Disclaimer: This headphone is owned by Inks, who is indeed the greatest contributor of this blog.
Along with full-sized T-series models, Beyerdynamic DT 1350 is also a part of Beyerdynamic's proprietary Tesla-driver technology implemented line up. Although it is a portable supraaural design, the manufacturer emphasizes the headphone's professional monitoring quality sound.
The goal of Tesla-driver is to achieve 1 Tesla, or 1 weber, which is according to the BIPM, producing an electromotive force of 1 volt if it were reduced to zero at a uniform rate in 1 second. In other words, Beyerdynamic is trying to accomplish very high electroacoustic efficiency by increasing the magnetic power of the driver. According to Mario Gebhardt, Head of Development for audio products at beyerdynamic,
"In beyerdynamic headphones, “Tesla” stands for our latest system technology that is distinguished by a very powerful magnetic drive and clean reproduction with a low degree of total harmonic distortion ... With the Tesla principle, the magnet is mounted as a ring encircling the coil and this enables us to divert more energy to the membrane, which is where it is required."Beyerdynamic's German patent, DE102009038593, indicates that the technology is comprised of a big annular "neodymium-iron-boron" magnet 120, which is situated at the outer rim of the voice coil, holding a base plate 110 and a cover plate 130, creating a massive magnetic system altogether, much bigger than that of any conventional dynamic transducer designs.
Moreover, the diaphragm 140 is made of a dual-layer compound foil, allowing "high inner damping of partial oscillations despite its low weight and contributes to the transparent, finely defined, and distortion-free sound".
In contrast to Sennheiser's ring-driver patent, DE102007005620, of which the magnet is situated in the inner rim of the overhung voice coil, Tesla-driver technology has fewer components(lower production cost), a larger magnet(greater efficiency), and an underhung voice coil(emphasis in the bass quality). And these fundamental differences clearly distinguish the unique design goals the two German manufacturers are embracing, and ultimately, reveal Beyerdynamic's technical advantage over its arch-competitor.
CON: Odd-order distortion in the bass, perhaps due to the underhung voicecoil & Audible ringing at 10 kHz.
ON SECOND THOUGHT #1: Although the tesla-driver achieves a high sensitivity of 109 dBSPL/mW @ 1 kHz even with a 80 Ω driver, there's a good amount of distortion in the bass, which clearly contradicts what the manufacturer claims, suggesting the sign of false claim.
ON SECOND THOUGHT #2: Unlike any other full-sized headphones, thanks to the dual-layer compound foil, DT-1350's resonance frequency is located in the mid-range, just like that of IEMs/earbuds. Although an amplifier is required to yield sufficient volume level, utilizing DT-1350's own impedance characteristic is perhaps useful to increase the mid-range vocal, which is slightly lacking in stock configuration, by 1 dB at most.