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Old 16th November 2009, 09:32 PM   #1
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Default main low end roll off

I'm designing a HT speaker system and have a crossover question. There is a crossover between the main channels and the sub/lfe, usually 40-80 hz.

Is the roll off of the main supposed to be a particular slope? Most systems must assume something since receivers normally just allow one to pick the frequency, not the slope. Should the box be .577, .707?

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Old 16th November 2009, 09:43 PM   #2
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Sub LFE frequency setting usually just affects the sub. The whole idea of the LFE channel is that it is separate from the main channels and is band limited by nature, i.e. the soundtrack on the DVD will only contain upto 80Hz.

The reason some amps allow you to change the frequency (it's still only changing the low-pass frequency on the sub channel) is because they have blurred the line between HT and music and allow music to bleed through to the sub channel, but HT and music sub crossovers are totally different.

So in short, if it is strictly a HT system then discard the sub filter if possible. If not try it first set to as high as it will go and if that sounds intrusive then set it to 80Hz. It may be that in DD5.1 mode the setting does not affect things anyway.
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Last edited by richie00boy; 16th November 2009 at 09:47 PM.
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Old 16th November 2009, 10:13 PM   #3
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re:'Is the roll off of the main supposed to be a particular slope? '
ideally the mains and sub xover slopes should be complementary, so you're right, sub makers must be making an assumption about xover order.
low end roll off is really a matter of personal preference - but then I think that if you have well designed woofers, subs are unnecessary
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Old 17th November 2009, 04:43 PM   #4
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I guess I didn't phrase my question right so I'll try again. I am building main speakers that will probably go down to 60 and perhaps 40. I can pick the crossover frequency so that is not the issue.

The problem is what slope should the mains roll off at? I can easily do .577 but if the system is expecting .707 the transition won't be smooth.

Put another way, does the typical receiver depend on a partial slope of the mains and partial slope of the receiver or does it assume a flat slope for the speaker.

Example: receiver sub slope 12 dB/oct, receiver main slope 6 dB/oct + main speaker slope 6 dB/oct.
OR
Example: receiver sub slope 12 dB/oct, receiver main slope 12 dB/oct.

Hope that's more clear.

Personally I would think that the receiver would assume flat speaker slopes because it would be impossible to get a correct response if the receiver is set to 80 Hz XO but the mains will go lower.
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Old 17th November 2009, 07:45 PM   #5
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And the answer is? BOTH!

As soon as I posted, the went on and I checked my Lexicon manual. Sure enough, for non THX setup the slopes are 24 dB/oct for both the speaker and sub outputs, while the THX setup at 80 Hz the sub slope is 24 dB/oct while the speaker slope is 12 dB/oct.

What this translates to is for non THX setups, the speakers are assumed either flat or having symmetrical roll offs, while for THX setups, the sub is assumed flat above 80 Hz while the speaker is assumed to roll off at 12 dB/oct below 80 Hz.
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Old 18th November 2009, 05:18 AM   #6
gedlee is offline gedlee  United States
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The thing is that the bigger assumption being made is that any of this matters. In a real room the sources are anything but the nice rolled off systems that we see in the data sheets. It's how these sources add IN THE ROOM at their choosen locations that matters and things like crossovers and slopes etc. simply aren't that significant in this scenario. I don't "cross-over" my subs and mains, they all overlap - by differeing amounts.
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Old 18th November 2009, 08:04 AM   #7
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Yea, considering the frequency, the room modes probably swamp out most of this anyway. But the question always bugged me and designing the speakers just brought it up again.

If one can do it "right" though, then why not? At least it's a good starting point!
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Old 18th November 2009, 09:02 AM   #8
gedlee is offline gedlee  United States
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Quote:
Originally Posted by Phil Olson View Post
If one can do it "right" though, then why not? At least it's a good starting point!
Well I'm not sure that it is "right" nor that its a good starting point. I am a strong believer in NOT even having a crossover to the subs. They should overlap the mains NOT crossover to them.
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Old 18th November 2009, 05:55 PM   #9
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Everything I've read says that you get too much bass doing that but to each his own. Lexicon actually allows that in their setup, (Full + Sub), but doesn't recommend it for that reason.

What I will do is set the mains box at 707 and since I use digital correction I can add the same slope on the sub. The capability is there for free so why not.
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Old 18th November 2009, 06:56 PM   #10
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Dr. Geddes, aren't you using bandpass subs though?
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