Suggestions on taming a 10db 30hz peak...

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  • Sonnie Parker
    • Jan 2002
    • 2858

    Suggestions on taming a 10db 30hz peak...

    Some of you may remember back when I finally got my HT room finished and got my BFD all setup that along the back wall I have a huge 10-12db rise at approximately 30hz. Some folks kinda like it while sitting back there but I know it's not uniform and correct so it has been bothering me somewhat.

    I've given some thought to it and in discussing this with a technical engineer at an acoustical lab he has made a suggestion that I use the stuffed rear corner as a bass trap by cutting out a port in that corner wall. He says this might work but he couldn't guarantee anything. Obviously he couldn't get into the technical details of what size port/hole because it would have envolved me giving him a lot more particulars and him having to do some figuring that I don't think he really wanted to get into. Anyway he left me scratching my head.

    I thought I'd try to give you guys the particulars and see if you might have any suggestions to if this would work and what size hole I might start with.

    Here's a layout of the corner:



    Basically you can see the size of the stuffed corner is 28" in width on the inside of the room with the 2 outside edges being approximately 22" in width. As you go inside the stuffed area it is not sealed off from the staggered studding throughout the walls thus any frequencies that enter into this cavity will travel not only into the cavity but into the walls as well. The ceiling part is sealed.

    Here's a pic of the wall with the section highlighted in lime green:



    Not easy to draw a straight line. Anyway... somewhere on that section my plan is to cut out a hole exposing the stuffed cavity. Since it's 28" wide and I need room to attached a fabric covering (or cover it with an acousitcal panel) I don't want to cut the hole any wider than about 20".

    The question is what height to start with.

    I know I could simply start out with say a 12" H X 20" W hole and do some testing and then go bigger if needed... but before I go cutting a hole in the wall I'd kinda like to know if you guys think that this sort of trap could nab a 30hz frequency and not cause a lot of other problems like snatching too much of my bass.
  • brucek
    HTG Expert
    • Aug 2000
    • 303

    #2
    Sonnie,

    I've never built any traps before, so I hope others can answer better than I do, but I think you're smart to try and trap some of this 30 Hz problem rather than BFD it away because you want to also keep the front row seats happy. No matter what you do though, it's really hard to get an even low frequency response at all seating positions (which you already know of course).

    Your theater length which I believe to be ~19 feet makes the number one primary axial mode at 30Hz (29.711Hz), so that's where the problem is originating from and will of course be the loudest close to the walls. Thirty hertz is a fairly low frequency to trap away. 8O

    Since your theater is already built and you're trying to use an existing corner volume of space to create a trap, perhaps it would be best to trial and error it. By this I mean placing your RS meter in the offending seating area and playing tones around 30Hz until you arrive at the center frequency of the problem. Then while playing that frequency and using the readout from the RS meter, modify the trap to be the most efficient.

    You're trying to trap a specific frequency, so I believe the membrane absorber type of approach which is used for broadband absorption won't work - they're usually for higher frequencies than 30 Hz anyway.. I think you'll need a Helmhotz type resonator which can be designed for narrow bandwidth to remove a single problem frequency.

    You'll need to first calculate the volume of your enclosure and then create the appropriate cutout. I have fooled around before with the site listed below and it seems pretty good once you know the volume. It has all the calculators included. It's a start anyway.



    brucek

    Comment

    • Sonnie Parker
      • Jan 2002
      • 2858

      #3
      Hmmm... nothing like a real challenge huh? Thanks for that link.

      A Helmholtz absorber is simply a box with a port on its front side to couple the enclosed volume of the airspace in the box to the air in the room. The depth of the enclosed airspace in the box behind the port and the width and depth of the port control the resonant frequency of the bass trap.
      This sounds kinda like what I'd be doing but wow I don't know how I'd ever calculate the volume with that corner not really being sealed away from the staggered studded walls. Trying to figure out metric and on top of that formula's has never been my forte.

      None the less... it sounds like I may be on the right track but as you suggest via a trial and error method to figure the port size.

      I think what I might do is take measurements from 16hz thru 125hz (1/6 octave) on the back row. Then I'll cut out a small port about 6" X 12" and retake measurements to see if any of those frequencies are effected. Ease up to 6" X 20"... remeasure... then 12" X 20" and so forth. Continuing only if I see that I'm getting closer to that 30hz problem.


      You know... it might even be that I could cut slots in the rear wall every few feet... using the walls as a trap since they are insulated. Could be a lot of covering up holes if it didnt' work though.

      Comment

      • ThomasW
        Moderator Emeritus
        • Aug 2000
        • 10934

        #4
        Sonnie,

        I use 1/4th cylinder floor to ceiling height bass traps in the corners behind each of my planars. They're 24" radius, filled with standard 6" fiberglass batting. They're quick, easy and low cost to build.

        There have been several threads on a couple of forums, so I've been harvesting links. The first is worth saving to disk.

        The one below has a nifty corner design. The site is British so substitute fiberglass batts where they say mineral wool (rock wool)
        Bass Trap Basic Design Principles for the DIY customer to implement in their own basstrap builds



        If you do some measurements and 'map' the room, and know the exact frequency, then helmhotz designs work very well.

        IB subwoofer FAQ page


        "Complicated equipment and light reflectors and various other items of hardware are enough, to my mind, to prevent the birdie from coming out." ...... Henri Cartier-Bresson

        Comment

        • Sonnie Parker
          • Jan 2002
          • 2858

          #5
          I'll investigate those links Thomas... thanks.

          I am trying though... to keep from taking up anymore room in my already tiny room. I'd really have to bring my sub out from the corner in order to get a bass trap in the corner and that really messes up my response. I could put one in the other front corner with no problem though. They wouldn't work in the back for lack of room and the angled corners to boot.

          Doesn't sound like there will be an easy solution but it might be fun trying anyway.

          Comment

          • Sonnie Parker
            • Jan 2002
            • 2858

            #6
            lol... I actually already had your first link bookmarked Thomas. I clicked on it and said to myself, I've seen this before... sure enough... I've actually used it in some of my acoustic post before. Of course all those materials are mostly good for 125hz and above with one type (Ottawa Fibre) extending down to 80hz.

            Now the second link is pretty interesting. It shows corner traps but indicates a heavier solid panel over a larger opening might trap the lowest frequencies. This could be something to consider. If simply using a port does not reach down and trap the 30hz area then it might be that I can cut out even a larger area and use a large resonating panel in front of the opening in the corner leaving some air space between the panel and the stuffed insulation. Still sounds like a lot of experimenting is in order.


            That third link... hmmmm... scientifically way too advanced for me. I mean really... look at my avatar. :B

            Comment

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