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Bass trap design?

Discussion in 'Home Cinema Buying & Building' started by ArsoN, Jun 23, 2005.

  1. ArsoN

    ArsoN
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    I'm gonna be starting work on my cinema in the next month or so, so I'm finalising my 3D sketches and I've hit a hurdle. I'm no acoustics expert, I've never dealt with anything like this before in my life (I have the basic down thanks to AVS forums). Now I was wondering, which design would be best (see attached pictures).

    [​IMG]
    [​IMG]
    [​IMG]

    They would be filled with as much rigid fibreglass/rockwoll/whatever as I can squeeze in. I'm also open to other suggestions.

    ArsoN.
     
  2. mattym

    mattym
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    there is a principle that suggest the baest type of corner absorbor has a gap behind it and the corner, this allows the air movement to keep going through the absorbing material, i will try to find the article that i read that said that!

    dont forget to check the powerbuys for corner basstraps, there are some in their at the moment....
     
  3. mattym

    mattym
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    found it:

    taken from this article

    http://www.rpginc.com/news/interview2.htm
     
  4. avanzato

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    The Square column is best from the designs pictured as it has the largest volume compared to the others.

    Helmholtz or panel traps are the other most used designs. They're easy to design but they do have a reputation of not working in quite the way the theory suggests, when actually built. The modex traps in the power buys are panel (membrane) traps and being professionally built do work as advetised. Though from my experience you'll need about 6 of them to start with.
     
  5. ArsoN

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    Aren't the Helmholtz absorbers supposedly tuned for certain frequency ranges? I've read that the hole sizes are tuned to certain frequency ranges, and will only work fully at the frequency range. I'll sketch in the square traps now, and see how it looks (visual impact is important, I like the column look). They'll basically be constructed as a minimal frame, filled with rigid insulation, then covered in acoustically transparent fabric. What about the traps around the ceiling perimeter? Would they be ok just being wedges glued into the corner? I'm stuck for ideas for other options there really.
     
  6. ArsoN

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    Another related issue, here's the image:

    [​IMG]

    How do you suggest I attack that corner? Hang a trap from the ceiling and bring it down as far as I can? Different type of trap? I can't move the door, thought about that already, I'm really stuck. :(
     
  7. mattym

    mattym
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    you could stick a huge block of foam in there, thats the method auralex would suggest, or if your making something then make a piece to go over the corner?

    Mat is correct about modex units, the more the merrier!

    helmholtz resonators work due to the depth of the cavity, different depths and materials will give different properties, for example a helmholtz mount for RPG Flutterfree can be as shallow as 1.11" for 250hz and theoretically as deep as 700" for that awkward 10hz!

    The most simple basstrap that works well is a sheet of fiberglass across the corner, depending on depth will depend on the frequencies.
     
  8. Mad Mr H

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    HI,

    food for thought and my opinions. (some may not like these ideas! I am open to suggestions)

    FIRST room acoustics are always different and depend on a large number of factors.

    The idea to "design" the room prior to starting is a good one to initally test for any issues which you could investigate prior to starting.

    HOWEVER

    I would question why at this stage you are considering rectifying "acoustic issues" which currently are not there.........

    Bass "collects" in corners and creates a "boom" style of sound, moving your seating position away from a corner or flat wall will reduce this.

    speakers mounted on walls with the bass driver at the bottom will also create a "boom" below them - consider inverting your wall mount speakers

    First post pictures
    1. Triangle set into corner
    2. Square set into corner
    3. Circle set into corner

    No 2. the "square" idea - quote from member "The Square column is best from the designs pictured as it has the largest volume compared to the others.
    " the volume statement is without question true BUT the design creates 2 internal corners and so 2 bass collection points - I would not use this idea.

    No 3. the "circle" again this would appear to create 2 internal corners but at least they are not 90 deg - not my fav.

    No 1. the "triangle" this creates 2 angles above 90 deg and I feel a good idea, the flat face will reflect hf so may create reflection issues.

    A good balance of acoustic systems is important.

    bass absorbtion or controlled reflection in corners is one area for thought.

    The other main area is removal of reflected sound, absorbsion panels to create a more intelligable mid/vocal range and a cleaner/crisp hf

    What do I use? I use standard "coving" round all corners, that means all four ceiling to wall areas - where you would normally put coving THEN I extend the coving down the corners of the walls. this also helps if you are to wall paper - it gives an edge to finish to and not a difficult corner.
    I also have 8 wall mounted absorbsion panels and will now consider 3 more ceiling mounted, floor is carpet with a medium/deep pile with thick underlay, concrete floor underneath all equipment, 3 rows of seating 2 on raised tiers again this breaks up the straight lines.

