Riser Help (acoustic nightmare)

Discussion in 'Home Theater Projects' started by Anthony Curtas, Jul 23, 2003.

  1. Anthony Curtas

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    I have threads going on HTT, AudioAsylum, and others, but I think I have the problem narrowed down and want your all's opinion.

    Last summer I built a riser for my HT. You can see it here:
    http://www.geocities.com/acurtas/ht/base_layout.html

    Recently, while tuning my subwoofer, I found a huge suckout from about 60 Hz to well above 80 Hz, and I couldn't place it at all.

    Modal distribution was not great, but wasn't bad. Sub and listenening position weren't on nulls.

    So on a whim, I turned my attention to the riser and panel resonators.

    The riser is simply a frame of 2x10's with 1/2" plywood on top. Two layers of regular fiberglass insulation line the inside. Note that I said regular insulation, the kind with thick paper on the back.

    Based on this website:
    http://www.mhsoft.nl/Helmholtzabsorber.asp

    I found out that my riser, unstuffed resonates at ~51 hz. Then I read that stuffing lowers the resonance point, so I figure I'm fine, right. Wrong. On all the plans I've seen for panel resonators, they use open fiberglass and acoustically transparent fabrics as to not reflect the sound. Figuring about 7" of insulation, that leaves about 2.5" of free space and a resonance of 100 Hz. Average those two values, and you get ~75Hz, which is exactly where my suckout occurs.

    I BUILT A PANEL RESONATOR BY ACCIDENT AND IT'S SUCKING OUT MY BASS!!!!!!! [​IMG]

    So now my question is, what do I do? What do you guys with risers do? Fill them with sand? Stuff the insulation really densely? Add multiple layers of plywood for stiffness?

    All help is appreciated. At least I feel like I'm onto the solution now.

    Thanks,
    Anthony
     
  2. Chad Anson

    Chad Anson Second Unit

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    Lots of folks, myself included (my riser is about 1/2 complete), use the following guidelines for a seating riser-

    * Stuff completely with insulation. Many use blown-in cellulose insulation to more easily fill the cavities. Use sand if you want to go all out.
    * For the top, I don't think one 1/2" sheet of plywood is enough. I'm going with two 3/4" sheets glued and screwed together. I've seen people sandwich a sheet of 1/2" plywood between two sheets of 3/4" sheets.

    Good luck!
     
  3. Anthony Curtas

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    That's my problem, I cheaped out and didn't get the extra layer or enough insulation to fill it up.

    When I built it, it was the last piece of the HT upgrade, which was already $200 over budget from extra paint, moulding, supplies, etc (this was just painting and construction, no gear in this budget). So I thought saving $50 was a big deal. I wished I'd spent it now.

    But I always thought it was a bit flimsy inbetween the joists, so some extra support is definitely in order.

    And hey, now I know how to build a resonator if I ever need one [​IMG]

    Thanks,
    Anthony
     
  4. Jay Mitchosky

    Jay Mitchosky Producer

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  5. Anthony Curtas

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    Eh, it's not that bad. 3 plywood panels are all that has to come off. The carpet is taped with a few staples. Plus, one of the plywood seams is not directly on a joist and I want to fix that and prewire for the bass shakers.

    So as long as I'm knocking out multiple projects . . . [​IMG]

    I'm just hoping it works and fixes the problem. While I usually trust math, this one may leave me wanting. We'll see.

    thanks all,
    Anthony
     
  6. Anthony Curtas

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    Well, the problem is fixed, a bit . . .

    http://www.geocities.com/acurtas/ht/base_layout.html

    I stiffened up the couch riser in the back, which was acting as a frequency
    damper (see chart at bottom of page). I added support joists and stuffed it
    with insulation to the brim. Then I added another 1/2" of plywood to the
    top. ~1" total thickness now -- screwed every 12", but not glued, in case it needs internal wire maintenance.

    The results are astounding. It did not completely fix the problem, but some frequencies are up 10dB!!! Well above the noise floor (~60dB)

    Everything else in the experiment was held constant (source, volume @ 40 Hz, mic position). Still some work to do, but overall, much better.

    Sound is better too, more articulte. I can definitely hear what I was missing.

    Id didn't completely fix the problem, so I'm going to try another crack at EQ'ing it, but overall, I'm happy with the results.

    thanks for the input,
    Anthony
     
  7. MikeWh

    MikeWh Second Unit

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    Thanks for posting the results, Anthony. I know it'll help me and others. Be sure to post any further improvements!

    I wonder if a firmer/denser surfacing material would improve the results-- like 1/2" or 3/4" MDF (instead of the additional 1/2" plywood). I'm thinking that it would greatly reduce the transmission of the bass into the riser, in addition to reducing the "drum effect."

    My thoughts-- if I tap on one side of 1/2" plywood, I can hear the deep thud very well through the other side. Do the same with 1/2" MDF, and it sounds completely different. What do you think?

    Also, I didn't notice until just now that the sub is actually sitting on the riser. How does it sound when it's not on the riser? I'm assuming you're getting a deeper or more hollow bass with it on the riser, as it is essentially driving the riser.... I'm just interested to see what the difference is.
     
  8. Anthony Curtas

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    Thicker is definitely better. If I had this to do over, I'd go with actual high quality subfloor (tongue 'n' groove) with maybe an mdf or regular plywood sheet on top. Roofing felt between the layers would help with damping.

    Also, if this were a truly permanent installation (I only plan to be at this house for another 5 years or so), I would have caulked the bottom of the riser and filled the whole thing with sand.

    As for the bass quality: the sub on the riser is no longer a problem. The tight bass I was used to is back. Hard to test now, since there are no good spots in the room other than the corner the riser is on.

    The biggest difference, is that now the riser feels like a floor and not a piece of furniture. It is VERY solid. If the entire floor had the same carpeting (other floor is concrete w/ vinyl tile and a big area rug with padding -- covers almost the entire area between couch and TV).

    If I stomp, I can still hear the thud, but it damps quickly. During reconstruction, I stomped on one side (fixed) and then the other (original) and the difference was amazing. One sounded like a normal floor, the other like a kettle drum.

    So I think this problem will prevail everywhere, but this is definitely a step in the right direction.

    Anthony
     
  9. Trevor_J

    Trevor_J Second Unit

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    Interesting results. Out of curiosity, what sub are you using?
     
  10. Anthony Curtas

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    DiY NHT 1259 sub I made a few years ago. Mostly MDF (baffle and rear are birch due to some cutting mistakes DOH!). Braced like hell too, thing weighs more than 100 pounds. Sealed design; nearfield measures flat from about 27 Hz through 100 or so.

    Driven by an NHT SA-2 sub amplifier. 130W into 6 Ohms, I think. I'd have to check the manual for exact specs. Blows me out of the room at minimal gain setting, so I'm not lacking on power.

    I think now that I have the resonator effect "removed" I can try EQ'ing to get the rest back. Otherwise, I'll look other places for sources of "suckout" and nulls.

    a
     
  11. Dennis Erskine

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    ...hum...flexible riser absorbing bass...the wrong bass frequency at that. Imagine what soft flexible walls are doing.
     

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