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Old 11th February 2008, 01:23 AM   #1
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Default Crossover Slopes In Context.

Sorry, I'm off on one of my rants again.

A recent post got me thinking about Crossovers, specifically first order crossovers, or 6db/octave crossovers.

Let's put 6db per octave in perspective, but before that, let's put 6 db in perspective.

3db is a slight but noticable change in volume. If you tweak your volume control up or down just enough to clearly perceive a change in loudness, that is about 3db. So 6db is two small tweaks upward or downward of your volume control.

Now let's look at 6db per octave - 1st order crossovers and for this example, let us assume a crossover of 2,000hz. One octave down at 1,000hz is like tweaking your volume control down 2 slight nudges. Dropping another octave to 500hz is like 4 downward nudges. Let's generously say that is about like turning your volume control down from 40% of a turn (4 on a scale of 10) to about 20% volume (2 on a scale of 10).

We are down two octaves and while the sound has diminished, it is still certainly playing and is certainly audible.

Now let's drop one more octave to 250hz (3 octaves or 18db total) which I say for purposes of illustration is about like turning your volume down to ONE (from 4 down to 1). That is pretty quiet, but you can still hear it.

Now let's go the other way; let's move an octave upward from 2khz to 4khz then another octave to 8khz, about like going from 4 on the Vol Cntrl down to 2. Now a third octave up to 16khz, and while the loudness is very low, it can still be heard; it is still at a functional level.

It would seem that if you intend to use a 1st order crossover, you pretty much need two full range speakers; a woofer that goes up very high and a tweeter that goes down very low.

So, briefly let's consider 12db/octave crossovers and ask the general question, how many DB does a speaker have to drop to be consider acoustically out of the circuit? How far down before any sound coming from it become irrelevant?

If you are pushing a speaker beyond its rated frequency response, is it just dropping off on its own, or is there real potential for distortion outside its rated frequency range?

With 12db crossovers, you drop pretty fast; 24db in two octaves and 36db in three. While the phase relationships cause by the crossovers might be better with 6db crossovers, how does one reconcile the extreme frequency range that is forced on the speakers?

Just curious.

Steve/bluewizard
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Old 11th February 2008, 01:56 AM   #2
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It's a sobering thought isn't it! If you do a vector sum on two sinewaves then one has to be down by 20dB to make less than 1dB difference in the summed total. 1dB difference in level may be hard to hear but I think it could make a difference in the sound of a speaker when it appears as a dip in the response.

This means the audible overlap region in a 1st order XO speaker is 8 octaves!
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Old 11th February 2008, 02:16 AM   #3
SY is offline SY  United States
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That's the least of the worries with 6dB crossovers. Besides the horrific lobing and power handling issues, the acoustic crossover is a mess unless the drivers are so flat that they don't need crossovers in the first place. I've heard some pretty good speakers with simple 6dB electrical crossovers, but they sounded good despite the crossover, not because of it.

Add in the complication of (generally) non-coincident centers and it's a dog's breakfast.
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Old 11th February 2008, 02:30 AM   #4
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It seems generally accepted that masking occurs at about 40dB. That is to say any signal down 40 db will not be heard in the presence of a 0 db signal.

In multiple driver designs, it's typically the high end of a given driver that makes for a problem. Cone break-up is very audible and objectionable. Some low order crossover designs find it necessary to add a trap at the break up frequency, this causes at least one frequency to be much further down in level than the crossover slope itself would provide.

I notice in speaker reviews that high order crossovers seem to be gaining some favor in commercial designs. They seem to be more necessary in drivers with exotic cone materials (ceramic, metal, kevlar etc).

One advantage of the Linkwitz Riley crossover is that the signal is down 6 db at the crossover frequency instead of the 3 dB down of most conventional designs. This extra 3 dB might be part of the reason that the L-R crossover is well received.
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Old 11th February 2008, 10:59 PM   #5
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Well, one of my questions was answered but the other one remains.

Assuming a typical 12db crossover, how much room do I need to leave when selecting a crossover point?

If my woofer goes from 30 to 3000, and my tweeter goes from say 2000 to 20,000, do I have a workable combination? Can I crossover at 2,500 and still be in the workable range of the speakers?

Someone said, I needed to drop down by 40db to be considered out of the circuit. For the moment, let's just call it 36db or 3 octaves with a 12db/octave crossover.

If this is true, it seems an impossible task to require either speaker to operate 3 octaves above and below the crossover point.

So, to what extent can I count on the natural roll-off of the speaker?

In short, I am trying to determine how to match speakers, woofer/Tweeter or woofer/mid/tweeter to a 12db crossover.

How wide a span do I need on each side of the crossover points to still be within the realistic working range of the speaker?

And what happens if the range is too narrow? Fried speaker? Distortion? ???

Thanks.

steve/bluewizard
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Old 11th February 2008, 11:29 PM   #6
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Quote:
Originally posted by BlueWizard
If my woofer goes from 30 to 3000, and my tweeter goes from say 2000 to 20,000, do I have a workable combination? Can I crossover at 2,500 and still be in the workable range of the speakers?
Hi Steve,

Some general rules I follow:

1. Cross the woofer as low as possible.
2. Do not cross the tweeter lower than 2X the Fs.
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Old 12th February 2008, 12:31 AM   #7
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Quote:
Originally posted by BlueWizard


So, to what extent can I count on the natural roll-off of the speaker?


