• Re: Bass traps for subs - AlphaGeek 22:21:21 09/12/04 (0)
  In Reply to: Bass traps for subs posted by bbenn on August 18, 2004 at 19:00:33:

  
  IMHO I think that you should test the room first (with sub in place) with a SPL meter and test tone CD before buying anything. You will probably find that you have more than one problem frequency.

  You will notice on the websites of the better acoustic treatment manufacturers graphs of which frequencies that their products will absorb. This, combined with the measurments, will help you make an educated guess on what type of design will actually help your problem.

  At least with the tube trap designs, absorbing progressively lower frequencies requires progressively larger tube traps.

  
  
  

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• Here is what gives regarding sub trap - bbenn 07:06:58 08/19/04 (3)
  In Reply to: Bass traps for subs posted by bbenn on August 18, 2004 at 19:00:33:

  The subject of sub trap came up after i read the attached product description from Acoustic Science makers of Tube Traps....

  http://www.acousticsciences.com/subtrap.pdf

  there was also a short write-up in one of the columns in the latest issue of Stereophile.

  I happen to have Lowther based Beauhorn's as my main speakers.

  bob
  
  

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  • Re: Here is what gives regarding sub trap - David Aiken 17:12:17 08/19/04 (0)
    In Reply to: Here is what gives regarding sub trap posted by bbenn on August 19, 2004 at 07:06:58:

    Like Richard, I wonder about that trap and it's effectiveness.

    The effectiveness of the ASC traps, including that one, is maximised by increasing size and internal volume, and by placement in corners. That trap is small compared to their standard traps and, if you're using their normal traps, it won't end up in a corner because you've already filled the corner.

    What it is, to my mind, is a subwoofer stand - which may or may not be beneficial - which happens to absorb some sound including low frequency sound. Like all ASC traps, it is a broad band device so it doesn't need tuning though its dimensions mean that it will be less effective at dealing with deep bass than a normal sized ASC trap, and its placement will also make it less effective. It would probably have some effect on the floor to ceiling mode, but 1 trap of that size isn't going to be hugely effective there.

    I'm sorry that my original reply ended up being a bit disjointed, but I was having difficulty with your question and I started from it rather than from the likely problem.

    I have no experience of Lowthers but from what I've heard, it's likely that they are starting to roll off slightly at the 70.6 hz mode associated with an 8' ceiling. Even so, they aren't going to be strongly down there and the contribution of your REL won't be excessive at that frequency. I suspect you'll only notice a room mode problem at that frequency with the sub if you already notice it, and we tend to be fairly tolerant of room modes in some ways, simply because we're used to listening to things indoors where they exist. Their adverse impacts also decrease as the frequency of the mode rises, and that mode will tend to be less noticeable than the lower modes associated with your other room dimensions which will be longer (unless, of course, your room is an 8' cube or smaller).

    Try the REL in your room and see how it goes without worrying about this sort of stand or anything else at first. Find the position that works for the sub doing a straight setup according to the instructions. See if you get any problems that really do demand treatment when you do things that way, then start playing. Try raising the sub off the floor and see what impact that has. Since the REL fires down, you'll need a smooth, hard surface for it to fire against for best results. I found raising my Storm III on a homemade isolation platform based on the Symposium platforms very effective (successive layers of MDF, hardboard, GatorBoard, hardboard, MDF listed in order from top to bottom). The platform extends around 4" each side of the REL and sits on tall cones so the top of the platform is maybe 4" above the carpet. I have tried lifting the platform higher by sitting it on solid boxes but didn't think that helped enough to try and make a taller platform for long term use.

    If you get some problems with the sub in the system, they're far more likely to be below the 70.6 Hz floor to ceiling mode, and you will need much larger traps, strategically placed in room corners, to deal with them or you can try some electronic equalisation instead (or as well).

    David Aiken
    
    

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  • That product doesn't make much sense to me - Richard BassNut Greene 16:45:39 08/19/04 (1)
    In Reply to: Here is what gives regarding sub trap posted by bbenn on August 19, 2004 at 07:06:58:

    Without measurements, who knows how well the product performs?

