In its August and September 2020 editions, Voice Coil magazine featured a two-part article by Igor Levitsky on The Advantages of Thin-Film High-Frequency Transducers. Part 1 and Part 2 of the articles are now available online. As an experienced speaker designer, Igor Levitsky details in the article the different design approaches for this speaker topology.
Igor Levitsky is highly recognized in the audio industry for developing some of the best planar magnetic speaker and headphone drivers for some of the most reputed brands, focusing precisely on this type of technology. Igor worked with Radian Audio Engineering in the development of the latest generation LT Series of planar ribbon drivers, reflecting the latest manufacturing techniques and materials.
Over the years, the audio community has acknowledged that speakers based on drivers with thin-film diaphragms deliver exceptional sound quality in mid- and high-frequency ranges. Many well-known loudspeakers from such brands as Infinity, Genesis, Apogee, Magnepan, Quad, KLH, MartinLogan, ESS, BG Radia, Wisdom Audio, SLS Loudspeakers, Adam Audio, Alcons, Philips, Foster, Sonigistix/Monsoon/Level 9/Eastech, and others have been praised for superior audio quality in mid- and high-frequency bands. Similar transducer technology has proliferated over the past 10 to 12 years in headphones where brands such as Audeze, OPPO, Hifiman, Meze, Mr. Speakers, Stax, and Sonoma have taken leading positions in the high-performance headphone market. Even Sennheiser, a juggernaut of headphones, when its engineers were tasked with developing the ultimate headphone, chose a thin-film electrostatic concept and developed its legendary flagship Orpheus, avoiding using any kind of voice coil-based drivers.
Thin Diaphragms
The key mechanism in thin-film drivers is a very thin diaphragm made from polymer film/aluminum foil laminate (or just pure aluminum foil ribbon) that is uniformly driven more or less by either electrostatic (based on forces acting on electric charge in electric field) or electro-magnetic (using Lorentz force on a current-carrying conductor in a magnetic field) motor system. Such diaphragms have extremely low mass, comparable with the mass of air load, vibrating along with the diaphragm. Mechanical and electro-acoustical mechanisms and relationships in those drivers differ in some fundamental ways from typical common cone or dome drivers.
Traditional drivers are, in general, a very good universal solution for a wide range of applications that has been tested by time. The first cone-based electrodynamic driver was invented by Chester Rice and Edward Kellogg at General Electric in 1925, and in principle remains the same to this day. However, as almost always in audio, gains in one area require compromises in other areas. Thin-film drivers do not possess universality, they are very challenged when it comes to the reproduction of low-frequency signals. Some electrostatic and large panel planar designs lack ruggedness and are not suitable for certain applications. They may have limited dynamic range and often have compromised dispersion characteristics.
On the other hand, since thin-film drivers can compromise on ultimate universality and applicability for the most demanding pro applications or bass reproduction, they get the advantage of using different principles and design solutions. Thin-film transducers use different technology and materials that provide less distortion and, in many cases, enable them to achieve better sound quality than voice coil-based drivers, which have a “burden” to meet requirements in universality, extreme output levels, ease of mass production, low cost, and so forth.
The fundamental advantage that thin-film drivers possess over typical electrodynamic drivers is that they do not use round voice coils with multiple wire-turns immersed in a magnetic gap. Such motors tend to have high inductance and magnet systems that are not symmetric and are prone to magnetic hysteresis, Eddy currents, and a slew of nonlinearities associated with inductance modulation when voice coil dynamically moves in the magnetic gap.
The August 2020 edition of Voice Coil includes the first part of this article, with the second part published in the September 2020 edition. In the second part of the article, Igor Levitsky looks at the detailed design aspects of the Radian LT2/LT6 ribbon transducers and their specific advantages.
