There has been a “renaissance” in AMTs over the last several years, and we now find AMTs used not only by home audio companies, such as MartinLogan, Gryphon, Elac, Wharfdale, Arion, Emotiva, and Burmester Audio (which uses the Mundorf AMTs), but also in pro sound, in both studio monitors (Adam, Presonus, HEDD, and Eve Audio), and AMT drivers for PA applications, such as the Beyma TPL-200/H and the Eighteen Sound AMT200P. Mundorf also has a pro sound AMT available.
There are five models in the Mundorf U Series, each with the same AMT width, but different heights. This month we focus on the U80W AMT (Photo 1). However, all the diaphragm area ranges from the various models are shown in Table 1.
The diaphragm material used in all the Mundorf AMT’s is a proprietary blend, as explained by the company:
“All of our AMT diaphragms are manufactured in-house by a unique manufacturing process of our own development. The mixture of our basic diaphragm material was optimized with focus on its acoustic properties, hereafter the manufacturing process was tailored to ensure the very best acoustical result possible with every single diaphragm material mix. Every detail of our diaphragm‘s geometry is based on a deep understanding of the mechanical requirements and physical properties of an AMT diaphragm. Only with this knowledge is it possible to achieve the perfect ratio of a low diaphragm weight, the highest possible diaphragm flexibility, and the most effective inner damping at a maximum of diaphragm stability. Even the slightest shift within this well-balanced ratio would lead to audibly worse results. This out-balanced ratio makes the Mundorf AMT diaphragm one of its kind. Due to its exceptionally high inner damping, it is characterized by extremely low distortion data. This applies particularly to the odd-numbered harmonic distortions K3 and K5.”
Features for the Mundorf U80W include a 63mm × 19mm pleated proprietary AMT diaphragm, an optimized rear chamber, a flat face plate with an optional secondary aluminum face plate, 94dB sensitivity, 80W power handling with a minimum second-order 2.2kHz high-pass network (3kHz first-order network), gold-plated solderable terminals, and a diaphragm area (Sd) of 31.4cm2.
Testing commenced using the LinearX LMS analyzer to produce the 300-point stepped sine wave impedance plot shown in Figure 1. With a nominal 4Ω impedance, the U80W has a 4.25Ω DCR, with minimum impedance of 4.3Ω and at 7.36kHz, which is not particularly significant since the amount of reactance in this type of transducer is minimal. Mundorf specs the resonance for the U80W AMT at 2150Hz, and as can be seen, there really is only a barely discernable resonance to AMT transducers.
For the next set of SPL measurements, I recess mounted the Mundorf U80W in an enclosure that had a 13” × 5.25” baffle area and measured both the horizontal and the vertical on- and off-axis at 2.0V/0.5m (normalized to 2.83V/1m) from 0° on-axis to 45° off-axis using the Loudsoft FINE R+D analyzer and GRAS 46BE microphone (again, supplied courtesy of Loudsoft and GRAS Sound & Vibration). Since the U80W comes with an optional face plate (Photo 2), the on-axis measurement is shown without the face plate attached, and with the face plate in place. All the remaining SPL measurements were made with the face plate attached. Also, due their asymmetrical diaphragm shape, AMTs have a vertical directivity that is significant enough to be taken into account, which is why I also performed a series of vertical plane measurements.
Figure 2 displays the on-axis frequency response of the U80W AMT, the black curve with the face plate attached and the blue curve without the face plate. On-axis, the most significant difference is 1dB to 3dB of gain between 3.5kHz to 6kHz with the optional face plate, otherwise not much change. For the device with the face plate in place, the response is ± 2dB with no major anomalies from 4.7kHz to about 40kHz, beginning its second-order low-pass roll-off at above 40kHz. Figure 3 shows the 0° to 45° on- and off-axis response in the horizontal plane. Figure 4 displays the normalized horizontal plane response. Figure 5 shows the 180° horizontal polar plot (in 10° increments with1/3 octave smoothing applied) generated using the CLIO Pocket analyzer and accompanying microphone (courtesy of Audiomatica SRL).
For the vertical plane of the Mundorf U80W, Figure 6 gives the 0° to 45° response, normalized in Figure 7. The CLIO-generated polar plot is illustrated in Figure 8. Last, Figure 9 shows the two-sample SPL comparison showing the two Mundorf U80W AMT driver samples to be matched within 1dB to 1.2dB or less throughout the entire operating range of the transducer.
For the remaining series of tests, I set up the Listen, Inc. AudioConnect analyzer and ¼” SCM microphone (provided to Voice Coil by Listen, Inc.) to measure distortion and generate time-frequency plots. For the distortion measurement, I again mounted the U80W in the same enclosure as was used for the frequency response measurements, and set the SPL to 94dB at 1m (2.98V determined by using a pink noise stimulus generator and internal SLM in the SoundCheck V18 software).
I then measured the distortion with the Listen microphone placed 10cm from the mouth of the AMT diaphragm. This produced the distortion curves shown in Figure 10. Note the extremely low third-harmonic level.
Following this test sequence, I then set up SoundCheck to generate a 2.83V/1m impulse response curve for this AMT transducer and imported the data into Listen’s SoundMap Time/Frequency software. Figure 11 shows the resulting cumulative spectral decay (CSD) waterfall plot. Figure 12 shows Short Time Fourier Transform (STFT) plot.
After reviewing the data, I found the Mundorf designed AMT definitely exhibits excellent performance, which is what you would expect from a respected high-end home audio manufacturer. For more information about the U80W and other Mundorf products, visit www.mundorf.com. VC
This article was originally published in Voice Coil, March 2022.
