port turbulent jet noise. any advice?

What kind of flares have you tried on the ends of the port?

You can only push so much air though a port of given diameter before it will make noise. You can use flared ends or you can make the port a larger diameter. The only other option is finding a way to change the speed of sound.

Take a look at this: http://www.carstereo.com/help/Articles.cfm?id=21

http://www.carstereo.com/help/Articles.cfm?id=21

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Informative, yes, but way too much work. All you have to do is model the box in WinISD and it will not only figure the correct vent and box size but will also tell you if the vent mach is too high.

You need a flared port, or a bigger (and longer) one

http://www.madisound.com/nht10subkit.html

The flare (a trumpet bell shaped zone on the inner and outer ends of the port) smooths the transition regions of flow and makes the port quieter.

You can alternatively try to fill the port with straws, but that will also reduce output.....

Yury said:

... but I'm looking for new, science solution.
I'm sure it's exist.....

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Yury,

Let us know if you are able to change some of the laws of physics, because I'm sure we can come up with a few we'd like to change.

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.

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.

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.

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.

Dave Bullet said:

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

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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.

Yury said:

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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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.