Port velocity vs port resonance

Raise the tuning a bit, flare the ends of the port, use a steep LPF and even a notch at port resonance to make it harder to excite.

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.

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.

I have built a few subwoofer's that have had port air velocities in the 25-35m/s range, from what I have found slot ports seem to be far more forgiving with port noise,

However I think port velocities in the mid forties would mean that there was compression of the air, that probably reduces output,

bringing the tune up to 13-14Hz could be the way to go, your probably not going to notice the difference.

I never found any issues with loss of output at all. My alignment was the best in that size box so I already knew I would have to use EQ but with room gain it wasn't an issue.

I now have multi tuned subs which are low, medium and high and and my low tune is still high on velocity and no issues with compression, output or chuffing.

Hi Tim,

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.

a large flare can change audible chuffing greatly for the same diameter:

http://www.subwoofer-builder.com/port-flares.htm