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Get familiar with the reaction system: Introducing the Reaction System
Original Great Heil AMT Test Sweeps . . .
Test Sweeps for subject units I just received:
No instructions concerning wiring or polarity so assumed the darker wire (Purple = Negative) Blue = positive.
I fail to observe any issues with these sweet mothers . . . You know what I mean 
Looks like 1KHz is the sweet spot for an XO, yes?
Attached as files for simplicity on my part - Let me know your thoughts/observations, please.
Comments
You really should run a distortion sweep to determine the crossover point.
Agreed! This is what someone posted
Distortion Sweep attached - includes FR, 2, 3, 4 & 5th order harmonic distortion (Black, Red, Purple, Green, Blue - respectively).
Mic = 6in away, On Axis @ 90 db:
Can you refine the resolution?
90dB @ 6 in isn't very loud, only 76dB@1m.
Removed
I must point out that these AMT's are open backed and 1.5 feet from a room corner.
I suspect if I put a board behind them they might measure differently as there wont be much reflectivity coming at the mic out in the room . . . may try that next . . .
OK, board installed and eggcrate foam applied behind this AMT:
New distortion graphs attached . . . Better?
OmniMic auto scales the Snapshot output so here is a screen grab instead.
Attached as file:
Pic of Test shelf set-up for continuity of discussion,
Attached:
I succumbed and bought a pair too. I’ve been intrigued by these for years. The sale price was just too enticing for my willpower and the headphones were an unexpected bonus. I’ll be doing measurements in VituixCad with a turntable. I’m out of town next week so it may be a while before I have results.
I really dislike Omnimic default resolution - not just the smoothing it applies, but the height by width ratio additionally hides things you should probably know when designing. 1/24th smoothing will show quite a bit more detail, and if you can squish the HxW ratio closer to 4x3 and change the vertical resolution to at least 5db scale we can get a better picture of the response.
Alternatively, you can export the 1/24th FRD and import in to a crossover design tool and take a snapshot from there.
I usually go with no smoothing and a 3 db/div height scale.
Why does OmniMic truncate the DISTORTION chart at 10Khz? Is it because all measured distortion drops to almost zero at that point or what?
Harmonic distortion is a multiple of the frequency, so 2nd order distortion at 10khz is 20khz and at 15 khz it is 30 khz. This is outside hearing and many microphones range, and based on what I've been told the software respects that.
Excellent explanation! - Thanks again, JasonP.
Also the Omnimic only operates at 48kHz sample rate, so at best it can measure to 24kHz, or 2nd harmonic of 12kHz, but is software limited to 20kHz range.
It's good enough for my ears but thanks for the ^ info as it adds to my understanding.
I managed to take some on axis measurements of my new ESS AMTs. Some notes:
Speaker Mounting: I decided to measure them in the open without any baffle (see picture of my kludgy setup).
SPL Measurements: I measured at 0.5 m with 0.7075 V (2.83V / 4). I have a mic SPL calibrator so the SPL levels should be accurate. Subtract 6 dB for the corresponding 1 m response at this drive level. Add 6 dB to calculate the corresponding 2.83V @ 1m response.
Distortion Measurements: Always take these with a large grain of salt as they are both position and level dependent. Zaph liked to take his tweeter distortion measurements at 4” while others take measurements at a full meter. YMMV.
What I take away from the distortion graphs is that it is predominantly 2nd order, which is a plus.
ESS lists the AMT1 at 96dB/1w/1m, and it's a 4 ohm impedance so ~99dB/2.83V/1m which is quite high, and roughly lines up with what you show.
Distance requirements for distortion can be quite a bit different than what we usually do for frequency response. The mic should be placed close to the driver to reduce room interaction, but far enough away to capture the full pass-band response and keep SPL hitting the mic below 120dB. Call it "semi-far field". Most condenser mics will run out of steam beyond 120dB and will introduce a distortion of their own.
Zaph measured at 4" (0.1m) for tweeters, and 0.5m for woofers to my knowledge. Hificompass measures at 315mm, or 12.4" which I think is a good compromise for keeping a consistent distance regardless of driver, at least until you get to large woofers or some real loud stuff. Measuring distortion at 1m doesn't make much sense in most cases, there will be a fair bit of room interaction contaminating the lower frequencies.
Comparing distortion results from different sources should always be taken with a grain of salt, differences in mic, equipment, drive level, distance, all affect the results for direct comparisons to be made, which is why sites like Hificompass are invaluable as they provide an apples to apples comparison with the same conditions across the board.
In any case, I think the AMT1 looks like a great tweeter, especially given its age. Due to its size on might want to keep a bit of a rising response in the high frequencies on-axis to compensate a bit for the narrow directivity.
Was it mentioned that each tweeter weighs 14.5 lbs?
And these are true dipoles: I measured the output of one backside and it was virtually identical to the frontside. It will be interesting to see the polar response when I get time to do the turntable measurements.
These were measured by 'Dirk' in my living room last winter and closely approximate everything others have experienced with the Heils. This is the midsize Heil. Anybody with answers to this is welcome to reply!
^ File size = 0 bytes, Marty.
Please insert the data before saving?
Ahh crud. My laptop is on it's last legs. I'll have to find a new way to post it.
I think you'd have better luck saving it as a .pdf and posting that.
Will do it this weekend, after the new laptop arrives. I can barely see the screen anymore (typing this on the iPad).