What is the likely cause of my problem?

Discussion in 'Archived Threads 2001-2004' started by Dom H, Nov 26, 2001.

  1. Dom H

    Dom H Agent

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    Room
    11ft * 12ft * 8ft (ish)
    Lots of soft furnishing, not too live but several large parallel surfaces, worst front to back walls.
    System
    Sony DB-930 Receiver
    B&W 602's
    Problem
    2 ch audio & 5.1 (dont usually use center) both affected:
    Peakyness and muddy sound (dialogue) at high volumes.
    Moderate volume sounds fantastic but get to a certain point on the volume knob and dialogue becomes confusing and muddy. Theres a definite peakyness, certain frequencies shriek out. Mainly male dialog, female voices are much nicer.
    Placing lots of absorbing material between the back wall and speakers solves the peakyness (standing waves?), dialogue is alot more understandable but still sounds muddy, not very natural sounding.
    I think part of the problem is the parralel surfaces and the other is the amplifier in hte sony receiver. I am looking into 2 Marantz mono amps for the mains.
    Room Layout (accurate)
    [​IMG]
    Many Thanks
    Dom
     
  2. ThomasW

    ThomasW Cinematographer

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    What's the object between the speakers?

    If it's a RPTV then that's the problem. You're getting comb filter cancellation from the frequencies reflected off the screen
     
  3. Dom H

    Dom H Agent

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    Its a 29" Direct view set sitting on a low unit. Top of tv is aprox inline with eyelevel.

    Any quick fixes?
     
  4. ThomasW

    ThomasW Cinematographer

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    Only quick fix is to pull the speakers farther out from the wall. Maybe throw a piece of carpet/thick towel/whatever over the picture tube see how that impacts the sound
     
  5. Bob_A

    Bob_A Supporting Actor

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    Have you tried using different receivers and/or speakers in your room?
     
  6. ThomasW

    ThomasW Cinematographer

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    The B&W 602's are a nice neutral speaker. The Sony receiver is ok, but certainly not up to the quality of the speakers.

    Given the fact that the room isn't all that large I doubt that you're overdriving the amp but that's certainly a possiblilty.

    Where is the volume knob when the problem occurs? If it's higher than 12 o'clock then you are probably driving the receiver into clipping, and hearing gross IM and THD.

    Another possiblity is if the seating position is immediately in front of the back wall. Reflections off the back wall can muddy the sound when they interact with the front wave.
     
  7. Dom H

    Dom H Agent

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    My previous speakers (same receiver) had the same problem, in fact every system I have heard in a small room has had the same sorta sound up loud.

    The volume knob is at about 10 o'clock plus where sound gets muddy.

    I seem to have solved the problem, putting a thick pillow behind my head always helped but I also moved the speakers towards me, away from the wall and to my right, keeping the left speaker out of the corner. It sounds SO much better, just to see I pushed my speakers right back against the wall and it was horrible. I did notice that if I carried on pulling them away from the wall it didnt sound as good.

    Could someone please explain to me why having speakers against a wall is a bad idea? I understand what a standing wave is and why early reflections are bad but I can't see what this has to do with the proximity of the speaker to the wall.

    Many Thanks, much apreciated.

    Oh, any comments on upgrading the amplification to the Marantz mono blocks, likely improvement?
     
  8. Guy Kuo

    Guy Kuo Supporting Actor

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    Proximity to a wall, ceiling, or floor maximally excites room modes which involve that surface. The room modes are typically in the mid to low bass region. If the low frequency room modes are getting energized by your speakers you get a preponderonce of bass. Add some soft material in the room absorbing high frequencies faster than low frequencies and you soon end up with a system in which low frequencies tend to linger much longer than high frequencies when a sound stops. Basically you get a RT60 which is very long for low frequencies and short for highs. In other words, muddy sound.

    Another thing I note from your drawing is that the speakers are asymmetrically placed relative to the left and right walls. The effect of that uneven placment is that the left and right channels will differ in their frequency response. This difference will reduce the tightness of the sonic imaging. If you can at all rearrange things to achieve left/right symmetry with respect to the side walls and speakers, it would help your situation.

    The advice given previously for keeping speakers away from walls and objects also bears some strong consideration both in terms if avoiding diffraction/reflection effects ruining imaging and reducing excitation of primary room modes.
     
  9. ThomasW

    ThomasW Cinematographer

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    BTW, do you have a RS SPL meter? What SPL do you consider 'loud'?
    The Marantz amps should be a bit of an improvement.
    I'd consider something like the Anthem MCA 2 Series II amps from Sonic Frontiers. These are a moderately priced neutral sounding amp
     

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