The 6 ohm Eminence Lab 12 is rated at 400 watts, tests were done at 800 watts, 49 volts into 3 ohms.
The B&C BC18SW115-4 is rated at 1500 watts, it was tested with 77.5 volts into 4 ohms.
Distortion was taken as the nearest whole number from the “distortion calculator” chart below, no attempt was made to sum all the various distortion harmonics together.
More detail in the methodology and distortion readings for other speakers can be seen in this thread, page six sums up most of the information:
PSW Sound Reinforcement Forums: LAB Lounge => JBL SRX718 - Basic Distortion Measurements
My test computer was carried away by a gust of wind yesterday, suffered a four foot drop, breaking the screen :^(. An expensive gust that occurred just before the last test I was going to attempt before it started to rain.
Unfortunately I will not be able to post any distortion screen shots until the computer is repaired.
The BC18SW115-4 BR 38 Fb averaged 120.6 dB, 2.69% distortion.
The BC18SW115-4 TH averaged 126.52 dB, 5.438% distortion.
The 2xLab12 TH averaged 124.33 dB, 6.375% distortion.
In a previous test, 2xLab12 in a ported cabinet averaged 117.12 dB, 6.25% distortion.
So much for “ tapped horns are cleaner than BR” myth.
Especially since the TH have more distortion up high, the 80 Hz harmonics march right up into the 320 Hz and above range.
Of course, if the BR were given enough power to be the same level as the TH, it’s distortion would be higher, though the upper end would probably still be clean.
The 2xLab12 TH is about 2 dB more sensitive than the B&C TH, so should have been within a dB of it with half the power, but the B&C was 2.188 dB louder overall.
Other than below 30 Hz,( all the cabinets need a 28 Hz or so filter) the B&C probably would be OK with at least 3000 watts peak.
The overall margin of level difference between the BC18SW115-4 in the BR compared to the TH is 5.92 dB, about the same as the sensitivity difference was in the magnitude response.
However the BR has lost LF relative to HF in the high power sine wave tests compared to the Magnitude Response tests, perhaps to port compression.
The B&C TH is the inverse, the upper 90 Hz area peak has gone away at high power sine wave testing.
Port compression is well known, the loss of upper frequencies, that is kind of “different”.
The Lab 12s, however, retained their more prominent 90-100 Hz peak, the only frequencies that broke the 130 dB one meter barrier, assuming I have Smaart SPL calibrated correctly.
I think it is within a dB or so of reality, and the BK-4004, used for the tests is rated for 148 dB at less than 1% THD.
Comparing the frequencies from 35 to 70 Hz:
The BC18SW115-4 BR 38 Fb averaged 119.53 dB.
The BC18SW115-4 TH averaged 127.27 dB.
The 2xLab12 TH averaged 124.28 dB.
In the lower range, the TH is louder than the BR by 7.74 dB, and the BC18TH has beat the Lab 12TH by 2.99 dB, using about 3 dB more power. The Lab 12s were pretty well maxed out at 50 Hz, while the BC18SW115-4 could still take another 3 dB or more overall, probably achieving 5- 6 dB more output.
I did high volume magnitude response tests to see if these frequency response differences show up with pink noise, using an amp capable of 85 volts just at clip level, the response curves for the BC18SW115-4 looked almost identical at their curves at -10 dB.
The Lab 12 had a slight dip in the range from around 45-60 Hz at high power compared to low, the power level was enough to cause power compression, as evidenced by the 45-60 Hz dip still evident even at low power until the voice coils cooled down.
During testing, at times I noted the Lab 12 magnet structure getting fairly warm compared to the ambient temperature, and a fair amount of heat coming from the vent, while the BC18SW115-4, getting almost double the power of the two Lab 12,
The amp used for the test was a Speaker Power “Torpedo” SP1-4000-DS, a “plate amp” designed to be put in a speaker cabinet, it is a good power match for the Lab 12s, (and will power four in Paraline without problem) but a bit light for the BC18SW115-4.
All this testing is expensive, I blew up a QSC PL3602 comparing it to the SP1-4000-DS, then broke my computer yesterday.
