Oh no! My big speaker boxes must be shrunk! - diyAudio
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Old 14th April 2002, 02:40 AM   #1
Ignite is offline Ignite  Canada
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Default Oh no! My big speaker boxes must be shrunk!

Well, well. Looks like I will be living in somewhat cramped quarters next school year, due to lack of alternatives.
I've rebuilt my mains to be small enough, but subwoofers are going to be a problem. I don't want to spend the dollars on another driver, and the only driver I have that is currently not being used is a Tempest. Not exactly a small-box driver. Right now it plays happily deep in a 5.5cu ft vented box. I really need to go to 2-3. So a sealed 2.5cu ft with a linkwitz transform is probably the best idea, but that means heavily reduced efficiency. has anyone got some other wild plan that might yield better results? I'm desparate. I want an f3 of 25-30Hz if I can do it.
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Old 14th April 2002, 03:06 AM   #2
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Try asking Dan Wiggins at Adire about it, if anyone can help it's him.
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Old 14th April 2002, 08:37 PM   #3
CHRIS8 is offline CHRIS8  United States
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Well, i know that you did not want to purchase another driver......but perhaps this can be done with future benefits!

Buy another identical subwoofer driver to use in isobaric setup, you will get to have the exact level of output and extension with half the internal volume.

For the future...you get to look forward to using BOTH subwoofers in a normal setup for 6db higher maximum output!

Otherwise, purchase an inexpensive driver that will behave well in a small enclosure.

-Chris
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Old 15th April 2002, 01:35 AM   #4
Wizard of Kelts
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I have a few ideas, a couple of which are fairly wild. I will just give the first couple of conventional notions first.

A) A vented box of 2.7 cu ft, tuned to 20 Hz, put into a corner. The box iself gives you a smooth response that is 3 dB down at about 32. However, a corner boosts the output at the vent frequency about 5 or 6 dB. When this happens, your curve will change so that you should be 3 dB down around 27 or 28 Hz. When it comes to loudspeakers, 6 dB = half an octave = double the box volume.

B) A fourth order bandpass box. This is generally recognized to be 3 dB more efficient than the vented system. So right there, you make up half the difference you are losing by halving the box volume, even without help from a corner. I ran it on WinISD, and got best results from the following: Front chamber: 1.2 cu ft, Rear chamber: 1.6 cu ft, Tuning: 52 Hz.

The response on the graph for the bandpass looks slightly ragged, but remember that it only amounts to a couple of dB between the highest and lowest points, and that is not particularly hearable in the real world. On the graph below, note the extra output below 30 Hz-it is right in the range you were aiming for, and it is above the 3 dB down point. That is your 3 dB benefit for the fourth order bandpass box.

Incidentally, I don't think putting the bandpass box into a corner will help your bass extension much. The resonance frequency is 52 Hz, so that is where most of the increased out put will go.

You asked for "wild ideas". I have a couple of suggestions that I cannot completely vouch for since I have not built a box according to those principles. However, both have patents on them, (they are by the same man, Thomas Clarke), and one of them was printed in the Journal of the Audio Engineering Society, so this is not "mad scientist" stuff, either. Both appear to offer advantages over conventional systems. More tomorrow when I look up a few things.

Here is the graph for the vented box and fourth order bandpass box. Blue line is the midpoint, green line is the -3 dB down line:
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File Type: png ignite's enclosure.png (4.8 KB, 348 views)
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Old 17th April 2002, 03:26 AM   #5
Ignite is offline Ignite  Canada
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Kelticwizard, thanks for your work on this. Interesting ideas. The 2.7/20Hz vented looks like it has a decibel efficiency over a 2.5 sealed as it is. Does a vented box actually react more to a room's transfer function?

2.5 Sealed: can be stuffed pretty heavily so I could get away with a smaller box for the same response or that size box and slightly better response.

2.7/20Hz Vented: Higher efficiency, but the big thing for me is that xmax is going to dip bigtime at 20Hz, so I can drive the sub with more power at less distortion giving me a better window for a linkwitz transform.

1.2/1.6/52Hz 4th Order Bandpass: Warming up to the idea.. not sure.

Once again thanks for the comments.
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Old 17th April 2002, 06:02 AM   #6
Wizard of Kelts
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Here is some info on one of the two systems I was talking about. It is the Augmented Passive Radiator enclosure. It is essentially a dual chamber system with a two passive radiators connected together. Lest this sound expensive, remember that a passive radiator can be nothing more than a piece of weighted styrofoam hung in an enclosure opening. This particular system seems to be adaptable to passive radiators that can be homemade at little cost.

I will admit that I have not made this enclosure or a handmade passive radiator yet, so I cannot say for absolutely certain that this is so.

I have part of one of the articles from the Journal of the Audio Engineering Society, but it is incomplete and I have no scanner. The articles are available for $5 from the Journal of the Audio Engineering Society. Order Page:

http://www.aes.org/journal/search.html

A) Augmented Passive Radiator Loudspeaker Systems, Part 1 Vol. 29, Number 6 pp. 394 (1981) Author: Thomas L. Clarke
B) Augmented Passive Radiator Loudspeaker Systems, Part 2 Vol. 29, Number 7 pp. 511 (1981) Author: Thomas L. Clarke
C) Comments on "Direct Low-Frequency Driver Synthesis from System Specifications" Vol. 32, Number 7 pp. 543 (1984) Author: Thomas L. Clarke

Here is the title and patent number if you want to look up the patent. Do a quick search on the following page:
http://www.uspto.gov/patft/index.html

Be sure to check "all years" in the years box on the query page. If you check out the patent, remember to download the image viewer at :

AlternaTIFF: http://www.alternatiff.com/ (tested: IE, Netscape, Opera)

interneTIFF:
http://www.innomage.com/interneTIFF.htm
(tested: IE, Netscape)
Both work better on IE 6.


By cutting the volume of your speaker in half, you lose 6 dB. That is the equivalent. The augmented passive radiator system is 3 dB higher than a vented system, so you are halfway back I include an illustrative page form Clarke's patent.

In a couple of days, I will give you info on a sort of augmented passive radiator version of a bandpass system. It is called the Acoustic Lever and it was invented by Earl Geddes, former audio engineer at Ford. More about that later. It might increase your efficiency even more than the 3 dB the passive radiator system is supposed to.

Anyway, just to make sure, in these illustrations of the front-mounted augmented passive radiator system, the driver is the smaller cone on top, the passive radiator assembly is the larger cone on the bottom.
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File Type: png passive radiator augmented.png (14.7 KB, 289 views)
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Old 17th April 2002, 06:37 AM   #7
Wizard of Kelts
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Does a vented enclosure react well with a room better than a sealed? I would say that a vented enclosure can be made to react well with a room down to it's tuning frequency. After that, it is pretty useless, (for alignments where Vas is equal to or larger than Vb, which is the usual thing).

Just thought that I would add that the Augmented Passive Radiator has a lot of variations possible. However, 2 of the most useful appear to be:

A) It has the same 3 dB down point as a vented system of the same size with the same driver, but has a 12 dB/octave rolloff from there instead of 24 dB/octave like a bass reflex. This adds a certain amount of bass in real world rooms where "room gain" is a factor.

B) It has a 24 dB/octave rolloff like a vented system, but the 3 dB down point is a quarter octave lower than the vented.

Plus, there are variations like 3 dB down points even more than a quarter octave lower than the vented system, but with a 30 dB or higher rolloff per octave. This last alignment is for the man who likes bass and isn't fussy about transient response.
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