TC Sounds LMS-5400 18" sealed 75L vs. 200L *special test* - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 23 Old 07-26-08, 07:57 PM Thread Starter
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TC Sounds LMS-5400 18" sealed 75L vs. 200L *special test*

TC Sounds LMS-5400 18" sealed 75L vs. 200L

This is a special test which was performed in order to clear some issues that were brought up at the AVS forum with regard to performance of different sized sealed enclosures, especially when it comes to the distortion behaviour. Some people claimed that a smaller enclosure would produce less distortion at the low end than a larger enclosure. Other people quickly corrected that it only applies to certain special situations and only suspension non-linearity was cured, though with some nasty side effects. After 10 pages of banter and bickering I thought to call it quits. Since I have always liked to actually measure things instead of only simulating, I packed my gear and went out to take some measurements. Since I had the driver, amplifier and the two different sized sealed boxes ready, things progressed quickly, and here we are.

The testing was done identically to my previous subwoofer tests. The following gear was used.
  • Driver: TC Sounds LMS-5400 18" (voice coils wired in series for a ~4.5 ohm DCR load) (same driver was used for both enclosures)
  • Enclosure1: Sealed 100L box with extra stuffing (EPS/styrox) to make it around 75 liters (can't calculate the exact volume, but Fb suggests around this)
  • Enclosure2: Sealed 200L box (used to previously house also two 18" passive radiator, now those holes were simply closed off with sheets of 1" thick MDF)
  • Equalizer: Behringer DCX2496

Driver:


Amplifier:


75L enclosure:


200L enclosure:


Test setup was standard 2 meter ground plane:


Off to the results...

To properly test whether a smaller enclosure volume results in less THD, I needed to equalize both subwoofers identical when it comes to their frequency responses (a must for a sweep THD test). That way the driver excursion and therefore output would be identical at every frequency. Only the power needed to produce the output at each frequency would be different because of different sensitivities at each frequency.

So first I measured the frequency response of the 200L box and adjusted the FR of the smaller box accordingly. Then I performed my usual battery of tests on both subwoofers. Though I'm not going to post all the results (for example group delay) because they would be identical to both of them.

First we have a frequency response comparison. You can three frequency responses: 200L, 75L Eq'd and 75L UnEq'd. The curves should be pretty self explanatory - smaller box results in higher Qtc and Fb which make the response roll-off higher/faster. With only three filters I managed to get the FRs pretty much identical.



Then we have the maximum output and THD (=total harmonic distortion, up to 48kHz/2) sweeps. 50 Hz was matched at 90 dB and 5 dB increment was used between sweeps. Sweeps were taken up to the level where the driver didn't suffer from high amount of compression and/or didn't produce obvious noises of distress. This meant 115 dB nominal level for both of them. Though the 115 dB sweep was terminated for the 75L system because the subwoofer started to produce a very unpleasant noise at around 35-40 Hz and below. The probable cause was amplifier clipping due to large amount of EQ boost at the low-end. I didn't want to take a risk of blowing the voice coil or the amplifier (or both) so I terminated the sweep. Even though the 115 dB sweep of the 200L system also reached high THD levels at the low end, it wasn't nearly as unpleasant to a human ear (not that much higher distortion components). The obvious difference between two systems can be seen on THD graphs. I didn't plot the two lowest level THD sweeps (90 dB and 95 dB) because measuring low levels of THD at the low end of the frequency spectrum is extremely difficult due to background noise typical outdoor environment has.









Here are the spectras of the recorded signals during the 110 dB sweep at 20 Hz. You can see the distribution of different harmonics there. The first one is for the 200L system, the second one for the 75L.





Then we have the CEA-2010 standard. It measures the clean (=uses a stepped distortion limit) maximum output of a subwoofer using a burst signal which mimics typical transient of a real world signal. The following results were recorded. The expected "shift" seems to happen between 40 Hz and 50 Hz.



Even though already these tests show pretty conclusively the result of the lower low end sensitivity and the required EQ boost, I decided to take a few extra measurements for the VLF enthusiasts out there (cough...Bosso ). I tuned the signal generator to 8 Hz (all sorts of problems occurred below this, not to mention extremely high THD levels). As the screen below shows, I managed to record ~90 dB @ 8 Hz for the 75L subwoofer. Anything higher and the amplifier would clip wildly, not to mention +100% THD produced by the driver alone. For the 200L subwoofer 90 dB resulted in ~1/2 the THD and 95 dB was also within the limits, albeit with high THD and suspension noise. It's pretty obvious that a single 18" sealed subwoofer isn't capable of producing usable levels of output at these frequencies (without a HUGE room gain).







As requested by Mark Seaton, I measured the impedance response for both systems. I also included the driver in free air. Naturally the unit for the y-axis is ohms, not desibels.



Conclusions

Even though some special drivers may benefit from the linearization effect of the air spring, there's no doubt in my mind that the net effect of all the variables causing non-linearities (=distortion) will pretty much always favour the larger enclosure volume. Although I have to admit that the difference I measured today was smaller than I expected. This might be because of the extremely beefy voice coil (xtralong 4" LMS coil wrapped around a titanium former) found in the LMS-5400 driver. It can tolerate the added power with small side effects. Some lesser driver might have showed much larger difference. But that's a discussion of another thread.

-Ilkka Rissanen
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post #2 of 23 Old 07-26-08, 09:29 PM
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Re: TC Sounds LMS-5400 18" sealed 75L vs. 200L *special test*

outstanding. Thanks illka.

my first impression is, I wonder how the results would have varied if the driver was one with a softer suspension, instead of the stiff LMS5400.

nonetheless, thanks for doing what you do best!


