Is it just me that wants to see these things tested using some proper equipment, and without putting them in them in a load of random battery powered devices?
We should do both. The normal end user doesn't care about some abstract characteristic curves, but if the sleeve doesn't have any advantage for many products, this will help when people decide if they should buy it. Unfortunately I bricked my RK8511 electronic load (meanwhile I fixed it, with a new power supply that can deliver the 25 A required for re-calibration), so I couldn't do a test with higher currents, but at least some numbers are out, see
here. Dave wrote that he will do tests with his electronic load as well, would be good to verify some of my numbers. I think it is very strange that it doesn't boost the output always to 1.5 V, but maybe I didn't measure it right? First time I did something like this.
Unless you're playing the same white/pink noise during both tests then Bob can say that you played music with different spectral content hence the variation. Or even that the volumes were different. Some headphone manufacturers might make one model available in more than one nominal impedance. I obviously don't think that any of this is the case...
Of course you should use the same test procedure for all tests with one device and I'm sure Dave does this. I played the same song on auto repeat, on max volume and with the same headphones (some pillows over the headphones helped to do still some work while the test was running
).
PS: does a SMU PC software have a button to measure and calculate an efficiency curve? I thought about building my own SMU (might need some time, because I already have to many hobby projects). Can't do 6 digits precision, high frequency etc., but better than 1% should be possible with cheap components and hobby-grade PCB layout. With the right software it could be very useful to characterize all sorts of components, maybe as an Arduino shield to allow writing all sorts of custom scripts for it. Couldn't find a finished and full featured DIY SMU project so far.