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December 2011

15" Woofer Survey… A Tale Of Three Woofers
Article By Jeff Poth

Difficulty Level

 

  "Bass is big. A 6.5 is not a woofer." This simple precept will serve you well if you want high fidelity reproduction in the bottom octaves. Sure, a small driver (or several) can make some bass, but if you want to emulate a large surface like a drum or the body of a cello, a full-size woofer moving only slightly will do a better job of it than a little driver really pumping . "But Jeff, you've positively reviewed several small speakers!" Emphasis on small, boys (and the rare female diy hifi enthusiast). Ya picks your poison, small has its upside just as big does. Most domestic situations are intolerant of 15"s and whatnot, and small can tend towards better high frequency performance from your midrange/midwoof allowing use of a small dome tweeter, and is less expensive and less demanding of large cabinets.

For those of us who are excessively hairy-chested and have understanding significant others (take a bow wifey), the 15" becomes an option. In my domicile there are 3 stereos. In my youngest daughter's room we keep the Zigmahornets with 4" fullrangers which work well for the small room and modest requirements. See my article Merrill DCA 4 And The Zigmahornet. The master bedroom has my 8" B20 TLs and is great in the sweet spot, delivering plenty of well-defined bass within the space thanks to the low tuned TL. See my article Pass The Bofu Building a transmission line cabinet for the Pioneer 8-inch full range driver.

But when you get to the big rig.... Ahhhhhh... there's no limit to output, you can sit in a broad area without giving up much, and things are just effortless. That effortless sound comes from using a 15" midbass with subs below it and a high end horn on top; this type of system would thump a medium sized bar or restaurant gig plenty well but is only asked to do home levels. Because I'm using a 15", the cone has approximately the surface area of six 6.5" woofers, and thus has to move very little to achieve the same SPL relative to one, or a few, 6.5"s or 8"s. This keeps distortion low. But even then, there are concerns to be addressed- what style of driver suits me best? On paper, for my needs, a hifi-specific, high-efficiency 15" is more appropriate than the pro drivers. The pro drivers have advantages in power handling and maximum output- areas where most any 15" in a home environment will excel. But when the rubber meets the road, we sometimes find things different than on paper. Accordingly, I shall take a spin around the block with 3 high-quality 15"s, JBL 2226h, Eminence Magnum 15HO, and BD BD15.

 


Woofer 1: JBL 2226h available at Parts Express and seen at JBL's website.

Just about everybody with an interest in music has heard the 2226h at one time or another. The 2226h is the reigning champion for the "King of Thwack" title- woofers don't tend to do better in terms of impact than this beauty. It has been used in a large number of JBL professional speaker systems, from installed theater systems to PA. This is a true classic, a woofer that has an advanced motor with a shorting ring and a carefully shaped polepiece, as well as a large amount of venting to ensure that the voicecoil doesn't burn up. This arrangement is described by JBL as Vented Gap Cooling and in addition to a shorting ring at the base of the polepiece, the polepiece is undercut and has cusps at the pole right next to the voicecoil. These cusps are said to maintain saturation of the steel and reduce flux modulation and associated distortion. The 2226h has a typical large-woofer impedance profile with a fair amount of voicecoil inductance, and a top end rolloff at about 1300Hz. Lucky for us, the rolloff has only modest breakup peaking due to the nice character of JBL's fiber-reinforced paper cone and quality design. The vast majority of pro woofers like this have nasty breakup modes, but the 2226h largely retains its composure at the top end of its response. Our 2226h also has a larger Xlim than the other woofers in the survey, maxing out at 40mm (hardest to damage). 2226h has stood the test of time, being extremely durable and exhibiting very good sonic quality.

 


Woofer 2: Eminence Magnum 15HO available at Parts Express.

