by GARY STOCK
The Featherweights are this year's big news in the stereophone held. Typically, they weigh in at two ounces or less, sit directly upon the central portion of the pinna (the ear's fleshy outer structure), are supported by a minimal headband assembly, and operate off very modest amplifier powers due to their high-sensitivity and impedance (generally in the range of 50 ohms or so). In conjunction with portable stereo cassette players, they just might be the technical development that expands the popularity of headphone listening beyond the audio enthusiast and into the workaday world, simply by combining portability, credible performance, and comfort for the first time in a reasonably priced product.
The idea of a low-mass high-performance headphone is not entirely new, of course. Sennheiser's classic HD 414, introduced in 1968, weighed an unprecedentedly low 4 1/2 ounces and employed an innovative open capsule approach in order to eliminate large ear cushions. Among the first of the lightweights, it enjoyed sales of 2 1/2 mil lion units and went on to become one of the world's best selling headphones. Other open designs from Beyer, AKG, and Koss also became popular during the '60s and '70s. Most of these stereophones fell into the four to seven ounce weight range, and all were intended primarily for home and studio use.
(above) Because of diffraction effects by the head and resonances
in the cavities of the outer ear, a loudspeaker which measures flat in a free
field will have this curve when it is measured in the ear canal. (Data courtesy
AKG after Blauert, 1974; Shaw, 1966; Jahn, 1960, and Wiener, 1974.)
Then at last year's January Consumer Electronics Show, a handful of companies introduced portable stereo cassette systems that used extremely lightweight headphones. Sony called theirs the Soundabout, then changed their mind and began using the term Walkman, while Technidyne called their system the Hip Pocket Stereo. The entire category became the splashiest high-fidelity success story of 1980, prompting more than 15 companies to show Featherweights at this January's show, by the way, including several that use match box-sized microcassettes. It also dramatically demonstrated the performance potential of low-mass headphones to both casual listeners and audiophiles, and spurred many of the large, well-established manufacturers of conventional-sized headsets to reexamine the technical tenets of the low-mass approach. Suddenly, there is a flock of featherweight headphones to choose from, and a lot of people are coming around to the notion that the best examples of the type can hold their own with almost any full-sized stereophone.
There are a number of technical reasons for the astonishing sound of the Featherweights. To some extent, new materials technology is involved: Recently developed high-energy magnetic materials such as samarium cobalt--the same material that has increased the output efficiency of a number of phono cartridges and speaker system drivers--are used in most of the Featherweights. These materials provide enough sensitivity so that the designers can then exchange some of that efficiency for more linear frequency response and/or reduced distortion. Koss Corp.'s Dave Thomas comments that "You try for maximum element efficiency so that you can throw some of it away using mechanical damping in order to get smoother response." More sophisticated plastics have also contributed to better performance by reducing the level of mechanical resonances within the element diaphragm. Even the cabling to the amplifier is said to have been improved; Sony makes a big point of the oxygen-free copper lead-in cable used in their MDR-series featherweight phones, noting that the reduction in series resistance and stray inductive effects noticeably improve the system's response.
The most significant reason for the remarkable sound quality of the Featherweights, though, may simply have more to do with the way they interact with the outer ear and the ear canal--and therein lies an intriguing controversy long debated by headphone designers but rarely discussed in the audio press. It might well be called the Comb Filter Controversy, and the roots of the question go down to the most basic points about how human beings hear sounds.
There is universal agreement that no one actually hears a flat frequency response. In large group tests where participants were asked to make different frequencies sound equally loud to them by adjusting levels, and in other tests where miniature microphone capsules were placed at the entrance of the ear canal, the resultant curves demonstrated that the structure of the pinna and the presence of the head radically alter the frequency balance of what we hear. This introduces both a general curve shape like that shown in the accompanying figure and groups of sharp narrow dips--"suck-outs"--caused by the cavity resonances and mechanical bending of the pinna. These patterns of dips collectively comprise a "comb filter" (so called because of its characteristic comb-like pattern on a frequency response chart) which varies enormously from one per son to the next. The notably large ears of actor Clark Gable may well have heard a response curve that would vary by as much as 10 dB at certain frequencies from that heard by the petite ears of, say, actress Mia Farrow. All of us carry around an elaborate comb filter attached to the sides of our heads, and we hear every sound throughout our lifetimes processed through that filter. Yet be cause we learn to hear both "live" and recorded sounds through this filter, we come to view its effects upon what we hear as normal; removing or changing its effects will perforce sound unrealistic to us.
