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Building my own bass trap...

 

Bronze Member
Username: Bigpoppaphile

SAINT CLOUD, MN United States

Post Number: 28
Registered: Mar-06
I just saw on diy channel the other day a home theatre show. During the show this guy was setting up his own theatre in the basement and they were doing the whole nine yards, custom woofer boxes and speaker mounts in wall, projector screen, seperate room for racked componants, etc.

What intrigued me was the professional they had helping him brought in these concrete form tubes you can buy at menards and he made bass traps out of those by filling them with sand and putting them in the corners. They painted them and also hid some in the corners of the guys custom speaker cabinet.

I was wondering if this would really be effective and also how the height of the column, width, and how full of sand it was would affect the frequencies it adsorbs. Is there some sort of formula one could use that relates to porting subwoofers that could be applied to the bass trap? It seems to me however they determine the size in diameter and length of the ports in speakers could maybe work here.
 

Gold Member
Username: Mixneffect

Orangevale, Ca. USA

Post Number: 1081
Registered: Apr-05
You may want to look into how an anechoic chamber is built, and design a room based on those principals.

I have seen many "UGLY" non effective bass traps/soundproofing techniques, materials, and overall esthetics.

If bass traps were that important to me, I would look into how an anechoic chamber is built, and build my own room based on those principals.

Basicly I would attack the real problem instead of trying to ineffectively "band-aid" the real problem.

:-)
 

Bronze Member
Username: Bigpoppaphile

SAINT CLOUD, MN United States

Post Number: 29
Registered: Mar-06
That would make the most sense and be the best solution, however, I live in an apartment right now and that would not be something I would care to do. If I had my own theatre room in a basement then yes.

So that is why I was wondering about the base trap.
 

Gold Member
Username: Mixneffect

Orangevale, Ca. USA

Post Number: 1084
Registered: Apr-05
Oh, Thats too bad. I mean you are in a bind because of volume restrictions and remodeling.

Other than building your own sound room, you will find that most wedges, and panneling are costly, and sometimes they only target certain frequencies.

Hope you find something that fits your need. :-)
 

Bronze Member
Username: Bigpoppaphile

SAINT CLOUD, MN United States

Post Number: 31
Registered: Mar-06
Yeah, I ran some tests yesterday on my day off with my test cd and radio shak meter. I have a huge dip right around 40hz, which sucks since that is what some bass notes from a bass guitar plays and spikes at around 120, 200, and a little at 400. No wonder I can never decide what level to set my subwoofer at. I either can't hear the low bass I want and then get too much upper bass and muddy up my midrange if I turn up the volume on the woofer. Going to try moving the location of the woofer today. I messed with the cross over knob on the subwoofer since my receiver doesn't allow me to set that, it helps a little...
not sure if bass trap is worth the time even though I seemed simple to make.
 

Gold Member
Username: Mixneffect

Orangevale, Ca. USA

Post Number: 1086
Registered: Apr-05
The size of the room has a lot to do with sound.

The shape of the room has a lot to do with sound.

The placement of speakers and the listening location of the listener also has a lot to do with sound.

Check this out before you start your modifications. I think it will help a lot.

http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/auditcon.html#c1

You may also find that your position (length) in relation to the speaker may affect the wavelength of the 40 Hz tones.
 

New member
Username: Einsteinjb

Philadelphia, PA USA

Post Number: 1
Registered: Sep-06
I don't know anything about the particular design you mentioned but there are many types of bass traps both DIY and professionally made that can be very effective, either for specific frequencies or for broadband absorption. Obviously custom designing and building a room isn't practical for many people. And BTW, an anechoic chamber is a great testing tool, but would NOT make a very pleasant listening room. :-)

Google "bass traps" and you'll find plenty of good reading. GIK Acoustics makes pretty affordable traps, as does RealTraps.com and many other sites. (And of course you'll find plenty of EXPENSIVE options as well.) Ethan Winer of RealTraps.com has quite a few informative articles on his site, along with some great DIY info.