The Magnum line of woofers from Eminence is very similar in most respects to the JBL Vented Gap Cooling drivers, with extensive ventilation, undercut polepieces (notably absent are JBL's "cusps" on the pole). What the Magnum series DID have were phase plugs- huge ones! On a 4" diameter voice coil, that's a heckahunka Aluminum, which serves as a shorting path and heatsink. In pretty much every respect, the 2226h and 15HO are interchangeable with the exception of some additional breakup peaking in the 15HO, but in most speaker systems, the extra couple dB will be effectively suppressed by a lower crossover point to a 15". This was the finest 15" made by Eminence at the time of manufacture, but has since been replaced with the Kilomax series, which have heatsinks rather than phase plugs and do not appear to be quite to the same standard in terms of motor design (but cannot be certain). The Kilomax are known to have some failure issues, which may be related to their ridiculous power handling, but dramatically lower Xlim than the equivalent Magnums. Magnum 15"s had about 30mm of Xlim, where the Kilomax are about 20. The Magnum is an extremely durable driver, similar to the 2226h above, and anyone running out of steam in a home environment with either of these is doing something very wrong (asking too much sub bass, for example- these aren't home theater subs).

 


Woofer 3: BD Designs BD15 available at

A high efficiency speaker designer, Burt Doppenberg specified his own take on a high efficiency 15" woofer. This is very different from our other woofers, having a smooth curvilinear cone vs. straight-wall with corrugations (as in the JBL and Eminence Models). The shape of a curvilinear cone tends to help the driver reproduce higher frequencies, which is desirable with a driver intended for wider bandwidth usage.  More importantly it sports an underhung 16 Ohm motor with a modest 3.5mm Xmax and a ring neodymium magnet. This is not a pro woofer, nor a subwoofer. This is a hi-fi midwoofer, designed with the ability to operate higher in frequency and with an notable absence of breakup peaking and (presumably) extremely low distortion. The underhung motor makes for extremely high fidelity within its limits, and the 16 Ohm impedance is a welcome characteristic, allowing this woofer to be used with OTLs, which often allow more power into higher impedances. Alternatively, a pair of them in parallel gives you a nominal 8 Ohm impedance characteristic with 102dB/W/m sensitivity. The ring neodymium motor also has the advantage of a conductive magnetic material; with some advantages to the nonconductive ferrite material used in our other two contenders (the aluminum shorting rings used in both the JBL and the Eminence mimic this in some ways). This driver has the strongest motor of any of the three due to the use of higher energy, more expensive neodymium. The powerful motor allows allows it to be similarly sensitive to the other woofers despite the higher impedance: it only takes about half the current to get going as either of the other two, since the impedance is doubled. Naturally, this also means a more damped bass (no free lunch). It also has about 40% less moving mass, compared to the JBL and Eminence.

 

Comparison Details
So, we're comparing two top of the line drivers for high-power professional midbass use, and one premium hifi-specific driver. All three have quality cast frames and are similar in price, ~$350 range (though both the JBL and BD will likely increase after this article). All three have been utilized in highly regarded designs, the JBL in their theater and PA speakers, Eminence in our own Dick Olsher's "Basszilla", and BD designs woofer in Burt Doppenberg'sOris "Swing" system. They are all sensitive in the ~97dB range, with the 2226h having the smallest Vas at about 180L and highest Fs 40Hz and Le at 1.75mH. The Magnum is in the middle for Vas at about 246, and the Fs is our lowest at 33Hz with a medium Le of 1.28mH. The BD maxes out the Vas of our sample with a solid 288 and a medium Fs at 36Hz, with the lowest Le of the bunch, most accurately stated at about 1mH (1.38mH @ 1kHz, .76 @ 10kHz). The low Le is of special note for the BD as it's a 16 ohm driver, and thus would normally be expected to be the highest, all things being equal. AE speakers makes 15" drivers with very low Le but alas, they were not to be tested, but should be on anybody's short list for high efficiency 15"s along with the JBL and BD above, the Magnum as well if you can secure a pair (discontinued).

As mentioned earlier, all will achieve earsplitting levels in a home environment. Max output goes to the JBL, as it has the highest Xmax and Xlim and is very slightly below the Magnum in stated power handling (600W vs. 650) though many factors come into play at extreme high power). The voicecoil overhang is approx 5.5mm for the JBL vs. 4.8mm for the Magnum, which is a trivial difference, the 40mm vs. 30mm Xlim is the big determiner here. Either are appropriate for extremely high power usage, but not so much for use as subwoofers due to the relatively small Xmax of either (this is one way that efficiency, and thus maximum midrange SPL, is achieved in pro drivers vs. home/subs). The BD falls short of its brothers in the maximum output category, with a 3.5mm Xmax and lower 150W power handling. While the Xmax differential wouldn't be a big difference when overhung like the others, this is an underhung driver and thus that limit must be respected, where the others are more of a "soft" limit. The price for exceeding this limit is dramatically rising distortion for the BD, though the hard cutoff does serve to protect the woofer when it exceeds Xmax, so Xlim does not need to be worried about overmuch.

