Port Mach & Flaring

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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.
 
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?
 
diyAudio Moderator Emeritus
Joined 2001
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.
 
diyAudio Moderator Emeritus
Joined 2001
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.
 
diyAudio Moderator Emeritus
Joined 2001
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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kelticwizard said:
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.
 
diyAudio Moderator Emeritus
Joined 2001
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.
 
diyAudio Moderator Emeritus
Joined 2001
Quote Bostarob: "How would one go about constructing a flared port?"

I think that instead of constructing them, you mostly buy them from Madiound and Parts Express.

2", 3" and 4" sizes. I have heard of a place that sells a 6" size, but I am not sure of it's name at the moment.
 
Volume 50 number 1/2 in the JAES.
On page 98 there is a memoriam of my great uncle, Arthur Gruber. [this is why I have this journal]
Ever seen a flared port with dimples reminiscent of a golf ball? I've seen a B&W with one. Surface roughness is supposed to somehow make better use of turbulence.
 
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