Cardioid using aperiodic ports (a la Fulcrum Acoustics)

Kevin McDonough

Freshman
Jan 3, 2016
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hey all


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

http://www.audioxpress.com/article/fulcrum-acoustic-unveils-new-ccx-subcardioid-coaxial-loudspeaker

http://www.fulcrum-acoustic.com/products/cardioid-subwoofers/


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
 
Hi Kevin,

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.

Cheers,

David Gunness
 

Kevin McDonough

Freshman
Jan 3, 2016
31
2
8
38
Hey

Thank you for your detailed reply, I wasn't aware you were even a member here never mind that you'd take the time to reply about your design! :)~:)~:smile:

k