For my use, Xmax is not much of an issue. This is a home environment and these are meant as midwoofers, I'm not trying to EQ for maximum bass output. I have some very good, very high output drivers to accomplish that job in my (currently a quad) of Trio 12"s with the Dayton sub amp. See my article Creative Sound Trio12: Reworking surplus cabinets to create a new subwoofer.

 

Test Environment / Performance
These were tested in my "Big Rig". Active bi-amplification with a 4th order filter at 1kHz allows these to perform in the range where their response is all smooth, and hopefully will make the listening test more ‘fair'.  The bass cabinet was tried both sealed and vented, and was best tuned to 36 or so giving a well-damped alignment for all drivers, with about a 5ft^3 cab volume giving a nominal sensitivity of about 97dB for all three drivers. Treble was via a horn-loaded dipole Heil AMT1, also crossed 4th order @ 1kHz. While the crossover might be better off with a more refined setup (that is to say, customized to the drivers in question, consistency is more important than absolute performance, for this evaluation.

Impedance Sweeps are below- unit consistency was great with all three woofer pairs, as exhibited by the overlaid L/R sweeps on the Eminence and BD. T/S parameters

 

BD15_scans.jpg (143298 bytes)
BD15 Scan

 

Magnum_15HO_scans.jpg (140690 bytes)
Magnum 15HO Scan

 

 2226h_scans.jpg (133648 bytes)
2226h Scan

The frequency responses of the various woofers appear to track fairly well to the published response curves (my rudimentary frequency domain measurements are a blend of near and farfield measures and don't lend themselves to publishing).

 

Sound
Interesting results here. Listened to, unfiltered, the BD has by far the best performance. This could be used as a widerange driver, a la Lowther, with a horn tweeter on top to make things sing. The smooth response and high frequency extension are beguiling from a design perspective, and the neodymium motor is a thing of beauty. Unfortunately, with the lowpass filter in place, and at the miniscule excursions I require (where distortion is extremely low for all drivers under test) the BD falls short of its brethren. The reason why is simple -- more bass.

 

Seriously?!?!
Yep, seriously. Bass extension is no laughing matter. While it's unlikely any of my "big" rigs would ever suffer from a lack of bass, the difference between the drivers was focused on this part. My setup simply didn't demand the strengths of the BD, and the others enjoyed more midbass impact and allowed the detuning of the subs further down in frequency and level. In a setup utilizing slower slopes, or more output, the BD might be more desirable, but at home levels, with a steep active filter, it just doesn't have any real-world performance advantage to my ears. It is important to note here, that the BD is 2dB more efficient than the JBL and 3 more than the Magnum. While it's 96dB/W/m, it is a 16 Ohm driver and thus is only using 50% as much power. This powerful motor leads to an overdamped bass alignment (no free lunch).

The higher frequency performance definitely comes into play, but with the steep filter, it's only a little bit of flavor, and the breakup of the Magnum is not as audible as was expected. The JBL and BD both sound more rolled off, while the peaking of the magnum contributes just a little bit of extra persona. I believe I can improve the HF performance with some cone coating (but wanted to do this comparison article beforehand). Even with a little bit of in or near-band breakup, the Magnum won out comparisons this time around.

 

Conclusion
All three woofers performed admirably, but there was a little more punch due to the better bass alignment, and a little crisper overall performance (probably due to the little bit of breakup) from the Eminence vs. the JBL or BD. This preference was specific to these cabinets and this system, and should not be taken as anything further than that. I was thoroughly impressed by all three, and system design, as always, will dominate the individual drivers.  Perhaps the most standout thing is the job all three manufacturers did in ensuring that their drivers left factory on-spec. This is nontrivial, and they tracked extremely close both to spec and to each other (in pairs). Any of these would make an impressive bass section for a home loudspeaker, used appropriately. I honestly expected the JBL to win out, with its combination of a fairly smooth top end and increased bass vs. the BD, and the Eminence having the breakup behavior, but between the heavy lowpass filter and the excellent bass performance of the Eminence, it just wound up being the best woofer for my rig.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     
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