I'd just like to chime in with something I learned while playing with Fusion 360 for my 30 day free trial like a year back.
This is a totally over-engineered enclosure meant to have a low pass at 1000hz:
has only 30db signal to noise ratio at 1000hz.
And this is only
the resulting noise made by the panels (outside only) that will result due to the air pressure difference inside relative to the outside, which moves the panels. At 2000hz its about 24db signal to noise (so 6db/oct lowpass is out of the question obviously, even 12-18db/oct lowpass is not ideal), at 500hz its 36db, at 250hz its 42db, at 125hz its 48db.
To improve on this I could add butyl on the inside and outside (you know the stuff they add to car chassis to deaden the sound), and put rockwool on the outside, and the pillow stuffing will hopefully do the trick inside. But then it looks like a rockwool teletubby. It would certainly not have any issues in THD+N as far as the enclosure is concerned though.
Ever since I figured this out I have been rather more relaxed about the amplifier distortion. I have this order of importance now:
- Enclosure panel movement noise.
- Enclosure movement resonances (great bracing also braces the braces themselves asymmetrically, so you don't end up with massive amounts of bracing components resonating at the same frequency).
- Enclosure geometry resonances.
- Enclosure deadening (so the surfaces produce less sound when moving, because the surface is covered in butyl, rockwool and pillow stuffing, in the most extreme cases. Most importantly internally opposite the cone, and any baffle area on the front facing the listener).
- Sound treatment of room.
- Driver distortion.
- Amplifier distortion.
- Magic XLR cables I christened to bassanism with water boiled on melted JBL magnets.
The higher the frequency and the higher the decibel the more important all of these are. So there can be cases where upgrading an amp or driver is the next step without touching the above steps, but chances are there's something to be gained for half the price by taking a look at the above steps.
The top steps are the best in dollar per db in reduction in THD+N.
Sometimes a cheap floorstander set can be fixed by just running a couple braces from the left panel to the back panel and leaving the right side alone, that breaks up the resonance so you don't have both sides (and potentially also the back) resonating at the same frequency.
And often the internal closed mid woofer will have woefully inadequate stuffing, adding some butyl to the back plate (both sides since the other side of that thin MDF panel "plays" noise into the vented enclosure) and adding a thin layer of rockwool and then putting the factory stuffing back, does wonders.
Same with car stereos, when you fix your door "enclosure" it goes from a ****ty sound to a quality sound instantly. And if you spend twenty hours instead of ten you get dollar per db THE cheapest quality improvement. The amount of 1000 dollar car stereo 2-ways I've heard sound **** is too many to count, all because the owner didn't bother spending a couple hours fixing the door with some butyl and wool. Then I stick a MiniDSP in there to get rid of 3db wattage lost in heating the passive crossover, and some butyl in the door, and suddenly I'm a sound quality GOD to them.
EDIT: Point number 8 I hope you understood to be a metaphor for "audiophile stuff for sound quality" :P