I have a brand new woofer that I got as a replacement, the original was
defective.
The replacement driver has been sitting around un-played. Seems like it is time to see how it changes with 'break-in'.
I got a DATS recently, so I'll use that. I did check it against an HP LCR with a bias adapter and frequency generator, it is fairly accurate and less cumbersome (although I still have a Macintosh IIfx and with a National Instruments GPIB bus card and LabView that I can fully automate the old HP with...
) But, I'm not looking for tiny signals so lots of simple test fixtures should work. I do think FR and distortion measurements might be a good addition, since many of these break-in mechanisms that are posited here seem that they would also be accompanied by distortion (like changing plasticity of parts). But this is a pretty boring experiment, where I really don't expect to change the world, so the distortion measurements will have to wait!
I did about 50 measurements, first to try to establish a baseline, then to see how temperature might change the measurements, followed by a 30 minute stress to see if the driver breaks in.
Group 01 I measured the driver right out of the box (the red dots).
Group 02 The heater came on, so I made ten more measurements over the next half hour while the heater gently blew hot air into the room (green triangles). Note, the heater went from low to high fan speed after the third measurement in this test.
Group 03 The heater goes off, and I keep recording measurements.
Group 04 I used a fan to blow a gentle breeze over the driver.
Group 05 I put the driver in the fridge for 10 minutes.
Group 06 1 hour after the fridge.
Group 07 20 more mins in the fridge.
Group 08 Warming for 2 hours after the fridge while taking measurements. Break-in is next.
Group 09 Right after 30 mins 30 Hz Break-in while driver is tangibly warm.
Group 10 Cooldown for 2 hours after the break-in while taking measurements.
Group 11 Let speaker rest overnight after break-in.
I wanted to see if the DATs signal is large enough to induce some sort of break-in. Whatever the impact of the small DATS test signal, I discovered the heater is a larger effect that that.
Also, the driver tests much different than Seas' spec in almost all parameters, I guess I'm not surprised since few manufacturers produce drivers and spec sheets that align. Even top of the line Seas and Scan-Speak drivers deviate. In my limited experience, JBL are the closest to published spec, and seem to have the best unit to unit matching. Seas are not very close, although I can't hear the matching issues and whatever the parameters are, the drivers are great... I certainly have lots of them for some reason...
From the measurements with and without heater, looks like room temperature variation is worth about 0.6% F(s), 1.3% Q(ts), 1.2% Q(es). The Q(ms) story is interesting since Q(ms) seems to be affected by the breeze from the heater vent. So does wind from a hand-held fan, which makes sense. I am surprised the airflow from the vent on the opposite side of the room had such an impact, but after thinking about the sensitivity of microphones in general, even a speaker should be quite sensitive to a breeze! Good thing I tend to play music louder than my heater so the impact of random air currents on the effective Q should be negligible! L(e) and R(e) are unsurprising.
I fully expected large changes in parameters after chilling in the refrigerator. I didn't expect it to take more than 2 hours to recover. I wonder if it would fully recover, or if it has some plastic hysteresis that would require large motion to change. The R(e) measurements show it certainly has not come back to equilibrium temperature. In any case, I ran out of patience and decided to move on to break-in stress.
I gave the driver a break-in stress of 30 Hz at 80% or so of maximum. After the break-in stress, the driver was noticeably warm to the touch. After 10 or 15 mins I could no longer tell it was warm. After 2 hours of cooldown, the driver seemed to stabilize, remeasuring this morning (in a cold room) after letting the driver sit overnight confirmed this.
After the overnight rest, F(s) dropped by 1.3% from T=0 value, some of that is likely the cold temperature in the room this morning. Probably closer to 1% after controlling for temperature; that's not much. Seas driver to driver matching is worse than that
(more on that in a bit
). Q(ts) drops by 2.2%, Q(es) by 1.4%, and Q(ms) by 3.5%, some of these changes are likely temperature.
Regarding driver matching, I have a pair of similar Seas drivers (actually the first version of these W18 woofers). The unit that I am breaking-in here is the
second version, they are different drivers with somewhat different FR and T/S parameters:
Despite that, I am hard pressed to hear the actual differences between v1 and v2 under normal use. The easiest way for me is to do a sweep on the two drivers with no filters and listen for the different tone of that big resonant peak when the cone breaks up above 4 kHz, they actually have a different timbre as the sweep runs through that resonance. I can hear it on different precision tracks if I remove the notch filter I normally use with these drivers. Other than these corner cases, both versions sound awesome when used with an crossover, with a notch filter at the resonance.
Here's the unit matching between the two well-used W18 v1 drivers I have, mounted in identical sealed enclosures (not free-air!!!)
R(e) is well-matched, which I expect for two (hopefully identical) lengths of wire at the same temperature. The rest of the parameters are all over the place. Winding the wire into a voice coil has so many geometric considerations, no surprise 3.5% difference. Same for all of the plasticy and bendy things that conspire to make the mechanical bits and pieces, I am not surprised at this level of mismatch in F(s) and the Q measurements, it is very common even if I can't hear the differences in use. I actually have no idea how the JBL drivers I used to measure were so consistent, like better than 1% for these electromechanical parameters.
Whatever happened in the first 30 minutes of stress last night on the new driver is way less than the driver to driver matching I get between samples. I'll subject the driver to more stress today. See if anything changes.
