[JohnR]

I've not tried a cardioid (or U-frame) sub/woofer yet, and have been meaning to for a while. Thinking back to my Subwoofer Origami article on dipoles, I wondered whether it was possible to accomplish the same thing as a U-frame but with force cancellation. I ended up with the following. I've not done the measurements to confirm that it works as expected yet, but since I just took photos, here they are...



Here's how it looks from above with the lid on, I'll work on the cosmetics for the next one... 



Hoped for: same type of dispersion pattern as a U-frame (with internal damping added), small reaction force on the cabinet, more woofage for the same footprint. Do you think it will work?

Measurements to prove or disprove in the next week or so.

[BPT]

Interesting. How about adding sides, thus channeling out of phase directly to the rear.
Chris H.

[JohnR]

Hi Chris, thanks for the comment. To be clear, the "front" (towards the listener) is at the point of the V i.e. bottom of the first picture.

If I'm understanding your suggestion, I think that would tend to push it towards dipole behaviour. The idea of the U frame is to use the frame to delay the out-of-phase signal at the rear, so that cancellation occurs at the back of the frame rather than to the sides.

[Blackmore]

Hi John,
I'd be curious how close to the back wall you could place the V cab before it became some sort of lossy bass reflex or completely lossy sealed cabinet. 

Cool design and you've been busy with the table saw and wood glue.   

[Russell Dawkins]

I would be interested in the effect of various types and amounts of stuffing in the "V" on the response and the polar pattern.

[markk]

Hi John,

Thanks for showing your project.

Please keep us posted as to how it sounds.

It may be interesting for you to read the Nelson Pass article in the Enjoy the Music magazine.

It is titled The Slot Loaded Open Baffle Project.  Here is the link 
 
http://www.enjoythemusic.com/diy/0911/slot_loaded_open_baffle_speaker.htm

He mentions "When you build a slot loaded cavity like this, you will run into a cancellation frequency that occurs when the wavelength is twice the depth, where the pressure from the part of the woofer which is deepest in the slot is one half the wavelength and cancels out the pressure from the portion of the woofer near the slot opening. Here the cancellation occurs at 500 Hz. "

It may be good for you to know where this cancellation frequency is for your project as that may influence the sound and blending of the rest of the drivers you are going to integrate this bass system to.  I think the deeper you place your woofers from the front of the slot the lower this cancellation frequency will be for your system.

Also he seemed to get more efficiency from the woofers facing into eachother and sending sound out of the slot in his diagrams and pictures so you may try and turn your unit around and reverse the polarity to see if it gets any louder or sounds better or different.

I hope you enjoy this article by Nelson Pass as it was a very interesting one for me.

Thanks,

Mark K

[johnk...]

Been a long time since I posted here but I saw this and took a look. I think that the V frame will work OK is correctly damped but one factor is that since the drivers are mounted on the sides there will be a loss of effective path length compared to front mounting.

Also, as must as I respect Nelson Pass's electronics designs I have to say that his conclusions for the slot loaded dipole thing are not supported by my research. I built a scaled down model in order to be able to obtain good acoustic measurement of such an arrangement. You can review my data here: http://musicanddesign.com/Nelson_Pass_Slot_loaded_OB.html

[JohnR]

Hi John, thanks for dropping in. I'll have to revisit your pages to figure out how to determine the correct amount of stuffing in a U-frame, I found that with no stuffing, the V frame was approx a dipole as expected. With stuffing the response to the rear dropped but not as much as I'd hoped. Also the front output at 20-40 hz dropped which I didn't expect. I'll do some more experiments before posting any measurements.

[zobsky]

Hi John, thanks for dropping in. I'll have to revisit your pages to figure out how to determine the correct amount of stuffing in a U-frame, I found that with no stuffing, the V frame was approx a dipole as expected.

... but with the added benefit of mechanical vibration cancellation

[AJinFLA]

Been a long time since I posted here but I saw this and took a look. I think that the V frame will work OK is correctly damped but one factor is that since the drivers are mounted on the sides there will be a loss of effective path length compared to front mounting.

Hi John, I hope you posting doesn't mean you summer sabbatical is over so soon. I imagine there's still some top down weather left up there.
I've actually been using that format (quietly) for a couple years, but with a "U", the drivers fully opposed for greater force cancellation and effective baffle length. I demoed just such a sub system last weekend at Capital Audiofest. Of course the damping is not as effective either due to the shorter propagation, but there are ways to use the asymmetry to an advantage and I'm still not convinced that it has to be, as just about any type of gradient system seems to work better IMO.

cheers,

AJ

[AJinFLA]

Hi John, thanks for dropping in. I'll have to revisit your pages to figure out how to determine the correct amount of stuffing in a U-frame, I found that with no stuffing, the V frame was approx a dipole as expected. With stuffing the response to the rear dropped but not as much as I'd hoped. Also the front output at 20-40 hz dropped which I didn't expect. I'll do some more experiments before posting any measurements.

Hi John,

How are you measuring, outside?

cheers,

AJ

[JohnR]

Hi John,

How are you measuring, outside?

cheers,

AJ

Hi AJ, yes, outside on the deck.

Part of the reason/excuse why it takes so long to do 

[AJinFLA]

Hmmm. That makes the loss at 20-40hz a bit of a puzzler. How much?
Did you perform a measurement without applied signal to the sub, to see the ambient levels of LF outside?
The stuffing can't physically restrict movement of the cone inside the apex of the V right?
Did you do any impedance measurements before/after?

[Russell Dawkins]

Maybe the stuffing is just reducing the level of the two drivers' resonant peak which would have been contributing to the output somewhere between 20 and 40 Hz.

[JohnR]

Hi AJ, 6-8 dB. I wondered about the physical restriction too, I'll try again. I wasn't able to do any more on that day, it was cold and dark by then

What would the impedance measurement show?

Russell, I'm thinking it's probably more likely what AJ suggests. I'll have another go.

[Russell Dawkins]

Impedance curves show the frequency and nature (Q and degree) of driver and system resonances. I suspect that in this case, with symmetry in two of three axes, the advantages of the configuration may go beyond force cancellation to include some degree of resonance cancellation.

Years ago (35!) I built a sealed subwoofer with two 12" drivers back to back, based on a design I saw in a English magazine like Hi Fi News. The operating principle was dubbed motional feedback, I think, and the idea was that with drivers having matched resonant frequencies the tendency to resonate at a particular frequency would be cancelled by the opposite driver trying to do the same. What was claimed was that the system would as a result exhibit almost no resonant characteristics, and this was demonstrated by an impedance curve through the bass range which showed a straight line ±1 or 2 ohms. As it turned out, that is exactly what it sounded like–I had never heard such a non-resonant bass before - all other speakers sounded boingy to some degree through the bass range. This led to considerable experimentation with other types of bass systems, one of my own devising employing a single driver, but none having the articulateness of the back-to-back pair.

[AJinFLA]

Hi John,

The impedance will show the shift in resonance w/ and without the stuffing.