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Contrasseur last won the day on September 28 2016

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  1. Modern voice-coil subwoofers are currently capable of maxing out the linearity of even the largest FEA optimized surrounds. They're much simpler to make and have far fewer moving parts. There's also no static friction to cause distortion at low levels, or kinetic friction to cause wear at high levels. Cost is much lower and reliability is much better. Plus they can often be useful over much greater bandwith due to less inertia. Danley's original ServoDrive transducers had 12-16mm xmax. Outstanding when 4mm was the norm, but pretty lackluster by today's standards. Surely a servodrive with modern materials and engineering methods would be quite impressive indeed, but the cost would certainly be unreasonable. I don't know how you could manage surround noise, let alone mechanical noise with such excursions. If Danley's patent expired, it's telling that no one has bothered to come up with a copycat product. If he still holds the patent, it's equally revealing that he doesn't bother making speakers with it while the idea is still protected.
  2. OK Josh (and other bass overlords), here's my hypothesis Underlying assumptions At such low impedance loads, neither amplifier's power supply can sustain max-burst levels for any significant period of time. The Speakerpower's power supply can produce more current and has greater output capacitance. The Powersoft's power supply produces much higher voltage rails. The drivers compliances have loosened up since testing with the K20. Rigorous testing also broke in the cones/spiders/dust caps, enough to shift breakup modes. This last assumption is a stretch, but it's the only explanation I can think of for the last conclusion. Explanation of max burst results - K20 wins on the high end At the high end, the max burst performance is voltage limited, not distortion limited. You estimated the K20 to be able to produce ~400v bursts. At system resonance, current draw is minimal, so the inferior current capabilities make no difference. Same at the high end where driver inductance drives up system impedance. Explanation of max burst results - SP2-12000 wins on the low end This is likely to be the breaking-in of the driver. Below system resonance, the compliance starts pulling the cone toward the center before the motor finishes pushing it all the way to it's peak positions. We already know that this is the main source of distortion at the extreme low end. The loosened compliance helped reduce distortion further, contributing to the higher achievable SPLs before breaking CEA-2010 thresholds. The lower system resonances also lowered the frequency of the impedance peak, reducing current and extending Xmax at a slightly lower frequency than in previous K20 testing. Better output capacitance could also prevent the rails from sagging, which could introduce some distortion in the K20 as well. Explanation of long term results - K20 wins on the high end Higher available voltage offers greater achievable SPL on the high end. Tightened compliance during these tests also helps high end performance. Higher impedance in the high end due to the impedance peak and driver inductance minimizes the effect of the lower-current power supply. Explanation of long term results - SP2-12000 wins on the low end More current capacity from the power supply offers more low-end maximum performance. Explanation of sensitivity measurements - SP2-12000 wins on the low end I don't see how this could have anything to do with the amplifers themselves. I think this is solely due to loosened compliance. Explanation of sensitivity measurements - peaks and troughs seem switched between tests Same with this anomaly. I don't see how going from one flat-response amplifier to another could cause any change here. I think the breakup modes have shifted. On both of these drivers, breakup modes are very mild, exhibiting well damped resonances that create soft lumps in the FR instead of sharp peaks. It doesn't look to me like the peaks have become troughs and vice-versa, it looks to me like all but one or two of the resonances have shifted lower in frequency. The one that I can't really explain is the blip around 150Hz. This can be seen in all your measurements of similarly sized boxes, which is sure to be a rear chamber resonance. I don't see how this could have moved if the enclosure is the same size. Maybe the damping material has settled, affecting speed of sound and therefore perceived depth of the rear chamber to the acoustic waves?
  3. If weight is evenly distributed, your suspension could probably handle it. If it sags slightly, there's no top plate for the VC to scrape into. If you've already bought the driver, give it a try and see what happens.
  4. Metal would require lots of tooling. Folding, welding, maybe even casting, just way too much for probably no net benefit. If wood won't cut it for you, then you need composites. 1/4" carbon fiber or fiberglass with epoxy is damn tough. Even 1/8" will go a long way. They can be laid up in the final shape, with virtually no tooling required. Adding bracing would be elementary. Some people even consider it easier than woodworking once you get the hang of it. Just carve out a foam mold, glass the whole thing, then remove your foam. Cost is quite reasonable also. For such a small enclosure, the materials cost will be miniscule compared to your current $2.5k electronics and transducers budget. Useless in the workshop? Draw it in Solidworks or SketchUp and send that bitch to a 3D printer. You can brace it within an inch of its life for no extra work or money. Put gussets on top of gussets inside your gussets. You only pay per gram in 3D printing. You'll probably be using ABS or something. Fiber reinforced plastics can be stronger than many metals, but I don't know if they're printable.
