Subwoofer crossover

[dieselpower1966]

I'm wondering if any one in the shack has experimented with the subwoofer crossover to see how far off they actually are?

for example if I set my sub crossover at 80, I know that it's not really at 80 because those things are not spot on. but how far off is it?

Now how about the crossover setting in the receiver? is that dead accurate or is that off as well?

feel free to comment, it could be an interesting topic.

Dieselpower1966

[eugovector]

It's not that it's off per se, but it's a slope. Think of it like this, the frequency doesn't drop off like a cliff (Full volume at 80HZ, 0 volume at 81HZ), it drops off like a hill (80HZ=0db, 81=-0.1db, 90hz=-10db etc, these numbers are not exact, example only).

[JRace]

It all depends on the slope
12dB/octave means:
80Hz - (0dB)
160Hz - (-12dB)
320Hz - (-24dB)

The crossover point is where the signal starts to roll off.

[brucek]

12dB/octave means:
80Hz - (0dB)

Actually, an 80Hz crossover is already down at 80Hz by either -3dB or -6dB, depending on the slope..


Crossover Slope comparisons 80-12-24 copy.jpg

brucek

[Mike P.]

Thanks for posting the graph. I'm learning something every day!

[dieselpower1966]

Brucek
I agree and know what you are talking about with the -3db -6 db depending on the slope.
I have read in A V review, Hometheater magazine, and what hi-fi magazines that just because the crossover knob on the sub is pointing at the 80 hz mark doesn't mean it's at 80 hz. Hometheater had reviewed a sub, and had it crossed over at the 80 hz on the test bench, but it was actually crossing over much higher something like 86 hz.
so how is one to know exactly where the sub is crossing over if they cannot believe the knobs on the subs crossover setting. and please don't say that it's a rough guess...
which brings me back to the crossover setting in the receiver. how do I know that it's actually crossing over at 80 hz. ?

[eugovector]

dieselpower1966 wrote:

Brucek
I agree and know what you are talking about with the -3db -6 db depending on the slope.
I have read in A V review, Hometheater magazine, and what hi-fi magazines that just because the crossover knob on the sub is pointing at the 80 hz mark doesn't mean it's at 80 hz. Hometheater had reviewed a sub, and had it crossed over at the 80 hz on the test bench, but it was actually crossing over much higher something like 86 hz.
so how is one to know exactly where the sub is crossing over if they cannot believe the knobs on the subs crossover setting. and please don't say that it's a rough guess...
which brings me back to the crossover setting in the receiver. how do I know that it's actually crossing over at 80 hz. ?

Yeah, if someone put the knob on crooked, you can get messed up. If a receiver is messing it up, I don't know what to tell you. Crappy receiver. So...

Turned the sub all the way up (or bypass if available) and use your receiver to crossover, or use REW and a microphone.

[bobgpsr]

how do I know that it's actually crossing over at 80 hz. ?

It can be very accurate in a AVR/Pre-Pro if done by a DSP. For sub plate amps the analog crossover filter that is set with the coarse control dial marks is lucky to be within 10% of setting.

Good enough for most users -- picky users, should use the AVR/Pre-Pro DSP, otherwise if using the plate amp (not perferred) will need to use external measuring equipment to tweak exactly in. Not worth it IMHO.

[salvasol]

eugovector wrote:

...Turned the sub all the way up (or bypass if available) and use your receiver to crossover, ...

I think this is the best option ... let the receiver handle the crossover frequency; don't worry if is exact or not (2 -3 Hz difference won't matter )

[brucek]

so how is one to know exactly where the sub is crossing over if they cannot believe the knobs on the subs crossover setting. and please don't say that it's a rough guess...

Well, normally you would use the crossover in your receiver to handle bass management duties, but it's easy enough to do a REW response sweep of the sub to check the crossover.

which brings me back to the crossover setting in the receiver. how do I know that it's actually crossing over at 80 hz.

Do an REW loopback sweep of the receiver to check its crossover. Not really needed though, as most receivers would be quite accurate. Close enough for home use anyway..

brucek

[JCD]

I think the answers have already been put out there but, to summarize:

1. The Crossover is NOT where the sub starts to slope off, it's the point where the sub's FR and the main speakers FR have the same SPL for a given frequency so that when the two wound waves are combined you get a seamless flat FR over the entire spectrum.
2. Unless you have some special circumstance, I can't imagine any scenerio where I'd use the built in crossover of the sub instead of the crossover in the AVR/Preamp. NOte: this assumes the AVR/Preamp has a sub out line.

JCD