By Alan Trevena
(Audio Engineering Society, Chairman of the Technical Committee on Loudspeakers and Headphones)
The Audio Engineering Society (AES) Technical Council has been in place since 1979. Its Technical Committee on Loudspeakers and Headphones has existed in its current form for more than 20 years and comprises 75 members worldwide. With their mix of academic and professional backgrounds, members provide expertise in transducer design, modeling, and measurement for applications across the audio industry. These committee members set the standards and help shape the audio field for future generations.
Loudspeakers remain at the heart of almost every audio system and provide the link between technology and the art of music. Essentially, loudspeakers convert an electrical audio signal into a corresponding sound. The first crude loudspeakers were invented during the development of telephone systems in the late 1800s, but electronic amplification via vacuum tubes began around 1912, making loudspeakers practical. By the 1920s, they were used in radios, phonographs, public address systems, and theater sound systems.
In the last decade, the loudspeaker industry has continued to expand exponentially, in part due to the recent growth in headphone sales and the increase in personal devices (e.g., smartphones and tablets). With such devices, there is more focus than ever on small and micro speakers and increasingly innovative designs to produce big sound from tiny devices.
The recent growth in headphones and personal consumer devices has led to increased use of small and micro speakers. This provides unique challenges to the system designer. While overall product form factor is shrinking, consumer expectation on audio quality (specifically low-frequency extension and loudness) has not changed. The focus on improving low-frequency extension in micro speakers is leading to new approaches (e.g., a “hybrid” diaphragm with complex compliance across the surface).
|This 40-mm neodymium high-output headphone element contains a neodymium magnet and low-mass mylar diaphragm combined to deliver high sensitivity and extended high-frequency response. (Photo courtesy of Parts Express)|
Factors Affecting the Industry
A major loudspeaker component is the driver and the material used to make the driver. Modern driver magnets are almost always permanent and made of ceramic, ferrite, Alnico, or rare earth materials such as neodymium and samarium cobalt. A trend in design—due to increases in transportation costs and a desire for smaller, lighter devices (as in many home theater multi-speaker installations)—is the use of the rare earth materials instead of heavier ferrite types.
When high field-strength permanent magnets became available, alnico, an alloy of aluminum, nickel, and cobalt became popular. Since it dispensed with the power supply problems of field-coil drivers, alnico was used almost exclusively until about 1980.
After 1980, most driver manufacturers switched from Alnico to ferrite magnets, which are made from a mix of ceramic clay and fine particles of barium or strontium ferrite. The size and type of magnet and details of the magnetic circuit differ, depending on design goals.
|Audeze EL-8 headphones are designed by BMWDesignWorks USA and available in open- and closed-back models, wood veneer accents, and features advanced planar magnetic technology. (Photo courtesy of Audeze)|
Then manufacturers switched to neodymium magnets, which have become the most widely used type of rare-earth magnet, replacing alnico and ferrite magnets in many modern technology applications where strong permanent magnets are required—including loudspeakers and headphones—because their greater strength allows the use of smaller, lighter magnets for a given application.
Following the spike in neodymium and dysprosium prices in 2011/2012, these key materials hit peaks of 800% and 2,200% of their 2008 prices, respectively. While dysprosium remains elevated over its mid-2000’s price, the market appears to have stabilized. This is, in part, as a result of additional mines coming online outside of China, but also the focus given to the China market and export quote process by the World Bank. This intervention gives hope that the cost of these essential materials will remain somewhat more stable. As a result, the trend to redesign neodymium-free drivers has started to subside as a result.
In general terms, signal processing is an enabling technology that encompasses the fundamental theory, applications, algorithms, and implementations of processing or transferring information found in many different formats. Digital signal processing (DSP) is the processing of digitized discrete-time sampled signals by computers or by digital circuits such as ASICs, field-programmable gate arrays (FPGAs) or specialized DSP chips. Improvements, and the growing prevalence of DSP, in consumer audio systems means the relationship between the driver and the system is changing. Excursion at low frequencies and performance at high output levels are digitally managed. Frequency response irregularities are smoothed with ease using simple DSP tools. Almost all consumer products now include built-in digital signal processing to optimize the acoustic system.
To Stay Informed
Numerous papers on loudspeakers and headphones appear in the Journal of the Audio Engineering Society (JAES) and are presented at AES conventions each year. There have been several dedicated conferences on both loudspeakers and headphones.
|The OPPO Digital planar magnetic driver generates sound by a thin, light diaphragm whose entire surface area is evenly driven. (Photo courtesy of OPPO Digital)|
The next one, “Headphone Technologies,” is being planned for fall 2016, to be held in Denmark. The conference targets scientists, developers, and practitioners involved in headphone technologies independent from having their personal focus in materials, electro acoustics, signal processing, or perceptual evaluation. Moreover, the conference will put a special emphasis on the emerging fields of mobile spatial audio and augmented reality (i.e., applications and solutions for headphones with sensors). For more information, visit www.aes.org. LIS