    I hope this is of some help I tried to take some pics but the room is too dark at the mo.

    One thing to always consider is how much you want to spend, the coving is cheap compared to other real bass trap systems BUT do you really have that much of an issue? if so maybe more consideration to your system design would help. by this I mean the actual setup and locations.

    best of luck.........

    added in edit

    the door in corner with sofa wont be an issue - door wont open with sofa there! so you wont use the room. (Just kidding)

    here I think you have to compromise, use the design down to top of door, accept the world is not perfect and see how it sounds, if a real issue then think in more detail.
     
  9. mattym

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    i think the main reason the major acoustic manufacturers use the triangle shape for corner bass trapping is for the reasons posted by MadMrH, though if the material on the face is absorptive it shouldnt reflect HF

    I would like to see piccies of your room MadMrH, it sounds a little different from the normal!

    Did you use coving in the corners when you built the room or was it done after, and if so, did you notice a big difference in the room after installing it??
     
  10. avanzato

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    The columns are to be made from porous absorber, ie fibreglass/rockwool, how does a material that allows sound energy to pass into it manage to create a 'bass collection point' ?

    IMO triangles are often used as they give a reasonable amount of surface area and volume without intruding into the room too much. They also look nice when installed and it's what people now expect a bass trap to look like, so that's what the companies offer.

    I added coving to my room to cover up a dodgy wall ceiling joint and it made no noticable change to the sound of the room.

    ArsoN: Rather than the wedge foam have you thought about building a soffit around the ceiling instead. They are useful in that you can run services (air con, lighting, cables) invisibly and can be fitted with acoustic treatments (using an acoustic cloth finish) giving you more acoustic options.
    mattym is right with the corner, with wedge foam a corner cube will be the most aesthetic finish. running a trap down to the top of the door will look stupid (i've tried it) and you're not losing much in the way of trapping by not having anything in the few cm that are left.
     
  11. mattym

    mattym
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    matt is correct with the Soffit, made correctly they can be very effective bass traps too
     
  12. ArsoN

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    MadMrH has given me something to think about. I think what I'll do is renovate the room as you normally would, put 1" rigid insulation covered in fabric around the bottom 1m of the room (as I see almost everyone does in a dedicated room), then set everything up and see how it sounds. I can always buy/build traps later if bass is a problem.
     
  13. avanzato

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    I can guarantee now that you will need bass traps and I would think about going for 2" thick fibreglass and even up to 4" thick on the front screen wall.

    I use RPG soundwave and I have the soundwave (each panel is 1.2m high) low on the wall but I'll be trying them higher up as RPG recommends some time soon. This is because there's some slap echo from the top half that isn't treated yet and I don't want to add too much absorber. I'll be adding thicker absorbtion to the front wall and bass traps front and rear when I've settled on a design for them.

    If you'd like to model your room before building try the 'room acoustics' calculator at http://www.hunecke.de/index-en.html it's not going to be exact but will give you some pointers.
     
  14. Killahertz

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    I prefer to think that corners provide maximum bass (modal) support, rather than 'collecting' bass - although I can understand the premise. Being the termination of all dimensions, and room boundaries, corners support all modes that are capable of being excited relative to the room's dimensions. Not that all modes are necessarily excited, or to the same level, that is relative to the placement and operational bandwidth of the means of excitation (loudspeakers, subwoofer, etc).

    Whether they boom or not is dependant on too many factors to say that they do carte-blanche. Further, in order to that we experience such, not only would a corner have to be maximally excited, but we'd then have to sit diagonally opposite it - which would be quite unusual, to say the least.

    Now, whilst that follows for room corners, it does not necessarily follow for all corners - that depends on the dimensions of the introduced 'corner' and the impacting wavelength. Thus, a square (perhaps free-standing) column used as an absorbent, even if solidly constructed (rather than than a skeletal frame), even of reasonable dimension, would be acoustically small compared to the wavelength of the target frequency range, and therefore be acoustically transparent, in terms of providing corner-based modal support (ignoring any absorbent effect that may be offered). Essentially, though, this is a theoretical exercise as a square-section column filled with absorbent is of little practical use (at the target frequency range).

    For freestanding devices i'd use designed resonant-cavity traps. Or, more preferably - if one's DIY is up to it - fixed resonant panels, running from floor to ceiling. However, for ease of use i'd recommend one of the many designed acoutical foams. It's not necessarily the cheap option, but it is simple, and their wide-bandwidth of operation means that they are effective. As importantly most are architecturally pleasing, so have a greater domestic acceptance factor.

