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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Audio Measurement and Testing => Topic started by: Jason Lucas on September 04, 2013, 03:54:21 pm

Title: Acoustic crossovers, stacking filters, am I over-thinking this?
Post by: Jason Lucas on September 04, 2013, 03:54:21 pm
So let's say you have a pair of powered tops and a powered sub.

As part of its factory processing, the powered sub has a low pass filter at 100Hz (I do not know if it's 12dB/octave, 18, 24, etc)

The tops do not have a high or low pass filter built in, but can use the "low cut" outputs from the sub.

But, if the sub were to be "aux-fed", you'd have to use an external crossover.

Since the sub already has a low pass filter, you wouldn't want to stack another one on top, right?

So, if using an external crossover, you'd only use it to high pass the tops, right?

But if the high pass is a different order (say 24 or 48dB per octave instead of whatever the low pass on the sub is) wouldn't the acoustic crossover be messed up? Or is it not worth worrying about?
Title: Re: Acoustic crossovers, stacking filters, am I over-thinking this?
Post by: Steve M Smith on September 04, 2013, 04:04:57 pm
I would be more concerned that they were both crossing over at the same frequency than what the cutoff slope was.


Steve.
Title: Re: Acoustic crossovers, stacking filters, am I over-thinking this?
Post by: Corey Scogin on September 04, 2013, 04:18:23 pm
So, if using an external crossover, you'd only use it to high pass the tops, right?

But if the high pass is a different order (say 24 or 48dB per octave instead of whatever the low pass on the sub is) wouldn't the acoustic crossover be messed up? Or is it not worth worrying about?

Ideally, you would measure the system and use whatever configuration yields the best results.  I think using an external crossover to only high-pass the tops is your best bet.  The crossover points may not need to be the same on the sub vs the mains.  Sometimes they should be different depending on the filter slopes and the acoustic response of the subs/tops.

Title: Re: Acoustic crossovers, stacking filters, am I over-thinking this?
Post by: g'bye, Dick Rees on September 04, 2013, 04:19:47 pm
I would be more concerned that they were both crossing over at the same frequency than what the cutoff slope was.


Steve.

Why?
Title: Re: Acoustic crossovers, stacking filters, am I over-thinking this?
Post by: Jonathan Johnson on September 04, 2013, 04:30:03 pm
Or is it not worth worrying about?

But how does it sound?
Title: Re: Acoustic crossovers, stacking filters, am I over-thinking this?
Post by: Don Boomer on September 04, 2013, 05:00:19 pm
But how does it sound?

Which is probably the only point that matters  ;)

The acoustic crossover point depends on a number of things but one of the most important is the relative levels.  Typically I see that the subs have been boosted by anywhere from 6-15 dB which shifts the acoustic point up in frequency.  There's nothing to say that filters have to be even or matched.  Asymmetrical, under and overlapped may be exactly what is called for.
Title: Re: Acoustic crossovers, stacking filters, am I over-thinking this?
Post by: Doug Fowler on September 04, 2013, 05:54:36 pm
Which is probably the only point that matters  ;)

The acoustic crossover point depends on a number of things but one of the most important is the relative levels.  Typically I see that the subs have been boosted by anywhere from 6-15 dB which shifts the acoustic point up in frequency.  There's nothing to say that filters have to be even or matched.  Asymmetrical, under and overlapped may be exactly what is called for.

Yep. 

Often, since the subs are so much higher in level than the tops for many types of music, the crossover frequencies may be underlapped to help flatten the response.  That is, if that's what you're going for.  It's more difficult to do this nowadays with the black box processing schemes, since you don't have access to that stuff.

Of course there are plenty of black box presets out there for full range tops + subs.

If you're doing stuff that requires a flatter spectral response (classical, acoustic, etc) it's a worthwhile project to find out what it takes to flatten your system through the sub / LF area.  Chances are you will turn your subs down (or not use any, depending on the tops), and the acoustic xover will shift down.

The first thing you should do, though, is align the top boxes to the subs.  If you have a cancellation around xover you'll be chasing rabbits trying to flatten that area.  Get it aligned first, then see what you are dealing with.

(not speaking to Don, obviously)