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post #3 of 23 Old 07-26-08, 09:35 PM Thread Starter
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Re: TC Sounds LMS-5400 18" sealed 75L vs. 200L *special test*

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outstanding. Thanks illka.

my first impression is, I wonder how the results would have varied if the driver was one with a softer suspension, instead of the stiff LMS5400.

nonetheless, thanks for doing what you do best!
Thanks!

To my understanding suspension stiffness (whether it be stiff or loose) itself doesn't cause any anomalies, assuming it's symmetric and "linear enough" throughout the operating range. So I'm guessing drivers with bad suspension (i.e. non-symmetric) would benefit from small enclosures. Of course assuming that they have the VC to handle the extra power needed. But for good drivers, larger box results always lower distortion at the low end.
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post #4 of 23 Old 07-26-08, 09:44 PM
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Re: TC Sounds LMS-5400 18" sealed 75L vs. 200L *special test*

Interesting, thanks Illka.

Now the question is, why is the THD higher? Flux modulation because of higher current?

OT, but whatever happened to the next batch of subwoofer tests - Ava18, Acoupower, etc?

Thanks
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post #5 of 23 Old 07-27-08, 01:22 AM
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Re: TC Sounds LMS-5400 18" sealed 75L vs. 200L *special test*

Yet again Ilkka, you show why you are without a doubt the man

When it comes to subwoofer banter, there is nothing better - in my opinion - then having someone as capable as you be able to measure the actual set of circumstances. Readers of this forum and others have benefitted from your testing time and time again. On a personal note, I now understand a bit better where you were coming from in your messages to me a while back - expect a PM from me soon, I will try to right the situation if you will allow it.

To recap some of your findings, larger enclosure = less distortion and more output without sounding displeasing to the ear. In regards to single digit extension, even a driver with the massive amount of displacement and the linear motor system used in this driver (which is head and shoulders above anything else you have ever tested), THD is greater than 100% at levels that still require significant amounts of room gain to bring it up to what most would consider adequate output, let alone reference levels, and cause severe amp clipping. In other words, even a cream of the crop 18" and super beefy amplifier can't do justice to that range.

Ilkka, based on some of the questionable FR measurements you may have seen on the net of single 15" or 18" drivers without nearly as much clean displacement as the LMS 18" and amps that are much more modest than the Crown CE being linear to even lower single digits in what I would consider medium to large rooms, without clipping, and without being displeasing to the ear, would you conclude that the measurement gear being used for those types of measurements isn't properly calibrated?


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post #6 of 23 Old 07-27-08, 07:33 AM Thread Starter
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Re: TC Sounds LMS-5400 18" sealed 75L vs. 200L *special test*

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noah katz wrote: View Post
OT, but whatever happened to the next batch of subwoofer tests - Ava18, Acoupower, etc?

Thanks
Nothing happened to them, they just haven't happened yet.

On a serious note, there has been some major problems in acquiring the drivers you mention. I already bought and paid two Ava18 drivers but the buyer got cold feet and pulled off the plug. Luckily he returned my money... Then there was the TC bankruptcy that affected (actually still does) heavily on Soundsplinter. Acoupower's Carlos hasn't replied to my messages for months so that looks thin too.

Hopefully with the help of Nathan from Funky Waves, I will be able to test the Acoupower driver soon. It is pretty expensive to get the driver here so the buy-in thread might be needed to bring back to live.

Also hopefully Kevin Haskins from DIYCable will be able to provide me a Maelstrom-X driver to test (once they are back in stock).

Mark from Mach5Audio has already sent me one of his IXL-18 drivers. It should be here soon.
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post #7 of 23 Old 07-27-08, 08:05 AM
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Re: TC Sounds LMS-5400 18" sealed 75L vs. 200L *special test*

can you test the new drivers that arrive to you in the 200L and 75L enclosures as well? since you've started already ... it will be nice to have comparisons on how each driver is affected by volume

thanks in advance
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post #8 of 23 Old 07-27-08, 08:07 AM Thread Starter
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Re: TC Sounds LMS-5400 18" sealed 75L vs. 200L *special test*

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mike c wrote: View Post
can you test the new drivers that arrive to you in the 200L and 75L enclosures as well? since you've started already ... it will be nice to have comparisons on how each driver is affected by volume

thanks in advance
Can't make any promises, sorry. Also not all drivers are suited for that kind of boxes. Some require much larger boxes to be able to perform properly. The LMS-5400 is pretty exceptional 18" driver to be able to work rather well in a 75 liter sealed box.
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post #9 of 23 Old 07-27-08, 09:20 AM
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Re: TC Sounds LMS-5400 18" sealed 75L vs. 200L *special test*

Superb as usual Ilkka.

I was disappointed to see such big differences (and with a driver like the 5400).
I can only assume that when the box is smaller it requires larger amounts of power,
that makes the coil very hot and effects the overall performance.
Makes you think about eq sealed subwoofers in general and my own rl-p18 in a (about)
110 liter cabinet.
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post #10 of 23 Old 07-27-08, 09:37 AM
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Re: TC Sounds LMS-5400 18" sealed 75L vs. 200L *special test*

Thanks for choosing this particular issue to clarify by your testing process. It adds much to our understanding of the moving cone / magnet audio reproducer.
Your tests using the LMS 5400 also answered a long standing question I had regarding whether or not my own LMS 5400 was bottoming from extreme excursion at low frequencies or was it the amp (XTi 4000) that was going into clipping or protection that made horrible noises. From your testing experience using a CE 4000 with the LMS 5400 and noting the clipping or near clipping of the amp at high levels of VLF I now can conclude it was the XTi 4000 and NOT the LMS 5400 that was the weak link in the audio chain. The LMS 5400 is capable of pulling huge amounts of electrical energy from the amp attempting to reproduce high volume VLF, more than the amp, in your case a CE 4000, could handle.
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