    PS: Just to add voice to that already pointed out: Helmholtz resonant absorption is not right in this instance. Such devices are very function specific, and potentially ideal (if designed, built, and tuned carefully) for treating specific modal problems.

    PPS: ArsoN - I like those 3D pictures. It's a great idea as a means of pre-planning, but also helps in 'explaining' issues like that raised in this thread :smashin:
     
  15. Mad Mr H

    Mad Mr H
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    Please read in full before reply - this may not be a well structured post as its a Sunday and im resting! My answers/questions will also be quick ones.....so may not be as full as I would like or have the time today. Some may be already obvious to you (and others)

    KEY to this post - All italic in green are quotes

    In your first post you say

    THEY would be filled with as much rigid fibreglass/rockwoll/whatever as I can squeeze in

    Please define "THEY" - Exactly how to you intend to build these, you then mention in post #5 the following

    They'll basically be constructed as a minimal frame, filled with rigid insulation, then covered in acoustically transparent fabric. Please define minimal frame - my concern is that it must be solid to avoid addition of acoustic issues.

    Post #5

    Aren't the Helmholtz absorbers supposedly tuned for certain frequency ranges?

    ALL acoustic systems will have a range at which they are more efficient, this applies to speakers as well.

    Post #7

    Mat is correct about modex units, the more the merrier! you can always start with one and work your way up, if you are lucky you will find a supplier who will take sale or return, ask them to suggest a number to suit your room and get 20-25% more - this will enable you to test the room beyond what you are trying to achieve and help show that too much acoustic treatment is a bad thing.

    Post #9

    if the material on the face is absorptive it shouldn't reflect HF - 100% agree, in my post #8 about the triangle system I did not know how these were to be built and wanted to highlight a possible issue rather than assume anything. Thank you for pointing this out.

    Did you use coving in the corners when you built the room or was it done after, and if so, did you notice a big difference in the room after installing it?? - weeks of testing went into this room, as it was being built. The building team have worked with me on many AV related projects (studios clubs etc.) and understood from the start that many items would be changed or removed as the room was built.

    coving was the first thing to go in and yes I noticed a difference - now as soon as you add anything into a room it will sound "different" . the question is 2 fold what are you trying to achieve? and is the "difference" a step toward or away from that goal? A studio is design to add no colour to the recorded sound and create an accurate recording area, the engineers then give the recorded tracks the style they want. In a home theater set up I feel the astmoshere created should be acurate and involve you in the film BUT may not need the same acoustic treatment as a studio.

    It should be noted that the difference was not huge - the room at this point was totally empty, the largest change of sound was the absorbtion panels which to me are the best value to achievement ratio of all acoustic items I have used - Second is my mains filtration system (thats another topic!)

    Post #12

    I think what I'll do is renovate the room as you normally would
    I suggest you start with an empty room, set up the audio system as you intend it to be finished. Set up speakers in correct locations, your final speaker cable, delays, NO eq, and everything as you think it will be. Then add the chair(s) and sit down and test it. I am sure it will sound poor, don't change anything to help it that is your starting point. this is bare room acoustics and may highlight very obvious issues, try some of the following just to help your own understanding

    Do these in a number of positions within the room but try and use the same positions (mark the floor). I suggest you sit and read for half an hour in the room, that will give you time to settle and adjust to the room.

    1. Clap your hands
    2. Talk to yourself or another - a male and female voice will be helpful, cup your hands behind your ears this will reduce the reflected sounds you hear, talk at a wall, corner, ceiling or floor
    3. stamp you feet
    4. clentch your fist bang bottom of hand near little finger on the walls, floor, ceiling
    5. hop on on leg (purely for my amusement :D )

    now the above will possibly tell you very little however you will have already started to realise the way the room reacts to different sounds and how the room will add, react or shape the sound. These are real items and not theory this is your true starting point. At this point high end reflection and long reverb times will be obvious. any "boom" from non solid walls etc will show.

    A bare room with flat parallel surfaces is as far from an anechoic chamber as you can get - I would not want to watch films in either - the point here is that you can go too far. (no one point out that below 200Hz the chamber is of little use!)

    Anything you do needs to shape the sound in a way you are happy with it, retailers may (not all) try and sell you loads, once you have the "buy it bug" they are on to a good thing. too much acoustic treatment is worse than too little , its wasted money - you spend most of your time working or living in "standard" rooms your "treated" room is bound to sound different as it will always be a change from the norm.

