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[g'bye, Dick Rees]

Yes I know, i never said not aligning it.... By physically reversing the sub you have already polarity reversed it

You still seem to be missing the fact that the time alignment and polarity are related.  With the delay properly applied you have realigned the waves to be in polarity despite the reversal of the middle box.  The delay is there to put them in sync.  THEN you flip the polarity of the middle box to get the desired result.  See what the delay is doing?

[Nathan Salt]

Hmm, I just ran it up in mapp. I've never done the stacked arrangement, only end fire and the standard cardioid. I thought physically reversing the sub would polarity reverse it which doesnt appear to be happening in mapp? I'm confused :s

[Christian Tepfer]

Yes I know, i never said not aligning it.... By physically reversing the sub you have already polarity reversed it

Think of it this way:

If you had 2 subs, one physically reversed and they would be radiating at the same point, cancellation would appear in the back and in the front.

The concept of the CSA (stupid name btw., can anyone come up with a better one?) is using physical displacement and reversal and electrical displacement and reversal in order to get maximum cancellation in the back and not much harm in the front.

[Nathan Salt]

I've done spaced cardioid and end fire where you space them to a 1/4 wavelength depending on what frequency you want the most control over. I understand how they work, in the case of the spaced cardioid using polarity reversal on the rear to cause cancellations in the rear. I've never done the stacked before so I assumed the physical reversal achieved the spacing + the polarity switch. So when I was reading you also flip the polarity I was like wtf?

[Eirik Joerstad]

Hi!

To confuse things further, wouldt it make sense to physically reverse the bottom sub if you are groundstacking three subs, assuming you want a symmetrical stack? This would be because of the halfspace / image sources createted by the surface. I have have done both, not sure if i can hear any differences.....


Eirik.

[Nick Hickman]

Nathan et al,

A sub radiates in-polarity everywhere so physically rotating it does nothing.  Offsetting the acoustic centre in space will change arrival time, i.e. shift phase (differently for every frequency), but the wavelengths at issue are well over 3m long.

The "rear source polarity-inverted and delayed to match the physical spacing between sources" alignment is a pressure-gradient system.  If the rear delay time exactly matches the physical source spacing, it's cardioid.  If it doesn't match, it's one of the family members (hyper-cardioid, wide-cardioid, whatever).

With perfect omnidirectional sources and ideal acoustic environment, the pressure-gradient pattern is consistent at all frequencies below the upper limit, and the frequency and phase response is consistent everywhere.  (Of course, there are downsides too.  No free lunch!)  It's the analogue of a pressure-gradient microphone (i.e. the directional microphones we use) and explains how a mic capsule a few millimetres across can have a large amount of directionality to a wavelength many metres in length (i.e. a low frequency).

Nick

[Jay Barracato]

Here is quick and dirty set up in my basement today (weather is lousy outside). Excuse the extra crud in the measurement like the heating unit running and all the duckwork rattling.

The first image is the raw measurement. I just used the delta delay function to set the delay and took a wild stab at the level for the reversed sub (-3db). One curve is with the polarity matching and the other (lower) is with the polarity reverrsed. The offset is the same for both. The second curve is the same data with a little smoothing (1/12 octave) to show the general trend.

[John Sulek]

No it is not....
If you have them facing the same direction you polarity reverse the rear one and delay it to cause cancellations behind. Or you can physically reverse it which is the same as a polarity reverse. If you do both you will get cancellations out front......

I stand ready to be corrected, but two different system designers with lots of Meyer experience showed me how to do this. I just opened up the Galileo file in Compass to double check...it has been a long week...lol.

[Mac Kerr]

Yes I know, i never said not aligning it.... By physically reversing the sub you have already polarity reversed it

No you have not. Since at sub frequencies the speaker is almost omnidirectional all you have done is offset the rear facing speaker in time, equal to the depth of the box. It is still in polarity with the front facing boxes. Polarity is not about what direction the speaker is facing, front or rear, they are both causing a positive pressure in an omni direction.

Mac

[John Sulek]

No you have not. Since at sub frequencies the speaker is almost omnidirectional all you have done is offset the rear facing speaker in time, equal to the depth of the box. It is still in polarity with the front facing boxes. Polarity is not about what direction the speaker is facing, front or rear, they are both causing a positive pressure in an omni direction.

Mac

Thank you Mac for that concise explanation.

[ProSoundWeb Community]