Port Length-Cut From Thread

[Mario]

SteveCallas wrote:

While the sims may not show a perfectly flat response with a larger enclosure, do remember to go large enough as to be able to use an adequately sized port (at least 8" diameter).

This is what I've got so far. I was really ecstatic to hear that Gen2 can be used in smaller enclosures until I saw the size of that port.



I would really, really prefer not to go larger than this, but am afraid there's still not enough clearance between the port and the driver I am thinking of using 3 ports 4" wide which would give me an equivalent of 7" port (6.9 actually). Any reasons why I should not go that route?

[SteveCallas]

This is probably not the right thread to discuss build specifics. But I'll quickly tell you that you don't want a port length longer than 36" - either increase enclosure size, raise the tuning frequency, or consider passive radiators.

[Mike P.]

But I'll quickly tell you that you don't want a port length longer than 36"

I'm not sure why you would limit a port to 36 inches when ports in excess of 50 inches have been used will no negative effect as long as a 4th order 80 hz crossover is used and the first port resonance is 125 hz or higher.

[SteveCallas]

I agree to disagree with those who claim no audible degredation with port resonance that low.

[Mike P.]

There are a least a dozen or more sub builds here with ports that exceed 36 inches. No complaints from any of them. One with a 50 inch port was close mic-ed and the response shown to have no issues. So what are you basing your "disagreement" on?

[Ricci]

Mike I have to side with Steve on this one. I've not seen the data that convinces me that a 50" port is not going to have some audible issues in the pass band. I'm interested in seeing it if you have some links. I've seen a lot of people claim that it is fine, but you know how that is. Perhaps through creative bending, placement and damping/stuffing of the enclosure you can implement one well, but I would still hesitate to recommend it.

63hz 2nd harmonic is 125hz
42hz 3rd harmonic is 125hz
32hz 4th harmonic is 125hz

Distortion coud excite the port resonance and a crossover does not help with that.

[Mike P.]

Post #39, Andrew tested a 55 inch port. It's my understanding there are no issues as long as a 4th order 80 hz crossover is used.

http://www.hometheatershack.com/foru...dations-2.html

[Ricci]

I remember that thread. I also remember not being totally convinced. Andrew's measurements were from 1" and did not include a complex sum of the port output mixing with the driver output measurement from a further distance such as 1m equidistant from the port and driver. They clearly show a very large spike in the upper port output. Also his enclosure has more attention to detail and also has a many folded port. Other people may not put so much effort into it. What if it's just a straight 50" pipe with no bends and not as much damping? The resonance will likely be stronger.

Someone should test this stuff...

[Mike P.]

Someone should test this stuff...

I'll agree on that.

[Mario]

I changed the effective volume to 370 l and the port is now 3'4" with 169Hz resonant frequency. I'm not really worried about resonance since I'm planning on getting a BFD. A simple notch filter would put that issue to bed (Isn't this the original purpose of BFD, surgically removing troublesome frequencies?).

I still don't know what's the minimal acceptable distance between the port and the driver. Any suggestions?

[Mike P.]

One port diameter or more.

[dynamowhum]

Mike is this when port and driver are mounted on the same baffle? Is it different when the port is perpendicular to the driver or if it is rear ported? Thanks.

[Mike P.]

For a sonotube the end of the port should be one diameter from the magnet, for box enclosures the port should be one diameter away from any side or back wall.