The 18SW115 is history...lets see what a 21" can do! - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 39 Old 01-04-14, 07:03 AM Thread Starter
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The 18SW115 is history...lets see what a 21" can do!

I really had no qualms with the performance of the tank-like B&C but after carefully studying the Data-Bass archives, an undeniable truth has descended upon me. The distortion measurements in particular have caused me to look toward a driver size that 8 years ago would have been unfathomable, unworkable, and too expensive to even contemplate. However, along comes neodymium, and along with increased xmax, superfluous BL, increased building skill and a bigger budget, the cabinet can be kept smallish and the performance improved...even in a dual opposed setup, which is becoming more and more enticing. I can imagine the overkill of a 2x21" setup in 1200 cu. ft. but for now I embrace merely one.

The moment I read about Ricci's dual 21SW152 setup I have been trying to figure out how to trade out of the 18SW, and that was only about 3 or 4 months after completing the build. Actually it was the discontinued 21SW150 that excited me because the BL was very high, with qes/qts both being low enough to make it very suitable for my type of application. However, Ebay STILL has that thing going for well over $700.00 and it has been there for about a year now. I looked at the 18 Sounds 21LW1400 but it is no match for the B&C below 20 Hz even though it can be had for much less. Failing to find anything in the league of the B & C 21's, I knew that group buys and vaporware really bug me, so anything outside of the pro sound arena didn't really excite me, especially in lieu of the T/S parameters and of course the Data-Bass archives.

I don't know how I missed the RCF LF21N451 but after reading the specs, I was thoroughly impressed and it seemed like the closest thing to a 21SW...the problem? The price, especially on Parts Express. It would be quite an investment to sell the 18SW for less than I paid and fund the difference for minimal gains, but out of the blue I found one that wouldn't break the bank. It really is a lateral move considering my venue of 1200 cu.ft. SPL is out of the window but extension is another matter. The 18SW at 3.3 cu.ft. gave out at about 14 Hz, with anything below that not really relevant to the conversation or sound signature. Perhaps the RCF 21 will get me to 10 Hz, with my bottom room mode being 40Hz. About 1/2 an octave of extension...that's about all that can be expected from the sweat of a reworked cabinet. 3.5 Hz for a dollar-cost average of $29 per, but...I'll take it!

I suspect that the T/S parameters will be off, as they were for the 18SW. Factory Fs said 32 Hz but after a 40 hour break-in, the cold Fs was actually 26Hz...measured with a home-built impedance analyzer. That means qes,qts,qms and vas were all different than the factory specs. Unibox showed me a qtc. of .48 with full fill and an Fb of 52Hz. It came in at about 52.5, so I achieved the critically damped response that I always crave in a system. I will need an Fb of about 60Hz to achieve a similar result from the RCF...hopefully I won't have to expand the cabinet beyond an additional 3/4" baffle. Much thanks to the humble seller for helping make all this possible. I await the driver!

MBoutte
south Louisiana
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post #2 of 39 Old 01-04-14, 07:10 AM
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Re: The 18SW115 is history...lets see what a 21" can do!

What net volume are you planning on?




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post #3 of 39 Old 01-04-14, 07:15 AM
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Re: The 18SW115 is history...lets see what a 21" can do!

Why are you trying to shoot for 10hz with pro audio drivers?

Only one that can do 10hz at all with some SPL is the TC Sounds 5100. But still a HT subwoofer would be far better at 20hz and down. The UXL from Mach Audio will win on performance against almost anything pro audio from 30hz and down. For single driver it is one of THE best drivers for HT subwoofer systems.

I am not saying you cant use your RCF but you can get higher SPL and lower distortion from a HT type of subwoofer. I am not trying to be mean about this just telling you what I have learned.

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post #4 of 39 Old 01-04-14, 07:27 AM Thread Starter
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Re: The 18SW115 is history...lets see what a 21" can do!

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What net volume are you planning on?
First I will have to measure the Fs and then the box frequency as is. 1 lb. per cu. ft. of polyfill so it is tightly packed.

3.78 cu. ft. [107 liters] should give an Fb of 59.6 Hz /qtc. ~.500
3.55 cu. ft. [100 liters] should give an Fb of 61.2 Hz /qtc. ~.514
3.30 cu. ft. [95 liters/current size] should give an Fb of 62.4 Hz /qtc. ~.523

The caveat is the T/S parameters...they will be different and I only hope in the same way as the 18SW was. It is a used driver after all. If the Fb comes in around 60 Hz, I will just leave it to the extra 3/4" baffle and call it a day.