I am almost finished building my own REALLY cheap DIY panels that I'm using for both broadband absorption and some basic bass trapping in my little 11' x 11'9" x 7'6" room, where I have all my HT gear as well as my home mixing studio set up. Can't tell you how effective they are yet as they're not finished but I'm sure they'll work as they're very similar to some professional products I've seen. I basically made 3 simple wooden frames, 4' x 2' x 4" (one of them is actually 3' long due to space restrictions in that one spot) with no backing at all, covered one side with polyester batting from WalMart, and I'll be filling them with 2 sheets each of 2" rockwool insulation from www.atsacoustics.com. Can't beat $36 for a box of 6 sheets plus shipping. I'll then cover the other side with more batting, wrap the whole thing in burlap (in the colour of my choice) again from ATS Acoustics, hang 'em up in the corners, badda bing, bass traps on the cheap. Can't wait to hear them. :-)
 

Gold Member
Username: Mixneffect

Orangevale, Ca. USA

Post Number: 1095
Registered: Apr-05
"You may want to look into how an anechoic chamber is built"

I didnt mean build an anechoic chamber in your living room. I mean just use an anechoic chamber idea and principles in mind when you build or retrofit a listening area for sound.

Hope this clarifys my advice. Sorry for the confusion.:-)
 

Bronze Member
Username: Philipt95148

Post Number: 35
Registered: Jun-06
Jeff,
ATS website shows Mineral wool, are they the same as the rockwool you used?
If so how do you find the stiffness, compared to 703? I mean are they sagging and how do you deal with that?
Thanks
 

New member
Username: Einsteinjb

Philadelphia, PA USA

Post Number: 2
Registered: Sep-06
mixneffect, I think your idea is interesting, but honestly I wouldn't use anechoic chamber designs for anything other than speaker testing purposes, even for inspiration. I'm certainly not an acoustic room designer but those who've heard what speakers actually sound like in anechoic rooms say it really doesn't sound good. It's purely a testing tool. :-) Besides, I'm reasonably sure the actual dimensions and size of the room aren't as much of a factor when you're building an anechoic testing chamber -- it's what you put on the walls (and possibly behind them and what you make them out of) that will make the room anechoic. Any room in which you succeed in utterly killing any acoustic reflections of any kind at any frequency is, in fact, an anechoic chamber. I would think enough of the right kinds of insulation on the walls would create that effect in almost any room.

I think if you had the luxury of designing and building a room from scratch for either 2-channel music listening or for home theater, you'd be much better served researching existing, proven designs for these types of rooms, which are very different from anechoic chambers. 2-channel listening rooms tend to be built to have some natural ambiance (room sound), while dedicated home theater rooms are generally made to be pretty dead, particularly on the front wall behind the speakers and screen and up to just above ear level all around. Music listening rooms also often feature hardwood floors while home theaters generally have carpeted floors and often acoustic padding under the carpeting, unless I'm much mistaken.

Anyway, Philip Tran, yes, mineral wool is the same thing as rockwool. ATS is the only place I've been able to find that sells it, though I've read about it on many sites. The stiffness is, well, not. I was prepared from what I read for it to be floppier and less rigid than 703 which is supposed to be fairly rigid, but when it arrived it was actually extremely soft and floppy. In fact as soon as I opened the box and tried to pull a couple sheets out, I discovered you need to be very careful with it because my hands and fingers (wearing latex gloves of course) left smooshed impressions in the material and it felt like it would rip if I pulled too hard. I was gentle though and didn't damage any of it. It really does not hold its shape on its own at all, so I wouldn't consider just wrapping it in burlap as I did with some cotton-based panels I bought from a guy on eBay that were also a bit floppy but not nearly as much as the rockwool.

However, when I placed two sheets into each of my frames and stapled the batting onto the frame to hold it in place, it actually stayed there quite nicely and does not sag at all. There's no room in there for it to sag. The batting is keeping it in place on both sides. The burlap wrapped around the whole thing just reinforces it, makes it look nice, and helps prevent me from poking holes in the batting or tearing it, which is also quite thin and somewhat delicate.

I'm really happy with these panels, BTW. They look really nice (if I do say so, and as long as you don't examine the tops or bottoms too closely to see my messy stapling job) and seem to work quite well. I've got two of them hung up in corners close to the ceiling, while the third simply stands behind my bedroom door. When I want to watch a movie or do some serious listening I just close the door and stand it in that corner leaning against the door. My room's pretty dead at this point with nice, tight bass response no matter where I sit. It's not perfect but way better than before I did any treatments. In a room this small, I feel killing the walls with a lot of absorption was the best solution. You can't get much good room sound in a room this small so might as well just kill it. :-)
 

Gold Member
Username: Mixneffect

Orangevale, Ca. USA

Post Number: 1102
Registered: Apr-05
I m going to try yet another way of explaining what I meant by my previous statements. lets see if this helps;

When you use the idea and/or principals of a certain object or system, it does not mean that you will actually build that certain object or system. You will just "keep in mind the principals."