    Could you add a design drawing of the room viewed from above and add seating position, screen size, doors, windows, room size, room height, or expanded elevation (all 4 walls in a long line) - By the way I like your pictures. :clap:

    Parallel walls are the cause of the problem, though to build isolated walls in 2 angled planes might not look nice, but is very wacky!
     
  16. ArsoN

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    Ok, I've taken a few screenshots of my plans as they stand now (still very much up for debate, hence why I'm posting here. You guys know a hell of alot more than I do, so I take any constructive critisism as I can get.

    First, the plan view: [​IMG]
    And a shot of the screen wall:
    [​IMG]

    There is a window behind where the screen will be going, but I intend building a small stud frame that'll slot into the opening, screw it into the sides, fill with fibreglass, plasterboard over, and make good. Room dimentions are 2.89m(W)x3.88m(L)x2.80m(H).

    I would love to have the seating position away from the wall, but as you can see, I'm very limited with space.

    [Edit] Also, I've decided to go with proper acoustic tiles for the walls, and bass traps for the corners (when I get to that stage), found a shop on eBay which seems pretty well priced.
     
  17. mattym

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    Dont forget to check the powerbuys for acoustic treatments, view all posts older than 7 days and you will see there are a few, maybe one of them may be too your liking!

    What software have you used to do your plans, they are very good.

    I will let the people who have experience in doing builds anser your question.... :smashin:
     
  18. ArsoN

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    I used a program called Sketchup. One of these things that, once you know how to use it well, you wonder how you ever managed to get by without it. :smashin:
     
  19. mattym

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    thanks.....i will look it up
     
  20. Killahertz

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    ArsoN,

    A couple of things strike me from your recent information, one is the similarity of the height and width dimensions. Even if they aren't exactly the same, their behaviour, acoustically speaking, will pretty much be. That means a variation of grouped (stacked) and disparate modes, giving a very uneven response in the modal (essentially low bass) region. Whilst wideband bass trapping will help even out the response, it may be insufficient to wholly ameliorate the response. However, whilst this may end up being the case, whether that response - in audible terms - is acceptable or not can only be decided by yourself.

    Another point is that the largest dimension of the room means that modal support extends no further than about 44Hz. OK, in this case (dimensionally speaking), that means fewer issues with stacked and disparate modes, however, it also means that you are relying on room gain to bolster output in the last octave. For music alone, this isn't so much of a problem as there is little real content below about 40Hz, movies are a different matter, though. Good solid walls, and a solid well-fitted door will help retain maximal room gain effect, but realistically, only if the target subwoofer retains realistic output into the last octave.
     
  21. patco

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    One of the most inexpensive ways to make a bass trap is to stretch a membrane at 45° across the corner. The prism-shaped enclosed volume can be partly filled with standard absorbent to widen the efficient bandwidth
     
  22. slingshot

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    Thought I'd post my experiences with bass traps, I've tried several things to calm a bad room (+12Db around 40-50Hz).

    I made some panels from 25mm rockwool slabs, this helped with treble but as expected nothing for bass, I then tried some corner triangle foam, which was alleged to work down to 40Hz but didn't make much difference. Then I bought a couple of Modex corner traps from Matt, these have helped but still think another 2 or 4 would be good (don't have the room though).

    Anyway that's what I found if it's any help.

    Slingshot,
     
  23. Mad Mr H

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    How did you measure the +12dB ???
    What equipment, and where in the room?
    What is your room size?

    +12dB is massive! are you sure you were running a flat eq? are you sure you have the bass amp set level with the rest? you are not running the front left/right full range? At +12dB I would suggest you look elsewhere for the problem as well as the room?

    Maybe you even have an earth issue? not high on my list but could be.

    I write this not knowing you so please dont think I am trying to preach to those that already know but +12dB if the rest is close to flat is honestly massive.

    In fact I am not sure I have ever seen such a large peak so low that was room related, could be but wow you could not enjoy a sound like that.

    with the 2 traps you have installed what actual change did that create?
    measured not percieved differance? a +/-dB change at specific Hz would be good?

    you dont have a large bay window or curved wall do you? that would do it!
     
  24. slingshot

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    Measurements were done with a radio shack analogue meter (prbabably some of it is the meter) at the listening position.

    There's no eq, that's stereo only so no sub, just using straight through. All the mains checks out OK, I've even connect everything up to a separate ring main in the garage just to test.

    The 12db is pretty consitent with how the room models since it's a frequency where pretty much I have a mode from length, width and height of the room (moving house is the next upgrade).

