Lowest frequency, thought i knew but now??

Discussion in 'Subwoofers' started by MattTheBeanster, Mar 23, 2006.

  1. MattTheBeanster

    MattTheBeanster
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    I'm currently setting up my sub so am rereading a load of material so as to make sure i do it the best i can.

    but i've come unglued on the speaker lowest frequency for the crossover.

    my b&w 602's are specified as

    -6dB at 40Hz and 42kHz 49Hz - 22kHz ± 3dB on reference axis

    so does this mean that their lowest is 40hz, so i can set my crossover there?

    stupid question i know but i'm having doubts. :)
     
  2. Jammyb

    Jammyb
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    no, you need to set your speakers to 'small' in the reciever setup, so your speakers won't be plumbing their depths anymore, then set the sub acordingly, somewhere between 80 and 120hz most people seem to settle for, have a play and see what sounds best.
     
  3. MattTheBeanster

    MattTheBeanster
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    sorry, i meant the crossover on the subwoofer as it's a rel and sounds best under-pinning the fronts, so they need to be set to large.

    if i'm sure what the lowest frequency given the specs then i can work out what to set the sub's crossover at and know for sure what the lowest for crossover on the amp is for the other speakers (which are set to small).

    basically i don't confidently understand -6dB at 40Hz and 42kHz 49Hz - 22kHz ± 3dB on reference axis.

    cheers
     
  4. skinnyfat

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    A lot of manufacturers use way out figures quoted like this;) . Same as rating an amp at 130 W @1KHz 0.5% TDH. (not exactly difficult to achieve). Jammy has given you a good starting point. Remember that crossover does not mean 'dead stop' for speakers and instant take over for the sub. If your x-over is set at 100Hz , your speakers could roll off ~120 -150 Hz.
     
  5. skinnyfat

    skinnyfat
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    oops, writing while yer posting:blush:

    Very basically, they're telling you that the speakers will be able to produce frequencies at 40Hz and at 42KHz BUT at 6dB lower than rated output. Someone :lesson: me if I'm wrong there:)
     
  6. slingshot

    slingshot
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    You're using the speaker level input to your sub, so as you say you need to set the X-Over on the sub, I'd suggest a good starting point would be to set the sub X-Over at 45Hz.

    What you should achieve is something like the sub rolling off above 45Hz, whilst at the same point the speakers are starting to work, I was going to try and draw a piccie but probably not upto it.

    Find some material that has low bass, or a sweep test and see how it sounds. If you notice a dip arround the X-over frequency either see if you can get rid of it by adjusting the phas on the sub, or slightly increase or decrease the sub level.

    Hope that helps

    Slingshot
     
  7. MattTheBeanster

    MattTheBeanster
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    ideal :thumbsup:

    many many thanks guys, that's just what i wanted to hear.

    I have the sub setup ok (now i know i do), but have to play with the center as for some reason it sounds better as large, but i know i lose the sub. Now i know the correct x-over i'll set it to small and try playing with the settings.

    again, thanks for the replies.
     
  8. slingshot

    slingshot
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    To be honest I've sometimes found the centre better set to large, seems to give better dialog especially with male vocals i.e. James Earl Jones and the like.

    I assume if you are using the speaker level connector only on the rel then you've switched you amps sub setting to off, so all bass get's redirected to the mains (and then on to the sub from the mains) ?
     
  9. Badger0-0

    Badger0-0
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    It needs to be bourne in mind that it depends on the speaker/sub setup.
    My MS902's go down to 55Hz, but my crossover is 100Hz, because the sub does it better. The specs only give you a clue, experimentation is all, IMO.
     
  10. MattTheBeanster

    MattTheBeanster
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    The rel can do both at the same time, so i can have the front on large and underpinned by the sub and all the others set to small with the amps x-over sending the low stuff to the sub.

    I'm playing around with settings at the mo but it's hard to find a one setting that suits all solution.

    As badger says experimentation :)
     
  11. rob_w

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    Just for laughs try stuffing the ports of the 602's with a pair of socks and set them to small on your processor, xo'd at 80Hz. Let the processor manage the REL's xo and see what you think. (ditching the speaker level stuff.)

    I used to have 602's btw.

    Rob.
     
  12. Nick_UK

    Nick_UK
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    You have to appreciate that deciBels (dB) are not a linear measurement. When something is "3dB down" it means that the power output at that frequency is half what it was at the reference output (usually 1kHz). A 6db drop means that the power has dropped by half again from 3dB, so the power at 6dB is a quarter of what it was at 1KHz. It's measured like that because our ears do not have a linear response to sound levels.

    The trick is to get your subs to "roll in" where your main speakers are "rolling off", so you get a smooth transition from your main speakers to your sub without any nasty peaks. Not easy to do this with paper specs - best done with a sound meter and some experimentation, or (failing that) with some audio tests.
     
  13. Nimby

    Nimby
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    -3dB is used as a convention by audio manufactures and users.

    You see it all the time to describe electronic equipment.

    +/- 3 dB means the variation from a flat curve could be double the difference.

    +3dB to -3dB = 6dB variation.
     
  14. AngelEyes

    AngelEyes
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    setting your crossover to 45hz will be way to low and leave a gap in your frequency range. As stated 40hz output at -6db will mean the speaker rollof will probably occur further up the frequency range than 45hz.

    Please correct me if I am wrong...

    Personally I would try 60hz or 80hz and plot a few frequency sweeps with an SPL meter for the speakers only and the sub only, to see exactly where the crossover is. :thumbsup:

    Once you can see where the main speakers rolloff you can change the crossover to match and integrate perfectly with your sub. :)
     
  15. Nic Rhodes

    Nic Rhodes
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    I would start with 80hz crossover with these speakers
     
  16. rob_w

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    In theory #if# you have a 24dB/octave crossover (4th order) on your sub then the -6dB @ 40Hz/24dB roll off of the ported b&w's will match correctly.

    However, at some point room gain will kick in, changing the roll off of the mains to a 12dB/oct slope at some point.(depending on room size)

    Thats why I suggested blocking the ports of the 602's - will make them roll off higher (around 80 - 100Hz) and change their roll off to a 12dB/oct slope.

    If set to small the processor will put a 12dB/oct slope on them making a 24dB/oct slope to match the 24dB/oct that it puts on the sub output.

    Hth

    Angel eyes - you have to remember that the xo point -needs- the mains and the sub to be 'down' about 6dB each in order to sum to a flat response. If you had mains -6dB at 40Hz and the sub -6dB at 80Hz you'd have a hump from about 40 - 80Hz...
     

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