Reflex port under the cabinet ?? - diyAudio
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Old 29th September 2003, 02:32 PM   #1
gary f is offline gary f  Canada
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Default Reflex port under the cabinet ??

Here is the idea,

I want to build a down firing port that would open on the bottom of the cabinet. This way, I could adjust the effective area of the port by adjusting the gap between the cabinet and the stand below.

I think this have been done before maybe by Naim if I am right.

What do you think!

F
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Old 29th September 2003, 04:27 PM   #2
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I think they call that a "slot-loaded port" or something like that. It has been done before, though what the details of the tuning is, I don't know. I do believe that if you go just a few inches above the ground or stand, the port reverts to normal tuning pretty quickly.
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Old 29th September 2003, 08:49 PM   #3
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Default Re: Reflex port under the cabinet ??

Quote:
Originally posted by gary f
Here is the idea,

I want to build a down firing port that would open on the bottom of the cabinet. This way, I could adjust the effective area of the port by adjusting the gap between the cabinet and the stand below.

I think this have been done before maybe by Naim if I am right.

What do you think!

F
Polk uses what appears to be something like that for a built-in powered subwoofer. They add a pyramid thingy with curved sides that points up at the port and call it a "Power Port (tm)."

http://www.polkaudio.com/home/techno...=3&speaker=164

It's not adjustable though.

"This technology provides performance that vastly exceeds any conventionally ported speakers in the same size and price class."

I've been reading a lot about loudspeakers and such lately. Oddly, they ALL vastly exceed the others.
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Old 29th September 2003, 09:26 PM   #4
gary f is offline gary f  Canada
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Quote:
I've been reading a lot about loudspeakers and such lately. Oddly, they ALL vastly exceed the others.
Yeah but mine will exceed EVERYTHING else of course....

The idea seems good for another reason than adjustability (see schematic)

The box act like a big flare, just when the air exit the port, there is a limited area to pass which is the circumference of the port multiplied by the height of the gap (point A). But after a while (point B) the air have more space so the speed goes down, like in a flare.

F
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Old 30th September 2003, 07:10 PM   #5
rabbitz is offline rabbitz  Australia
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Default B&W down firing port

B&W uses downfiring ports on some of their subs - wild looking port too with golf ball type dimples.
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Old 30th September 2003, 09:36 PM   #6
SimontY is offline SimontY  United Kingdom
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And Yamaha do this with their bass drivers on some of their subs!!

They claim positioning is [somehow] less critical because of the smooth flow of bass or something like that...

Go here and click on 'speakers' to see quite a few!
http://www.yamaha.co.jp/english/product/av/index.html

I should add I've not heard any of these subs, though I think we sell one at the shop I sometimes work at...


-Simon

ps. those golfball ports are cool, every speaker I've heard that uses them had good or excellent bass
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Old 1st October 2003, 02:29 AM   #7
GM is offline GM  United States
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>I want to build a down firing port that would open on the bottom of the cabinet. This way, I could adjust the effective area of the port by adjusting the gap between the cabinet and the stand below.

>I think this have been done before maybe by Naim if I am right.

>What do you think!

====

It's been around since almost the beginning and very effective when properly implemented. It's best used on a cab tuned too low, then load the vent to 'pump it up'. As you surmised, since this raises the box system's efficiency, it lowers its vent mach, like flaring does.

====

>those golfball ports are cool, every speaker I've heard that uses them had good or excellent bass

====

Indeed they do, they act as vortex generators to increase boundary layer 'stiction', slowing the vent speed even further than normal flaring does.

GM
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Old 1st October 2003, 03:07 AM   #8
mm is offline mm  Canada
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I built a pair of speakers using this principle...you will not be disappointed.
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Old 1st October 2003, 07:03 PM   #9
SimontY is offline SimontY  United Kingdom
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Quote:
Indeed they do, they act as vortex generators to increase boundary layer 'stiction', slowing the vent speed even further than normal flaring does.
Without reading many of your other posts, I would have taken that sentence as a joke!!! You're the man

I've been really blown away by the new B&W 703 speakers. The bass is simply phenominal - tight, controlled, tuneful, extended, with so much punch - truly excellent! Driven with a modestly priced Rotel pre/power combo you really don't need much cranking to get punched in the stomach, but in a nice way! I could talk about these speakers all day, suberb imaging, amazing detail, not harsh at all....


-Simon
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Old 21st October 2003, 04:21 PM   #10
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I believe slot ports are calculated like any other but use a bit different end correction factor and the effective length is measured down the physical middle of the duct, no matter how it's folded.

But I don't think this setup would behave like a normal slot port. Wouldn't the top of the cabinet you place this on act like a wall of the port, changing the effective length and tuning? I've no idea how you could predict the results.

It would be interesting to measure the output with the box upside down, compared to the normal orientation.

cheers
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