View Full Version : How does one determine box volume for using a passive radiator?

01-03-2006, 07:50 AM
I've read much about the pros and cons of passive radiator systems and have owned a couple, one of which, the Altec Lansing 886a, I liked very much. I have also read how one of the chief advantages of a passive radiator is that using a passive radiator rather than a simple port allows for a much smaller net internal displacement for the same tuning. My question is this: does anyone know how to determine the optimum alignment, internal volume-wise for such an application?

My plan is to use two 8-inch drivers, Vas, 2.765 cu. ft., Fs, 39.67 Hz, Qes, .572, Qms, 3.87, Qts, .498, with a 12-inch passive radiator possessing a radiating area of 480 sq. cm and a moving mass of 200 grams. The radiator's native, unadorned moving mass of 200 grams will tune a 1.6 cu. ft. enclosure to appromimately 31.23 Hz, which is almost exactly the Fb arrived at using the 4th order equation; however, the 4th order equation for box volume produced a net displacement of 5.53 cubic feet, for which the moving mass would be only 44.556 grams, so, obviously, something other than a 4th order alignment is required to configure this system properly--but I simply don't know what it is. Anyone have a clue?

01-03-2006, 11:06 AM
You'd definitely need a lower-mass passive radiator, for that size enclosure.

I'd look at maybe multiple passive radiators- if you tried, for example, two of the aforementioned 12" PRs in the larger box, it should get much closer to correct alignment than one.

Also, don't worry about tuning PR enclosures slightly lower than vented boxes... they seem to "support" the bottom end a bit better. Should still give decent low end "fatness".

In addition, you can frequently get away with slightly smaller enclosure volumes with a passive radiator, for the same low end extension as a vented box. This is due to the slightly stronger "support", as mentioned above.

If you want to play around with alignments, I've found that Harris BassBox is a good way to compare vented and PR alignments side-by-side. Easy to see how the tuning similarities and differences manifest themselves...


01-03-2006, 12:30 PM
Thanks for the input--however, the mass required is still far below that provided as a minimum by two of the PR's. Perhaps these drivers just don't
lend themselves to this approach. The manufacturer specifies .38 as the Qts, 2.04 cubic feet as the Vas, and 29 Hz as the Fs. With these values, the Vb is more manageable, but physical measures (provided by another on-line hobbyist) of representative examples produced the figures I presented earlier.

Weirdly, the "recommended" ported enclosure for one of these drivers is 1.4 cubic feet tuned to 34 Hz. It was upon this recommendation that I had based my idea for a passive radiator-based system with two of the 8-inch drivers and a soft dome in an MTM arrangement and the 12-inch radiator located below the array, or, alternatively, on the back of the enclosure. It seemed like a good idea at the time, but seems less so with each passing moment.

To utilize these drivers with a 200 gram moving mass PR would seemingly require a reduction in enclosure volume that is somewhat more than slight--drastic is more like it. Yet, the systems I have seen--and which sounded good--utilizing PR's have generally had enclosures rather on the slender side--apparently more on the order of sealed box alignments, for which, in the case of these drivers, the manufacturer recommends 0.7 cubic feet (however, that's with a box resonance of over 57 Hz).

Basically, I'm baffled.

01-03-2006, 01:26 PM
I think whatever program is calculating PR alignments for you, is giving falsely-large enclosure sizes. There's no real reason, a PR alignment should EVER have to be LARGER than a vented alignment. Usually, they can be up to 10% smaller, and maintain the same response "shape".

I'd try a different method of calculating PR box size, or manually FORCE the program to calculate an alignment using about the same size enclosure as the vented alignment. Shouldn't be THIS complicated!

One method I've found workable- try figuring out the air volume necessary for a sealed alignment, with a Qtc of between .57 and .61. Then, take that volume, input it into the PR calculations, and play with the PR tuning frequency until it's "maximally flat". Usually, that gives GOOD results!


01-04-2006, 10:44 AM
Thanks, once again, Gordon. I guess I should explain that I'm one of those weird "old school" guys who never does anything with a computer I don't understand how to do myself. I had to become an accomplished graphic artist and work at it professionally before I'd get into computer graphics, for instance. I had to learn to flatten a PA system by ear before I could trust a spectrum anylyzer and a pink noise generator. I had to learn to set the exposure according to observable conditions and film characteristics before I'd rely on the built-in light meter. Now I know how to determine the mass required of a passive radiator using nothing more than my ancient TI-30 SLR calculator and a manually derived Fb, but I don't know how to arrive at a Vb for this intended application--I'm NOT using a program that I can FORCE to do anything, but I CAN derive a sealed alignment for these drivers with a Qtc in the range you have kindly specified and go from there, I think.

01-04-2006, 11:16 AM
How about just giving us the manufacturer(s) and model number(s) of the transducers or else fill in as much of the data you can below so we can help you out.

01-04-2006, 11:46 AM
Mechanical parameters:

Fs: 39.67 Hz
Qms: 3.87
Vas: 37.215 liters
Cms: .576 mm/N
Mms: 30.94 g/0.03094 kg
Rms: 1.885 Kg/s
Xmax: 4.5 mm
Xmech: 6.75 mm
Dia: 16.97 cm
Sd: 226.2 sq. cm
Vd: 0.102 liters

Electrical Parameters:

Qes: .546
Re: 6,98 ohms
Le: 2.359 mH
Z: 8.037 ohms
Bl: 9.48 Tm
Pe: 60 w

Passive Radiator Parameters:

Fs: 16.8 Hz
Qms: 3.0
Vas: 5.2 cu. ft./147.16 liters
Cms: .45 mm/N
Mms: 200 g/0.2 Kg
Rms: 7.03 Kg/s
Sd: 480 sq. cm
Xmax: 10 mm

01-04-2006, 11:55 AM
BB6P doesn't like your parameters:

Seems to like the PR fine

01-04-2006, 12:30 PM
The parameters I listed earlier were derived by testing two Dayton DC200-8
woofers and averaging the results; these measures were supplied by a speaker enthusiast on-line. The published parameters for the DC200-8 are
fairly different and seem to vary considerably from unit to unit.

01-04-2006, 12:42 PM
Ok. I'm not real sure why people think they can average TS numbers but whatever.

01-04-2006, 01:58 PM
quote from whizzer

"I had to learn to flatten a PA system by ear before I could trust a spectrum anylyzer and a pink noise generator."

WOW,I don"t believe that I could dial in a flat reference by ear! :blink:


01-05-2006, 05:45 AM

Thanks; I truly appreciate the effort and information. I'd like to know the constants the program used to derive this Vb, but it looks suspiciously like twice the averaged Vas (tested) of the individual drivers. I'll play with these figures and see what I can come up with. Gordon had recommended that the sealed alignment was a good starting point. I had hoped for a somewhat smaller enclosure, but a graph showing a total Q of .8 looks a lot like the response curve of a passive radiator system, and using that figure for a sealed alignment yielded a Vb of a shade under 2.7 cu. ft., while a maximally flat vented alignment (using the test-result parameters) called for an enclosure of approximately 5.5 cubic feet, so I would conclude that this is a do-able project.


Yeah, but my ears aren't what they used to be. Now they're a lot more decorative than functional.