Port velocity vs port resonance - AVS Forum
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post #1 of 11 Old 08-01-2012, 04:04 PM - Thread Starter
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Hi guys

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

Any thoughts on this?

Tim
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post #2 of 11 Old 08-01-2012, 04:27 PM
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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.
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post #3 of 11 Old 08-01-2012, 06:16 PM - Thread Starter
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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?
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post #4 of 11 Old 08-01-2012, 10:00 PM
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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.

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post #5 of 11 Old 08-01-2012, 11:42 PM
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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.
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post #6 of 11 Old 08-02-2012, 05:40 AM - Thread Starter
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Thanks guys

<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!
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post #7 of 11 Old 08-02-2012, 09:33 AM
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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. smile.gif
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post #8 of 11 Old 08-02-2012, 11:08 AM
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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.
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post #9 of 11 Old 08-02-2012, 12:29 PM
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Quote:
Originally Posted by TimVG View Post

Thanks guys

<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!

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.

Mark Seaton
Seaton Sound, Inc.
"Make no little plans; they have no magic to stir men's blood..." Daniel H. Burnham
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post #10 of 11 Old 08-02-2012, 02:24 PM - Thread Starter
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Thanks for your comments

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.

Tim
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post #11 of 11 Old 08-02-2012, 09:51 PM
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a large flare can change audible chuffing greatly for the same diameter:

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

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