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Cardioid using aperiodic ports (Fulcrum Acoustics)

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kevinmcdonough View Drop Down
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    Posted: 25 July 2017 at 8:06pm
hey all

really interested in this kind of thing......




Using aperiodic ports at the rear of the speaker to create passive cardioid dispersion. 

Anyone any links with more info to see exactly how this is done? 

Forgetting about the cardioid aspect for a bit, as I understood it these kinds of "vents" lowered the impedance spikes of cabs and made them perform more like sealed boxes or infinite baffles, by giving a restive "air leak". 

But a sealed box doesn't have the greatest bass response obviously. So does that mean if I was doing a sub version, I'd really need to have traditional ports as well as the vents to get the driver to play low enough?  Or would just adding a very large vent area be enough to accomplish the amount of tuning that a port would otherwise create?



K


Edited by kevinmcdonough - 25 July 2017 at 8:28pm
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bob4 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bob4 Quote  Post ReplyReply Direct Link To This Post Posted: 06 August 2017 at 12:31am
Afaik geithain uses the same method in their studio monitors
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kevinmcdonough View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kevinmcdonough Quote  Post ReplyReply Direct Link To This Post Posted: 06 August 2017 at 3:35pm
Dave Gunness himself (designer, and made famous with Gunness Focusing among other things) was kind enough to reply to the same questions I posted on another forum. His reply below, for anyone who's interested....

Quote The difference between an "aperiodic port" design and our passive cardioids is the acoustical mass of the ports and the amount of resistance. The acoustical mass in our passive cardioids puts a second-order low-pass filter a little below the frequency below which we want cardioid behavior. The resistance is just enough to damp the helmholtz resonance to "slightly underdamped" (Q of about 1). An aperiodic port design has almost no acoustical mass and is dominated by resistance, so there is essentially no helmholtz resonance and the resulting first-order low-pass corner is well above the passband; so it operates simply as a resistive leak that damps the sealed box resonance. If you look at the impedance of our passive cardioid subs, you'll see a trace of the double peaks of a ported box, but the dip between them doesn't go nearly as low. That's the effect of the port resistance.There is still significant reduction of excursion around the helmholtz frequency, and the summation of the rear radiation and front radiation increases the output over about 1 1/2 octaves. If you try to stretch the cardioid behavior beyond 1 1/2 octave, there will be a penalty in low frequency extension and maximum output.

Adding "conventional" ports to the front of the cabinet would essentially short out the rear ports, which would destroy any cardioid behavior.




Edited by kevinmcdonough - 06 August 2017 at 3:36pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Crashpc Quote  Post ReplyReply Direct Link To This Post Posted: 06 August 2017 at 4:40pm
Doesn't sound right to me. Like saying A, not B... Aperiodic ports were here for ages with us. And for sure these have different performance. But connecting it with boasting about directivity alone is rather a long shot. Quite a drag in my eyes.

Edited by Crashpc - 06 August 2017 at 4:43pm
Nikon and Canon people should not be married to each other. Why did you let this happen?
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kevinmcdonough View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kevinmcdonough Quote  Post ReplyReply Direct Link To This Post Posted: 06 August 2017 at 4:52pm
Well he wasn't saying that the ports were aperiodic, someone else had described to me that that was how they worked. However as he replied in his message, his ports are somewhere in between, not purely traditional ports or purely aperiodic ones.

And its more the fact that they are set up in a particuluar way at the rear of the cab, so their radiation cancels with the cone sound and creates the cardioid aspect.

Kev
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 06 August 2017 at 9:32pm
not sure if this is the same idea taken to the extreme of having the entire back of the cabinet open with an acoustic resistor in series with the volume compliance:

http://www.linkwitzlab.com/frontiers.htm#A

are the vortex vents used by seventies-trousers an aperiodic port too?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 07 August 2017 at 4:32am
Originally posted by Crashpc Crashpc wrote:

Doesn't sound right to me. Like saying A, not B... Aperiodic ports were here for ages with us. And for sure these have different performance. But connecting it with boasting about directivity alone is rather a long shot. Quite a drag in my eyes.


I trust Dave’s data very much. Even if I disagree with some
Of his methods in principle I can’t deny the results.

Also careful not to use the word vortex in relation to ports eh? Oh sh...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 07 August 2017 at 5:09pm
what's the effect of making the box out of thin but well damped wood instead of thick, stiff wood. surely this would damp the q of the box in a similar way to introducing resistance in the port and also dramatically reduce the weight...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 07 August 2017 at 5:48pm
Originally posted by toastyghost toastyghost wrote:

Also careful not to use the word vortex in relation to ports eh? Oh sh...

My first thoughts too! LOLLOLLOL
Matt Stolton - Technical Director (!!!) - Wilding Sound Ltd
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Hemisphere Quote  Post ReplyReply Direct Link To This Post Posted: 07 August 2017 at 6:50pm
Originally posted by snowflake snowflake wrote:

what's the effect of making the box out of thin but well damped wood instead of thick, stiff wood. surely this would damp the q of the box in a similar way to introducing resistance in the port and also dramatically reduce the weight...
Google 'BBC thin walled speaker' there's masses of discussion about stuff like this floating around. Some of it relates to a very specific vein of research the BBC conducted in the 70's for midrange enclosures, but there's a fair bit of off-shoot debate about the possible benefits you mention.


And an ongoing thread in the Diyaudio Full Range section front page.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 08 August 2017 at 1:06pm
Originally posted by Hemisphere Hemisphere wrote:

Originally posted by snowflake snowflake wrote:

what's the effect of making the box out of thin but well damped wood instead of thick, stiff wood. surely this would damp the q of the box in a similar way to introducing resistance in the port and also dramatically reduce the weight...
Google 'BBC thin walled speaker' there's masses of discussion about stuff like this floating around. Some of it relates to a very specific vein of research the BBC conducted in the 70's for midrange enclosures, but there's a fair bit of off-shoot debate about the possible benefits you mention.


And an ongoing thread in the Diyaudio Full Range section front page.


very interesting, thanks
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Timo Beckman Quote  Post ReplyReply Direct Link To This Post Posted: 13 August 2017 at 10:46pm
You guy's might find this interesting?


After this one there's a gradient sub array (https://timobeckmangeluid.wordpress.com/2017/08/17/gradient-sub-array-or-reversed-end-fired-sub-array/) with the same subs (back sub delayed to the front sub and after the phase alignment reverse polarity) . I hope to find the time to do a measurement on 1 Fulcrum CS121 at Thursday / Friday if the weather allows it. That will show the difference between the front and back in level.....



Edited by Timo Beckman - 17 August 2017 at 3:35pm
The sound will be as good as the band play's
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