It is far too much
Really, think it over. Been there, done that. Making it bigger only causes the urge to damp lower resonances, which indeed need more damping. Smaller volume leads to higher frequencies to damp e.g. less damping material.
The energy density has to be very high for air and damping materials to behave nonlinear. A midrange driver probably even couldn’t produce the levels involved. Furthermore I doubt that possible nonlinearities of the damping really would be noticed as the driver itself would be way beyond it’s limits.
Another aspect: a smaller mid enclosure brings less acoustic output of panel resonances. Now that is probably less relevant in a classic threeway. But still the problem of relative low frequency resonances persist.
I’ll end my argument here and won’t spoil this topic any further, it’s not that important. Looking forward to posts with measurements!
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Mark - I agree that 12 liters is more than enough (quite a bit more), and is probably past the point of diminishing returns. I simply think that the midrange enclosure should be as big as practical, and in this case, with this design, it is practical to go as large as 12 liters. I am sure that 6 liters or 4 liters would be fine, but there is no harm in being larger than necessary.
When i talk about the energy density, I am not concerned about non-linearities. I just want to bring down the SPL within the enclosure as quickly as possible, both for acoustic resonances (standing waves), and for the large regions where there is no acoustic resonance. The fibers/foam absorbers work better when there is more of it.
And you make a good point Mark about a larger midrange enclosure having the potential to have a larger signature... yes that is a good point.
In the end I think this whole issue is "small potatoes"... it may make a small difference, but it is far less important than getting the basics right: Smooth flat frequency response on and off axis, good bass response, low distortion, managable impedance. The fact that you have used small midrange enclosures with good results, while others have used much larger enclosures, demonstrates that this is a small issue... I suspect if we discussed this over a couple of beers we would soon be in agreement... And yes we are pulling the discussion off the rails...
At half wavelength (cabinet dimension = lambda/2) there will be the first mode resonance. You can think of it as a reflection, that is a good analogy.
I guess it depends on the damping... if you are talking about true structural damping, then yes. But if it is something like thick felt or foam then I would think it would not reduce the cabinet volume.
You talk about leaving some breathing room behind the driver. Do you find this helps on midrange drivers? I know that for bass drivers it is considered a best practice to keep stuffing away from the cone. But for midrange frequencies I am not sure. I know that I stuffed my mid driver enclosure full of long fiber wool without a dedicated bubble... I made sure the stuffing near the driver was less dense, but it is right up against the magnet. What is your experience?
Your new rule of thumb for driver Vb to be greater than or equal to Vas/2 is yet another guideline that seems to make sense. As I mentioned earlier, I asked this question about a year ago, and I got a lot of different recomendations.... but the general trend was "bigger is better".
(Wow 3 posts in a row... )
To play Devils advocate with myself, and back up what Markbakk is saying, take a look at the Vandersteen 2c Signature 3
Vandersteen Model 2Ce Signature III | Vandersteen Audio
The 4.5 inch midrange driver is crossed at 600 and 5k, and I would judge the enclosure to be 2 or 3 liters at most.
To play Devils advocate with myself, and back up what Markbakk is saying, take a look at the Vandersteen 2c Signature 3
Vandersteen Model 2Ce Signature III | Vandersteen Audio
The 4.5 inch midrange driver is crossed at 600 and 5k, and I would judge the enclosure to be 2 or 3 liters at most.
Small correction: I’m using the RS125-8. The Vas mentioned it correct for the RS125-8 though.
Yes, I'm talking about solid panel damping material here, like bitumen sheets or similar. We can't get the panel resonances below the passband and I don't think we can get them above. So I would go with both bracing and panel damping in this situation despite the fact that the addition of the damping will in fact reduce the panel resonance frequency which the bracing has just raised. I believe planet10 advocates for no damping but I'm not convinced that's the best strategy. An added bonus to the damping material is that it will also provide a little bit of extra attenuation towards the transmission of sound to and through the panels as well.
As a general rule, yes I have been leaving breathing room behind my midranges and have been very happy with the results. But at the same time, I haven't done A/B comparisons with insulation right up against the back of the driver so I don't think I can draw an objective conclusion about the efficacy of the condition. If the acoustic absorber is of low enough density, it may indeed be fine. But in the interest of eliminating any sense of congestion or lack of openness to the mids or a boxiness or whatever kind of subjective word you want to use, I think a larger enclosure that allows for both unimpeded room directly surrounding the back of the driver and then enough room for insulation, bracing and panel damping material that will sufficiently attenuate internal and external cabinet sound sources to meet one's goals is the way to go where possible.
My thinking may have taken some inspiration from B&W's mid sphere/tube design where the sphere radius is about equivalent to the diameter of the driver cone and the sphere is just very lightly stuffed. Quoting from their model 800 Development Paper:
"A sphere of around 300mm proved to be the best size (for a ~150mm diameter mid). Any larger and imaging seemed to be impaired. Any smaller and the unit sounded ‘closed in’. This is a difficult phrase to explain succinctly, but is akin to having traces of the effect obtained by cupping the hands round one’s mouth when speaking."
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