Multiple ports Tempest

[Stew320]

Hi there,
I'm new to vented enclosures. I'm building a sub using a Tempest in a net 197 litre box tuned to around 20hz for home theatre application.

I am having difficulty finding 6" PVC pipe for the port. I have 3" PVC pipe in stock. Would the efficiency of the box be any less using 3x 75mm x 440mm ports instead of 1x 150mm x 600mm ?

Also is it wise to bunch multiple ports together?

[Svante]

The efficiency will be the same, but the maximum level at the lowest frequencies might drop a little, depending on the margins of the original design. I have not checked your dimensions, but they seem reasonable and assume you have done some calculations to keep fh constant.
If you want the same max output level, use 4 tubes, this would give the same area and you should use the same effective length.

[Stew320]

Cheers!

Just what I wanted to hear.

Stew

[Grahamt]

Instead of PVC you can use 6" sonotube which is readily available at hardware stores.

[johninCR]

Originally posted by Svante
If you want the same max output level, use 4 tubes, this would give the same area and you should use the same effective length.

Multiple ports with the same total cross sectional area does not exactly equal a single port of the same length. This is due to there being more friction for the air to overcome with multiple ports.

Check out http://www.jlaudio.com/tutorials/por...ex.html#basics for the correct method for computing multiple port lengths.

[Svante]

Originally posted by johninCR



Check out http://www.jlaudio.com/tutorials/por...ex.html#basics for the correct method for computing multiple port lengths.

Unfortunately, the second method on that page does not give the "correct" result either. It would if the ports were spaced infinitely far apart. And the reason is not the frictional losses as stated, but the end correction. The end correction is a way of including the mass of the co-oscillating air outside the tube in the total length of the tube. Assuming that the opening behaves like a baffled piston this mass can be modelled as a plug of air with the same area as the tube and a length of 0.85 times the tube radius. The equation on the page has some similarities with this.

Now, if the tube diameter is smaller, this correction gets smaller, and so a longer tube is needed. That is, provided that there is no interaction between the tubes. However, if the tubes are mounted close to each other, there will be coupling between them, and the end correction will grow. This is why "method 2" is not correct either.

So the truth is somewhere between the two solutions given on the web page. There are further complications, for example if the port is not baffled (like on the inside of the box) the end correction becomes smaller.

So, be sceptic towards web sources that state that they give "correct" solutions. And the required port length change has nothing to do with frictional loss, it can be explained by a change in the co-oscillating air mass.

However, frictional loss has some impact on the design. Primarily the Q of the helmholtz resonator will be smaller, and this will lead to a slightly lower level at fh. Also the risk of turbulent sound is higher. So there are no advantages with splitting a tube, apart from practical ones.

[johninCR]

Svante,
Realistically how far apart should the ports be to effectively have infinite spacing. I'd think that once you get more than 1.5 or 2 port diameters of spacing between ports that the interference between ports would be negligble. Is that not correct?

[Svante]

Hehe, this is tricky stuff, and just to show you that it is, have alook at:

http://www.acoustics.hut.fi/asf/bnam...papers/w01.pdf

The answer is there, I bet.

The answer is that I have no rules of thumb for the port spacing, you might be right in the 1.5 to 2 port diameters and I would guess that it is reasonable, without knowing.

[Stew320]

I think a 6" pipe will be the better soloution.

[johninCR]

Ouch, that stuff is to involved for my taste. I was just guessing at 1.5 or 2 by trying to visualize the interaction of air outside of 2 ports. Maybe it should be higher with low tuning and moving more air. I'll just remember to space multiple ports at practical distances. Thanks for pointing out the finer details though.