  5. I think you can still do the PR just fine, but SME's right, you have to account for time domain issues. Group delay is related to the curvature of your final SPL. This means that it's treating say, 35Hz very differently from 40Hz. Intuitively, ask yourself "How is it going to tell the difference between these two frequencies, and how is it going to attenuate one but not the other?" This is the exact same concept that gives rise to the Heisenberg Uncertainty Principle. Knowing frequency with 100% precision requires infinite amount of time. Makes sense right? Frequency is cycles PER UNIT TIME. Now your system sees an input curve. As voltage rises, it doesn't know if it's going to be a 40Hz sine wave, a gaussian function (bell curve), a 39Hz sine wave, or any other infinite number of shapes until your 40Hz sine has completed it's full cycle. That's 1/40th of a second, or 25 milliseconds. Let's say our sine wave stopped at 25.6ms. It was a 39Hz sine wave. A perfect brickwall 40Hz filter would eliminate it, but it wouldn't be able to tell the difference until that last 600 microseconds. Such a slope is so steep that 40Hz frequencies would be delayed infinitely until it could be certain there was no 39.99999...Hz content, then the 40Hz tone would pass through unadulterated and any <40Hz tones would be eliminated. Real systems aren't perfect brickwalls, but attenuate by a certain amount. To attenuate power by 16x (12db) for every octave, we've created a second order rolloff. Sealed systems eventually progress to this steepness. A ported system will eventually have a 24db/octave slope. These slopes (and therefore delays) will stack with whatever active processing you add to it. Option 1 Eq flat to 40hz with only a 4th order rolloff. This means no high pass. Loud notes below 35Hz will fart, but you'll still only have your 4th order rolloff and time domain behavior will be quite acceptable. Option 2 Soften the corner of your final slope. Think Bessel filter, Zaph's quasi-EBS/QB3 alignment, Ricci's 25Hz ported BMS 18N862 system etc. i.e., a gradual increase in slope the further you drop in frequency. Response will be flat to say ~60Hz, slowly roll off to maybe -6dB at ~40Hz, then your high pass filter drops frequency response below tuning. Now all frequencies from 30-60Hz are delayed, but no frequency is delayed by more than one cycle, so time domain behavior will be acceptable. You can keep your high pass filter to eliminate below-tuning farts, you won't hear any ringing at the low corner, but 40Hz output is reduced. Option 3 Eq flat to 40Hz and put in your 40Hz high pass filters. No farts, no reduced 40Hz output, but very high group delay. Maybe as much as 1.5 cycles, maybe more. Now let's throw another wrench in the system. Room effects. Room effects will add all sorts of peaks and valleys everywhere in the low end. Even if Option 3 had barely acceptable group delay before you put the speaker in the room, it probably won't afterward. That's why a lot of speaker designers opt for Option 2 or Option 1. You'll have to decide where you want the tradeoffs to land.
  6. Dig around as in eBay or dig around as in back alley dealer who probably stole them?
  7. Not at the size enclosure you're using, and the frequencies you intend to use them at. The distance between woofers is acoustically small. Even at 300Hz, the woofers are just about quarter wavelength of each other. The lower you go, especially down to 40 and 30Hz, the distance between sources, even wrapping around the cabinet, is negligible compared to the wavelength they produce. They will be pushing in phase unless you wired one wrong. Again, each woofer will eat up a little internal volume. Your proposed enclosure is ABSOLUTELY TINY so even going neodymium might have a measurable effect on the interior dimensions. Still, maximum output will be increased for the reasons previously discussed.
  8. What does this mean for your enclosure? Below Fb, you will see no gains in sensitivity. Adding more woofers will give you more displacement and power handling. This is where the increased maximum output comes from, and can only be achieved with the application of proportionally more power. Comparing Ricci's 21SW152-4 single driver to double driver systems, sensitivity is only increased above 60Hz. Fb will go up in frequency. Adding more woofers gives each woofer less air space, so each one's air spring is stiffer, yielding a higher resonance. Notice Ricci's 21SW152-4 single driver system has an Fb ~50Hz, but the dual opposed is 65Hz despite adding a little more volume. This means you won't see any of the Fb efficiency gains until a much higher frequency. If you look at the Xmax Investigation thread, you'll see that Xmax is also increased at Fb. A driver can often remain relatively linear all the way up to it's mechanical limits. If your single driver system was getting an Xmax boost at 40Hz, your quad driver system might not get the boost until 70Hz. Ricci's 21SW152-4 data show only a 2dB gain in CEA-2010 burst performance around 50Hz going from one driver to two. Everywhere else, a 5dB gain can be seen. At Fb your maximum performance will increase by adding more drivers, but it will not be double. Above Fb, you will see great gains in performance and exactly the kind of scaling you'd hoped for. Josh has said many times that this punchy midbass can be perceived as low sub-bass. In reality, it's just extra midbass. Without EQ an undersized system will be very punchy and top-heavy. After flattening the response, your sub-bass will no longer be overshadowed by the mids and you'll notice all the rumble your system is capable of.
