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Does bass extension matter for rear (80 vs. 120Hz speakers)?

Discussion in 'Speakers' started by ChrisAG, Mar 31, 2003.

  1. ChrisAG

    ChrisAG Supporting Actor

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    I'd like to expand my 6.1 system to 7.1, for which I would purchase a pair of dipoles for the sides to compliment the monopole rears. The choices, to match my fronts, would be Mordaunt-Short 903S bipoles or 506 THX dipoles.

    The delima is that while the 903S' cost less than half as much and are smaller, they are only rated down to 120Hz. The THX versions are larger and more costly, but go down to 80Hz.

    If I get a receiver with a fixed crossover of 80Hz, will that leave too much of a bass 'hole' in the rear if I buy the 120Hz speakers? The rear centres will be MS 902 monopoles, which can go down to 55Hz. I'd rather not get a subwoofer for the rear at this point.
     
  2. ChrisAG

    ChrisAG Supporting Actor

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    So no-one knows the answer to this one, hmmm?

    Remember, you don't need to own Mordaunt-Short speakers to contribute to this thread! [​IMG]
     
  3. Edward J M

    Edward J M Cinematographer

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    My $0.02:

    Yes, a speaker rated -3 dB at 120 Hz that is high passed at 80 Hz will create a dip in the frequency response for that channel between approximately 125 Hz and 65 Hz.

    How audible that will be in your system depends on a variety of factors, and cannot be easily answered. One factor certainly will be whether the rear surround channel contains signal information below 120 Hz on any given DVD. If it does, I would suspect the rear channel would sound a little "thin" for that passage on the DVD.

    My rule of thumb is to high pass a speaker at least 1/2 octave above its F3 point to avoid this depression in the FR. Remember, the selected filter point is not a brick wall - the high passed speakers will still be required to play well below the filter point, at a progressively decreasing rate of probably 12 dB/octave or 18 dB/octave. When selecting a high pass filter point, it should be a given that the natural FR of the speaker will remain flat to at least 1/2 octave below the selected filter point. For example, I use an 80 Hz high pass, and the highest F3 of any of my surround speakers is 55 Hz.

    Regards,

    Ed
     
  4. JimmyK

    JimmyK Second Unit

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    Jim
    OK, I'll give it a shot.

    Before I upgraded my rear speakers, I was using some small Radio Shack speakers (sealed enclosure, if that makes any difference) that probably didn't do much below 100hz or so. My processor had a fixed crossover at 80hz. Also, I am only using a 5.1 system.

    I never really noticed a "hole" in the bass in this setup. I suspect that most of the bass in program material comes from somewhere in the front channels or the LFE channel, so as long as there wasn't a "hole" up front, it wouldn't be very noticable if at all in the rear. Others may disagree.

    I'm not sure if this would be different in a 7.1 setup.

    If you can buy the speakers from a dealer that will allow you to exchange the speakers if you don't like them, then try the less expensive ones first. If exchanging is not an option, then I might want to err on the side of caution and get the bigger speakers (assuming you have the room for them) just in case it does make a difference.

    I hope something in here has helped you.

    JimmyK
     

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