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Old 20th June 2002, 11:05 PM   #1
Ignite is offline Ignite  Canada
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Default Port Mach & Flaring

Ok, I am planning on building a home theatre kit for a friend, and as always, the subwoofer is a big design consideration. With my design a 4" port would have a mach of 0.29, or 2 with mach of 3" of 0.13 and a port any larger would be almost unusably long. Now, if the ports were flared such as the "Precision" Ports from PartsExpress (#268-350 and #268-352) how high a port mach could I get away without hearing whistling? I'd love to only have to use a single port, especially since the 3" flared ports don't come long enough.

FYI: This is a tempest sub. I realize I may have to go for a very large port / number of ports.

Also, are there any issues associated with going with a large number of small ports? I could very easily get a low low tuning frequency with 5 or more 1.375" wide ports without having hugely long tubes.
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Old 21st June 2002, 12:37 AM   #2
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You could find a a 4 inch or 3 inch flexible tubing. Probably at a hardware store. Then coil it. However, if the tuning frequency is lower than 25 hertz. You might want to use passive radiators.
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Old 21st June 2002, 06:59 PM   #3
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Thats an awesome idea, I hadn't thought of that.

Still, I'd love to get away with a single flared 4" port but with 0.27 port mach I don't know if the flaring will prevent the sort of chugging whistling sound ports with too high a mach sometimes make. Does anyone have any idea how much the flaring will cut down? Or any first-hand experience?
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Old 22nd June 2002, 05:15 AM   #4
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Passive Radiators solve a lot of port problems, but create some of their own.

For one thing, the Passive Radiator itself has a resonance, and this resonance interferes with the box resonance if the passive radiator does not resonate far enough below the box resonance point. Theoretically, you should be able to tune the Passive Radiator so low that it is not a factor, but in the practical world it ususally seems to be. As a result, while a ported system is generally a 24 dB/octave cutoff after resonance, the passive radiator seems to be a 30 dB/octave cutoff, and the F3 often is a little higher, though not much.

The steeper the cutoff slope, the higher the distortion. However, it should be pointed out that the lower the frequency, the less distortion is perceivable.

The Passive Radiator itself costs something. I think Adire has a 15 incher, and if you buy 2 of them, the cost becomes almost as much as the Tempest. The area of the Passive Radiator(s) should be at least twice the area of the speaker.

Don't rule out the Passive Radiator, but don't jump into it without knowing it's disadvantages.
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Old 22nd June 2002, 05:23 AM   #5
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On the question of the flared ports: I would have to check my sources, but I am almost certain that I have read an article fairly recently that states a flared port is roughly equivalent to a straight port that is twice it's area distortion-wise.

Note that it is twice it's area-not twice it's diameter. So a flared 2 inch port is roughly comparable to an unflared 3 inch port as far as distortion and port noise goes.
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Old 22nd June 2002, 05:37 AM   #6
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Ignite:

By the way, I just came across this chart which shows the difference between a box containing the Peerless 832732 woofer in a ported box and a Passive Radiator box. Note the difference in the fourth impulse. Ported is on the left, Passive Radiator on the right. Input is in the faint red line.

There is a definite difference.
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File Type: gif peerless woofer br and pr.gif (32.5 KB, 347 views)
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Old 23rd June 2002, 09:05 AM   #7
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Quote:
Originally posted by kelticwizard
On the question of the flared ports: I would have to check my sources, but I am almost certain that I have read an article fairly recently that states a flared port is roughly equivalent to a straight port that is twice it's area distortion-wise.
That is awesome. So from the definition of area, a flared port of radius r is around equivalent to a straight port of size f(r) = (2*(r)^2)^0.5. So 4" port flared is equivalent to about 5.66", which has an acceptable mach of 0.13. This project has a finite dollar sign, and I really don't want to have to get a 18" passive radiator or something.
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Old 23rd June 2002, 02:06 PM   #8
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Hello Ignite,
I saw some info somewhere on the net recently that asserted that a semicicular port cut out of the bottom of the front panel is the best.
Sorry I have no idea of a link.

Regards, Eric.
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Old 23rd June 2002, 03:48 PM   #9
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Ignite:

Correct on the 4" flared =5.6" unflared port.

If you want to use passive radiators, (which you have made clear you wish to avoid), I just want to emphasize that the area of the passive radiator(s) should be at least twice the area of the driven cones.

Peerless with it's XLS series recommends a 12" XLS in front with two 12" PRs on the left and right side. This is an example of using 2 PRs of equal size to the driven cone. But again, PRs have their own advantages and disadvantages.
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Old 28th June 2002, 05:14 AM   #10
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I have an extensive article on flared ports that I should read. How would one go about constructing a flared port?
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