    Anyway I haven't had chance to do the the measurments, but the corner traps have got me back by almost 3db, I'll do the full sweep again when I get an hour and check.

    Slingshot
     
  25. slingshot

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    If anyone's interested I've just spent an hour re-testing the room, I did a set of readings with the Modex bass traps (I bought of Mattym in the power buys area), then just took them into the next room and redid the reading.

    The readings that changed were:-
    40Hz 0db
    45Hz -1db
    50Hz -1db
    56Hz -2db
    63Hz 0db

    I believe the traps were supposed to be tuned for about 40-50Hz. The peak I mentioned is actually at 56Hz so I've gained something, now if only I could have a stack of bass traps too the ceiling I wouldn't need to move house.

    Slingshot
     
  26. mattym

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    slingshot im sending you a pm m8....
     
  27. Mad Mr H

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    I would ask that you talk in public please.

    Reason I say that is the above list of "changes" would make any bass trap not really a viable purchase. But I know that's not right.....

    HOWEVER - the frequencies that are being looked at here are right at the bottom of the spectrum - I would struggle to believe that most people could honestly test these levels correctly.

    WE use test equipment up to £8,000.00 to check these issues.

    I would like to hear the response, Bass traps are employed the world over and so do work, some designs are better than others and some will work better in different situations.

    I am fairly sure the test results above may not be a true reflection of the product performance, could you also most some pics of the room?

    If you spent less than £500 on a test meter the response im guessing wont reach down as low as you are trying to see (hear!).....

    Im sure you used a "radio shack analogue meter", think thats what you had. About £30.00 and a "needle" based meter - NOT A CHANCE will that be of any use, the digital version is slightly better but we only give those to customers so they can check on venue levels to monitor H&S (health and saftey) issues under the H&S at work act.

    I honestly dont think the above figures are a fair test
     
  28. slingshot

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    The PM was just sending room dimensions so Matt could model it.

    MadMrH a couple of points :-

    First up I've already stated :-
    'Then I bought a couple of Modex corner traps from Matt, these have helped but still think another 2 or 4 would be good (don't have the room though).'
    So I have said when listening they made a difference.

    Secondly :-
    You asked me to measure them in your post on 18-07-2005, and I have quite clearly stated a couple of times I used a cheap RS meter 'Measurements were done with a radio shack analogue meter (prbabably some of it is the meter) at the listening position.', which I know aren't that great and are widly inaccurate.

    So maybe not a fair test but I'm simply commenting on my experiences with bass traps, I don't feel I need to remove or edit any of my posts, I thought I'd been clear but maybe not enough:-
    Even though the measurements say otherwise there is definatley an audible improvement when using the Modex bass traps (heard by both myself and girlfriend, double blind tests are a bit difficult to do when it involves moving a couple of large bass traps in and out of the room)), I'm fairly happy with the Modex traps however as stated I think another 2 or even 4 would help in a room my size. I think even Modex suggest several tuned at different frequencies.

    I hope that cleared it up.

    Slingshot

    P.S. Also if I haven't made it clear Matt is a top bloke to deal with if anyone's giving the powerbuy a go.
     
  29. mattym

    mattym
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    MadMrH

    No need to worry, nothing sinister was going on, i knew that Slingshot was happy with the 2 he had already from following up that i always do. As i am not a seasoned campaigner so i am trying to learn as mush as possible about the technical side of the this, so part of my message was to get more sweep figures. I also asked for room dimensions, slingshot has expressed an interest in wanting more traps, so im trying to make sure he gets the amount he needs, the dimensions will be run through our software to make sure he gets what he needs.

    Im quite confident that the traps work fine, we have several acousticians who continually specify modex corner and modex to tackle specific issues, the continued custom of these acousticians for these products speaks volumes!
     
  30. Mad Mr H

    Mad Mr H
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    It is very difficult at times to tell the "tone" (please pardon the poor pun!) a post is written in. this is one reason I am NOT a great fan of many forums.....
    (this one is usually ok)
    SO

    LET ME MAKE THIS VERY CLEAR

    I have spoken to Mattym and think the ideas behind the products he sells are a very worthwhile investment.

    I thought an earlier post #25 by slingshot if read by people with "limited" understanding might put people off the "bass trap" products/ideas - I was only trying to show that I thought the products would perform much better than shown BUT that without the use of expensive equipment and labour this is very difficult to prove. I am sure they DO work better. I think a true set of readings would be of great benefit to sales of these items.
     

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