I think this is exactly the point. Pretty much every driver is a 2nd order bandpass. Sometimes the cabinet provides a little Q peaking and sometimes the upper band of the driver is trashy due to breakup, resonance etc etc.

My belief is that the best sounding speakers exploit the natural 2nd order response of the drivers and use a shelving filter if the XO is too low (for high pass) or add a 2nd order on top of the natural 2nd order to achieve LR-4, for example. Of course you have to keep in mind the phase responses of everything because amplitude is only half the picture.

When you don't consider the driver response then it will modify the expected crossover performance and the result is a non-flat summed response at the listening position. If you're XOing your tweeter with a 1st order at 2KHz and fs is at 1KHz then you'll have 1st order response to 1KHz then 3rd order below that. You'll be pumping in bass to the tweeter at -12dB at 500Hz, -24dB at 250Hz. This will increase doppler modulation of the tweeter output and reduce the max acoustic output (headroom) i.e. make it easier to fry the tweeter.
Woofers really don't care what you send them they just have a habit of making nasty noises outside their comfort zone. For instance if the woofer had a resonance/break up mode at 4KHz that provided 10dB of gain then the woofer could become audible again at that frequency with corresponding effects to the sound quality.

In reality, with passive XO's you have to accept the compromise of crossover region width vs crossover complexity. I think the 40dB down requirement is pretty much unachievable as you indicate although it's probably real in careful listening conditions. 20dB down for 1dB total contribution gives you some hope and lets face it, if our speakers were 1dB we'd be ecstatic right?.

These are a few of the reasons I gave up on analog crossovers some years ago.
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Old 12th February 2008, 12:59 AM   #8
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Quote:
Originally posted by BlueWizard
Assuming a typical 12db crossover, how much room do I need to leave when selecting a crossover point?
Well that's the trick, crossover design and driver selection are usually done together. Some drivers are more tolerant than others. Look at the manufacturers data and remember that the design will not be ideal.

Quote:
Originally posted by BlueWizard If my woofer goes from 30 to 3000, and my tweeter goes from say 2000 to 20,000, do I have a workable combination? Can I crossover at 2,500 and still be in the workable range of the speakers?

Someone said, I needed to drop down by 40db to be considered out of the circuit. For the moment, let's just call it 36db or 3 octaves with a 12db/octave crossover.

If this is true, it seems an impossible task to require either speaker to operate 3 octaves above and below the crossover point.
Using 2000 and 3000 as limits and crossing over at 2,500 is not a bad start (Remember to use geometric centers; square root of (X * Y) equals 2450Hz.). You are quite unlikely to achieve the 30-40dB an ideal design would require, compromise, build, listen and adjust. Repeat as needed

Quote:
Originally posted by BlueWizard So, to what extent can I count on the natural roll-off of the speaker?
See the manufacturers data sheet, most drivers will match published specs and frequency response plots reasonably well. Remember that the enclosure also causes roll-off at the low end.

Quote:
Originally posted by BlueWizard In short, I am trying to determine how to match speakers, woofer/Tweeter or woofer/mid/tweeter to a 12db crossover.

How wide a span do I need on each side of the crossover points to still be within the realistic working range of the speaker?
This is why two way, three way and four way designs exist, more drivers equals less overlap, more complexity, more costs.

Quote:
Originally posted by BlueWizard And what happens if the range is too narrow? Fried speaker? Distortion?
No damage will happen to the drivers. However your ears may experience less than perfect sound, possibly painful and ugly sound.

steve/bluewizard: These questions are complex enough that the usual advice for a newbie is to build a kit, learn, decide how much time and effort you want to invest. Next, acquire some tools, such as design software and a method of measuring your designs progress.
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Old 12th February 2008, 05:57 PM   #9
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All very good information and I thank you for it, but I think you are all too deeply into the puzzle.

All I want to know is how much overlap on the raw speakers?

Before I can even start designing a set of speakers, I need to choose the drivers. I've always assumed you designed the crossovers and cabinet to fit the drivers, not chose drivers to fit the crossover.

So, I'm at the starting point, I'm searching through catalogs of speakers, I see speakers that look nice, are priced right, and have reasonable good specs and reputation. That's where I'm at in this hypothetical situation; I'm still looking at the catalog.

For the moment, let's say my goal is a 2-way system with a 12db crossover.

How do I chose the speakers relative to their frequency response?

If I have a 1khz overlap, is that enough? Should I have a 2khz overlap? 3khz? Do I need one octave of response above and below the crossover point? 2 octaves? 3 octaves? Or do Octaves not even matter, maybe is is a matter of raw numbers, 'X' hz above and 'Y' hz below the XO.

So, let's start there; I'm not at the point of designing cabinets or crossovers, I'm on the internet looking through catalogs of speakers trying to find a good match for a woofer and tweeter.

In seeking a good match between a woofer and a tweeter, how much overlap in frequency response do I need?

That is the fundamental question that needs to be answer before anything else can be done.

Steve/bluewizard
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Old 12th February 2008, 06:26 PM   #10
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Quote:
[i]
How do I chose the speakers relative to their frequency response?[/b][/B]

Quote:
Originally posted by Cal Weldon
1. Cross the woofer as low as possible.
2. Do not cross the tweeter lower than 2X the Fs.
That is a fundamental starting point. Find drivers that meet that criteria. Example: Don't try and match a 10" woofer with a 1" dome tweeter. Find a tweeter with a low enough Fs to match where you want to cut off the woofer. The woofer cut off can be determined by the beaming and where the cone break up starts.
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