Mutual Coupling for Ports

[Collo]

When you add a second sub to your system, you get an extra 3dB because you doubled the power. If the drivers are close together you get another 3dB from mutual coupling.

My understanding is that the two drivers act as a single driver that is larger than the sum of the areas of the two drivers

It's not often in life that you get something for nothing

Here's my question.
Can you do the same with ports.

Maybe you could put 4 drivers in a cabinet, space them out a bit and fill the area between them with flared ports, generating a mega-driver!

mischieviously yours,
Collo

[Collo]

even better with seven drivers.....

[Cordraconis]

Well, as a port works as a resonator, (and obviousy has a moving air mass ), this means it emits sound and therefore can be coupled for more output.

However ... it only resonates at a certain frequency (DUH!), and therefore it will only couple over a very limited bandwidth.

First I wanted to use 3 ports for my sub design, and position them as a mini-line array given my 'pillar design', but eventuelly I abandonded it for one huge port-as-bracing as you well know.
I would also have needed an exceptionally high column for the array to work.

For me, the proof that there is mutual coupling *outside* the cabinet, is the fact there is more deeeeeeep bassssss on one side of my sub, than on the other. (I layed it down on the flour to reduce the visuality even more for my mother. ) I'm not complaining tho: it's also against the back wall, so more output from floor-and-wall mirroring AND my computer table resides in the 'coupled area' of the port.

OTOH, with 30-something frequencies, soom modes become MUCH more troublesome in my experience. I should measure those out one day, but now I'm more than happy with what I got, but still ... Even so that I'm thinking about useing multiple smaller subs next time I build (a) sub(s) -- probably within a few years when I moved out.




Bottom line is: yes, but too small bandwidth. Maybe in some bandpass designs???

[simon5]

I think it doesn't work like that.

I mean it's the driver who's pushing air inside and out. I think manufacturers would already have used that idea.

You could do a small MythBusters experiment, Collo!

[Ron E]

Mutual coupling is a separate concept from the geometric reasons (array) why you get +6dB on axis for two drivers in parallel.

True mutual coupling is an effect of radiators on each other, and Richard small proved that cone-port coupling is of negligible consequence in most circumstances in his papers in the JAES in 1972. I think the surface areas need to bevery large before mutual coupling is a factor.

[Collo]

Ron - Could you tell us a bit more about the geometric reasons?
I expect that the drivers need to be within a certain distance of each other for the effect to happen.

Simon - we get the Mythbusters here in Aussie too. You're probably thinking of the "Brown note" experiment....
http://www.meyersound.com.au/brownnote.shtm

Cordraconis - you're right about the limited bandwidth - if there was any effect, it would be a bit of a one-note wonder. Might be OK for SPL competitions though....
I see you are already dreaming about your next sub. Welcome to the world if DIY

[Ron E]

Actually, two sources mounted near each other simply add their volume velocity so you get 2x pressure or +6dB. Mathematically, sensitivity is related to (Sd*Bl/(Re*Mms))^2 Consider that doubling Sd and halving Re and doubling Mms gives a factor of 4.

If you mount the two woofers several feet apart, you get comb filtering off-axis at higher frequencies, but should still get +6 on axis with the center of the drivers below a frequency related to the distance between the two..

[Collo]

thanks for that...

I'll just stick to one or two drivers in a cabinet then.

[DSP_Geek]

Originally posted by Collo

Here's my question.
Can you do the same with ports.

Long story short: no.

Take a port, divide it into two, you have two ports, each with half the volume velocity of the original port. Add the volume velocities of both ports together, and voila! The original VV comes back.


Francois.