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Old 13th December 2004, 11:17 PM   #1
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Default bipole and reflex port question..

I am designing a port loaded loudspeaker. It is actually a bipole with drivers on front and back.

questions are:

1) should I put the port on front /back / or both?

Thanks!!!
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Old 14th December 2004, 01:39 PM   #2
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You can put them on the front and back, or top and bottom, or anywhere, with two things to consider: the port action lasts for one octave above the tuning frequency, (Fb), and it would be best if the ports are one tenth of an octave apart or less at that point.

There is a general rule in audio that two sources are coupled to each other if they are one tenth of a wavelength or less apart.

Wavelength = 13,500 inches / frequency

Example A: Box tuned to 30 Hz.

Port action lasts up to 60 Hz. Wavelength of 60 Hz = 13,500 / 60 = 225 in. It is best if the ports are within 22.5 in of each other.


Example B: Box tuned to 40 Hz.

Port action lasts up to 80 Hz. Wavelength of 80 Hz = 13,500 / 80 = 168.75 in. It would be best if these ports are within 17 in of each other.

One tenth of a wavelength of a wavelength is an optimum value, you can go longer than that without too much trouble. However, remember that at 25% of a wavelength, there will be 3 dB cancellation between the ports at one octave above the tuning frequency. Of course, at one octave above the tuning frequency is the point where the port stops putting out anything anyway. But the effects can be slightly audible, say, one half wavelength above Fb.

So try to make your ports within 10% of a wavelength apart, or reasonably close to that, and there will be no audible effect of separating the ports from each other.
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Old 14th December 2004, 09:12 PM   #3
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Default hmm...

but if i am using 2 similar drivers one on front and back...

if i have a choice of 1 on front, 1 on back or both (ports) which makes more sense?
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Old 14th December 2004, 09:56 PM   #4
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If I were building a bipole, I would have one port and put it on the front or back, whichever suits your fancy.

Incidentally, superior results have been reported for raising the speaker a couple of inches and putting the port on the bottom. At least one speaker builder I know refuses to build them anyway else. He says the coupling between the port and the room when it is done that way is very, very noticeable and beneficial.
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Old 14th December 2004, 10:06 PM   #5
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Default ok...

in this case the required volume is very small ( bipole fullrange drivers)...

would it be better to just do a TL enclosure?
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Old 14th December 2004, 10:19 PM   #6
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Just for curiosity's sake, what kind of vlume are we talking about?

I don't see why it necessarily would be easier to do a Transmission Line, in fact they generally are more complicated and have more variables.

Whether the sound is worth it or not is up to you. Both Transmission Lines and Bass Reflexes have their adherents and their detractors.
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Old 14th December 2004, 10:34 PM   #7
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Do an MLTL simulation with MJK's mathcad worksheets. You'll likely end up with a tall slim simple-to-build box with a port on the bottom. I plan to do a bipole version of my current project, an MLTL two-way, this winter, and need someone to find out how to siMJKulate with two drivers and report his findings on how to do it here .
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Old 14th December 2004, 10:38 PM   #8
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You may want to take a look at this thread.
diyAudio Full Range Reference Project
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Old 14th December 2004, 10:55 PM   #9
bzdang is offline bzdang  Canada
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Good thread! And amongst other things, a link to Martin's how to on modelling with two drivers.
http://www.quarter-wave.com/General/Two_Drivers.pdf
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