When someone asks you to hold your hand up like a "hi-five". It doesnt mean that you are going to hi-five anyone. :-) Know what I mean?

Now as far as size of the room, shape of the room and distance from your speaker to your ear is definetly an important factor in how the sound that you will hear will actually sound like. I hope this makes more sense.

Size of the room has a lot to do with wavelengths, and how they will react to the room. The same goes with shape. For instance you will get a boomy, sound if you have a rectangle room and you run your speakers the length of the room. The same goes with your listening distance. A square room has its dilemas as well.

When you take these things in consideration, you will see that anechoic chambers are built with these parameters in mind, plus more.

Yes I have listened to speakers in an anechoic chamber. For one your ears pop when you enter the room, so I dont recomend building an anechoic chamber as a listening room.:-(
 

New member
Username: Scott_r_foster

Jacksonville, FL US

Post Number: 3
Registered: Nov-06
Tube traps work like organ pipes.. in reverse.

The idea is to build a device that resonates at the "problem" modal frequencies of your room. It is in effect a Helmholtz bottle. In a tube trap, pipe length is the critical dimension, but you also want to calculate the Q of the chamber [add fuzzy stuff] and the size of the port [the opening you leave at the end of the tube].

Here is a link to a calculator for Helmholtz devices:

http://www.whealy.com/acoustics/Porous.html

I would caution you to consider a broadband approach to absorption before embarking down the resonate / Helmholtz path. Resonate absorbers of all types must be tuned to match the room [requires post installation measurement], and even when tuned can re-emit the sounds they capture, which merely displaces the problem resonance in time [such units will hum at their resonante frequency and can create a new problem to replace the one they "solved"].

Mineral fiber panels that are thick enough to work on lows as well as highs [broadband absorbers] are both simpler to make, and install, but don't have to be tuned, and can't create re-emission problems.

Good Luck!
 

Bronze Member
Username: Leonski

Post Number: 97
Registered: Jan-07
one-frequency sub problems are difficult but not impossible to solve::
you can, for example, do a 'sub crawl' where you put the sub where you want to sit....no kidding.....than move around the room to proposed sub locations and listen. If you find a spot with nice, even, response, swap the sub over and have a seat! this actually works.

There are also online 'room mode calculators' which will compute resonance modes for simple cubic /cubic retangular rooms and allow you to see problem frequencies in advance.

Harmon White Papers has an article about subs and placement in which they claim that MULTIPLE subs are better than a single unit. I suspect that with a limited budget, you are better off with 2 lesser subs than 1 mondo.....at least for a total of 1000$US and up......for less, it probably isn't worth it.

Some sub placement guys like corners and others have recommended the sub close to the listener, which makes a certain amount of sense, there being no standing wave peaks or 'suckout' to worrry about in such a near field enviroment.

Square rooms are bad as are rooms of even multiple dimensions, like 12x24 with an 8ft ceiling.

Careful attention to detail and placement can make all the difference.
 

Gold Member
Username: Thx_3417

Bournemouth ...

Post Number: 3966
Registered: May-05
Build an anechoic chamber in you're home, not a bad idea, but you'd have very little room of space to play around with once its built with little room to swing a cat around.

I'm thinking of adding bass traps to the rear corners of the room in the near future with supplies from (Studiospares) based here in the UK. They seem to be doing an offer on acoustic room treatment at the moment at low prices.
 

Silver Member
Username: Leonski

Post Number: 125
Registered: Jan-07
Anechoic space in your home? BAD idea.
Have you ever been in such a space?
All the normal q's provided by ambient noise/sound are GONE. You pop your ears a lot, thinking they are plugged.
I have been in a very sound controlled recording studio. Great for recording, bad for listening, which is done in the control room, which is NOT such an anachoic space.
You couldn't even hear your steps, had trouble localizing some sounds and voices were....different, there being no ambience to go with the 1st arrival. Uncomfortable.
 

Silver Member
Username: Leonski

Post Number: 126
Registered: Jan-07
One other thing::
A 'bass trap' is really a Helmholtz Resonator.
You can test the idea quickly by blowing over the top of a bottle and hearing the resultant tone.
Changing the amount of liquid in the bottle changes the pitch. Such traps would seem to be useful for sucking out a frequency peak, particularly in the bass region......
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 10719
Registered: May-04
.

