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-   -   Porting a fullrange enclosure? (http://www.diyaudio.com/forums/full-range/23936-porting-fullrange-enclosure.html)

HiggityHank 9th December 2003 06:04 PM

Porting a fullrange enclosure?
 
I've done a lot with subs, and know that sub-bass is non-directional, but this is my first fullrange speaker project, and coincidently my first home audio project. I had a little help the other day with my measurements (thanks guys) and am now just having a little trouble with the porting. I was thinking of using a 2.5" port, which WinISD says only needs to be 5.85" long to achieve my desired 35hz tuning.

My only real question is, how can I face the port, does it really matter if it's not facing directly at the user? With a tuning this low, I believe I could get away with downfiring it, or even upfiring the port, but I really just don't want it facing out... something about ports on cabinets just doesn't look right to me. Also with the option of up or downfiring the port is that I can use a larger port, with more length. For example a 4" port, 16.75" long. (from what I've heard, larger ports sound better... but again, my fullrange experience is limited)

Any help would again be greatly appreciated.

(if it matters, the exterior dimensions of the cabinet are 10"w - 30"h - 13" deep with a full double baffle)

wigginjs 9th December 2003 07:25 PM

The placement of your port shouldn't make any difference, although I think I would avoid a downward firing port because the floor might impeed airflow. I think a more accurate description of larger ports is that they are "safer". The larger the port the less likely it is to produce "port noise" (basically a whistling sound caused by a large volume of air moving through the port, usually occuring around the resonance frequency of the enclosure). WinISD actually reports the calculate "port velocity", green numbers being "safe", red numbers meaning there will probably be port noise. Generally speaking I would only recommend using a 2.5" port with speakers 6.5" in diameter and below, but it really depends on the specific driver.

wigginjs 9th December 2003 07:26 PM

Another quick note, the length of your tube should be measured WITHOUT including the portion of the tube located in the baffle. (ie. you've calculated 5" as your tube length, and you have a 1" thick baffle, your tube needs to be 6" long).

Nuuk 9th December 2003 07:38 PM

My full-range ported cabinets used a downward firing port about three inches in diameter and worked great.

I have tried four inch diameter ports but they don't sound any better than the three inch IMHO.

HiggityHank 9th December 2003 07:53 PM

Am I perhaps tuning this enclosure too low? I'm working with a Peerless CSX 176 and a Morel MDT39. I also plan on having seperate subs... should I aim for a higher tuning like 40Hz?

My only real concern with the 4" port would be pipe resonance, which I believe wouldn't be a real issue at only 16-18" of length with proper bracing

Also, is there some huge magical difference between premade port tubes and ABS or PVC pipe? I understand flaring the ends to reduce whistle, but is it really necessary to spend $10+ on something vs. using I've already got kicking around the house? (and can get more of for pennies, or less)

roddyama 9th December 2003 07:53 PM

I always liked forward facing ports only because there are fewer issues to contend with, but that's me. The port will work fine in any direction as long as the port opennings are unobstructed. That if it's facing downward, it is at least a few inches off the floor, or if it's rearward facing, you place the speaker far enough from the back wall. Common sense prevails here.

BTW, use the length the program gives you or your tuning will be off. However, you can experiment with radiused or flared edges which will change the required port length.

roddyama 9th December 2003 07:58 PM

Quote:

Originally posted by HiggityHank
Am I perhaps tuning this enclosure too low? I'm working with a Peerless CSX 176 and a Morel MDT39. I also plan on having seperate subs... should I aim for a higher tuning like 40Hz?

My only real concern with the 4" port would be pipe resonance, which I believe wouldn't be a real issue at only 16-18" of length with proper bracing

I would try to keep the length-to-diameter ratio les than 3 to 1. You might want to try a 3" port.

Guss 9th December 2003 08:50 PM

Is the ratio that important ? I mean what if I want to double port my cabinet...I would need 20'' long ports with 2'' opening. Pretty ridiculous for a 5.5 woofer project but I can't get rid of port noise and now I don't want to have any so :smash: Should I brace the port ? It sounds contradictory to me but who knows!

I noticed that smaller diametter port, at higher volumes, just pass the air, as if there was no more sound coming out of the port. I'm not talking of unloaded situation but when air travels too fast in the tube. Is having longest largest or mutiple port will avoid that?

roddyama 9th December 2003 09:13 PM

The ratio is important. In general, I try for the largest dimeter that will give me a L-to-D ratio of between 1 and 2 (this is not to say that higher ratios won't work). This will usually keep the air velocity in the port down to an exceptable value. Higher ratios and you run the risk of turning the port into a Helmholtz resonator. Mutiple ports work like a single port of the same area and length with some resistive loading added.

HiggityHank 9th December 2003 09:38 PM

Quote:

Originally posted by m0tion
Another quick note, the length of your tube should be measured WITHOUT including the portion of the tube located in the baffle. (ie. you've calculated 5" as your tube length, and you have a 1" thick baffle, your tube needs to be 6" long).

Never heard this one before... Something tells me that would throw off my tuning, unless the entire baffle thickness was used as a flare, which in this case, it won't be. I can't see that 1.5" of extra port would be needed to clear my 1.5" baffle... but I guess I could be wrong.

Experts?


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