Port Noise, Wind Speed.................
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 8th April 2003, 04:48 PM #1 OMNIFEX   diyAudio Member     Join Date: Mar 2003 Location: Locked Up In The Amp Rack 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! __________________ OMNIFEX
 8th April 2003, 05:38 PM #2 bostarob   diyAudio Member   Join Date: Apr 2002 Location: New York 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)
 8th April 2003, 06:01 PM #3 Elso Kwak   Banned   Join Date: Dec 2001 Location: Zamboanga, City of Flowers, Mindanao 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.
 8th April 2003, 06:32 PM #4 hancock diyAudio Member   Join Date: Jan 2003 Location: Zurich 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
 8th April 2003, 06:36 PM #5 hancock diyAudio Member   Join Date: Jan 2003 Location: Zurich 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
OMNIFEX
diyAudio Member

Join Date: Mar 2003
Location: Locked Up In The Amp Rack
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,
__________________
OMNIFEX

 8th April 2003, 11:05 PM #7 mikee12345   diyAudio Member     Join Date: Nov 2002 Location: NZ 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-
 10th April 2003, 12:50 AM #8 bostarob   diyAudio Member   Join Date: Apr 2002 Location: New York 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
 10th April 2003, 05:10 AM #9 OMNIFEX   diyAudio Member     Join Date: Mar 2003 Location: Locked Up In The Amp Rack 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. __________________ OMNIFEX
 11th April 2003, 02:09 AM #10 bostarob   diyAudio Member   Join Date: Apr 2002 Location: New York 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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