MBoutte
south Louisiana
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post #5 of 39 Old 01-04-14, 08:30 AM Thread Starter
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Re: The 18SW115 is history...lets see what a 21" can do!

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Why are you trying to shoot for 10hz with pro audio drivers?

Only one that can do 10hz at all with some SPL is the TC Sounds 5100. But still a HT subwoofer would be far better at 20hz and down. The UXL from Mach Audio will win on performance against almost anything pro audio from 30hz and down. For single driver it is one of THE best drivers for HT subwoofer systems.

I am not saying you cant use your RCF but you can get higher SPL and lower distortion from a HT type of subwoofer. I am not trying to be mean about this just telling you what I have learned.
The TC 5100 is still only 18" and its distortion characteristics are worse below 20 Hz than the 21" pro sounds that were tested...Against the B&C 21", there is a marked difference. You are not being mean...just stating your opinion.

In response, I can only say that:

First, the box required will be much too large...

Second, most HT subwoofers have higher Le and a smaller coil, which more readily disturbs the response, especially under high heat/compression.

Third, 1196 cu. ft. is very small, with the bottom room mode being 40Hz. The 18SW responded to 14Hz but no lower. The RCF will have more displacement at a similar xmax rating. My guess is that the geometric rating will be conservative.

Fourth, SPL is not the real aim here, extension is...and in a space this small it is entirely possible. Sure it won't be totally palpable but it will be possible, unlike the 18SW.
Distortion below 20 Hz in virtually every case, was higher in HT subwoofers of the 18" variety because they must travel much farther to produce that band than a 21" driver. I did not in particular like the distortion levels of the SI HT-18d2 at and below 20 Hz. The 21" pro sound drivers are a bit cleaner at higher sweeps, especially the B&C 21SW152. The UXL-18 is better but the driver was supposedly damaged during testing...it delivered 108db @ 20 Hz with THD of 29% (3rd HD limited). The Dayton RS-HO 18" also disappoints at and below 20 Hz. In all instances, THD is rising rapidly below 20 Hz, even with all available excursion. Multiple iterations will do a better job but my application calls for a single driver and I pause at spending the same amount on an HT 18" subwoofer, when a 21" pro sound can do a better job in the 10 - 20 Hz band. It is the reason that I won't be returning to an 18" driver for infrasound. It simply has to work harder...and that goes for the LMS 18" as well...outright heresy I know, but for many moons I have coveted that driver, but that was "many moons ago".

Fifth...for me, very high BL is THE top requirement, with low qes and qts. Only the UXL-18 gets near these requirements but I would need to see clearer testing of distortion below 20 Hz. In any event, a very high BL 2x21" pro sound arrangement seems like the best avenue for me in the future.

Sixth...critically damped is my preferred alignment, which gives a very musical presentation and I'm not personally enamored of a "theatrical" bottom octave that might present undue listening fatigue over long hours. I like music too much for that. However, I do like good thump in the right places and at the right time for home theater.

Seventh...weight. Neo drivers simply can't be matched, especially when I tend to want to overbuild the cabinet.

Finally, the high Fb of pro sound drivers will make response below that more even and predictable, even though the room will have much to do with final response. In the case of the RCF 21, the Fb will fall on top of the 60Hz crossover point I use for both HT and music.

The above observations are indeed polemic and may incite a few but the application must suit the venue. I do like the FTW-21 and SI's 24" sub but both have unsuitable T/S parameters for my tastes. I will also say that my application is suitable for a smallish room, with a dual opposed 21" likely giving as much as one would want in a space so small. Once pro sound drivers swing a bit farther on the xmax, there will be little left in HT subwoofer-land to entice me.

MBoutte
south Louisiana
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post #6 of 39 Old 01-04-14, 09:58 AM
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Re: The 18SW115 is history...lets see what a 21" can do!

Polemic for some, but I wouldn't be among them. I'm the forum heretic of sorts - I love it when someone thinks outside the box, as it were, and bucks convention. The picture I'm getting in my head is of something with an end result similar to a subwoofer I just recently reviewed; the Bag End ID18-Pro. Your description conjures an image of what the design meetings at Bag End might have sounded like. After hearing the ID18-Pro I definitely see the merit in your approach, and your proposal should solve my one issue with the Bag End sub; total output. Let the build begin!

