It’s basically when the air speed velocity exceeds the ports ability to pass the air without causing wind noise from compressing the airspace in the port.
Chuffing IF present, is almost always masked by the rest of the soundtrack. Meatheads that blast the 10hz square wave in the EOT intro which has no other real sounds to cover it up, can expose chuffing. In practice it’s not a problem, and any subwoofer with a port is susceptible, but it takes tons of excursion. I only use ported subs, and have never heard any chuffing. People that run single ported subs extremely hot can encounter this too. Imo, that’s more operator error than design flaw. Sealed subs take many times more power and excursion to reproduce deep bass, and that ALWAYS comes with distortion, and they still can’t produce the same output. So imo, the balance leans towards ported.
Ok, back on the main road…