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Old 18th March 2007, 02:49 PM   #1
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Default Port Shape and Size-Questions from a Newbie

I've done quite a bit of searching on this in the Archives but still don't feel satisfied with what I found.

I've seen ports in a variety of shapes (round, oval, square, rectangle, Karlson slots) and would like to know what does each shape impart (if any) to the sound as it exits the port?

In my personal project Very Small OB i've noticed that every time I increase the backpressure on the driver I get more and more bass volume. The first few times I increased the backpressure I could hear lower and lower in the frequency range though that stopped at about 40hz. I can hear slightly lower but it has almost no volume so it doesn't count for much.

Can I damage the driver by restricting the port too much?
If I can restict it further what will give away that I've gone too far other that really bad sounds?

My personal experience is limited to my above referenced project with Planet10 modded FE126E drivers. Thanks in advance for any input.
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Old 18th March 2007, 03:34 PM   #2
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Can of worms. Changes exist in the performance of different vent shapes, some (most) subtle, others less so. Generally, round or oval vents are better in terms of fluid air motion, or so I understand it, but in a normal BR box, you've be hard-pushed to hear a difference -you might get some small measureable changes, assuming the tuning remains constant. Karlsons are a whole different ball-game, and nobody has yet come up with a way of properly modelling them yet -they're one of the last bastions of the cut & shut school of speaker design. When dialled in right, they can be very effective indeed at supressing additional resonances, but they have their own issues too. This is all highly simplified, and doesn't even get onto waveguides, horn loaded vents etc., but you get the idea.

You won't damage the driver by restricting the port too much per se but you will get a) port noise, and b) eventually less LF gain as you'll end up with a leaky sealed box, sometimes (and not entirely accurately) called an aperiodic cabinet, which can actually be very effective, but have a different set of priorities. These boxes are less efficient, & need more power, so go easy with the volume knob as you don't want to push them too hard.
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Old 18th March 2007, 10:04 PM   #3
Geoff H is offline Geoff H  Australia
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Hi dj_oatmeal. The tube on the back of your baffles extends the distance from one side of the cone to the other, itself improves the bass response over the original OB. Restricting the port size is converting it to bass reflex, which enhances the response around resonance. A too small a port will tune the box to a frequency too low for the driver. Seems like 40Hz is as low as it wants to go.

So I guess your question is do you want BR, OB, IB, or some variant. They all have their own virtues. Only you can decide. I suggest you look in the full range pic gallery.

The tube you have been experimenting with IMO is a good way to load a bass helper on a OB, but is too restrictive for a wide range on an OB.

Geoff.
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