    To avoid exciting the annoying first-order floor to ceiling mode you need to place either your subwoofer driver, or your ears (or both), in or near the null half way between the floor and ceiling.

    I actually do this at home in my living room with a tube subwoofer using an upfiring 15" driver located four feet off the floor.

    That null is very narrow (as all standing wave nulls are).

    Placing a subwoofer on a stand so the driver is two feet off the floor has only a small audible (and measureable) effect on excitement of that standing wave.

    Unfortunately many times the main speakers will contribute so much of the excitement of a 71Hz. standing wave that even placing the sub driver in the null as I do won't completely eliminate that standing wave.

    Fortunately the floor to ceiling axial standing wave is the easiest room mode to correct with parametric equalization because any setting for a person whose ears are XX inches off the ground will work well for other listeners anywhere in the room whose ears are a similar distance (XX +/-6 inches) off the ground.

    Bass traps are most effective in high pressure zones such as corners so if your sub was well away from walls or corners (not that common) a bass trap under the sub would not be as effective as it could be when placed in a corner.

    Lifting the subwoofer 18" and placing a bass trap in the room will help ... I only wonder how audible the improvement would be.

    
    
    

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    • Re: That product doesn't make much sense to me - bbenn 18:57:58 08/19/04 (0)
      In Reply to: That product doesn't make much sense to me posted by Richard BassNut Greene on August 19, 2004 at 16:45:39:

      Thanks for all the excellent imput and advice :-)
      
      

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• What gives? - David Aiken 00:18:38 08/19/04 (0)
  In Reply to: Bass traps for subs posted by bbenn on August 18, 2004 at 19:00:33:

  First question: do you need a trap for the sub? If the highest frequency handled by the sub is lower than the room mode associated with your longest room dimension, you don't need a trap for the sub. You may need traps for the mains, but not for the sub in those circumstances. If the highest frequency of the sub is above that room mode, you'll need traps for both the main and the sub frequencies.

  First note: The frequencies for which you will need bass traps will be determined by the room modes, not the sub or mains unless they don't have particularly flat response and they have response peaks you are trying to tame. Equalisation is a better way of doing dealing with response peaks.

  Second question: what sort of bass trap are you making? Many bass traps (the ASC traps and Jon Risch's clones, Ethan's traps, and Jon Risch's quick and dirty traps don't need tuning. They will work well at all frequencies over relatively broad ranges. The only bass trap you really need to tune is a Helmholtz resonator and they tend to be very narrow band. Tune it wrong and it won't cover the frequency you want, so you need to be very sure of both the frequency you want to absorb and the tuning of the trap. The other traps are a better approach as they will deal with a wider range of room mode problems.

  Second note: you said you want to manage the floor to ceiling mode which is 70.6 Hz. A good starting length for a Helmholtz resonator is half the room dimension you want to deal with, so a 4' tube would be fine, and a bit of stuffing in that would broaden the range over which it absorbed slightly. Look up DIY room lenses for construction details.

  However it sounds like what you need to work out first should be what the problem actually is. 70 Hz is getting high for a sub to cover and if you have problems there, you would have had them before getting the REL. In addition, the correct crossover frequency for a REL is quite low - I have the crossover on my Storm III set for 22 Hz with my Dynaudio Contour 1.3 SEs, and it has rolled off quite strongly by the time things get up to 70 Hz. I don't want noticeable sub output there because it will colour the mid to upper bass sound and become noticeable.

  You haven't said what your mains are, but since you said you want to tame the floor to ceiling mode when you set the REL up in the room, it's obvious the REL isn't causing the problem and it's a room problem that's already present. Given the way that RELs work, it's probably that when you get the REL integrated with your system, it isn't going to be adding enough at 70 Hz to make that particular problem worse. Unless your mains are rolling off very noticeably in the room already at 70 Hz, the REL should not produce a noticeable change at that frequency. You should only notice it's effects much lower down.

  
  David Aiken
  
  

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