Help With Bass Traps!

[scarboro78]

Hey Guys,

I've been some reading up on building my own Bass Traps... but I'm a little confused as to whether or not plywood is used in building a bass trap. Do I use Plywood on the backside of the Basstrap and only cover the one side with the Burlap Material where the #703 Fiberglass would be underneath? Or do I have to use plywood at all?.. Please someone help shed some light!

[reshp1]

You want as much of the absorbing material exposed (under fabric of course) and to have a air gap between the absorption and a reflective surface (i.e. the wall). Backing your bass traps with plywood would hurt you on both counts. Some people make a frame to make hanging easier, some people simply wrap up the fiberglass panels in fabric christmas present style. Place them across the corners for maximum effect, that's where the bass frequencies build up and where you can have a gap behind the panel without giving up too much usable space.

[scarboro78]

Ohhhhh I see... Thats pretty interesting.. So in actuality, all the framing and construction isnt totally necessary? As long as there is dampening of the sound happening before it reflects off the wall? (Stop me if im totally off). If I wanted to get totally ghetto (which is quite possible).. If thats true then I could theoretically stuff the fiberglass into long pillow cases or something of the sort? Hmmmm.... And By the way.. how much of the fiberglass is necessary? one sheet per frame (or pillowcase..lol) ???

[reshp1]

Yes you can just stuff it into a "pillowcase" There are certain types of bass traps that target specific frequencies that do require a frame and enclosure, but for simple broadband absorption you can just simply wrap in fabric.
I can't find the link, so someone help me out if you have it, but Ethan Winer did a experiment and showed that more thinner (3") sheets spread out over a wider area performed better than stacking the sheets (6") in less places. I wouldn't go thinner than 2" though (just my gut feeling, I have no evidence to back this up).

[RICK FITZPATRICK]

I wouldn't go thinner than 2" though (just my gut feeling, I have no evidence to back this up).

Because of the 1/4 Wavelength absorption limitation of resistance absorbers(1/4wavelength=max particle velocity), a panel 2" thick spaced 1" off the wall should perform almost equal to a 3" panel placed directly to the wall. (This has been tested and confirmed) Here is the caveat though. Let me introduce you to a wavelength. At 100 hz, a wavelength is 11.3 feet long. 1/4 wavelength would be approx. 34 inchs. This is why resistance absorbers have limited absorption efficiency at frequencies lower than 250 hz. Although all thickness's do absorb down to 125hz(lowest range that can be tested in a lab), the lower the frequency, the lower the coefficient. Here is a link to a comparison chart showing coefficients of all thickness's and brands of resistance absorption materials.
http://www.bobgolds.com./AbsorptionCoefficients.htm
fitZ
PS as a reference, the low open E string on a guitar is 82.4068923hz.

[Ethan Winer]

Rick,

> as a reference, the low open E string on a guitar is 82.4068923hz <

Not the way I tune it!

--Ethan

[RICK FITZPATRICK]

Not the way I tune it!

You know as well as I do Ethan, the musicians only have to agree on a pitch, regardless of its reference. I routinely tune my E by ear, and am always flat to a tuning fork. But, who cares. Its only a reference for musicians to play together. I played for years with a steel player who tuned to a rotary tuner, and didn't give an inch when asked to tune to a the bass players tuning fork. The bass player refused to tune to the Steel. These were ALWAYS out of tune, and I was caught in the middle. Talk about frustration. I was constantly tuning the highs to the steel, the lows to the bass or somewhere in the middle ARRRRGGGGGGGGGRRRRRRRRRR!!!
fitZ
BTW, is there a connection between olive branches and trojan horses?

[scarboro78]

Wow... how did you guys go from stuffin pillows with fiberglass to breaking down wave lengths and "stuff"? lol.. Just kidding guys.. I luv the info! But now one more question.. It seems like the most I guess efficient way of draping your walls or corners is by hanging your frames (or in my case pillows) one inch from the wall. BUT.... I've also seen people angling theyre frames to offset the 'squareness' of they're rooms which I'm guessing is to reduce the amount of reverb and standing waves and such.. Now, when.. and where would this apply? If necessary, would I do this on the celing side opposite the monitors?.. or would I do this on the side of the monitors, on the ceiling above?... Or do I do it at all??

[Todzilla]

This whole thread is assuming the bass trap is an open faced wad-o-fiberglass design, right? Not to knock that approach (I use it extensively), but it doesn't apply to the membrane type of trap a la Ethan's DIY traps.

