I got a bunch of these things a couple years ago after reading about their response. I finally got around to making one as a source follower circuit (it's about half way down this page http://www.linkwitzlab.com/sys_test.htm#Mic) and used it with my sound card to measure the freq response of some items I have around, a very old Panasonic bookshelf speaker, a Grado 80 headphone, eGrado headphones, an Anker bluetooth $30 speaker, and an old set of Pioneer HPM-100's. Here's a spreadsheet of the results. I've been thinking about getting one of the programs that sweep these things automatically, I'll get to it. But what I find interesting is the rising rate of audio levels in the headphones. The HPM of course have the best bass response, but seem a bit erratic in the overall levels. The Grado 80's seem to have the best 'presence' of the lot. I can hear things in them that don't even exist in speakers or other headphones. I drove the headphones directly with the 'tone generator' output of my Moto 2005HC service monitor and the speakers through a small 15W class D amp I use for the computer. The Anker BT I drove right from my computer's BT, it has it's own amp. I'm surprised how high they respond, I really can't hear much of anything past about 12KHz. I need to make up a TL072 or 074 preamp for the thing. This was just using the line/mic input on my Zonar 192KHz soundcard, which has a much better frequency response than the RealTech that came with this HP W10 computer. I just thought it was interesting, I've seen some old articles here about using these mic elements. I may make one just to try over the air. The holes in the lines are where I didn't take data.