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There's always plenty of heated debate regarding ATC's dome mid so I finally decided to put together measurement data and post it here to perhaps remove some of the subjectivity and also the bashing for the sake of bashing.
I should also thank Ralph who's FR and impedance data I have borrowed. Ralphs measurements followed my own very closely so thats reasurring regarding accuracy, it also saves me much time capturing, editing, converting and uploading that data.
Just a couple of very quick notes:
The distortion test was performed outside and at 90dB average measured at the mic which was exactly 2m from the driver. The driver itself was mounted in a Perceive Satallite enclosure with no crossover in place for both distortion and CSD tests.
Cumulative Spectral Decay and distortion measurements were taken in ARTA and STEPS software(v1.1.0) using a Behringer ECM8000 mic, both ARTA and STEPS were calibrated using a true RMS DMM, the mic sensitivity was taken as per manufacturer specs.
On Axis:
The amplitude response is not as smooth or as flat as might be expected for driver that costs so much or is held in such reverence. However the phase response is remarkably smooth between 300Hz-4kHz. The above plot is a 'nearfield' LF response merged at 700Hz with a 'farfield' high frequency response taken at 0.5m. The first major breakup occurs at about 4.5kHz and is also evident in the impedance plot below. Below 1kHz the response rolls off smoothly.
As mentioned, in order to use this driver in a system requires considerable EQ. This isn't easily achieved with a passive crossover, and the driver benefits from an active implementation.
Off-axis:
The off-axis response is where the driver really shines. It's easy to eqalise a driver to be flat on axis, but not so easy to equalise it for a flat power response as well. Only a driver with a consistent power response can be equalised to achieve both.
Below is an example of the ATC's consistent off-axis performance. EQ has been applied to flatten the response somewhat, and the consistency in amplitude response as the measurement microphone is moved off-axis is easily seen; at 45 degrees the amplitude response remains within +/-1dB between 400Hz and 2kHz. Only above 2kHz does the driver start to beam and the power response fall away consistent with size of the driver's diaphragm.
Note that the vertical scale is 1dB/division. By augmenting the ATC mid with a controlled directivity HF driver, a smooth roll off in the system power response can be achieved. By 4.5kHz things are getting nasty, so to use the driver up to around 3.5kHz as is found in ATC's own speaker systems suggests quite steep crossover filters are needed.
Impedance of 8ohm version
The impedance plot is well behaved with just a small glitch at 700Hz and a larger one at 4.5kHz. The driver resonates at 320Hz which is only marginally outside the claimed operating range of 380Hz to 3.5kHz. Normally a midrange driver is not used close to it's resonance frequency. But in the case of the ATC mid, it works very well down low, sounding better crossed around 300-400Hz than higher up. Of course, when used below 400Hz, a steep filter is recommended or power needs to be limited not exceed Xmax.
Distortion:
CSD:
Hope that helps demystify the fog of hype/BS that surrounds this driver. Make of the measurements what you will and please note the test conditions - it makes all the difference.
If anyone is interested in other measurements/conditions I'll should be able to provide them given time.
There also seems to be a distinct lack of measurements for the Scan R2904 ring radiator, if anyone wants those I do have them.
EDIT: Uploaded wrong file for distortion measurements, now fixed.
I should also thank Ralph who's FR and impedance data I have borrowed. Ralphs measurements followed my own very closely so thats reasurring regarding accuracy, it also saves me much time capturing, editing, converting and uploading that data.
Just a couple of very quick notes:
The distortion test was performed outside and at 90dB average measured at the mic which was exactly 2m from the driver. The driver itself was mounted in a Perceive Satallite enclosure with no crossover in place for both distortion and CSD tests.
Cumulative Spectral Decay and distortion measurements were taken in ARTA and STEPS software(v1.1.0) using a Behringer ECM8000 mic, both ARTA and STEPS were calibrated using a true RMS DMM, the mic sensitivity was taken as per manufacturer specs.
On Axis:
The amplitude response is not as smooth or as flat as might be expected for driver that costs so much or is held in such reverence. However the phase response is remarkably smooth between 300Hz-4kHz. The above plot is a 'nearfield' LF response merged at 700Hz with a 'farfield' high frequency response taken at 0.5m. The first major breakup occurs at about 4.5kHz and is also evident in the impedance plot below. Below 1kHz the response rolls off smoothly.
As mentioned, in order to use this driver in a system requires considerable EQ. This isn't easily achieved with a passive crossover, and the driver benefits from an active implementation.
Off-axis:
The off-axis response is where the driver really shines. It's easy to eqalise a driver to be flat on axis, but not so easy to equalise it for a flat power response as well. Only a driver with a consistent power response can be equalised to achieve both.
Below is an example of the ATC's consistent off-axis performance. EQ has been applied to flatten the response somewhat, and the consistency in amplitude response as the measurement microphone is moved off-axis is easily seen; at 45 degrees the amplitude response remains within +/-1dB between 400Hz and 2kHz. Only above 2kHz does the driver start to beam and the power response fall away consistent with size of the driver's diaphragm.
Note that the vertical scale is 1dB/division. By augmenting the ATC mid with a controlled directivity HF driver, a smooth roll off in the system power response can be achieved. By 4.5kHz things are getting nasty, so to use the driver up to around 3.5kHz as is found in ATC's own speaker systems suggests quite steep crossover filters are needed.
Impedance of 8ohm version
The impedance plot is well behaved with just a small glitch at 700Hz and a larger one at 4.5kHz. The driver resonates at 320Hz which is only marginally outside the claimed operating range of 380Hz to 3.5kHz. Normally a midrange driver is not used close to it's resonance frequency. But in the case of the ATC mid, it works very well down low, sounding better crossed around 300-400Hz than higher up. Of course, when used below 400Hz, a steep filter is recommended or power needs to be limited not exceed Xmax.
Distortion:
CSD:
Hope that helps demystify the fog of hype/BS that surrounds this driver. Make of the measurements what you will and please note the test conditions - it makes all the difference.
If anyone is interested in other measurements/conditions I'll should be able to provide them given time.
There also seems to be a distinct lack of measurements for the Scan R2904 ring radiator, if anyone wants those I do have them.
EDIT: Uploaded wrong file for distortion measurements, now fixed.
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. Personally, I'd rather be pleased.