by P. Milton
Stereo and earphones have been with us for a considerable time. The earliest account of a binaural sound experiment I have found was by E. Hospitalier in L'Electricien in 1881, which described the sound system installed in the Paris Opera House. A pair of microphones was used, one at each side of the stage, supplying a number of pairs to telephone receivers.
Apparently, the listeners received some degree of localization, even if the sound quality was not sensational.
It was a short step to providing the earpieces with a head band, but for the next 70 years or so, headphones were relegated to communications and were only required to provide a side-tone for CW reception, or at most, intelligible speech.
The situation might have remained as it was, but for an enterprising young man named John Koss, who saw the possibilities for private listening, thus founding an industry.
He described the beginnings of stereophones like this: "When I left the service in 1952, I opened a small business in Milwaukee, leasing television sets to hospitals. It was fairly successful, by the standards of being able to play golf and have dinner out with the family occasionally, but after four or five years I became involved with Martin Lange, a wizard in electronics who talked me into the tube checker business.
"Dealing with servicemen and distributors was not exciting enough for me, and at that time we saw that the stereo craze was getting under way, and the big names of the high fidelity business, like Harman-Kardon, Scott, and Fisher, were entering a new phase We thought that there was something that we might get into. It gave us a good excuse to go out and buy some hi-fi equipment, and I appreciated that.
"We bought a changer, added some electronics, and put some side wing speakers on it. It was good, but I thought that we should have some sort of gimmick. We could see, even then, that other products would try to dominate the small amount of room available in the home, and we would be competing with television and the electronic organ, so I thought that we should provide some sort of private listening arrangement. Perhaps we could have a plug at the back for a set of headphones.
"The headphones at that time were all communications types, and when we tried a set, we found that the concept was good, but the sound was terrible. We kept listening and trying to figure out why we could not change the electronics to make the phones sound right. After many attempts, we finally realized that the problem was with the headset and not the electronics.
"A lot of developments are the result of happy accidents.
In the corner of the room there happened to be a triangular box which had about 20 small speakers scattered all over the front to try to get the effect of a large woofer. My technical knowledge was limited to the fact that a woofer was big and a tweeter was small, but I could see that even those speakers were much bigger than the one-inch ones used in the headset.
I pointed to the box and said that it was too bad we couldn't have something the size of those.
"About half an hour later, the intercom rang and Martin called 'I've something for you to hear.' He had removed a pair of speakers, attached them to a headband, ripped the cushions from an old army headset, and glued them to the front. He had pasted cone-shaped pieces of cardboard to the back of the speakers and put the new headphones on my head.
"The difference was amazing. There were plenty of lows, and although plenty of work was needed, we had beaten the major problem. We managed to develop a set of headphones in time for the show in Wisconsin in 1958, and I started to sell the idea of private listening.
"Everybody thought the idea was super and wanted to buy phones to put them on their units. I very patiently explained that they were only available as a complete package. So they said 'Right, now how do we buy those phones and put them on our amplifiers?' The decision was easy. We saw that the world was full of record player companies, so our place was to be a stereophone company." How 'Phones Work Stereo headphones are sufficiently new to be in a state of rapid evolution. If we regard the actual drive system as a "black box," headphones can be divided into two main types, which have come to be called circumaural and supra-aural.
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Fig. 1--This telescoped view illustrates a conventional dynamic driver element
using a voice coil and molded diaphragm.
The circumaural types rely on a complete seal, in the form of a liquid- or foam-filled doughnut-shaped pad which surrounds the ear. The resonance of the moving system is placed fairly high, and at low frequencies the diaphragm produces pressure changes directly in the ear canal. The low-frequency performance can be extended to below 30 Hz, but it is very dependent on the efficiency of the seal. Long hair or spectacle frames can cause a drop in output below 200 Hz by between 10 and 20 dB. Completely sealed headphones isolate the listener and provide an acoustic environment in which all the sounds of reality, except those which are felt directly, are reproduced. The price to be paid is in extra weight and pressure around the ears, compared with the supra-aural or open-air types.
The present trend in design seems to be towards the lighter, open variety of headphones. The diaphragm is normally vented to the outside, via suitable acoustic damping, and is spaced from the ear by urethane foam pads, which provide a more or less controlled resistive leak around the ears, or soft cushions which sit on the ear and perform almost the same function as the completely sealed types. The acoustic cavity is different, but they have the advantage of avoiding leaks caused by spectacle frames.
