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Old 4th February 2008, 01:51 PM   #1
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Default Narrow, shallow rectangular port

First, some background - feel free not to read this if it bores you

I am busy designing and building a 2.1 system for music and a little bit of theater. The left and right channels are full rangers, using the Fostex FE167E, mounted in a 15l ported box. The front channels cross over to a single 12" sub at 95Hz with a fourth order active Linkwitz-Riley crossover.

Phase adjustment and sub level adjustment controls are included in the crossover I have designed.

The front channels will be powered with a LM3886 based amp for now, and the sub with a 200W amp of my own design. So far I have designed and built the sub, the amps and the crossover and am busy with the front channel boxes.

Now, my question.

I already own a pair of well braced 15l internal volume boxes made of 19mm MDF, which I would like to use. The FE167E data sheet has an example of this driver in a 15l box, with a port tuned to 67.25Hz. I would like to keep this frequency, as it works well with my sub. In this example, the port is a 5cm diameter by 5cm long circular port. Port velocity peaks at about 6m/s according to WinISD pro.

WinISD tells me that I can replace the circular port with a rectangular port 16mm x 82mm and 19mm deep - and get the same port tuning. Here, the velocity is still ok - peaking at about 12m/s. The advantage of doing this is that the box material is 19mm thick, so I can just cut a slot in the front baffle and get the port I need.

I mainly want to do this for looks. Are there any disadvantages to this narrow rectangular port approach that WinISD can't tell me about?
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Old 4th February 2008, 04:21 PM   #2
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Not AFAIK in this particular instance. You'd technically get a touch more turbulance, but as the vent air velocity is within reasonable limits, that shouldn't be audible.
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Old 5th February 2008, 05:45 AM   #3
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Thanks Scottmoose

Does the port need to be approximately equal distance from the speaker all the way along? For example, could I build something like the picture I have attached, with a vertical slot port?
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File Type: png vertical_port.png (8.1 KB, 192 views)
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Old 5th February 2008, 06:32 AM   #4
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The only problam i see with that port is that it is going to make the baflle eveb flopier than it is... you do need to ensure it is well braced.

dave
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Old 5th February 2008, 09:04 AM   #5
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Thanks plantet10. The front baffle is very well braced, but I will feel how stiff it is after I have cut the slot.

I am going to try it out, and post measurements and photos when I'm done. If it doesn't work, I can always just make a new front baffle.
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Old 5th February 2008, 09:26 AM   #6
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Quote:
Originally posted by cabbagerat
Thanks plantet10. The front baffle is very well braced, but I will feel how stiff it is after I have cut the slot.

I am going to try it out, and post measurements and photos when I'm done. If it doesn't work, I can always just make a new front baffle.
Worth a try. If it doesn't hang together it is easy enuff to rebuild that box -- i'd suggest many-ply void free plywood.

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Old 14th February 2008, 08:41 AM   #7
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I slotted the boxes, and they sound really good now. There is no audible port noise at normal listening levels (as predicted), and it looks fairly nice, too.

I'll so some measurements and take some photos soon, and post results.

The boxes are very stiff, but currently have no fill or damping. What is recommended for BR boxes of this size?
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Old 14th February 2008, 11:03 AM   #8
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If you can't hear any box problems, & you like the way they sound, then I simply wouldn't bother. If it ain't broke...

If you simply can't help yourself (DIY audio does tend to get addictive), try lining the top, back & one side-wall with wool carpet underlay.
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