In this blog I test the Original ESS Heil Air Motion Transformer™ tweeter. A pair of these units were sent to me by a friend. During measurement, the tweeter required that I block the rear output so that rear wave did not interfere with the front wave creating comb filtering in frequency response. I used a thick towel to block the rear output.
The tweeter is available direct from ESS here. It just so happens that the tweeter is on sale for $175 USD each (regular $349 USD).
- Weight: 14.5 pounds
- Height: 6″
- Width: 6.75″
- Depth: 4.25″
- Impedance: 4 ohms, Re: 3.9 ohms
- Power: 40 Watts RMS, 160 Watts Max
- Response: 800Hz - 20kHz
- Sensitivity: 96 dB, 1 watt, 1 meter
- Fits ESS Cabinet and Driver Models:
- ESS: AMT-1. AMT1, AMT-1A, AMT-1B, and many more
It's worth noting that the driver can cover a wide frequency range reaching down to 800Hz with reasonably high sensitivity of 96dB.
The dipole radiation pattern should provide a very spacious sound presentation.
Measurements
Starting with frequency response we see a relativity flat frequency response from the claimed 800Hz all the way to 18kHz where the response falls nicely with no breakup. The response continues to roll off with 28kHz extension using a -10dB downpoint.
Implementing this result into a finished speaker design would be quite straight forward with minimal correction (if any).
Off-Axis (Horizontal)
The horizontal off-axis was measured at 5 degree increments out to 90 degrees using 1/12 octave smoothing. The coverage narrows in the 3kHz and 6kHz region. However upper treble remains wide providing an approximation of a 90 degree listening window. Generally the power response will remain even into the room compared to some direct radiating solutions which transition from omnidirectional to narrow over their bandwidth.
Personally I would consider this a good result considering the driver's unique design.
Vertical coverage is quite narrow which would require careful placement of the driver ensuring proper listening height. Personally I wouldn't focus too much on the vertical result. There might be some limiting of the soundstage spaciousness in the vertical (floor and ceiling result) but this is not a critical sound quality factor in my opinion. For those interested, this result is due to the physical height of the radiator surface, being a tall ribbon type of driver. The positive trade-off is the low distortion which we shall see later.
Impedance
The impedance shows the FS centered at 900Hz with an extremely flat inductive free impedance response above this region. The impedance is free of high Q resonances through it's pass band. Really great! Also confirmed is the 4ohm load. The flat nature of the response will provide a non-reactive (easy) load for any amplifier. It's been my experience that these types of drivers (planar included) are amplifier immune, meaning usually they bring the best out of any amplifier with minimal sound quality change when swapping out different amplifiers.
Time Domain
The CSD waterfall plot shows very a very clean result. The FS shows good damping. The upper treble is devoid of any resonances.
Burst decay shows the same story, even with a 35dB vertical scale.
Distortion
Distortion at the 85dB 1m test signal is extremely low across the bandwidth. (0.05% for H3)
Signal to noise improves at the higher test signal level and we see distortion improve even further, with H3 and H4 remaining extremely low. (0.07% H3 for 2kHz region)
This is an extremely good result.
The distortion profile also indicates a trouble free design across the spectrum, with no anomalies which may indicate some sort of flaw in the design or manufacture.
Intermodulation Distortion
With -80dB dynamic range in the upper treble, you're amplifier will likely have more distortion in the upper treble than this tweeter. A very fine result!
The 95dB result is also excellent with no part of the spectrum coming unhinged.
Subjective Listening
My listening was limited to mono in my test area. In general, AMT drivers have a very clean and open sound, and this was no exception. My brief listening test confirmed that the measurements correlate to great overall sound quality. The driver offers similar clarity and dynamics as a compression driver. Unfortunately due to my limited listening time I could not tell if this driver was superior to a good compression driver. I can certainly assert that this driver would excel over many dome tweeters, even with a waveguide, offering lower distortion and wider bandwidth with ease.
Conclusion
The design of the tweeter is commendable, considering it's unusual physical configuration, it provides a flat frequency response, wide frequency bandwidth, and low distortion. As a designer myself, I can fully appreciate when a design comes together to provide an excellent balance of sound quality performance attributes. In terms of DIY, it would be a breeze to implement as a 2-way using a 10" or smaller woofer. The final speaker result would especially be appealing to those using low power amplification. Other considerations would be room treatment. Special attention to the front wall acoustical treatment ensuring the rear radiated energy does not adversely affect overall on-axis clarity. I would suggest broad spectrum diffusion in this instance along with at least 1m wall distance to delay the secondary image.