SubSonic Filter - Home Theater Forum and Systems - HomeTheaterShack.com

Old 09-14-09, 02:02 AM Thread Starter
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Ryan

Join Date: Jul 2009
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SubSonic Filter

If a 24dB/octave subsonic filter is set to 20Hz, does that mean 21Hz is unaffected, 20Hz is the least attenuated, and at 10Hz you are down 24dB?

Thank you.
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Old 09-14-09, 10:04 AM
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David

Join Date: Apr 2008
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Re: SubSonic Filter

Is it passive or electronic? Generally the frequency setting given for a passive filter is the -3db point, meaning you are -3db @ 20Hz (and -24db @ 10Hz.) Electronics list the settings in various ways depending on the make/model of the unit. What Xover are you using?
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Old 09-14-09, 10:33 AM
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ec

Join Date: Jan 2008
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Re: SubSonic Filter

Depends on the type of 24dB filter. Most common in this situation would be Butterworth or Linkwitz-Riley, probably in that order. Butterworth is -3dB at corner frequency, and L-R is -6dB. Rest of the frequency response is asymptotic to the ideal you allude to, flat in passband above corner, 24dB/oct slope below corner. Just have to smoothly add in that corner point, and you pretty much have it. If interested, look up the concept of damping and how it relates to frequency and phase response.

EDIT: Also, Butterworth and L-R are just two discrete points in a whole continuum of possible 24dB/oct filters, with damping corresponding to Q=0.707 and Q=0.5, respectively.

Last edited by ec114; 09-14-09 at 10:43 AM.
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Old 09-14-09, 04:59 PM Thread Starter
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Ryan

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Re: SubSonic Filter

I saw a post the other day where someone's SSF was set to around 19Hz and they were tuning to 20Hz, Steve said it was a waste, that he needed to tune higher.

I guess what are the rules of thumb... SSF 5Hz below tuning? 10Hz? I know more needs to be known...enclosure, power given... so plotting should be done for tweaking.
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Old 09-14-09, 07:00 PM
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Glen

Join Date: Mar 2009
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Re: SubSonic Filter

After looking at numerous hpf and endlessly modeling a few drivers, but not knowing detailed specifics, 1-4hz below tuning seems prevalent and useful. Between 2-4hz, cone excursion often drops within limits without affecting the breakpoint of the low end too badly. Perhaps someone else will shed more light on this.
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Old 09-14-09, 07:18 PM
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David

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Re: SubSonic Filter

To know with certainty you need to model the system and find out where the driver exceeds Xmax, but generally setting the subsonic at approximately the same as the port tuning frequency is sufficient to protect the driver without significantly limiting output.
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Old 09-14-09, 09:24 PM Thread Starter
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Ryan

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Re: SubSonic Filter

3 alignments, each plotted twice, once without filtering and once with Highpass butterworth n=2 Fc=25Hz

[IMG] http://i248.photobucket.com/albums/g...5/FRvshpf2.jpg [/IMG]

[IMG] http://i248.photobucket.com/albums/g...e/mix5/x-1.jpg [/IMG]

rather than seeing 25Hz on down beeing affected, I am seeing both sides being affected
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