Resonators and traps function differently from one another. A HR's results are reasonably narrowband due to its tuning frequency determined by construction details and size. If you can tune and place a HR properly, no small task, you can bring down a room's modal frequency peaks in the specific area where the HR is sited but you will not do much more than tame a simple frequency in one area of the room. HR's work best in very large rooms where the resonant frequencies of the room are fairly closely placed. A HR will do little for the overall problems most small to medium sized rooms have with tubby bass due to too many reflections caused by room dimensions and the tendency of bass wavelengths to get "trapped" in areas where two or more surfaces meet. You cannot place a Hemholtz Resonator in a tri-surface corner and expect good results. For that matter, due to size restraints, you can't place an effective HR in a tri-corner and have any real low frequency attentuation. Also, read the post from Tuesday, November 14, 2006 - 11:51 am for more discussion regarding HR's.


Traps work on the principle of absorption. By passing the low frequency pressure wave through a porous material the energy of the pressure front gets converted into heat and its energy is dissipated by friction. The problem with most traps is they cannot be thick enough to actually be very effective in the most problematic frequency range encountered in most consumer type, existing construction style rooms. Effectiveness beneath 80Hz is very difficult to achieve and even that will make serious demands on the decor of the room.


Placing the sub close to the listener will not guaranty good bass response. The standing waves within a room are determined by the dimensions of the room relative to the frequencies you are trying to reproduce in that space. If you happen to be sitting in the center of a 35Hz standing wave, you can place the sub at your feet and still not have reasonable bass repsonse in that frequency range. The wavelengths still require a set distance to propogate within the room and the cancellation point will remain the same within that room. The only way to alter that for a given frequency is to change the room dimension causing the standing wave reflection. If that's not possible, you are going to have to change your seating location and then the position of the subwoofer. There are some work arounds to this but they are too room dependent to be used as hard and fast rules.



.
 

Silver Member
Username: Leonski

Post Number: 127
Registered: Jan-07
Isn't there something to bass traps besides sheer absorbtion/bulk? That makes a good, overstuffed sofa a bass trap, no?
Other than a shotgun approach, how do you control such a bass trap? I have seen corner placement in the shape of tubes and triangles....

I couldn't imagine how much detail you need to go thru to design a HR. The above post advocates a broadband approach and is the obvious way to go for anyone short of a person with a massive amount of time and proper measurement equipment...

I'm going to check out that link.
ONE DAY I have promised myself I will build a proper HT room and this is the kind of stuff I'll need. Also, the HK White Paper on subs/placement is very good. I suspect many room mode problems could be reduced by better placement.

The nearfield sub is advocated by Dr. Hsu himself.
While this is certainly 'no guarantee', many people have decent luck with it. Some others swear by the sub-crawl technique.

There seem to be very few rules, indeed. I think it would be a good idea to avoid mid wall sub placement in rectangular/ symetric rooms. There are a number of online room mode calculators which can help in this regard.
 

Silver Member
Username: Leonski

Post Number: 128
Registered: Jan-07
I just took an (obviously) quick look at the accoustics calculator site.
The control room calculator may be just what the Doctor ordered for a home listening room. Doom on you, though if you have an irregular space, like my current listening room with 7 wall planes, a vaulted/pitched ceiling and hardwood floor on a concrete slab! The stereo is on a short wall and when installed had an echo from the backwall which was tamed by wool tapestry hung about 2 inches from the surface.
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 10727
Registered: May-04
.

Making a room work well for acoustics will eventually be a decision between how the room should function and how the room should look and work. Effective bass tuning cannot be done without seeing the pieces that are doing the control. They can be dressed up and disguised but they are not small and will not be hidden behind, or within, a sofa.


Yes, an overstuffed sofa is a bass trap of sorts if the covering material is not reflective. However, a sofa is seldom placed where the bass problems exist so its effectiveness is minimized. More often than not a sofa is better at damping mid to high frequency reflections rather than taming bass response. Two and three junction points between large surfaces of a room are the culprits in most bass reflection problems (other than simple room dimensions) and those areas are where you should first turn your attention when tuning an existing room.



True bass traps are only working on the principle of absorption. Combination pieces have been created which do other functions as well but most reasonably priced, generic "over the counter" bass traps are going to require substantial size to be effective at anything beneath 20-300Hz. Passing the pressure wave through alternating layers of damping/air/damping is the most effective treatment and is typically managed by placement of the absorptive material away from the wall or corner by a few inches. To give some idea of how much material is required to damp very deep bass frequencies, place "anechoic chamber" in a search engine and look at the amount of material required to damp a large room down to approximately 20Hz. That will normally require damping material 48" thick across every surface of the entire room.


