Ripole cavity resonance question

[bvan]

I'm wanting to build a pair of ripole subs into some 3-way OB speakers. I like this design mainly for its force cancelling properties as they sit on floaty wood floors.

There will be a pair of 12" Peerless XXLS per side, and I'd like to run them up to about 200hz. XO is digital and can be 4th order, plus I could tame some resonance with Eq.

The smallest I can make the cavity is 32cm front to back, and side to side. I'm guessing this means I cant push the resonance much above 200hz?

The problem is though, I'd like to have the lower mid OB driver in a slightly wider than 32cm baffle so that its dipole roll off doesnt start too much above 200hz.

My question is, if I make the whole speaker say 44cm wide, and the dipole (drivers lying horizontally as in Orion 4) then have a width of 44cm, does this lower the ripole resonance? Or is the ripole resonance not affected by the distance between these two parallel sides but only by the depth of the chamber, i.e distance from front/backwall to mouth of cavity, if that makes sense?

Cheers
B

 

[bolserst]

My question is, if I make the whole speaker say 44cm wide, and the dipole (drivers lying horizontally as in Orion 4) then have a width of 44cm, does this lower the ripole resonance? Or is the ripole resonance not affected by the distance between these two parallel sides but only by the depth of the chamber, i.e distance from front/backwall to mouth of cavity, if that makes sense?

I'm a little unclear on your dimensional descriptions...a drawing would be helpful.
In general the cavity resonance is a function of both the contained volume and the size of the opening. However once the cavity gets so narrow that it starts acting more like a quarter wave organ pipe that a helmholtz resonator, the resonance frequency will mainly be dictated by the depth of the chamber.

Attached are some 1m ground plane measurements I made some years ago comparing an H-dipole, N-dipole, and Ripole. These should help you get a handle on what the resonance frequency and magnitude might be in your case.

 

[bvan]

Thanks for that. I'm not sure from your measurements what exactly accounts for the small differences in the resonance frequency though.

Let me describe my confusion like this: in a design like the Orion 4 which I'm considering

I could go either N-baffle Ripole or N-baffle Linkwitz (not necessarily trying to restrict and lower fs). The way I see it the air in the cavity is constrained in 3 dimensions: A) driver to opposing face, i.e 3, 6 or 12"; B) between the two parallel side walls (12"); and C) between the back/front wall and the opening of the cavity, also 12".

If I were to widen the baffle I'd be increasing B only. Would this lower the resonance peak, or is that more a function of C, the depth of the chamber, as in the length of a bass port? I've never really understood how this length effects resonance frequency as in my head I imagine resonances only forming between two parallel sides, not between one side and an opening...

Hope I'm explaining myself ok

 

[adason]

nice measurements bolserst

 

[bolserst]

If I were to widen the baffle I'd be increasing B only. Would this lower the resonance peak, or is that more a function of C, the depth of the chamber, as in the length of a bass port? I've never really understood how this length effects resonance frequency as in my head I imagine resonances only forming between two parallel sides, not between one side and an opening...

The cavity resonance is not a due to reflection between parallel sides. It is a Helmholtz type resonance which develops because the air inside the cavity acts like a spring and the air at the opening of the cavity acts like a mass. You can think of it as a vented or ported box with port length zero and port area the size of the opening.

With this in mind, you can get an estimate of what the cavity resonance would be by using a calculator for a vented box. Just set the volume of the box equal to the volume of the cavity, and the area of the port equal to the area of the cavity opening. Then solve for the tuning frequency that gets you a port length of zero. This is more useful for trends off of a measured case you have built than calculating an exact cavity resonance, but it will get you close.

For the examples I posted measurements for, a vented box calculator would estimate the resonances to be:
H-Baffle = 270 Hz
N-Baffle = 225 Hz
R-Baffle = 270 Hz

For your example where we start with N-baffle dimensions(A=6" , B=12", C=12"), if you increased B to 18", the cavity resonance would likely drop by about 10% to around 205 Hz.

 

[bvan]

Thanks bolsert, that makes sense to me. I think I'll widen the baffle then add some inserts into the cavity walls to reduce the volume in the hope of pushing the resonance as high as possible.

I'd read another thread on this topic where someone posted that even if the resonance peak is Eq's flat, that the decay times at these frequencies will still be longer than wanted, so I dont wont to have to rely on DSP to fix things if this is the case.

Cheers
B

 

[bolserst]

I'd read another thread on this topic where someone posted that even if the resonance peak is Eq's flat, that the decay times at these frequencies will still be longer than wanted, so I dont wont to have to rely on DSP to fix things if this is the case.

Agreed. Just like trying to "fix" room modes with equalization, you can reduce them in level, but the long decay time remains. From listening impressions with N-baffle cavity resonancies near the crossover point, I found I preferred the sound when I notched out the cavity resonance -3dB to -6dB or so rather than equalizing it flat.

 

[bvan]

Cheers, I'll keep that in mind.

 

[Rudolf]

I'd read another thread on this topic where someone posted that even if the resonance peak is Eq's flat, that the decay times at these frequencies will still be longer than wanted ...

I don't believe that. Did he show any evidence of longer decay times ... 😕

From listening impressions with N-baffle cavity resonancies near the crossover point, I found I preferred the sound when I notched out the cavity resonance -3dB to -6dB or so rather than equalizing it flat.

This makes much more sense to me. Note that the radiation pattern of thoses frames widens rapidly above the dipole peak (resonance frequency) . While the on-axis response stays the same, the total energy input into the room is increased. This can be compensated by reducing the on-axis level accordingly.