What crossover type ?

[fatmarley]

Hi Guys,

Can anyone explain to me how you work out what type of crossover I am using ?

Here's the predicted response in LspCAD:



And here's the 1mtr measurement (I didn't bother to do a near field of the woofer)


Sorry for the stupid question...

[Chris Daly]

Hi Fatmarley
Decibels per octave is the usual measure of crossover rolloff, which in case of your very nice graphs indicates that the woofer rolloff is approx 18db per octave, and tweeter approx 12db per octave. An octave is a doubling of frequency.

Cheers / Chris

[fatmarley]

Hi Chris,

Thank's for replying. I still don't understand how to work it out though

I can see what a what a decibel is and an octave, but i'm not sure how you work out what the slope is.

[Chris Daly]

Look between two frequencies that contain the active region for both woofer and tweeter ( relative to your graph the third line in from 1000 ) from 2000 to 3000 hz which is an octave ( a doubling of frequency ) and then compare to the decibels on the left hand side,

Cheers / Chris

[fatmarley]

I'm still obviously doing it wrong, because the woofer looks like 12db per octave and the tweeter about 6db !!!

When I get a chance i'll put the picture in window paint and draw where i'm measuring from. I feel like a right plonker...

[awpagan]

I might be able to help on this
From the graph it looks like 2500Hz on -14db start on crossover so 1 octave would be 5000Hz.
At 5000Hz it's at -28db so 14db per octave.
Take into account the frequency of the woofer at 5kHz without crossover.

The woofer probably has a spike in sensitivity in the higher order frequencies.

[fatmarley]

It looks like -26db at 5000Hz to me (sorry for being pedantic)

So assuming I am right (2nd order acoustic on woofer and first order on tweeter) - Hows that supposed to effect the sound/phase etc ?

Would I be better off trying to change the tweeter to second order, to match the woofer ? Does anyone care ? Are people put off helping because of my user name (fatmarley) ? Perhaps I smell ? Are you supposed to leave a gap between the last letter in a sentence and the question mark ? Am I drunk or just a bit silly/tired/bored ? So many questions and so few answers...

[awpagan]

hmm could be -26db needed my glasses.

do you have the graphs/datasheet for the drivers you are using?

IMHO I would leave the crossover as it is, looks fairly flat at crossover.
Isn't that what your looking for? (ps no space after word for ? mark)

[mdocod]

Looks like 1st order electrical on the tweeter. Notice the -12dB from 6K down to 1.5K (2 octaves where the tweeter is probably naturally closer to flat), then it turns steeper when the natural rolloff of the tweeter kicks in, increasing the rolloff to something like 12dB/octave accoustic.

The woofer appears to me to be on either a 2nd or 3rd order electrical, I would have to see the response of the woofer without the network in place to know for sure.

Either way it looks like a well thought out x-over for the drivers. Measurement confirms the BSC is close to ideal.

[Shaun]

Originally Posted by fatmarley

Can anyone explain to me how you work out what type of crossover I am using ?

I don't know LSPCAD, but I'm sure it should be able to do this:

Select a crossover target curve. There'll all sorts of options: type; crossover frequency; slope; Q; etc. Try a few and see which matches what you've got. Start with the standard types (Butterworth, Bessel, Linkwitz-Riley, etc.). Easiest would be to identify the slope or closest slope. Then tweak the crossover frequency to bring it closer to your own. Then play with the types to find which transition area (i.e., the area between flat and the slope) matches yours. If, after this, you don't yet have a perfect match, it means that your crossover is not one of the standard types, and you may then have to use a custom option to get the target to match your crossover. But in this case it becomes a more complex description (as opposed to, say, 2nd order Butterworth).