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avatar307 4th December 2003 07:46 AM

Calculating volume - bandpass enclosure
 
Alright, really simple question that I'm sure just about everyone here can answer without a second thought. I am working on building a 4th order bandpass enclosure for a couple subs I have, which each need about 2 cubic feet of enclosure volume. Two questions.

1) Most of the bandpass boxes drawings I've seen show the front of the sub facing the ported side of the box. However, most of the bandpass boxes I've seen have the rear of the speakers facing the ported area, and the fronts facing a pane of plexiglass. Does the orientation of the sub matter? Are boxes designed with the fronts of the subs facing a sheet of plexi still 4th order enclosures.

2) When calculating the volume of a bandpass enclosure, do I include only the volume on the ported side of the sub, or do I include the volume on the sealed side as well? When I look at a 4th order enclosure that is all wood, with the front of the speaker facing the ported side, it seems to make since to me that you'd include the volume of both sides. However, when you turn the sub so the rear is facing the ports and the front facing plexiglass at an angle.... for some reason my head wants to say I should only include the volume behind the sub.

Yes, yes, I know, stupid questions. However, I want to make absolutely sure, so I don't end up building a big box that I have to throw away because I can't figure out what part I screwed up. LOL

Thanks!
Chris

sreten 4th December 2003 03:21 PM

Q1 yes they are still band pass.
The plexiglass is there so you can see the flash bass driver.

Considering a bandpass sub with a sealed section and a reflexed
section :

The sealed section determines the basic bass roll off which is
second order high pass filter. The reflex section adds a second
order low pass filter at a higher frequency which gives you the
bandpass characteristic. But both roll-offs are 2nd order.

To effectively use the total volume of the subwoofer the sealed
section should use the majority of the volume. The volume of the
reflex section only needs to be big enough to allow a sensibly
sized port (small ones will overload).

The car sub way of orienting the driver is a good way of maximising
the sealed volume and minimising the reflex volume in a box with a
fixed partition.
Other than this the orientation of the driver doesn't matter.

The sealed volume is used to calculate the basic 2nd order bass
alignment of the driver. The reflex volume is used to calculate
the tuning frequency of the port.

Adding the two together gives you the total sub volume.

Note that the sealed section should be stuffed with Dacron / BAF,
but the reflex section must be completely lined with foam to
suppress higher order modes and let the port do its job.

:) /sreten.

avatar307 13th December 2003 09:02 PM

Thanks for the reply! Sorry for my delay in getting back here to respond, but the time I got time again I had lost the post. :att'n:

In reference to lining the reflex portion completely with foam, style of foam would you suggest? When I first thing of foam, the softer, black foam comes to mind. Then there is the hard, white styrofoam. I guess the difference is not only the material used to make the foam, but also the air content of each.

I had planned to filled the relfex portion with some type of polyfill material, most likely the stuff used for filling pillows that can be picked up at Wal-mart. This I would assume is near the same thing as a Dacron or BAF type filler?

However, I know there would be considerable problems with this material being ripped apart and blown out the ports, so foam makes a lot more sense.

When I think of sound insulators, I usually think of the black foam. I expect I could find this material where materials of that type are sold for eliminating echoing in large open rooms. The typical image of the hard, white styrofoam makes me think of macking material. Definately not something I want to try to cut, and I fear it would also be too dense. The other material I think of is the sheets of R rated foam you buy for a little extra insulation under vinyl siding and such.

Anyone have any thoughts on all of this?

Thanks!
Chris

kelticwizard 14th December 2003 01:21 AM

WinISD and other freeware programs model 4th order bandpass. However, some people have complained that these freeware simulation programs don't model bandpass very well-however, they maybe be talking abut sixth order bandpass, (both chambers ported to the outside).

Anyway, which woofer are you thinking about using? I can model it here and post it, as can other Abuzz members.

It is not necessarily true that the ported side must be smaller in volume than the sealed side-however, most manufacturers do end up building them that way. The larger the ported side is in relation to the closed side, the greater the senstivity-you can relaisitically go up 6 dB over the speakers rated SPL!

However, such a sub will not go quite so deep. It is a tradeoff. On the other side, if you make the ported side smaller in volume than the closed side, you will have a lower cutoff frequency but your sensitivity will be very low.

Let us know what woofer you are interested in.

BAM 14th December 2003 01:41 AM

I read somewhere that orienting the driver with the magnet in the ported enclosure side helps lower out-of-band noise through the port when used without a crossover.

kevyjo 14th December 2003 01:48 AM

I guess it might serve as sort of a higher frequency "muffle" if the magnet and woofer assembly were in the way between the cone and the port. I would bet though in the real world it wouldn't make much of a difference, and either way would work.

kelticwizard 14th December 2003 01:59 AM

Doing it that way might help to make the ported chamber irregularly shaped, which I suppose would break up standing waves and higher frequencies.

Maybe it makes a diff, maybe not. Can't hurt.


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