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Old 6th July 2005, 09:02 PM   #1
BAM is offline BAM
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Default Series-Tuned Bass Reflex Questions - technical

Hi. I have an interesting question about bass reflex enclosures. What do you get when you build a bass reflex enclosure that is tuned to one frequency, but then have the port exit into another chamber tuned to a different frequency by another port which exits to the outside?

Is there a set of equations that can determine the resultant tuning frequency for a series-tuned enclosure like this? How would one determine what frequency the second chamber is tuned to if the driver is only in the first chamber? And, would there be any guidelines for determining how large that second chamber should be?

If so, I think I have an idea, possibly even a patentable one, for getting really big bass out of a really slim, small speaker enclosure.

Also, if there are any books or AES papers that can help me on my search for this series-tuned enclosure idea that I have, I'd like to know.

This prompts my question: In this web page, there is a sample set of design calculations for a series-tuned double-chamber reflex subwoofer enclosure. At the bottom of the page, it tells what the resultant tuning frequency of each chamber is, but not how that number was determined. That is part of what I am really looking for.

I have a feeling that those calculations which use an equivalent electrical circuit to describe acoustic system behavior might be of use here.
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Old 6th July 2005, 09:14 PM   #2
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It's called "double bass reflex" and it can do really good things, like reduce excursion at your second tuning frequency, thus increasing power handling.

From what I hear, they are very tricky to get right, and they're not completely general. IIRC, if your normal BR box is tuned to 40hz, the double bass reflex box will have a tuning at 40hz and a tuning at 80hz and will be just enough larger to accomodate the extra wood and ports. The excursion vs frequency plot will have two low points instead of one, and the impedance curve will have 3 humps instead of 2.

Also try Google...
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Old 6th July 2005, 10:15 PM   #3
RJ is offline RJ  United States
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Not hard at all;
http://www.diysubwoofers.org/prt/dual_chamber.htm
I used UniBox and designed a ported box with 2 ports.
You can also use WinIsd and just model it for two ports.
The baffle with the third port is then placed 1/3 the inside length of the box. Say your box is 30" high, the baffle would be placed at the 10" mark. One chamber would be 20" and the second chamber 10". 3 ports all the same length. It's true, you won't get over excursion below resonance. I tried really hard one time and all I got was a lot of wind come out the ports.
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Old 6th July 2005, 11:36 PM   #4
nate is offline nate  United States
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I have also been facinated with this. The company that has used this alignment with the most success has been Audiophysic. The Tempo (old version), Spark (current and previous version) as well as the Yara all use this type of design.

http://www.audiophysic.com/produkte/spark/index_e.html

About a month ago, I built a test cabinet to try out the double reflex design. I modeled a box using win isd for my eton 7-372 monitors which turned out to be .7 cu ft with a 4" x 8.75" port tuned to if I remember correctly 50 Hz. I then added another identical cabinet to the bottom of this. I used the same size and tuning. The results were very positive. I did some measurments and the actual tuning was closer to 70 Hz for the first cabinet and then 40 for the second cabinet. Upon comparison, the series tuned cabinet produced much deeper bass, with good tonality, then the smaller single reflex box. I also learned, from both listening and measuring, that there is a dip in the response just below the tuning frequency of the first chamber around 40-55 Hz.

http://www.audiophysic.com/produkte/spark/index_e.html

As time allows, I plan on building quite a few more test cabs. Hopefully using a little more science....

Also, I've heard that LSPcad can effectively model series tuned enclosures.

Nate
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Old 6th July 2005, 11:39 PM   #5
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Oops, I meant to paste this as well.

http://paginas.terra.com.br/educacao...negro/english/
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Old 6th July 2005, 11:43 PM   #6
Ron E is offline Ron E  United States
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Double chamber reflexes, with ports in the large and small chamber and one between the two will always have a notch in the response near the stopband - not really Hi-Fi, but since the ear is nearly impervious to relatively narrow, sharp dips (like TL's have) you probably won't notice.

A series-tuned reflex can be modeled with LspCAD - I doubt there is any real advantage to it or loudspeaker manufacturers would run to it.

TL's and TQWT's and all other designs that have been touted as giving you bass for nothing have all proved to be wishful thinking by the designer and/or to have serious downsides. There may be sonic effects from the inherent cabinet bracing but otherwise TL's have no more/better bass than a ported box. TL fans said as recently as 5-7 years ago that TL enclosures had a 6dB/octave rolloff. I remember an article in Speaker Builder where a guy using cookbook formulas (presumably from one of the Weems' books) thought he could get an extra octave out of some TL boxes he was putting in his truck - imagine his chagrine when they performed no better than the predictions for (smaller) sealed or vented boxes Our knowledge of TL's has improved recently due to some very professional amateur efforts and we now know many of the original claims for them are false....

If you want to develop equations, get some college (typically senior or graduate level) textbooks on electroacoustics and circuit analysis.

Acoustics by LL Beranek
Acoustical Engineering by Harry Olson
Theory and Design of Loudspeaker Enclosures by JE Benson

These should give you a good start.
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Old 6th July 2005, 11:59 PM   #7
Jorge is offline Jorge  Brazil
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Take a look at Martin King's website.

http://www.quarter-wave.com/

Note: to use his worksheets, you will need a fairly recent version of MathCad, but there are some tables also.
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Old 7th July 2005, 01:26 AM   #8
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This type can be modelled, using Adire drivers, on LSPCAD for Adire, which is freeware.

Notice that all three boxes about to be shown have different port configurations.

Note: If you can find an Adire driver with Lower Qts than the one you woant to model, , you can just add a series resistor to the Adire driver to raise the Adire's Qts to equal any non-Adire driver you have in mind. Tips on how to do that in coming post.
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File Type: jpg double reflex ype 1.jpg (9.7 KB, 473 views)
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Old 7th July 2005, 01:27 AM   #9
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And here is Type 2, also modellable on LSPCAD for Adire.
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Old 7th July 2005, 01:29 AM   #10
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And here is the Weems/Augspurger, (Weems gives full credit to Augspurger), type.

Here are two posts on it, #34 and 35, with instructions from Weems' Great Sound Speaker Manual.

http://www.diyaudio.com/forums/showt...3570#post93570

the thumbnail also shows how to add a series resistor to raise the Qts-which you can then model on LspCAD to convert the Qts of an Adire woofer to one identical to a non-Adire woofer you might have in mind.

And here are several pages devoted to it by diyAudio member Claudio Negro:
http://paginas.terra.com.br/educacao...negro/english/
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