I'm going to build a LLT subwoofer (12,3 cuft, 11hz tune) for which I'm going to use a slot port.
It seems I'm either going to have a high port velocity around tuning, or a very low first port resonance (<100hz)
The middle ground would be a max port speed just above 30 m/s and a first port resonance of 110hz
In this low register, what exactly am I aiming at for air velocity? Playing with tuning a bit doesn't seem to make much difference. What does seem to help is increasing the box size, but is 30 m/s at 10hz really that bad at full excursion? Will there be noticeable chuffing?
With the addition of a proper high pass filter @ tuning, your figure of 30 m/ sec should drop considerably. I would shoot for 11 m/sec or lower if possible. This isn't always possible due to the driver parameters and required enclosure size.
"What the heck was that crashing noise?" Me: "oh, just tossed the shampoo bottles off the shelf on the opposite side of the house. "
Just to mention I built a 11 cu ft enclosure for my FI Q18's which was tuned to 11hz. I never used a hpf, had 1000 watts input power and modelled port velocity of around mid 40's and 1st port resonance of 133hz which is slighly higher. I used a 100hz crossover for these and no port chuffing at all even playing very low stuff.
I can understand why you should play safe but I was very surprised with my findings. One thing I should mention is that 11 cu ft isn't ideal for the old Q18 as they require a much larger box but it wasn't an option for me but once EQ'd I was good to around 7-8hz.
<11m/s is impossible without the 1st port resonance ending up in the audible range.
mid 40's and no chuffing? Thanks for your input, that's an encouraging number.
Keep the input coming!
By every measurement of ported subs I've made, that's a pipe dream. Pun intended. I'm not sure how others are measuring compression, but using highly consistent measurements, you start seeing some in the 10-15m/s range and more as you go above that with the port flare/end geometry having some useful effect on compression and being key with audible turbulence/chuffing.
Port resonances are important to pay attention to, but location, port size and relative volumes are all factors as to the significance.
Mark Seaton Seaton Sound, Inc. "Make no little plans; they have no magic to stir men's blood..." Daniel H. Burnham
I think a safe route in that case would be aiming for a 1st port resonance of 80hz and keeping speed velocity down. In case the resonance would be audible I'll try a notch filter or simply using the sub in a limited bandwith (10 to 30/40hz). I have other subs that can cover the range above that if need be.