TOKYO -- Once a conversation limited to radio and audio recording professionals, headphone quality has become an important factor for ordinary music fans as high-resolution audio content becomes more accessible. This has led to a plethora of high-priced headphones hitting the market.
In pursuit of the perfect sound, audio equipment manufacturers are looking beyond conventional materials for the key components in their headphones.
Headphones can be classified into several types according to their structures. By far the most common for personal use are called dynamic headphones, which essentially work like miniature speakers. They receive music data in the form of electrical signals which vibrate a component called a diaphragm to produce sound.
The diaphragm is the most crucial component in terms of sound quality. To reproduce clear sound from high-resolution audio sources, the diaphragm has to be made of a material that has two key qualities.
Firstly, certain materials produce characteristic sounds -- materials that reverberate excessively produce a tinny or scratchy sound, while those that dampen the vibrations make a dull sound. For a clear sound, the diaphragm material needs to strike the right balance between the two.
Secondly, in order to reproduce the depth of the sound, the material needs to be able to transmit sound at extremely high speeds.
Traditionally, aluminum and plastic film have been used. These materials, however, have drawbacks -- aluminum is prone to lingering metallic noise, while film transmits more slowly.
Manufacturers are in constant competition for that elusive breakthrough that can both retain a clear, lively sound, while maintaining a speed that allows it to carry.
JVC Kenwood stepped away from advanced, composite materials and focused on wood. Last December, the company launched two high-end models of headphones that use diaphragms made of birchwood, retailing for approximately $555. Wood, a material whose acoustic properties have been harnessed for musical instruments for millennia, meets the two conditions perfectly.
The company has sold in-ear style earphones containing wooden diaphragms since 2008 and the products are renowned for high sound quality. The development of supra-aural headphones, however, faced a significant hurdle. Headphones require larger components, which made it difficult to control the minute movements of the diaphragm.
Innovation allowing wooden sheet to be cut much thinner solved the problem. Birchwood is processed into sheets 80 microns (0.08mm) thick, which are then formed into a specified shape for the diaphragm. The company has succeeded in reducing the thickness to 50 microns by improving processing methods such as the cutting angle. Both headphones and earphones using wooden diaphragms are renowned for their rich timbre and deep bass sounds.
Onkyo has taken an altogether different approach, becoming the first audio equipment maker to develop a magnesium-foil diaphragm. More importantly, the central and peripheral parts are made up of one piece. Magnesium is less prone to tinny sound than many other metals and is also very rigid and lighter than aluminum and titanium. But despite these advantages, it is difficult to process the metal in small enough dimensions.
Onkyo drew on its experience of using magnesium in loudspeaker diaphragms, and succeeded in processing the uneven sections joining the diaphragm to the driving component.
At certain frequency bands, magnesium diaphragms had a habit of producing unnatural sounds, but the problem has been eliminated by the new design, according to the company.
Onkyo has also developed a diaphragm using cellulose nanofibers -- ultrafine plant fibers produced by unraveling the bundles of microscopic fibers in wood pulp. They are only 3-4 nanometers wide, or about one ten-thousandth of the width of human hair. Their weight is one-fifth that of steel but they are five times stronger. The material can deliver clean, well-balanced audio and keeps unwanted noise to a minimum, producing a characteristic soft sound.
Onkyo aims to market upscale headphones using the material later this year at a price of over 100,000 yen ($922).
The company also plans to supply modules to other manufacturers on an original-equipment-manufacturing basis.
The headphone market has a tendency to shrink on a unit basis, but is expanding in value due to the growing popularity of upscale models. According to Grand View Research, the global market for earphones and headphones will reach $17.55 billion by 2022. While many consumers are happy to make do with the earphones that come with smartphones, a growing number want to enjoy high-resolution music with the best possible sound quality. This polarization means the market for upscale audio equipment is likely to continue growing.