And after all the testing with the BC18SW115-4, although it will definitely put out more level than the Lab 12 or 4015LF speakers, unless I get some more powerful amps, I’ll hardly see any increase in the LF output of my system.
The many pleasures of DIY.
My condolences on your losses.
What took out the QSC?
It would be nice to see the difference in the keystone vs a normal box width terminated TH mouth.
I completely understand your situation, testing equipment the right way takes so much time just to get accurate results. It must have taken an enormous amount of time to build and test all your stuff.
Now others can see and review your findings ... To bad it came at the cost of some of your equipment. What are your thoughts on the doubling distortion figure for the 18" TH vs the 18" BR? Could it be from the TH being less damped than the BR.?
Anyways thanks for the info.
Hopefully you can get the QSC warrantied
QSC - Warranty Registration
The damage was not too bad, one blown output transistor, $100 repair plus a 100 mile round trip X 2.
The MacBook was out of warranty by a bit too, but physical damage would not have been covered anyway, as I found out yesterday.
I will have to remember to use a "safety belt" on the computer for outside testing.
The QSC protection circuit did not respond quick enough to avoid damage when presented with a dead short with the inputs wide open with a very robust pink noise signal.
I normally turn off amp power and zero the inputs after each gig, but I had missed doing that with the PLX-3602. Five of the six amps I had on the last gig were all turned down, I happened to plug in to the one that was not zeroed.
I plugged in the input, plugged in the output, plugged in the AC, click clack click clack of the protection circuit.
A clip lead connecting the VOM test leads to the load resister bank had shifted, causing a dead short. If the amp's inputs had been brought up slowly, the protection circuit probably would have saved it.
Oh well, can't help bad luck...
All the TH research, design time and testing have occupied most of my time since the beginning of February.
Not sure what you mean by the TH being “less damped” neither the TH or Thiele BR use any damping material in the cabinet.
Acoustically, I would think the TH dampens the speaker more than a BR.
The Keystone TH is almost 6 dB more sensitive overall than the BR, more in the upper region of the pass band.
To get that sensitivity, seems both sides of the cone are “working harder”, the extra loading stress seems to translate to more distortion.
The Lab 12 had a bit more distortion than the BC18SW115-4, but it’s distortion figures did not change as much between BR and TH. The BC18SW115-4 is a very stiff cone, but the Lab 12 is cone is even stiffer.
The Lab 12 suspension is looser than the BC18SW115, it is probably a bit above Xmax at the 30 and 50 Hz tests.
I did not test the distortion of the 4015LF in the Keystone, but from my listening tests it was obviously more distorted than in the BR cabinet I measured it in previously.
The 4015LF cone feels almost “mushy” by comparison to the BC18SW115.
Stiff cones seem to help lower distortion in TH.
The Eminence 3015LF is popular in TH, it is lighter (and therefore more sensitive) than the 4015LF. I’d expect it’s distortion to be higher also.
Frequency, phase and transient response differences all make for different sounding speakers. Distortion is one of the “big four” regarding speakers, yet is seldom mentioned.
It’s funny to see people worry about .01 distortion in amplifiers, when good speakers, pushed to rated power levels, frequently exceed 10%.
The testing I did can easily be done by anyone with an RTA, it would be interesting to see more results from different DIY and commercial cabinets.
I've been using a free dropbox account to handle the backup of measurement files and programs like Hornresp across Mac and PC systems. Here is my referral link to dropbox it'll get us both some extra space. Another awesome and free thing that I use is evernote, these guys should get a nobel.
Dang Art. Careful with the equipment man. Of course its just not a complete day of testing without frying a sound card or mechanically breaking an $800 driver. Both things I have done.
TH distortion is more variable than what you see with traditional direct radiator systems. Some areas are very very clean and others within the fr will have much higher thd than the equivalent direct radiator. The varying loading and pressures on the cone, the xmax variance and more importantly the acoustic boosting at certain freqs causes the thd profile to be highly range dependant unlike a traditional direct box that has a predictable but comparatively smooth thd profile. TH's have much greater swings in thd level through out the operational range.
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