  9. If your speakers are all playing in phase, you won't have any cancellation. Here's what you have to remember. For sealed systems, efficiency below Fb is solely determined by the compliance. In Ricci style builds, i.e. undersized boxes, the speaker compliance is negligible compared to the small box's compliance. Since there's no impedance peak below Fb, motor strength plays virtually no part. Near Fb, sensitivity is still determined by the tiny box size, but impedance is much higher for more efficient woofers. Less power is drawn, so efficiency is greater. Above Fb, the sensitivity is determined by the reference efficiency of the woofer. This is the spec published by manufacturers. Note the 98dB sensitive 21IPAL doesn't reach this sensitivity until 100Hz. For sealed systems, Ricci's mantra is maximum full-bandwidth output from a small size, so he optimizes the limiting region below Fb. He picks woofers with enough thermal handling and excursion to get as much as possible down there. That's why he chose the horribly inefficient RE XXX's, because they can handle absurd power levels and have more displacement than just about any 18". However, I'd never say he's not concerned with efficiency, which is why he upgraded to the RF T3-19s. Their ridiculously powerful motors gave them more efficiency at and above Fb. Efficiency was only icing on the cake though. He only made the swap because he wouldn't be sacrificing any below Fb performance, slightly improving it in fact. Small caveat, the "inductance hump" will warp the shape of the sensitivity graphs. However (and you'll find the same answer from simulations), efficiency is entirely unaffected. Inductance warps the impedance in exactly the same way. Any observed increases and decreases in sensitivity are accompanied by equal increases and decreases in impedance. This is why Le is not in the sensitivity and reference efficiency formulas.
  10. Our Bass Overlord was kind enough to publish his findings on the subject. The impedance peak gets lower as padding is added. Since voltage is held constant, and P=V^2/Z, more current flows and more power is drawn from the amplifier. Power in has increased. You can see the sensitivity drops as well, so power out is reduced. Efficiency is power in vs power out, so on both fronts efficiency has been reduced. It's usually not that big of a deal though. Ricci's case study shows impedance peaks down by maybe 30%. This change reduces efficiency only in THE MOST EFFICIENT bandwidth for the system. Sensitivity in a sealed box is only down a couple dB, so in many cases, maximum output is not affected since most systems run out of excursion before thermal handling at these frequencies. As you can see in Ricci's conclusion, it's usually always a good idea to have some damping in the system. It's just about finding the right amount for your application. Remember, a resonance peak out of the intended bandwidth can be audible if the low pass filter doesn't attenuate it enough.
  11. Yeah, based on your application your results make a lot of sense. The padding is what's killing your impedance peaks. It reduces system efficiency by reducing both impedance and system sensitivity. It's a necessary evil though because if you plan on using this driver above a few hundred Hz you'll need to tame box resonances. You can try seeing how much padding you can remove before box resonances become problematic. "Ricci style" isn't terribly concerned with efficiency, and sensitivity even less so. It's more about putting a crap-ton of power into the low corner where box size determines sensitivity, and using components with enough displacement and thermal durability to handle the juice. Your lower impedance peak is further reduced by using such a small enclosure, which pushes system resonance upward. Again, unavoidable in such a small enclosure. Your lower impedance peak doesn't matter much anyways though, since a resonant system won't have any appreciable output that far below tuning anyways.
  12. What PR are you using? What driver are you using? Are you using any padding or damping materials? What is your intended tuning frequency? Does this correspond to the measured impedance minimum between the peaks? This part is normal behavior.
  13. We know that in sealed boxes, impedance peaks increase Xmax within their bandwith. See the Xmax Investigation thread for more. There is some evidence that Xmax is increased at the impedance peaks in resonant enclosures. Caveat: The site has very few data points for both sealed and resonant enclosures of the same driver, and most of those change other factors as well. Comparison to simulations has limited accuracy, especially at output extremes, so take the following conclusion with a grain of salt. It appears that distortion is reduced and displacement-limited output is increased near the impedance peaks in resonant systems. Greater than predicted by the effect of the port alone.
  14. Painting in broad strokes here, The height of the impedance peaks are related to the height of the impedance peak if it was a sealed box. Higher BL product = higher impedance peaks, higher damping = lower impedance peaks. The impedance minimum between the peaks corresponds to system tuning. This is an excursion minimum for the driver where most of the output is coming from your port/PR. This resonance splits the sealed box's impedance peak into two. The relative heights of the two impedance peaks corresponds to how much of the sealed system's impedance peak is above or below system tuning. Tuning well below Fs results in a larger in-band impedance peak and a smaller below-tuning impedance peak. See Ricci's data on the XXX Ported system or the TC Sounds LMSR-12 at 13Hz for examples of extreme low tuning. These are generalizations. Simulations will tell you a lot more.
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