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Fracture_clinic View Drop Down
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    Posted: 14 March 2019 at 1:36pm
If some ported direct radiating kicks are on top of some scoops, should the scoops phase be inverted.

forward motion of cone in kick = sound

forward motion of cone in scoop = negative pressure in chamber (no sound from scoop mouth)

I once read about the majority of the sound from scoops comes from the mouth below 70hz. (Tried and tried but I can't find that forum anywhere).

So my question is really, will flipping the polarity on the scoops (i.e cause pressure in the chamber at teh same time the kick cone moves forward) align the phase between the two with a crossover point of 75hz.

I'm going to test this in the next few weeks both, by ear and with a mic. Just wanted some thoughts from SP first!

Nice one.

FC

edit: spelling

Edited by Fracture_clinic - 14 March 2019 at 1:36pm
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JBK View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JBK Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2019 at 2:03pm
All scoops suffer from a dip in frequency response due to phase inversion between front wave and rear wave wich comes out delayed by the horn path. The frequencies in witch the cancellation occurs will depend on the scoop design (+TS parameters of the driver I guess). usually the scoop are lowpassed below this dip so it should not really be an issue.
flipping the polarity of the scoops may or may not gives better result, depending on the horn path length of the scoop. (it will be an improvement only if the horn path length match the wavelength / 2 of the attenuated frequencies, please someone correct me if I'm wrong) so I won't recommend it (well, you can try and decide if you like it better)
The better method would be at least to delay the kicks and the rest of the system according to the scoops horn path (delay the amount of time that sound will travel trhough the scoop's horn)
This will not be a perfect match, to proper time align you got to measure and play with different filter slopes and delay, but at least delaying the rest of the system from the scoop horn path lentgh will get you in the right ballpark.
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toastyghost View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2019 at 2:37pm
I’ll keep repeating this forever - do not use the horn length. It is always going to be wrong.

If you want to use the flip polarity to try and get it quiet method, then at least use Bink’s Warbles so that you’re checking more than a single frequency. You can find them on Merlijn van Veen’s site.

The short answer to your question is ‘it depends’
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Post Options Post Options   Thanks (0) Thanks(0)   Quote fatfreddiescat Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2019 at 3:43pm
If you have access to an allpass filter on your LMS then you could try using that to flip the phase between the horn and the direct radiating portion of the scoop.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2019 at 5:56pm
Originally posted by fatfreddiescat fatfreddiescat wrote:

If you have access to an allpass filter on your LMS then you could try using that to flip the phase between the horn and the direct radiating portion of the scoop.


An allpass doesn't 'flip the phase', it adds a phase rotation. Which would push them to 360 degrees out of phase potentially, give or take 60 degrees, depending on what frequency you're looking at.

In other words, summation, but a lap behind.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote fatfreddiescat Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2019 at 6:45pm
Originally posted by toastyghost toastyghost wrote:

Originally posted by fatfreddiescat fatfreddiescat wrote:

If you have access to an allpass filter on your LMS then you could try using that to flip the phase between the horn and the direct radiating portion of the scoop.


An allpass doesn't 'flip the phase', it adds a phase rotation. Which would push them to 360 degrees out of phase potentially, give or take 60 degrees, depending on what frequency you're looking at.

In other words, summation, but a lap behind.
Yes it will, could be unwrapped with the right processor, still maybe the lesser of evils.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2019 at 7:40pm
Originally posted by fatfreddiescat fatfreddiescat wrote:

Originally posted by toastyghost toastyghost wrote:

Originally posted by fatfreddiescat fatfreddiescat wrote:

If you have access to an allpass filter on your LMS then you could try using that to flip the phase between the horn and the direct radiating portion of the scoop.


An allpass doesn't 'flip the phase', it adds a phase rotation. Which would push them to 360 degrees out of phase potentially, give or take 60 degrees, depending on what frequency you're looking at.

In other words, summation, but a lap behind.

Yes it will, could be unwrapped with the right processor, still maybe the lesser of evils.


But you aren’t doing that without a proper dual channel measurement system, in which case you should be able to align the other cabinet relatively easily using ‘normal’ methods.

The trick is to use the scoop as it should be, and acoustically crossover somewhere in the 80s to 100. A ‘properly’ designed scoop can push that dip out of the usual band, but then this leads to scoop regulars to say it doesn’t play ‘deep’ because they’re not used to having upper bass present in that passband.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote fatfreddiescat Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2019 at 9:03pm
Originally posted by toastyghost toastyghost wrote:

Originally posted by fatfreddiescat fatfreddiescat wrote:

Originally posted by toastyghost toastyghost wrote:

Originally posted by fatfreddiescat fatfreddiescat wrote:

If you have access to an allpass filter on your LMS then you could try using that to flip the phase between the horn and the direct radiating portion of the scoop.


An allpass doesn't 'flip the phase', it adds a phase rotation. Which would push them to 360 degrees out of phase potentially, give or take 60 degrees, depending on what frequency you're looking at.

In other words, summation, but a lap behind.

Yes it will, could be unwrapped with the right processor, still maybe the lesser of evils.


But you aren’t doing that without a proper dual channel measurement system, in which case you should be able to align the other cabinet relatively easily using ‘normal’ methods.

The trick is to use the scoop as it should be, and acoustically crossover somewhere in the 80s to 100. A ‘properly’ designed scoop can push that dip out of the usual band, but then this leads to scoop regulars to say it doesn’t play ‘deep’ because they’re not used to having upper bass present in that passband.

I can't recall seeing an 18" scoop with a notch above 90Hz, not saying they don't exist just not seen one, possibly for the reason you have given.
If crossing over near the notch then the 180 degree phase shift really needs accounting for, second order low pass should more or less even out the phase to around, the front of the cone being full cycle behind on the horn as you know happens with an LR4 crossover.
I guess using a second order LP matched to the phase wrap of the scoop with a fourth order HP may get the job done, as should both be in phase ish throughout the crossover region.

Re dual channel measurement system, can be had for under a 100 notes including laptop, IBM T30 for example has stereo line in and out, REW, a cheapy measurement mic and some cheapy cables.
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toastyghost View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2019 at 11:54pm
They may be in phase but i can almost guarantee that most scoops will sound awful setup like that. Unless you like really grunty bass and an abundance of distortion through crossover.

There is more to designing crossovers than making phase slopes match.

Edited by toastyghost - 14 March 2019 at 11:55pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote fatfreddiescat Quote  Post ReplyReply Direct Link To This Post Posted: 15 March 2019 at 8:00am
Distortion? Not sure where that is coming from other than two boxes unloading one another at some point? Just about every discussion in scoops involves setting a crossover around the lower edge of the cancellation dip, some overlap the the two bands over the dip, either way there will be response ripple and lobing if not sorting the phase angle.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tonskulus Quote  Post ReplyReply Direct Link To This Post Posted: 15 March 2019 at 8:59am
DSP.  I cannot live without it anymore. 


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