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Old 18th November 2004, 10:19 PM   #1
quasi is offline quasi  Australia
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Question Port location in a bass reflex box

Does Location Matter?

In a bass refelex speaker box where there is more than one woofer, does the physical location of the port matter?

Having recently built this type of enclosure I uncovered some interesting stuff.

What does the forum think?

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Old 18th November 2004, 10:39 PM   #2
quasi is offline quasi  Australia
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Ok ...just read my own post and it reads like a loaded (no pun intended) question.

During the frequency response test what I found was that the woofer closer to the port had a much higher excursion that the other near the box resonance frequency.

Also the impedance curve was not as well defined as I would have expected.

This made me quite unhappy. So I redesigned the front panel so that the port to woofer (centre) distance was the same for both woofers. My perceived problem went away, i.e. both woofers now behave exactly the same.

That seemed to make sense to me. But now I'm thinking all the dual woofer boxes that I have seen to date are not symetrical, i.e. the distance between each woofer and the port are different. So now I'm wondering are the designers of these trying to achieve something like maybe balancing or extending response?

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Old 18th November 2004, 11:01 PM   #3
Ron E is offline Ron E  United States
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This actually happens a lot, it is most noticeable in boxes shaped like pipes - those much longer in one dimension.
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Old 15th March 2005, 09:52 AM   #4
quasi is offline quasi  Australia
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Default Port Location;

If the air inside a box is tactile i.e. compresses and expands with the woofer's movement and if sound has a defined speed inside the box then the distance from the woofer to the port must matter.

But I have been unable to find a formula or a guide on what this effect is.

Or is this a non issue because most boxes are not big enough for this to be a problem? Some articles that I have read have stated that the actual location of the port is uninportant as long as the dimensions are correct. What about dual woofer boxes with only one port at one end? The port to woofer distance is different for each woofer.

Any ideas ?

So many questions........that won't go away ...my head hurts...

Sigh
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Old 15th March 2005, 10:35 AM   #5
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I would have thought that since the woofers act in unison to compress the air inside the box as a whole, the distance to the vent would not matter. The box is operating in pressure mode not propagation mode, so the 'time to travel' theory should not hold. However, your finding is interesting. Had you not moved the vent to a central location and found a change, I would have said that the driver specs are simply quite different.

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Old 15th March 2005, 01:41 PM   #6
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It is also surprising considering that so much of this kind of thing is measured not in inches, but in wavelengths.

The port carries frequencies from the box resonsance frequency to one octave above it. The port output is greatest at the box resonance frequency, and gradually rolls off until there is little left at 100 Hz. If your port is tuned to 50 Hz, (a fairly high box resonance frequency), then the port is active from 50 Hz to 100 Hz, essentially inactive above that. The cone output supersedes the port output from 70 Hz on up, though it is aided by the port output until 100 Hz or so.


The wavelength of 100 Hz is over 11 feet. So wherever the port is located in a practical sized box is likely to be a small percentage of that. If there are two woofers in the box, the difference in length between them is likely to be negligible compared to the wavelength of the frequencies passing through the ports.

I suppose it is possible to build a seven foot tall box with one woofer on the top, one on the bottom, and the port on the bottom. There, you would have an appreciable difference in wavelength. Unless it is something like that, though, I must admit I am surprised-but I do not doubt your observation. Just surprised the phenomenon exists.
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Old 16th March 2005, 05:20 AM   #7
quasi is offline quasi  Australia
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Default Yes kelticwizard;

That's what I thought too, until I saw what I saw. That is the speaker next to the port had a much higher excursion than the one further away. Both woofers by the way were next to each other with their frames no more than 50mm apart.

I examined a friends PA setup with 3 PA drivers (dunno why he had 3) and saw the same thing but now with 3 speakers. Guess what, the driver excursion decreased as the distance from the port increased. I.e. the middle woofer moved more than the furthest woofer but less than the closest woofer.

This cannot be explained by wavelength as you suggested so what is it?

Is it the actual localised complaince of the air volume.

If driver distortion increases with excursion then it may be worth pursuing the reasoning and subsequent fix.

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Old 16th March 2005, 06:18 AM   #8
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Default hypothesis:

The nearer the driver to the port, the poorer the coupling. The air has to "turn around" at a much sharper angle from the driver nearer the port. Less of a load on the back = higher excursion.

Sensible?
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Old 16th March 2005, 01:21 PM   #9
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Default Re: Yes kelticwizard;

Quote:
Originally posted by quasi

I examined a friends PA setup with 3 PA drivers (dunno why he had 3) and saw the same thing but now with 3 speakers. Guess what, the driver excursion decreased as the distance from the port increased. I.e. the middle woofer moved more than the furthest woofer but less than the closest woofer.

Raises a lot of interesting questions. To keep from burdening the discussion with too many questions at once, I will just ask this one for now: were the woofers hooked up in series or in parallel? Does this phenomenon change when you switch between series and parallel hookup?
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Old 16th March 2005, 02:04 PM   #10
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Higher excursion in the woofer tends to happen if the port is too close because the woofer begins to function the inside of the port like an open baffle. Take an extreme example: if you have a port the same diameter as the woofer and it terminates close to the back of the woofer, the woofer will basically see itself as inside an open tubular baffle and pretty much ignore your nice big box you so carefully calculated. The open baffle will provide negligible cone reinforcement thus allowing it to flap in the breeze.
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