[scarboro78]

Originally Posted by Todzilla

This whole thread is assuming the bass trap is an open faced wad-o-fiberglass design, right? Not to knock that approach (I use it extensively), but it doesn't apply to the membrane type of trap a la Ethan's DIY traps.

ummmm.... OK Im lost.. lol. Thanks Tod! When you open faced wad-o... what exactly do you mean? And when you say membrane type... what exactly do you mean?? (I'm feeling real newbie'ish right now)

[RICK FITZPATRICK]

OK Im lost.. lol. Thanks Tod! When you open faced wad-o... what exactly do you mean? And when you say membrane type... what exactly do you mean?? (I'm feeling real newbie'ish right now)

Open faced wad-o refers to panels of Rigid Fiberglass or Mineral wool used as resistance absorpers, vs membrane type, which is a resonance absorber. This is a sealed box, with a rigid back, and the front is a panel free to vibrate at a specific center frequency determined by panel density and cavity depth, and is used to absorb problem low frequencys. These are difficult to calculate, and can also introduce more problems into your space than solve. Best left to professional acoustical designers. For home studio builders, resistive absorbers are the best bang for the buck solutions as they are broadband, easily hung on walls and ceilings, and will absorb low frequency(bass traps) when used in multiple panels hung in vertical batts 1 or 2" apart, with the face parallel to the mixing axis at the rear of the room. Also absorbs lower frequencys when stacked or placed across corners as room modes terminate at the corners of a room. (sorry Tod ) Also, you have a bunch of FREE bass absorption thanks
to your walls, which in essence are MEMBRANE absorbers between studs. Drywall is the panel and the cavity between the studs is a resonant airgap.
fitZ

[scarboro78]

Hmmm... interesting.. But that still doesn't answer my question as far as hanging bass absorbers at a vertical angle as i've seen many people do.. When and where would this be done?

[RICK FITZPATRICK]

Hmmm... interesting.. But that still doesn't answer my question as far as hanging bass absorbers at a vertical angle as i've seen many people do.. When and where would this be done?

Also absorbs lower frequencys when stacked or placed across corners as room modes terminate at the corners of a room.

The corner doesn't know if the panel is at an angle or not
fitZ

[scarboro78]

lol... so people do it to make their room look cool? ::

[RICK FITZPATRICK]

Let me clarify some things. Room modes are LOW FREQUENCY. They reflect out of corners. Place a resistance absorber ACROSS the diagonal of ANY corner, and it in essence becomes a "bass trap". IF this corner happens to be a WALL/CEILING intersection, then YES, the panel is hanging at a VERTICAL angle.
Since wavelengths approaching the dimensions of a room are long, it stands to reason, they are LOW FREQUENCIES(BASS) If you hang a resistance absorber at an angle off a wall, it still will not absorb longer wavelengths BECAUSE of this limitation. So where does that leave for it to work as a bass trap? CORNERS! The VERTICAL angle has nothing to do with it. It is placing it at a diagonal across ANY corner or filling the corner that gives it more depth at a place WHERE the reflection of low frequencies OCCURS. It also stands to reason, the depth from a corner to a face of a panel placed across it is what lowers its absorption bandwidth as you are increasing the depth from the face of the panel to a boundary. However, it STILL requires THICKNESS of the panel to REALLY lower the absorption bandwidth of panels on a wall. Remember, a 100hz wavelength is about 11.3 feet LONG! 1/4 wavelength is about 34 inchs!! That means for straight panels to REALLY be effective as bass traps, they would have to be at a thickness approaching this depth.
Furthermore, you need to understand the concept of SABINES and absorption coefficients. Since an open window is a PERFECT absorber, one square foot of OPEN WINDOW, is equal to ONE SABINE of absorption. The absorption coefficients of resistance materials are based on this principle, and is FREQUENCY related. A coefficient of 1 at a given frequency means that ONE SQUARE FOOT of it will absorb ONE SABINE at that frequency(I believe-see disclaimer ) However, it is more complex and is beyond the scope of my little explanation and knowlege to expand on. You might want to do a search on the net for a thorough explanation, as this is an ONGOING research issue. In fact, there are
MANY issues in this regard that are subject to expert opinion and analysis. Lab testing around the world is an interesting starting point for further investigation. Acoustics is a DEEP rabit hole from which I barely escaped, and have no desire to re enter. So please understand, my little explanations are barely scratching the surface of known data on the subjects, and I stick my neck out every time I try to help guys here with understanding the basic concepts.
Now for my disclaimer. I am NOT an expert and my .02 is subject to correction by those that ARE.
fitZ

[apl]

You need to read about bass traps at www.ethanwiner.com and www.realtraps.com.

