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I build rotary subwoofers that produce ULF (subsonic) sound levels (youtube) . The amplifier that I use is 12V 350W and supplies up to 25 amps.

I would like to create a reasonably high quality, strong passive filter that has a cut off frequency of 22 Hz.

I would also like to use nth order because it must have a very steep roll-off, e.g. nothing over 30 Hz max. My only concern is that the roll off will affect frequencies below 20 Hz. Is this possible without having to use digital controllers?

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  • \$\begingroup\$ yes active filter design tool at Ti.com easy peasy \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 31 '17 at 22:02
  • \$\begingroup\$ @TonyStewart.EEsince'75 true, but he's explicitly asking for a passive filter \$\endgroup\$ – Marcus Müller Aug 31 '17 at 22:04
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    \$\begingroup\$ For passive filters it is not easy to have high order LPF with good dampening with just air, better to make it flat response with servo control using reflector on cone and active filters. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 31 '17 at 22:07
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    \$\begingroup\$ Chris, it's much easier to read if you pay more attention to the capitalization of words (especially at the beginning of sentences). Since good readability increases the likelihood of people reading your question, I took the freedom of improving your question with respect to that slightly :) \$\endgroup\$ – Marcus Müller Aug 31 '17 at 22:07
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    \$\begingroup\$ "cannot go over 30 Hz" is easy with a low-power active or digital filter feeding an amplifier, but very hard with a high-power passive filter. And no, you're totally wrong about efficiency. The passive filter is inefficient. Only amplifying the signal that you sub needs is power-efficient. \$\endgroup\$ – Marcus Müller Aug 31 '17 at 22:53
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Just throwing together some numbers:

Per passive RC filter stage, you get 6 dB roll-off per octave.

You need your transition to happen between 22 Hz and 30 Hz, that means you want some let's say 40 dB attenuation in ca half a decade. That demands, very roughly, a steepness of 80 dB per decade. So, a 13-stage passive filter.

So, we can simply rule that out practically.

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  • \$\begingroup\$ thanks, even 13 stages would be fine. this is a specialist unit and would like customers who want these units to be satisfied :) \$\endgroup\$ – Chris Hudlin Aug 31 '17 at 22:47
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    \$\begingroup\$ no. 13 stages is just necessary. It's not a good filter. The losses would be humongous, the device would be huge, the accuracy would be bad, the phase linearity would be terrible, the price would be high, the thing might be about as heavy as the subwoofer. If you got customers with any idea of what a good device is, then they won't accept this. \$\endgroup\$ – Marcus Müller Aug 31 '17 at 22:49
  • \$\begingroup\$ so what would you propose to keep the output clean and have a steep roll-off but also power efficient? \$\endgroup\$ – Chris Hudlin Aug 31 '17 at 22:55
  • \$\begingroup\$ Tony and I have been repeatedly telling you. I really don't think I need to repeat this,but: Have two separate amplifiers, one for the sub, one for the rest. These amplifiers have active or (probably better, even) digital input filters (or they are actually DACs and get digital signals instead of analog ones, and then only get the right frequency ranges to begin with).That's cheap,it only requires each of the amplifiers to produce power in the bands that their speakers are working in,and you don't burn lots and lots of watts in the passive output filters.You get adjustable phase delay,too. \$\endgroup\$ – Marcus Müller Aug 31 '17 at 22:58
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    \$\begingroup\$ An active lowpass filter built around some opamps (Or a DSP) placed in the signal line BEFORE the amplifier is what you want. I would be careful about very high order filters, they usually sound dreadful, but your choice. Personally I would be very tempted to get a cheap loudspeaker management system and have a play with it, there are not expensive boxes these days and will let you fiddle with filter order, phase and delay settings easily. \$\endgroup\$ – Dan Mills Sep 1 '17 at 15:22
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How about this filter @Marcus? One Quad Op Amp with same specs. ( but terrible Group delay during cutoff.)

Ultimately the design specs are wrong and the OP needs to become familiar with how to write specs by learning how these designs work well rather than arbitrary numbers...

Amplitude, distortion , Group Delay error, passband ripple, band-reject f, and attenuation all affect sound and need to be specified. enter image description here But for giggles here's another result of an active filter

enter image description here

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