches off the wall. Increasing the gap further does in fact continue to lower the frequencies absorbed, but there's a point of diminishing returns.

Also, the best place to put rigid fiberglass, when the goal is to control modal resonance, is in the room corners and tri-corners. In that case the absorption is in the best place - where the waves are focused - and the air gap is also very deep in the center of the corner.

--Ethan
 
I think this whole concept is getting out of hand. I believe this is because misconception of principles and coeffecients. On page 164 of Alton Everest's Handbook of Acoustics, it states
"........Low -frequency absorption can also be improved by spacing the absorbent out from the wall. This is an inexpensive way to get improved performance--WITHIN LIMITS.....Spacing 1"material out 3" makes its absorption approach that of the 2"material mounted directly on the wall....."

Yea, but at what frequency. Related to GUITAR, here is the real deal.




http://www.bobgolds.com/AbsorptionCoefficients.htm
Product thickness mounting density 125hz 250hz 500hz 1000hz 2000hz 4000hz NRC
703, plain 1" (25mm) on wall 3.0 pcf 0.11 0.28 0.68 0.90 0.93 0.96 0.70
703, plain 1" (25mm) 16" air 3.0 pcf 0.65 0.94 0.76 0.98 1.00 1.14 0.90
703, plain 4" (102mm) on wall 3.0 pcf 0.84 1.24 1.24 1.08 1.00 0.97 1.15
703, plain 4" (102mm) 16" air 3.0 pcf 0.65 1.01 1.20 1.14 1.10 1.1 1.10
This tells me that at 500 hz, 1" 703 that is spaced out from the wall 16" improves its absorption coeffecient by a WHOPPING 1/100 of a percent. BIG FUCKING DEAL Do you think you could hear it?

Here is a portion of Eric Desarts chart found on this thread(Excuse me Eric, but at least I gave the link.)
http://johnlsayers.com/phpBB2/viewtopic.php?t=2429
ABSORPTION VALUES VERSUS MATERIAL THICKNESS AS % OF WAVELENGTH.
Compare column 3 , giving the material thickness as a % of the wavelength, with the corresponding absorption in Column 2.
You can compare them with the Engineering rules given before.
It's clear that the traditional 1/4 wavelenght (25%) approach does NOT apply.
This approach only relates to PLAIN waves at STRAIGHT incidence for DISCRETE frequencies:

OC 703 · 1.0" · 25.4 mm
0125 Hz · 0.11 · 000.9% ( This is less than 1/10 of 1 percent absorption, Rick Fitzpatrick )
0250 Hz · 0.28 · 001.8%
0500 Hz · 0.68 · 003.7%
1000 Hz · 0.90 · 007.4%
2000 Hz · 0.93 · 014.8%
4000 Hz · 0.96 · 029.6%

OC 703 · 2.0" · 50.8 mm
0125 Hz · 0.22 · 001.8% (Still less than 2 percent-Rick Fitzpatrick)
0250 Hz · 0.82 · 003.7%
0500 Hz · 1.21 · 007.4%
1000 Hz · 1.10 · 014.8%
2000 Hz · 1.02 · 029.6%
4000 Hz · 1.05 · 059.2%

OC 703 · 3.0" · 76.2 mm
0125 Hz · 0.53 · 002.8% (Less than 3 percent-Rick Fitzpatrick)
0250 Hz · 1.19 · 005.5%
0500 Hz · 1.21 · 011.1%
1000 Hz · 1.08 · 022.2%
2000 Hz · 1.01 · 044.4%
4000 Hz · 1.04 · 088.8%

OC 703 · 4.0" · 101.6 mm
0125 Hz · 0.84 · 003.7% (less than 4 percent-)
0250 Hz · 1.24 · 007.4% less than 8 percent..
0500 Hz · 1.24 · 014.8% less than 15 percent -Rick Fitzpatrick)
1000 Hz · 1.08 · 029.6%
2000 Hz · 1.00 · 059.2%
4000 Hz · 0.97 · 118.4%


Considering, ON GUITAR, from the Low open E string to an "A" note at the 5th fret,on the high E string, which is 440hz, that is about 80 percent of the available octaves and notes on a guitar fretboard. That tells me, that 80 percent of the guitar fretboard is BELOW 500 hz.. Soooooooooo......Even Eric Desart is telling me that 4" 703, absorbs less than 14 percent of 1 Sabine for every square foot of 703 @ 500 hz, and drops off like a stalled Cessna as the frequency gets lower, so you tell me.

fitZ :rolleyes:
 
Here are the notes and frequencys as related to guitar
String Bass Low E(1) 41.20344615

Guitar Low OPEN E(2) 82.4068923

Guitar mid E(3) 164.8137846
Apx. Start of Female Voice Fundamental (Bb Trumpet)

E(4 )high OPEN E 329.6275692

E5(5) @ 12th fret 659.2551384

A
55.00
110.00
220.00
440.00
880.00
A#
58.27
116.54
233.08
466.16
932.32
B
61.74
123.48
246.96
493.92
987.84
C
65.41
130.82
261.64
523.28
1046.56
C#
69.30
138.60
277.20
554.40
1108.80 THIS IS THE HIGHEST NOTE ON A FENDER STRAT and is 1/10th over 1000 hz :eek:
D
73.42
146.84
293.68
587.36
1174.72
D#
77.78
155.56
311.12
622.24
1244.48
E
82.41
164.82
329.64
659.28
1318.56
F
87.31
174.62
349.24
698.48
1396.96
F#
92.50
185.00
370.00
740.00
1480.00
G
98.00
196.00
392.00
784.00
1568.00
Ad
103.83
207.66
415.32
830.64
1661.28

What this tells me, is almost ALL FUNDEMENTAL USFULL MUSICAL NOTES are less than 1500 hz. :p So what if 703 absorbs @ 10,000 hz. What musical notes are YOU playing:rolleyes:
fitZ
 
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