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?
Thanks
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?
Thanks
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.
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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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
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
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.
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.
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.
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.
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.
If one can do it "right" though, then why not?
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.
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.
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.
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?
Your crossover point on your mains is defined by what Their F3 is and to get a smooth crossover between your sub and your mains you need to make sure the electrical/mechanical crossover of your mains matches your sub which is all electrical from the AVR.
Here is an 80Hz crossover setting with speakers that have F3 of 50, 60 and 70Hz.
An externally hosted image should be here but it was not working when we last tested it.
The 50Hz speakers are going to have the best slope....the 70Hz designs should be crossed over higher.
You really have to plot your slopes or measure in room to know what your best crossover should be.
Agreed, you are proving what I had discovered. Since most controllers assume flat freq response, the farther you are away from the f3 of the speaker the better the response.
In the Lexicon, if one sets the XO at 70 HZ both the mains and the sub will roll off at 24 dB/oct. If the mains are also rolling off at 12 dB/oct at 70 Hz, the actual roll off of the mains will be 36 dB while the sub is at 24 dB. If the mains can go down to 50 Hz then the 12 dB roll off is lower down and won't contribute as much error.
Another solution would be to add 12 dB slope to the sub so they are both effectively 36 dB/oct. Then they will match.
Of course THX solves all that by assuming the mains are 12 dB/oct at 80 Hz. In that case the THX XO rolls the sub off at 24 dB/oct but the mains roll off at 12 dB/oct which is added to the speaker to achieve an overall 24 dB/oct.
I should petition Lex to allow for 12 dB roll offs for the mains as an option!
In the Lexicon, if one sets the XO at 70 HZ both the mains and the sub will roll off at 24 dB/oct. If the mains are also rolling off at 12 dB/oct at 70 Hz, the actual roll off of the mains will be 36 dB while the sub is at 24 dB. If the mains can go down to 50 Hz then the 12 dB roll off is lower down and won't contribute as much error.
Another solution would be to add 12 dB slope to the sub so they are both effectively 36 dB/oct. Then they will match.
Of course THX solves all that by assuming the mains are 12 dB/oct at 80 Hz. In that case the THX XO rolls the sub off at 24 dB/oct but the mains roll off at 12 dB/oct which is added to the speaker to achieve an overall 24 dB/oct.
I should petition Lex to allow for 12 dB roll offs for the mains as an option!
You get exactly the amount of bass you set it up to have, but yes you do have a lot of bass headroom, which is a good thing.
Yes, I do, but I don;t see any problem with that. They do have a nature LP if thats what you mean and yes all subs need some form of LP somewhere. Its the idea of "crossing over" that I object to.
I see your point after reading the white paper on your web site. If I understand, lets say the mains go down to 60 Hz and are set up on the controller as "Full" so that channel gets the full signal bandwidth. You would also have a sub that has a XO set to say 80 Hz so everything below that also goes to the sub, (in Lexicon terminology it's called Full + Sub).
With that setup you are getting full low freq response from the mains so adding the other subs would mean that what ever they added would be extra output. What am I missing?
Now if you had your mains XO'd at 60 Hz and and a dozen subs also XO'd at 60 Hz to even out the room modes then you can set the sub output level to match the mains. That does make sense.
Nice web site by the way!
With that setup you are getting full low freq response from the mains so adding the other subs would mean that what ever they added would be extra output. What am I missing?
Now if you had your mains XO'd at 60 Hz and and a dozen subs also XO'd at 60 Hz to even out the room modes then you can set the sub output level to match the mains. That does make sense.
Nice web site by the way!
Thanks - I've lots more planned, but very busy in China at the moment.
What you are missing is the 3D nature of the sound field in a room. Its not like looking at the 1D FR on a piece of paper. Multiple sources add in some places, and subtract in others, so its not simply an "extra output" situation. You have to look at it statistically in three dimensions. Then you will get what you are looking for - a reasonable assesment of LFs in real rooms. This is why I object to any discussion of FR as a 1 dimensional parameter at LFs in real rooms. Its simply a gross over simplification of a much more complex problem.
Relax, I wasn't suggesting it was a problem. Though there are some, not everyone's out to discredit you.
The low pass is what I was getting at, and now I understand what you were talking about. It just needed clarification. The way you originally phrased it suggested nothing of the low pass, and I did not think that's what you meant. I was beginning to think you had gone mad suggesting no filter on subs. Thanks,
Dan
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