Taming Display Cabinet buzz excited by Subw?

Discussion in 'Home Theater Projects' started by Mary M S, Aug 17, 2004.

  1. Mary M S

    Mary M S Screenwriter

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    Any input on the matter would be greatly appreciated.

    This is my setup. LOCATION cannot be changed. 60” LCD screen backed into corner of room with PC-Ultra wedged right behind it. We have left literally only approx. a 1-2” gap between the back of the display cabinet and the sub cylinder itself. The plastic back of display is pie/wedge shaped and is acting like a resonance chamber. The wide end of this triangle pie is of course at face of screen with full-length speaker boxes integrated onto display sides running full-length left/right side of screen. The worst audible vibrations are resonating through the display back and rattling in the speaker ends, outer left/right edge of display.

    Display rests on wood component rack. Metal rollers underneath rack on wood floor (we can not pad the rollers since I pull the whole thing out when added/deleting new components) The cabinet base is 6 component shelves; metal racks for units top two rows, wood base of cabinet is shelf for bottom row.
    Tamed the component rattles in wood rack and the 50pd center speaker resting on wood shelf above display by using various pads.
    Affixed strips of dynamat at spaced intervals down back of display plastic cabinet itself, (staying off all air vents) to dampen the cabinets resonance.

    Not a question of pulling the sub back, as I tend to slightly under integrate sub calb. (after the new wears off) for best blend HT and music and the Ultra is currently running at a fraction of capability. Sub is calb running at ¼ sub gain. –2dB in the Outlaw sub trim, which integrates into room very well (if we can tame last cabinet vibrations. Tuning the sub to 16 or 12Hz tamed the vibrations, but if I bumped up dB’s (to allow for SPL loss due to lower tuning point, vibration levels were back).

    One test CD that excited the most vibration/buzz in room was Fight Club Track 3. I was happy to note after dynamat application to back of display that most of the rattle and sympathetic vibration (almost a buzzing) through the display out to speaker ends was tamed. The right side speaker still has a rattle, which sounds inaccessibly internal to perhaps a mount-point of speaker itself inside cabinet.

    The TRADEOFF. Turns out that the back of the display (figures!) must perform a function as a sound board/box for its own speakers which the dynamat is having adverse damping effects upon. Tweaking trim treble/bass in display menu alleviates the issue, but I have lost quite a bit in sound Q.

    Husband states tradeoff is "good enough" although he freely admits his hearing is not as sensitive or bothered by certain loss or gains in audio quality as my own.

    I want to start stripping off dynamat from center of back of display to see if I can allow the back to resonate more freely as a sound box, for its own onboard speakers; leaving the outer dynam. strips (towards the speakers) in place to tame the vibration, which exits audibly at left and right edges of cabinet due to subw excitation. Or remove dynamat completely and come up with some sort of barrier between the sub (1 to 2” inches away from the back of display) which interferes with destructive frequency waves exciting the immediate back of the display. He tells me let go and let live. I cannot, - I want both the new sub and the original sound quality out of my extremely above average display speakers.

    Any alternate ideas?
     
  2. Chu Gai

    Chu Gai Lead Actor

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    Well don't be so sure about not putting anything under the rollers. HD and Lowes both sell those hard plastic things that you can put under furniture so you can slide stuff around. The plastic itself has this fairly dense rubbery material that the furniture sits on.

    Possibly the back of the display can be removed by unscrewing it and then putting thin rubber washers or O-rings to sandwich between the two plastic parts and then just rescrew.

    Make sense?
     
  3. Andy Goldstein

    Andy Goldstein Stunt Coordinator

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    the "box" the speakers are mounted in, in this case the tv itself, should not resonate. it should be acoustically inert, adding nothing to the sound of the internal speakers. maybe you are used to the coloration that the resonating cabinet is adding to the overall sound, but it should NOT resonate. or maybe i'm wrong, and i should scrap all my 3/4" mdf speaker cabinets and build new ones out of cardboard or 1/4" plywood. [​IMG]

    ag.
     
  4. Mary M S

    Mary M S Screenwriter

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    Chu and Andy thanks!

    I’ll just lay out my ignorance for all to view (nothing ventured).

    I could be way off mark, the sound via the display speakers appears slightly muffled? Reduced? As if dynamic range had been compressed slightly in the upper and lower ends of FR.

    The Display was actually capable under certain well-mixed feeds of creating a very nice illusion of spatial dimension and depth to a soundstage? That appears gone.

    Additionaly, First thing noticable in the changed sound was the fact that vocals seemed harder to understand. If I am viewing late at night (due to small home and many late nights with paperwork) I could pull the volume back to (bars on TV) 3 or 4 (depending on channel feed) keep the sound low and still generally hear all dialogue on any given channel. It appears now that I need more bars (dB) to make human vocals intelligible. 5-6 and more on a very compressed ? feed poorly mixed.

    There appears a loss of formants? In the mid range. When I look up human vocal freq. range I see from various sites approx.: 250 Hz to 3 kHz or 85-1,100Hz. So this is at least partially my prob freq?

    Since the buzz/rattle in the display at certain freq. sounded not just like nuts and bolts rattling due to simple vibration. But rather - partially as if certain frequencies were using the back of the display as a sound box which amplify the particular note/range? (Note: the dynamat worked very well for this issue reducing the back displays 'amplifier' effects + ridding worst of hardware rattle)

    I have noted that speaker enclosure construction is inert as possible. Therefore my surprise when it seemed as if the dynamat damped the FR of the speakers in display as if the display was itself the soundboard of a quitar! Which I just damped by laying a blanket over, reducing its resonance.

    I could be looking the entirely wrong direction, maybe the corner loading of the display which places the speakers along opposing 90 degree walls had a productive reflective? affect on the soundstage of the display. With the dynamat absorbing reflections out of the corner, this positive reflection is gone?

    I have never charted FR of my room. In all the demo grab of DVD/CD material for the new sub have not taken time to order something like the manuf recommended Autosound 2000 (need to research this as I believe it might only cover the lower FR. and I need a disc with boarder range for plotting all speakers response in room). Which I understand, I might in gross form plot using my SPL meter (since I have no measuring devices) to help me see what’s going on.

    I did not consider myself ‘worthy’ of a new sub purchase since I had not taken the time to understand their construction and integration into HT and music. I’m having to play catch-up since the ‘make do’ dead sub forced this buy before I was ready.

    Definitely there is at least some (or more than I credit) vibration of parts. And I dearly hope to talk husband into taking back off display, to try adding the washers for that portion of problem. This would really take two sets hands due to size of unit, and I have a feeling he might kill me if he caught me with my little screwdriver disassembling the LCD!
     

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