Amp and Subsonic filters

Discussion in 'Archived Threads 2001-2004' started by Rudy D, Feb 25, 2002.

  1. Rudy D

    Rudy D Extra

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    I have a Crest Audio La901 amplifier. The manual says it has a built in 6db per octave high pass filter cornered at 8 hz to provide subsonic frequency protection. Is this good enough. I am looking to build my own sub, and was interested in a BFD. I read it did not have a subsonic filter. Will my sub be safe with my amplifier and a BFD. Or, can anyone recommend an eq for the same price as the BFD. Thanks,
     
  2. Harry Lincoln

    Harry Lincoln Stunt Coordinator

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    Rudy, is the subsonic filter adjustable or is it fixed at 8hz? If it is fixed then it would be to low for a DIY sub. As far as I know most DIY subs are tuned to around 15 - 20hz. The purpose of a subsonic filter it to cut all frequencies below the tuning point, so if your sub is tuned to 16hz your filter should be at 16hz.

    The BFD does not have a subsonic filter. By building a sub with a lower tuning point (eg. 16hz vs 20hz) you will reduce the chance of bottoming out the driver because there will be a lot less information that is below the tuning point. I have two shiva sonotubes tuned to 16.5hz powered by 185w amps, I have measured 128db peaks from the subs and have never bottomed them out, I do not use a subsonic filter.

    It all depends on the type of sub you intend to build, the size of your room, and how loud you want to play it.

    Harry.
     
  3. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    The corner frequency of a filter is typically the frequency at which the signal is attenuated 3dB. When you tune a subwoofer to, say, 16Hz its because you want flat response down to 16Hz. So it wouldn't make sense to use a hi-pass filter with fc=16Hz. You'd lose that flat response. Instead you want to set the corner frequency far enough below your tuning frequency so that it does not significantly attenuate the frequencies where you want flat reponse.

    The filter Rudy described is only a 1st order - 6dB/octave. So the rolloff into the filter band is very wide. Even with fc=8Hz you'd still be losing almost 1dB at 16Hz (assuming Butterworth filter). That's likely why they used such a low corner frequency.

    In this application I would prefer a 4th order Butterworth filter centered at 12Hz. This would attenuate the signal by less that 1/2dB at 16Hz. Still, a 1st order filter is better than nothing. And there's not likely many high amplitude signals below 16Hz anyway.

    These numbers assume that I've done my math right. My text only has the appropriate equations for low-pass filters and I'm winging it from there.
     
  4. Rudy D

    Rudy D Extra

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    I am confused about first order, and fourth order. Also, what is a Butterworth filter? What is the answer for my original question? Do I need to get an EQ or something with a better, or adjustable subsonic filter? Thank you very much for your help and input. Rudy
     

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