Here's a site with a few good acoustics articles; http://www.realtraps.com/articles.htm



Room tuning is partly science and partly talent. The personality of the acoustician comes to the fore and not everyone might agree with the choices one or another designer will make. In the case of the Realtraps site, I agree that positioning speakers within the short dimension of the room in order to fire into the length of the room will typically give better results that the alternative. Pay attention to the 38% rule discussed in this site. This and the rule of thirds will typically give the best results for a simple set up's starting points. I often refer people to the the Wilson Audio W.A.S.P. (speaker) placement technique as a starting point also.


I know what Dr. Hsu recommends and in some cases it will work well. But as a blanket statement, there are few blanket statements that can be made regarding bass response in a generic room. Take my word for it, I've just dealt with the issue; if you are seated in a null point due to a standing wave, you cannot crank the level of the sub's amplifier high enough to overcome the basic dimensions of the room without clipping the amplifier and overdriving the speaker. Null points and troughs are not things you move around a room, if you cannot move your listening position, placing the sub along side you at the null will only frustrate you.



.
 

Silver Member
Username: Leonski

Post Number: 129
Registered: Jan-07
No beef::
I've been in a recording studio, that while certainly not anachoic, was so dead that it was creepy. The only 'live' area was the hard floor upon which the babygrand was intended to be placed. My dad built this space, after conning his way into a couple LA studios and getting the gist of intent/construction. I know this studio was in use for at least a decade. NO parallel walls. Extreme thickness. Double hung glass to control room. Lead sheet on the door for damping, even though it was a solid core exterior door.

If you saw my listening area, you'd either smile and say 'what an opportunity' OR, 'go find yourself a good set of headphones'.
By necessity, my sofa is about midroom on the long axis. Every feature is asymetrical including the ceiling, which is vaulted.
If I shipped you the plan you'd see what a nightmare it is.

As for hanging something spaced from the wall, that is the FIRST thing I did, nearly 2 decades ago, in order to tame an awful echo.

As for nulls and troughs, that is why the Harmon White paper recommends double subs. It would be tough to be in the trough or peak where there are 2 sources of LF.
I wish I could afford a pair of subs, just in the interest of science!
 

New member
Username: Scott_r_foster

Jacksonville, FL US

Post Number: 4
Registered: Nov-06
As explained above, building a bass trap is not difficult.

In fact building bass traps that perform well below 100 Hz is not difficult.

This is a very effective design:

http://forum.studiotips.com/viewtopic.php?t=535

This is a very efficient design [scroll down and view the pdf with detailed instructions for building your own].

http://forum.studiotips.com/viewtopic.php?t=534

Both perform very well:

http://forum.studiotips.com/viewtopic.php?t=536

Also as explained above proper placement yields better perforamance. Here are lab measurements of the Ready Acoustics RT424 [a 4" thick upholstered bass trap with a OC703 core] . In one instance the panels is placed in a corner with the backside edges touching room boundaries [wall/floor or wall/wall]. In the second case a small gap [about 2"] is introduced separating the panel from the room boundaries. Notice how the absorption peak between 60 and 100 Hz collapses.

Conclusion: placement is important.

more details on this suite of tests here:

http://www.readyacoustics.com/index.php?go=acoustics-advice.acoustic-data

Upload
 

Silver Member
Username: Leonski

Post Number: 130
Registered: Jan-07
What'cha think?
Simple and appears effective AND can easily be made to look good. The local home supply store should have an ample supple of the right stuff.....
 

New member
Username: Hondafanatic17

Thompsontown, PA USA

Post Number: 4
Registered: Dec-07
Not to sound like an idiot but whats a bass trap do?
 

New member
Username: Scott_r_foster

Jacksonville, FL US

Post Number: 5
Registered: Nov-06
It converts acoustic energy into heat [thus a "trap"]... and is sized and configured to be able to work on low frequencies with efficiency [thus a "bass trap"].
 

Bronze Member
Username: Nency

Post Number: 98
Registered: May-09
Although acoustic foam products are useful for absorbing midrange and high frequencies, they are relatively expensive: Sculpted foam two inches thick costs about five times more than type #703 one-inch rigid fiberglass board which is just as effective. (Rigid fiberglass is similar to the fluffy type used for home insulation, but it is much denser. A sheet of #703 one inch thick is equal in sound absorption to a much thicker batt of regular fiberglass.) Likewise, pre-built commercial bass traps are readily available, but they too cost many times more than the raw materials needed to build your own.
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