Continuing this discussion from the One Day Ticket for Stax Galaxy - The STAX Compendium thread. An excerpt from post #42: To see if this was really the case, I decided to conduct an experiment. First, I measured the HD800 at a very loud level (about 100db). To see how much distortion is caused by the headphones and how much is caused by my mic + preamp + adc combo I then put foam between the headphone and the coupler plate, to decrease the level at the microphone. I used 1 very porous foam material (like a sponge) to break the seal and thus lower the level. Finally, I also used the thicker material that my coupler is made of (described in this post). I calibrated the volume level by using the specced mic sensitivity (15mV/Pa) and line in sensitivity (2Vrms) in ARTA. (Remember that my setup has no gain between the mic and the input) I don't think this calibration is very accurate at all, but I simply do not have the means to calibrate it more accurately (also added this to the other post). Here are the results in frequency response: As you can see, breaking the seal dramatically lowers the bass response. This was to be expected. Because the FR is changed so dramatically, we now absolutely have to check the "use harmonic frequency as reference" box. Without this the below results would be useless. Here are the distortion plots for each measurement: Yuck! ZOMG! WTF IS UP WITH SENNHEISER? 1% DISTORTION THROUGHOUT THE MIDRANGE!? "Why did it get lower?" But wait ... there's more Note how the D3 distortion in the bass stays mostly constant. This is actually the headphone. This one is higher - look at the FR. This is because this is actually louder up to 800Hz. Now the noise seriously cripples the measurements. The only thing that we can say for sure is that the HD800 probably produces less than 0.3% of distortion at 100db - and very likely much less than this. Conclusion: Know the limits of your measurement rigs. Mine should never be used to measure distortion at 100db. I even have my doubts if it is accurate at 90db (it probably isn't) and most plots would most certainly be limited by microphone noise. I'm sure the measurement rigs used by the other folks have lower distortion and lower noise than mine, but I'm still not certain that the results for super low distortion headphones, like the STAX electrostatics, are accurate. I simply don't believe that the SR009 has 0.2% D2 at 90db throughout the midrange. It is probably lower than 0.05% (Even the HD650 has less than 0.2% distortion. Those plots look more like less than 0.05% and limited by noise). We would have to drastically lower the noise of our measurement systems (The noise floor of my Schiit Vali is easily audible to my ears, yet my measurement system doesn't pick it up). The first step here would be to use larger microphone capsules, 1/2 inch or even 1 inch (those are maybe too rolled off up top), but those cost hundreds of dollars alone. Looking at the specs I really like the ACO 7012, B&K 4191 (and maybe G.R.A.S. 40AC and Gefell MK 202) capsules. This basically means that we should look at the distortion measurements with a grain of salt. When multiple different headphones all measure way too close in terms of distortion (every headphone in the STAX galaxy thread measures at a nearly perfect 0.2% D2 at 90db), it is very likely that these plots aren't limited by the headphones.