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[Steve Shafer]

18 Sound has some very nice looking plans on their new website.  I came across a dual 15" sub cabinet that looks very promising.  However, there are some design concepts that i wanted to learn more about before making saw dust.

http://www.eighteensound.com/staticContent/applications/kits /18Sound_kit15_dual.pdf

First off, the puss/pull driver arrangement.  I have seen similar driver reversal on dual driver sub cabinets before by EAW and Peavey.  Are there really significant reductions in distortion by using this arrangement?  

Second, in the 18 sound design, the drivers are not front loaded in the traditional manner.  They appear to be set back in the cabinet quite a bit farther than dual driver subs that i am familiar with such as JBL SRX, EV QRX, EAW SB 850.  

What are the design goals and advantages of setting the drivers further back like this?  Or is this just to make room for the one driver that is "reverse" mounted on the baffle.

I think this DIY project could be a real winner.  The plans seem well thought out and are very detailed. It would be nice to have a dual 18 version as well

my .02

Steve S

[Antone Atmarama Bajor]

     I have heard Tom D. say that the Push/Pull driver arangement cancels even order Harmonic distortion.  The distortions have to to with the Difference in surround compliance when the drivers move positive and negative.

    I have heard that THD can be reduced in the 10s to teens of dB.

    The reason for the cab face being recessed like that is a no brainier.

    They do that so there is room to place a grill to protect your speakers.

    Push/Pull configuration stops working in the frequency ranges where mutual coupling is lost,  To do it really well one should try physically time aligning the drivers arrival so that harmonic distortion is reduced in the upper range.

    I had some good results with this sub configuration.

 http://srforums.prosoundweb.com/index.php/m/80905/8490/?srch =First+Sub+Design#msg_80905

 http://srforums.prosoundweb.com/index.php/m/82403/9820/0///8 490/#msg_82403

    The main inconvenience about this design is that the Opposing Drivers magnet protrudes.

    You could do something like this Like M&K (I made a sealed sub like this for my friend using lab 12's I can make the plans available.)  The M&K is ported the port dumps where the down firing driver is.




    Hope this answers some of your Questions.

Antone-

[Ivan Beaver]

If you look at the measured specs you have to realize that they are playing the spec game-as lots of other people like to do-espically in the sub area.  They are applying a 2.83V signal to a 4 ohm load.  That ends up being a 2Watt input into the cabinet.  So you have to take the measured sensitivity ratings and subract 3dB from each one in order to get a "real" number.

It is not that they are lying-just being deceptive to customers who think they are seeing something that they are not.  All the data is there (they aren't hidding anything), you just have to know how to interpet it.

I like they way provide measured responses for different driver along with the impedance plots and the DSP settings and measurements for a corrected response.  You can see the amount of boost being applied so you can figure out if your amps are big enough and the loudspeaker capable of that power at a particular freq for your needs.

I wish more manufacturers would give that sort of data-instead of just single numbers like sensitivity of say 100dB.  Yeah but at what freq.?  In this case- in one model the sensitivity rating could either be 107 OR 93dB (corrected for 1W input) depending on what freq you want to look at.  That is only a difference of 14dB. or a factor of 25:1.  You normally use an amp that has 25 times the needed output don't you? Not meaning you in particular-just in general.

[Peter Morris]

As others have said the push–pull design reduces distortion.

Also as you noted there is a cavity in front of the drivers. This provides some extra loading and increases efficiency a couple of dB especially around 100 - 200 Hz.

Dynacord / EV have something similar - Planar Waveguide Horn, and Stage Accompany call it TST loading and use it in their XL series of bass bins, although with these designs there is more “loading” producing up to 6 dB of gain.

Peter