Is 12dB/octave sufficient for subwoofer? - diyAudio
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Old 25th February 2005, 03:17 AM   #1
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Default Is 12dB/octave sufficient for subwoofer?

for the low pass section, many people use 24dB/octave (using 2x 12dB/octave linkwiz circuit)

Is 12 dB/octave sufficient? Basically to make 24dB/octave filter, a 4-gang pot is required and those are hard to come by.
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Old 25th February 2005, 03:27 AM   #2
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12db/o is often insufficient for the low pass section of the filter. You only need a pot if the filter needs to be adjusted. Fixed resistors are a better choice anyway, after you zero in on the frequency.
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Old 25th February 2005, 08:39 AM   #3
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fixed resistor is a good idea, but i want it to be adjustable. Any where to source for a 4-gang pot?
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Old 25th February 2005, 09:06 AM   #4
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Whether you use 12dB/oct or 24dB/oct depends on your main speakers. Sealed required 12dB/oct to match them, vented 24dB/oct.

What I did was use 2 dual gang pots. I also put one output socket after the first stage to give me a 12dB/oct connection if I wanted it. I mostly use the output socket after the second stage though as my speakers are vented and the steeper cutoff also sounds better to me.
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Old 26th February 2005, 08:39 AM   #5
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My main speakers are ported, does this means i would be better off with 24dB/o?
I see some comercial subwoofers with 18dB/octave and 36dB/octave, how come is that?
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Old 26th February 2005, 09:11 AM   #6
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What I've heard (and believe)...

24db/octave is much better. You don't want to hear the sub in the range where it breaks up.

If your mains are ported, you will probably be better off to stuff old socks in the ports when you add the subwoofer.
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Old 26th February 2005, 11:16 PM   #7
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What you need in the way of a crossover is entirely dependent on the *actual* response of both your mains and your subs as well as the xover frequency.

It is impossible to make a general rule for the slope.

One method to arrive at the proper relationships is to measure the mains and the subs - subs without any xover. Then look at the response. Then listen to the mains.

Are you going to permit the mains to roll off "naturally" or are you going to select a high pass filter? (you might do well like this with a small sealed box "mini-monitor on top, for example)

If it is going to roll off "naturally" the the Q of the mains near F3 plays a role in the overall sound and xover choices.

Many people feel that an electronic highpass degrades the sound sufficiently to want to avoid that. Ymmv.

Add the proposed high pass filter, listen again. Measure the resulting response.

Why go through all this, you may be asking? Speakers are non-flat devices and what counts is the *acoustic* response, not the imagined "text book" response...

Now you can start to add the subwoofer in... is the subwoofer's response *flat* from your selected F3 xover point? Did your main speaker's response end up down 3dB and then 12 or 24 dB where you expected it to be? Does your subwoofer's response *rise* as it goes higher in frequency, does it have any peaks?
(most do)

After you answer all these questions and find solutions, you can try to make a crossover that yields a reasonably flat transition through the xover region. Probably more important is the "Q" of the resulting sub woofer system, especially near the xover point... that's what will help it appear to blend seamlessly - along with the choice of xover frequency...

In general the *higher* in frequency the xover is the more critical the subwoofer becomes... lower in frequency, the less critical overall, but the mains have to work harder, have higher distortion and if headroom is also an issue, that is reduced compared to a higher xover...

Stated simply, applying a 12 dB/oct xover at say 100Hz. doesn't assure that you will *get* any roll off at all at 100 Hz. - if for example the driver is climbing at 6dB/oct in that region... and if it is, then applying a 24dB/oct xover will get you an 18dB/oct result.

Of course you can argue that a first order xover is sonically and electrically the best as far as the seamless recombination of the two...

Good idea to measure and select the xover slope and frequency, and any compensation elements (as desired) based on the actual frequency and phase response.

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Old 27th February 2005, 02:29 AM   #8
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Undoubtedly, to get the best results, one would measure the actual system and design a custom crossover. For some of us simple folk (or busy folk), that's not really an option. What we do is buy a sub amp or an HT system with a built-in electronic crossover and then alternate guessing and tweaking. If one uses the G&T method, a 24dB/octave lowpass filter on the sub is a Good Thing.

Below is what Darren Kuzma of PartsExpress wrote in his review of the 300-804 plate amp, which has a 24dB/octave lowpass filter. Interestingly, PE does not advertize the feature in their catalog blurb for the 804, probably because their more expensive plate amps have a 12 dB/octave filter.

<QUOTE>
There are several problems associated with the common 12dB/octave filters found on other amplifiers. The shallow slope does not allow good integration with mid-sized main speakers. It is difficult to achieve a good transition without getting overlap in the lower midbass region, which can cause a tubby or boxy sound. Also, many subwoofers have large response peaks in the 800Hz-2kHz region. If not attenuated enough, these peaks can "bleed through" into the output from the subwoofer, causing what could be described as "ringing" sound. Not only is this annoying, but it can result in a fatiguing system and a subwoofer that does not "disappear" into the room.
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Old 27th February 2005, 05:39 AM   #9
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Thanks for the detailed answer, gentleman.
Got a few questions I want to clarify:

Quote:
Stated simply, applying a 12 dB/oct xover at say 100Hz. doesn't assure that you will *get* any roll off at all at 100 Hz. - if for example the driver is climbing at 6dB/oct in that region... and if it is, then applying a 24dB/oct xover will get you an 18dB/oct result.
Why would my driver want to do that? typically subwoofers(in a box) are flat from their tuned frequency to their upper limit before it naturally rolls off. If it starts climbing at certain upper frequencies, that wouldnt be a woofer driver anymore right?

From what I understood, the woofer and box design will determine the lower freq roll-off, and basically the crossover will take care of the upper freq roll-off, right?

And the most important thing is to make sure the sub upper roll-off matches the mains lower roll-off in order to blend seamlessly? And that is why 24dB/o is required when my mains are ported?
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Old 27th February 2005, 05:52 AM   #10
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Quote:
Originally posted by skyraider
Why would my driver want to do that? typically subwoofers(in a box) are flat from their tuned frequency to their upper limit before it naturally rolls off. If it starts climbing at certain upper frequencies, that wouldnt be a woofer driver anymore right?
No, many subs are more efficient in the upper range by 6dB or so.
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