[RICK FITZPATRICK]

You need to read about bass traps at www.ethanwiner.com and www.realtraps.com.

These are OPINIONS as well as facts. Here are more:
http://forum.studiotips.com/index.php

[RICK FITZPATRICK]

Here is what I mean.

[notCardio]

If the drywall and studs are acting as membrane absorbers, does that mean a concrete wall has little or no absobtion qualities. Or does it depend on whether it's concrete block, or solid concrete?

[apl]

Originally Posted by Cardioidpotent

If the drywall and studs are acting as membrane absorbers, does that mean a concrete wall has little or no absobtion qualities. Or does it depend on whether it's concrete block, or solid concrete?

Yeah, that changes things, too.

[scarboro78]

Cool.. I just saw that on one of the links above.. So that seems simple enough as far as building bass traps into the corners. But I dont think you guys understand what Im talkin about when I ask about traps being hung vertically.. I mean vertically from wall to ceiling.. Kind of whats going on the corner form wall to wall... but up across the top of wall to the ceiling. Is there any value in this?

[Ethan Winer]

Rick,

> The bass player refused to tune to the Steel. These were ALWAYS out of tune <

I'd have quit the band.

--Ethan

[notCardio]

The answer to your question is yes. Check out Ethan's site and you will see pictures of real world examples of what your describing. As they say, a picture is worth a thousand words, and Ethan's got 'em.

[RICK FITZPATRICK]

If the drywall and studs are acting as membrane absorbers, does that mean a concrete wall has little or no absobtion qualities. Or does it depend on whether it's concrete block, or solid concrete?

Walls only act as membrane absorbers if there is drywall on BOTH sides. IF, you have a studwall in front of a concrete wall, you do NOT want drywall on both faces as this creates a three leaf system, which can actually reduce its effective LF isolation qualities. However, IF the total airgap between drywall and concrete is hermetically SEALED(doubtful), then the whole wall in theory, should act as a dampened(studs and insulation) panel absorber. This would also imply that the concrete would also have a sealant applied to it and all flanking paths sealed in the airgap. What effect the studs would have on the panel I do not know, but I do know this. The transmission loss in two leaf systems is determined by its mass, rigidness, and airgap. It stands to reason that as a membrane absorber, a single wall with both faces shiethed with drywall, also has an absorption coefficient and resonant frequency determined by the mass of the membranes, the depth of the airgap, distance between studs, and distance between floor and ceiling plates. These wall cavities in essence form a box, hence a membrane absorber. A wall cavity with studs spaced 16"oc will have a higher resonant frequency than that of one with studs spaced 24"oc.
It also stands to reason, that a two leaf "box" comprised of a stud wall with one face of drywall and the other of concrete, and the airgap between them is sealed, also creates a "membrane absorber". The question is what affect the studs have on the drywall(membrane) vibration characteristics and frequency. I have seen old studio pictures of membrane absorbers(bass traps) 20 and 30 feet long, but the panels were one continuous panel with NO studs. Usually this was done by splining plywood panels together at the seams.
As to absorption of concrete vs concrete block. ALL materials have an absorption coefficient. Even stone. However, rationality tells me it would be VERY VERY low. On the other hand, structural transmission of sound in these materials is VERY effective. More so and faster than sound in air. This is ONE of the reasons why concrete floors in studios are isolated from the floors in the control room as sound travels faster in concrete than air. Disclaimer still in force.
fitZ

[RICK FITZPATRICK]

'd have quit the band.

Ethan, although the tuning issue was a constant thorn as we played, most of the time the difference was so slight, only the musicians could really tell when comparing a given tone between instruments, and the BASS player was astute when it came to tonality and tuning. Myself, hell, I was having a ball playing, cause it was a dynamite ass kicking country band. Although, basically country, we played anything. For 2 years we played the regular 9 to 2 at Sacramento's most historic country bar(everyone in country music played there at one time or other) but on Friday and Saturday, we hosted the all night jam sessions afterwords. And believe me, these weren't country jams either. Some of the best musicians I've ever witnessed played these. Come sundays though........ZZZZZZZZZZZZZZZZZZZZZZ!!! Burn out time till the next tuesday.
Some of the best musical times I ever had were at these sessions. Sometimes they went from Friday night, till Sunday with oour regular 9-2 on Saturday in between. Damn. Sometimes we went to local studios afterwords to record someones ideas that they came up with improvising on the spot during the jam sessions. Talk about fried. But sure was fun
fitZ