BTW; the two links in your tagline that lead to the tcsounds.com site are broken.

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post #7 of 39 Old 01-04-14, 01:00 PM Thread Starter
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Re: The 18SW115 is history...lets see what a 21" can do!

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Polemic for some, but I wouldn't be among them. I'm the forum heretic of sorts - I love it when someone thinks outside the box, as it were, and bucks convention. The picture I'm getting in my head is of something with an end result similar to a subwoofer I just recently reviewed; the Bag End ID18-Pro. Your description conjures an image of what the design meetings at Bag End might have sounded like. After hearing the ID18-Pro I definitely see the merit in your approach, and your proposal should solve my one issue with the Bag End sub; total output. Let the build begin!

BTW; the two links in your tagline that lead to the tcsounds.com site are broken.
The links were shut down on their end but everything that was there is in the member database under TC Sounds...my old 2x15 system before it became 3x15. Those were all low qtc. (<.5). I have seen various reviews on the Bag End subs and I always thought it was an interesting way to design a sub, taking the Fb out of the working range. I just got the RCF 21 a few minutes ago and it is HUGE!!!

This won't actually be a build per se, I just have to glue 3/4" of birch on the existing baffle and then get the circle cut out...everything else is mostly cosmetic, moulding and such.

It seems the ID Pro used the SIGMA PRO 18A-2 by Eminence...it only had 6 mm of xmax and 18mm xmech...with 1140 Sd. The RCF has 13.2mm geometric xmax, 29mm xmech and 1730 Sd. Fairly larger I would say. I am hoping it has a little more xmax than the geometric rating, which is usually the case with most neo pro drivers...perhaps around 16 -18 mm. I sure wish that Data-Bass had tested this driver against the B&C 21. Crikey! I just want to look at it for awhile!.

MBoutte
south Louisiana
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post #8 of 39 Old 01-08-14, 04:30 AM Thread Starter
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Re: The 18SW115 is history...lets see what a 21" can do!

So far ... 6 dB stronger at 11 Hz ... 91dB for the RCF 21 and 85dB for the 18SW...

corrected values...

102 dB @ 8 Hz
100 dB @ 9 Hz
107 dB @ 10 Hz
106 dB @ 11 Hz

Shaking power all the way down to just touching 10 Hz ... I don't know if I'm at the limits of the driver but lot's of racket between 10 and 14 Hz... The 18 SW struggled at 14 to stay palpable ... the RCF 21" does 14 Hz with ease... At 11 Hz, something in my closet is ill at rest.

UPDATE 1/7:

Recorded 104 dB @ 8Hz and some closet rattle. However, just one more dB on the master volume and the driver was in distress so 82dB uncorrected at 8Hz is the very edge of the envelope. The meter is a RS 33-2055 and I used the correction tables per. I will check again to be sure the amp wasn't clipping but I think I did in fact reach the limits at 8Hz.

So no surprise it digs deeper than the 18SW...by almost a full octave no less. I would enjoy seeing what a 21SW could do, with a sliver more xmax in the bag. Who knows, a good deal on one might come to fruition soon. The only other "ominous" option is the infamous FTW-21 but the specs are all wrong for a smallish box. I am on the edge of what is possible with a 21", smallish, critical Q enclosure. Not much left to get besides the 21SW or some other yet to be discovered gem, because it is all 21" from here on out.

Subjective listening? With a box frequency above the 60 Hz crossover? Is there really a sub in the room? That is what you would think until some untoward subterranean growl pierces the air. The dynamics are all there, just like my former builds ... snappy dynamics, no bloat, no theatrical bottom octave causing undue listening fatigue. You always want more, which is how it should be. You should want more but never really get it, because then all the extemporaneous and emotive power of sound remains surreptitious enough to tease and excite. Nope. In my system, the bass arrives and departs like a darting running back through a porous secondary on Superbowl Sunday.

MBoutte
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post #9 of 39 Old 01-08-14, 05:15 AM
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Re: The 18SW115 is history...lets see what a 21" can do!

Just curious, what amp are you using?




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post #10 of 39 Old 01-08-14, 05:19 AM Thread Starter
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Re: The 18SW115 is history...lets see what a 21" can do!

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Just curious, what amp are you using?
QSC USA900...bridged mono operation.

MBoutte
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