Originally Posted by Erich H
There's a commercial subwoofer that uses something 'similar' to an Eminence Lab-15 with a 4" Precision Port. It was tested by DataBass and the end result showed that chuffing wasn't really an issue until it was driven really hard, and if I recall, even then didn't seem like a big issue.
The design very likely had a high pass filter in place, and as tuxedocivic points out this will help the velocity and chuffing issues quite a bit. It's also possible (although probably unlikely) that there was some sort of dsp limiting built in. Even with the likely hpf and possible limiting the results are pretty bad as shown in LTD's post. The port alone is not necessarily entirely to blame as LTD points out, it could have been partly due to thermal issues and amp limits too, but the port is clearly too small and was likely mostly to blame.
Originally Posted by Erich H
In your link he says this:
So for home theater usage, the total allowance would be 55% more than what most programs would consider possible chuffing issues?
The only thing that seems odd about that write up is that there seems to be a decent amount of estimating going on after the point where they hear even the slightest chuffing with their ear next to the port. Basically adding 25% if your 8' away, then adding in a percentage for content being played.
The Flare It program on that website is very good, the data was empirically tested with a number of different examples. The conclusion is that 10 m/s is generally considered safe and anything above that will result in losses and at some point audible chuffing. The reason he gives masking allowances is because people want small enclosures with small ports and his masking recommendations are estimates of what he thinks the average person is willing to tolerate wrt audible chuffing. As he notes there are things you can do to mask chuffing and make it less objectionable, but as Notnyt very correctly points out, if there is audible chuffing there are losses. In fact by the point where you can hear audible chuffing the losses are already pretty bad.
In Flare It there are two limit lines, the core limit and the chuffing limit.
The core limit refers to the ability of the interior of the port to pass air competently. The chuffing limit refers to the ability of the port exit and entrance to introduce the air into the enclosure (at the entrance) and into the room (at the exit) without audibly chuffing, which is violent air vortexes at the port entrance and or exit resulting in inefficiency and noise.
The core limit can be defined by the Reynolds number of the port, which takes into consideration the length, shape, aspect ratio, surface smoothness and air velocity (although I have no idea how Collo came up with his core limit information). If the Reynolds number is more than zero there are losses, although the losses don't become apparent until the Reynolds number is quite high. At some point when the Reynolds number gets high enough the efficiency of the port drops dramatically and the port becomes very restrictive, beginning to dramatically block the amount of air that can pass. This is why you should not exceed the core limit in Flare It.
The chuffing limit, as previously explained, is the ability of the port exit and entrance to allow a laminar smooth flow of air without creating vortexes bad enough to cause audible chuffing. A healthy sized flare on both exit and entrance can ease this transition tremendously. The flare can be either a roundover or a chamfer up to about 45 degrees or so (so slot ports can be flared too). The problem is that losses start well before chuffing becomes audible so it's best to keep velocity at or under 10 m/s.
I assume the reason you ask is because you want to make ported kits. This is pretty easy to do, just design them to keep velocity under 10 m/s at xmax with the required high pass filter in place and no one will ever have a problem unless they don't use the high pass filter (in which case they will probably damage the driver anyway). You could probably get away with as high as 18 m/s and maybe even more without audible chuffing (you can test it pretty easily, just build it and listen to material with tones at the tuning frequency at full power) but it's best to keep it under 10.
I can't remember if WinISD will simulate flared ports with a high pass filter in place but the upcoming version of Hornresp will be able to (new feature), I think TL.app can do it, Akabak can certainly do it, MJK's worksheets can do it and Flare It will give acceptable port sizes but not a simulation.