Port Noise, Wind Speed................. - diyAudio
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Old 8th April 2003, 03:48 PM   #1
OMNIFEX is offline OMNIFEX  Jamaica
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Default Port Noise, Wind Speed.................

Hello All

What is the rule of thumb of the maximum turbulance
exiting from a port before the noise becomes
audable.


I generally use rear ports, so, I don't have this problem.

However, I may get my hands on six double eighteens
(Used) which are front ported.


Numbers would be great!

Thanks!
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Old 8th April 2003, 04:38 PM   #2
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minimum diameter (inches) > 39.37(411.25(Vd)/(fb^.5))^.5
Vd = cone displacement in cubic meters
fb = tuning frequency (Hz)

What's the surface area of an 18" something like .125 m^2
multiply that by the xmax in (be sure to convert to meters)

estimating can't kill ya...
hope this helps
-andy

(taken from the loudspeaker design cookbook - Vance Dicakson)
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Old 8th April 2003, 05:01 PM   #3
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Lightbulb Port Noise

Hi OMNIFIX,
The Bose 901 speaker have aerodynamically shaped ports to elimanate the port noise even at high volume.
Looks a bit like a yet-engine exhaust.
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Old 8th April 2003, 05:32 PM   #4
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The rule of thumb I read was to keep the max wind speed below 5% of the speed of sound: 17 m/s. I wish I could remember where I read that. I do remember that it was one of the big names in acoustics back in the 60's. Some JBL guys did some distortion measurements at different volume levels for different degrees of rounding off of the port ends. After making some calculations to convert to m/s the JBL charts did seem to back up the 5% rule. As I recall distortion was a few % at that point for either a straight or rounded port. Where as for 5m/s distortion was pretty minimal for a well rounded--still high for the straight port. Don't quote me on that, though--better go to the source, which was published in the JAES in 2002.

John
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Old 8th April 2003, 05:36 PM   #5
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Oh, a couple other things...wide-band noise is certainly one result of vent turbulence, but you also have to worry about non-linear distortion and vent compression (the acoustic resistance of a vent rises with velocity). Putting the vent in the back won't help you out there, unfortunately.

John
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Old 8th April 2003, 06:57 PM   #6
OMNIFEX is offline OMNIFEX  Jamaica
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Quote:
Originally posted by hancock
Putting the vent in the back won't help you out there, unfortunately.

John
Yes. I'm aware with that. (Thanks!) I have an
oppertunity of getting 6 Double Eighteens
(Dirt Cheap) However, the vents are in the
front, and, I needed a ballpark measurement
when I scale them in my program, the precentage
of port noise I will get at 1200 watts a box.
(Conservative Power)

Thank You Gentlemen

Best Regards,
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Old 8th April 2003, 10:05 PM   #7
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no one can know how much noise it will produce,,-can they ?
u can just minimise it.
winisd says to keep it below 3% speed of sound

www.jlaudio.com

bose-
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Old 9th April 2003, 11:50 PM   #8
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Isn't port velocity inversly proportional to port diameter? If you make the port or ports large enough (Dickason's equation) it shouldn't be a problem.
-andy
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Old 10th April 2003, 04:10 AM   #9
OMNIFEX is offline OMNIFEX  Jamaica
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bostarob,

You absolutely correct.

The only downfall of having a very large port, it reduces
the internal chamber for the speaker.

Say you have a 14 cubic foot box,(Internal counting
the driver) and your using a 24 x 22 x 12 inches in
depth port. You will have no port noise, however,
your internal chamber is now 9.5 cubic feet.

So, there's always a trade off on large ports.
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Old 11th April 2003, 01:09 AM   #10
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Are you sure? Then why is it that when you have a ported speaker with the vent hanging outside of the box, (for testing purposes) Fb is barely effected? I'll try and find something more substantial.

hmm.. Dickason talks about pipe resonances as the only disadvantage of having long port. He does say that "Larger cross secional areas will always produce better linearity in any given situation. For high powered applications, such as the speakers designed for stage performance, it is desirable to use vent areas as nearly equal to the driver area as possible." He also says that problems asociated with pipe resonances are not nearly as troublesome as those of undersized ports...But he does not mention that port volume subtracts from total box volume. If I am overlooking something please tell me.
-andy
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