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Old 3rd October 2004, 02:08 PM   #11
Ron E is offline Ron E  United States
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There are three main interrelated effects from having a port that is too small. One is the turbulence caused by rapidly moving air (re: Reynold's number). The second is the losses caused by the sharp edges which cause separation of flow, especially upon sudden expansion. The third is compressibility effects due to the velocity of flow nearing the speed of sound. 5-10% of the speed of sound is close enough to start having this effect.

The first and third are solved by making the port larger, and the second is solved by smoothing the transitions with flares or some other sort of turning vane type device - look up the polk audio "Power Port". Bose has a patent on a certain port shape that may interest you...

Simulate your box in a program which shows port air speed vs. frequency... Better yet, derive the equations yourself . Define the problem before you try to solve it.
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Old 4th October 2004, 05:10 AM   #12
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Another thing you can do to improve the performance of your port is to use both flares and dimples (like a golf ball). B&W use this technique and I have heard of one member on this forum using a B&W box with such a vent, but using the Peerless 10" xls. The vent is small with a generous flare and dimples, and apparently has no audible turbulence.

Have a look on the B&W site and you will find they have some information on the concept in one of their papers on the deveiopment of the Nautilus. There is a 2 page description with diagrams so I won't try to repeat it all here.

This really brings you to the limit of what a diyer can achieve - generous flares and dimples. B&W put a level of research into this that no individual is like to have the means to achieve. Apparently the behaviour of a port is difficult to model in terms of the aerodynamic effects. Standard techniques used for other aerodynamic studies don't have to deal with the rapid change in direction which occurs with sound waves.
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Old 4th October 2004, 05:56 AM   #13
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Quote:
Originally posted by johninCR


Yury,

Let us know if you are able to change some of the laws of physics,


I can't. sorry.
but do you agree - small, or bigger port makes distortion to another frequency? so - it's not the best idea
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Old 4th October 2004, 06:11 AM   #14
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Just wondering, is there some golden rule for maximum airspeed in the port before it starts making noise ?
With my lspcad I can simulate the airspeed but these numbers don't say mee very much.
I always heard people say make the diameter of the port at least 1/4 of the diamater of the woofer, but if you have a woofer with extreme cone excursion this might not be sufficient.
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Old 4th October 2004, 06:24 AM   #15
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that's right.
I'm just try to do not a big cube subwoofer but I wish to get perfect parametres. I'm understand, I have to make it bigger, or less power, sound pressure, or higher resonance frequency....
but I wont. just experimenting. different form of port, port param.. etc. since I make port smaller diam - 50mm, I got acceptable sound. ok. but I loose sound pressure 50% anyway - it's working and I'll experimenting again.
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Old 4th October 2004, 07:52 AM   #16
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If you can add more than one port to the enclosure.... Why don't you just add another one?

From memory you can make it the same diameter and length for the same tuning frequency. People might argue that you need to compensate for the increased resistance, as for example 2 x 2" ports will have more surface area exposed to the rushing air than a single port.

I might be wrong in "the same length / same diameter" - but I'm sure someone can correct me - or use WinISD.

David.
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Old 4th October 2004, 08:03 AM   #17
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done
same problem, a little better.
I wouldn't make it 3 pcs, don't like it, but probably - it's should be ok. now I try to safe my box desing
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Old 4th October 2004, 08:48 AM   #18
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Quote:
Originally posted by Dave Bullet
I might be wrong in "the same length / same diameter" - but I'm sure someone can correct me - or use WinISD.
If you add a second identical vent the Fb will be raised significantly.

Tuning is determined by:

* box volume
* total vent cross sectional area
* vent length

Adding a 2nd vent increases the cross sectional area, hence you need to make the vents significantly longer to keep the same tuning.
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Old 4th October 2004, 09:04 AM   #19
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Quote:
Originally posted by paulspencer


If you add a second identical vent the Fb will be raised significantly.

.

that's right
but if I make port rectangular section, I've get less length, egg-glass port will economize it again. now I have:
27 litres box inside
20 cm round port length
50 mm port diam
30 hz tuning fr. it's enough for a little box.
I used formula: L=(2354*D^2/VF^2)-0.85D
looks like truly
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Old 4th October 2004, 10:03 AM   #20
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Quote:
Originally posted by Yury



that's right
but if I make port rectangular section, I've get less length, egg-glass port will economize it again. now I have:
27 litres box inside
20 cm round port length
50 mm port diam
30 hz tuning fr. it's enough for a little box.
I used formula: L=(2354*D^2/VF^2)-0.85D
looks like truly
Tuning is correct

A rectangular port is no different unless it is closer to the floor which may extend the effective length of the vent, allowing it to be shorter.

Your vent should be flared on both ends to perform at its best.

Another way to reduce port noise which has not been mentioned is to use a rumble filter. This reduces the velocity further and can make a big difference.
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