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Old 26th February 2009, 08:13 AM   #1
Tarzan is offline Tarzan  Belgium
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Default Reflex port question

Hello all,

We do not care for the moment of exact measurements; it's the idea that needs clarification.

OK, we have a closed horn cabinet with a bassreflex port.
The length and surface of the exit mouth is known.
This will give a certain volume in the pipe that makes the reflex port.

If we now make another port (instead of the original and calculated one ) and so that the surface of the exit mouth and the length of the tube or whatever (let's call it BRVolume) is the same as in the first, calculated version, will the result be identical?

I believe, yes.

If now the BRVolume will change (x2 to even x5) but the surface and length remains the same, will the result still be the same?

If not; why not?


Another question I'm not sure off is the following.
Is there a problem if the reflex port mouth is closer to the listener then the speaker itself?
Imagine a horn with a bass port that is situated at the end of the horn whilke the speaker is at the beginning.

Here again I believe this will not give a difference as it is ussed in some PA cabs.

Probably I answered all my questions myself but to be sure, advise is very much appreciated.

Cheers,
Zilog
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Old 26th February 2009, 10:39 AM   #2
Tarzan is offline Tarzan  Belgium
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Hm,
after reading my own post I got mislead aswell.

The volume of the port as original will increase by a factor 2 to 5.
The erea where the air will be forced trough and that is visible on the front of the cabinet is the same.
The length of the port in the cabinet remains the same either.
It's only the volume that changes.

Hope this will make it a bit clearer.
Zilog
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Old 26th February 2009, 02:09 PM   #3
GM is offline GM  United States
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If I understand the question correctly, it can be modeled in Hornresp or Akabak.

GM
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Old 27th February 2009, 06:33 AM   #4
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If the port diameter remains the same, and the port length remains the same, how can the port volume change?

The only way the volume (as in cubic inches) can change is for one cabinets to be on top of a mountain, and the other cabinet to be at sea level.

The volume of a cylinder is the area of the base (a circle) times the height.

If the port dimensions remain the same, but the speaker cabinet size changes, then the tuning of the port changes. That is, the frequency that the box/port combinations resonates (the tuning frequency) at will change with cabinet volume.

It may seem as if I don't understand what you are asking, and that impression would be right; I don't.

Maybe I misread, or maybe the whole thing is too deep for me, but if the diameter and the length stay the same, the volume doesn't change.

?????

Steve/bluewizard
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Old 27th February 2009, 07:39 AM   #5
GM is offline GM  United States
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The vent need not have a constant cross sectional area (CSA) along its length such as an acoustic low pass filter.

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Old 27th February 2009, 07:40 AM   #6
Tarzan is offline Tarzan  Belgium
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Here again I have to hit me on the head.
The way I explain what I try to ask isn't clear enough; my fault.
To start again:

The cabinet remains the same.
The speaker ditto.
The erea of the port that's visible at the front and where the air leaves the cabinet remains the same.
The shape of the "mouth" will change for instance from a square to an oblong or even a triangle or whatever
.
The length of the port inside the cabinet remains the same to.
Only the volume behind the port will change.

Here is some confusion (see attached file).

Asume that in the original design there is a tube with a volume of let's say, 3000 cubic cm with a port mouth erea of 100 square cm.
The length of the "tube" is 30cm.

The new port will have (?) a volume of air of 12000 cubic cm. Allthough the mouth erea remains 10 square cm and the length of the port also 30cm.

Look at it as seperate internal box (blue) that's created in the original cabinet with the same port depth, same port opening but different height (= cabinet).
Instead of having a box that has the diameter/surface of the mouth opening we just make an opening in the front with the original surface erea and add a plate inside the cabinet as high as the cabinet. To facilitate the mechanical construction.

Is the amount of air pushed through the mouth in both cabinets the same?
I would say "yes"
Is the tuning of the cabinet the same?
I don't know.
Here I have to take the advise of GM and try out the soft as recomended.

Hopefully I made it clearer today.
Thanks, Zilog
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Old 27th February 2009, 07:51 AM   #7
Tarzan is offline Tarzan  Belgium
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Hi GM;

Your latest comment came in after I pushed the submit knob.
Aha, that sheds some light.
I have to find some info on that.

Still I like to know,

if when a "volume" is created behind the front of the cabinet and the air is allowed to "escape" through a few small openings in the front and from that "volume",

how to calculate the erea of these small openings so that the cabinet will behave "the same" as the original design.

OK, no real data yet but I have that idea since a long time and now there is a need for it, if the practise is possible.

Zilog
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Old 27th February 2009, 08:08 AM   #8
Tarzan is offline Tarzan  Belgium
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Hm, the icture didn't show up.
Here is another try:
Attached Images
File Type: jpg 000.jpg (60.6 KB, 180 views)
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Old 28th February 2009, 02:41 AM   #9
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Quote:
Is there a problem if the reflex port mouth is closer to the listener then the speaker itself?
Imagine a horn with a bass port that is situated at the end of the horn whilke the speaker is at the beginning
I don't see a problem with the port at that location. In fact, it is probably preferable there than at the rear of the cabinet, if there is any concern about group delay.
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Old 28th February 2009, 03:57 AM   #10
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Default I can only think of two things that will be affect

One is a certain area of the FR will drop since the round tube is acting as a sound port and the second thing is the possible baffle flex and resonationg
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