Recorded music heard through loudspeakers will sound natural if the loudspeaker response is fairly linear, since the ear processes the sound of the speaker just as it would any naturally occurring noise. With headphones, however, deriving a real-sounding response becomes considerably more difficult, since the mere presence of the ear piece near the ear radically changes the nature of the comb filtering. Headphone designers have three basic choices in the development of a stereophone, as far as controlling ear/headphone inter action is concerned. As shown in the adjacent figure, the earcup can surround the entire pinna, an arrangement called the circumaural approach; sit directly on the pinna's outer surface, the supra-aural approach, or fit within the entrance to the ear canal stethoscope-style, an arrangement rarely used in hi-fi designs which might be called intra-aural, though no specific term is used for this approach. Each of these approaches has its own virtues and drawbacks; it is these fairly arcane acoustical concepts, rather than the simplistic "comfort versus performance" questions usually raised, that are responsible for the numerous different headphone designs available today.
Circumaural headphones have the in disputable advantage of sealing against the side of the head, usually with a soft, heat-sensitive plastic cushion that molds itself to the head with the help of body heat. This, in turn, permits a more or less direct coupling between the driver element and the eardrum, and therefore bass response down very close to d.c.; Koss' Thomas notes that it is not difficult to reproduce 8 Hz in a well-sealed circumaural headphone. More important to headphone design theorists, however, is the notion that a circumaural headphone surrounds the entire pinna and thereby should leave the comb-filtering effects of the outer ear pretty much unchanged, and thus sound is fairly realistic.
-------- CIRCUMAURAL SUPRAAURAL INTRAAURAL
Supporters of the supra-aural approach concede the advantages in bass performance of sealed designs, but hold that the presence of a closed cavity adjacent to the pinna alters the severity and frequency of the dips that collectively make up the outer-ear comb filter. By moving the headphone capsule into the near-field (a fraction of a wavelength) of the ear, these engineers argue, reason able bass performance can be maintained, pressure on the tender outer ear reduced, and the comb-filtering effects of the pinna preserved, though in some what attenuated form due to the damping effect of the foam ear cushions on the pinna's many cavity resonances.
Sennheiser's Vice President of Engineering Horst Ankermann comments that "It is better in many cases to simply damp out some of the less predictable resonances than to allow them to be changed in frequency and then magnified by a sealed ear cup." The contribution of the Featherweight type to the stereophone design art, and the reason for its extraordinary performance, may be in its ability to sidestep--at reasonable cost and with reasonable compactness--the verified disadvantages of both circumaural and supra-aural design types. The exceptionally small capsules used in most of the Featherweights fit even closer to the ear canal than most conventional supraaural elements, thereby augmenting bass performance, and they may also exert far less of an effect on the pinna's various resonances than either the conventional closed-chamber circumaural phone or the resonance-damping large cushions of a typical supraaural unit. In this sense, most featherweight units have taken what might be called a "quasi-intraaural" approach that preserves some of the major comb-filtering influences of the outer ear along with very good bass performance and comfort.
The featherweight headphone is not the only direction headphone engineers are exploring, of course. AKG has made elaborate studies of comb-filtering effects in the development of their "free aire" K-240 and K-340 headphones, which use passive diaphragms and several chambers to simulate a series of comb filter resonances. Both the piezo ceramic diaphragm approach developed by Pioneer and the electret approach used by Audio-Technica and others rep resent new directions for the future. Substantial work on developing artificial heads (so-called "couplers") that will permit accurate testing of perceived headphone frequency response is underway through a number of sources [including Audio's own Equipment Profile program--Ed.], and headphones that sound better on a wider variety of real heads should be the result. The subjective listenability of the better Featherweights, despite their modest size and cost, however, will give stereophone de signers a good deal more to think about in their efforts to derive more realism from speakers that sit on your head.
(Source: Audio magazine, May 1981)
Also see:
Headphones: As Close as You Can Get (Apr. 1991)
Headphones: History and Measurement (May 1978)
Headphones around the house (May 1974)
Illusions for Stereo Headphones (Mar. 1987)
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