If a complete seal is difficult to achieve, then the philosophy of "If you can't beat them, join them!" applies to the open-air types. The response falls off rapidly in the bass and corresponds to a velocity characteristic. The art of design, in this case, is to select the right diaphragm resonance frequency and to adjust the acoustical and electrical circuit so that the output is reasonably level down to between 100 and 200 Hz. Slightly less bass than would be normally required with speakers seems to be acceptable with headphones, and this tends to be reinforced by current preferences for a tight and well controlled low end in speaker design, compared with the boom boxes which were popular just a few years ago.
The original stereo headphones used small loudspeakers as driving elements, and these are still used in the very cheapest units. The miniature speaker works well, particularly if it is properly sealed, to preserve the bass response, but for more accurate sound, a stiffer and more controllable diaphragm is necessary.
The Koss dynamic headphones use a molded-dome diaphragm, similar to a dome tweeter in shape, which is driven at its edge by a relatively large voice coil. The outer suspension is a single outward-going roll, molded integrally with the dome, which adds axial flexibility and some measure of extra radiating area. The diaphragm assembly is self-supporting and unlike a speaker, which is supported at the edge and at the center, is mounted at the edge only and allows the coil to move freely in the magnetic structure.
Good headphone design does not end at the driver. Comfort and weight also play their part in the acceptability of the product. It is little use having perfect reproduction, if it can only be heard for short periods. Figure 1 shows the basic construction of the dynamic driver from the latest example of the art, the Pro 4-AAA from Koss.
ES-Type 'Phones
Electrostatic headphones have the attractive advantage that the mass of the diaphragm is comparable to that of the air next to it, and such a diaphragm presents a much better acoustic match than the relatively massive diaphragms of the dynamic types. The electrostatic charge is evenly distributed across the diaphragm, and the movement is essentially that of the electric charge and is essentially linear.
If a single-sided electrostatic transducer were used, the force acting on the diaphragm would be proportional to the spacing between the electrodes and second harmonic distortion would be produced. The problem is overcome by using a push-pull arrangement. A large, constant charge is placed on the diaphragm via a very long charging-time constant, and the audio signal is placed on perforated plates on either side of it. Under these conditions, the force acting on the diaphragm is determined only by the magnitude of the charge on it and the signal appearing across the plates. An exploded view of an electrostatic driver appears in Fig. 2.
The uniform drive and superb transient response of electrostatic headphones make them a natural alternative to dynamic types, and nearly all of the best available headphones are electrostatics. They can be light, comfortable, and mercilessly accurate, but the necessity for an outboard power unit tends to make them substantially more expensive and not quite as easy to use as other types.
Fig. 2-An ultra-thin membrane provides the diaphragm within the driver of
an electrostatic 'phone. ELECTROSTATIC TYPE STEREOPHONE
Fig. 3-Construction of the Yamaha Orthodynamic headphone driver.
Other Types
Yamaha has combined the advantage of the low impedance of a dynamic design with the even drive of an electrostatic diaphragm in their "Orthodynamic" driver. The heart of the arrangement is an extremely thin (12 micron, or 0.00047 inch) corrugated-polyester diaphragm, with an integral, spirally wound coil. The coil is divided into sections, each wound in alternate directions, and the assembly is mounted between two waffle-shaped, sintered magnets (Fig. 3). The magnets are pierced so that sound can pass and are magnetized in annular segments, corresponding to the sections of the coil. This forces the lines of flux in a radial direction along the surface of the diaphragm, thus ensuring an even drive. The headphone cup is vented around the edge to reduce the stiffness of the assembly, and isolation and damping are provided by pads of felt and urethane foam.
The British physicist Oliver Heaviside did not own a pair of electrostatic headphones, but he too was concerned about the impermanence of an electrostatic charge and the necessity to provide a continuous polarizing field. In 1885, he theorized that if a magnet produced a magnetic field, then an electrostatic field should be produced by an electret.
Although Heaviside coined the word, it was not until 1919 that the first electret was made, more or less accidentally, by Mototaro Eguchi who was examining the electrical conductivity of cooling oils and waxes. He had poured a mixture of carnauba wax and a resin into a shallow, foil-lined dish and had hung a metal disc on silk cords just above it. A field of 10 kV/cm was applied while the wax was cooling, and a negative charge was measured on the surface connected to the positive 'electrode when the polarizing field was removed.
After a few days, the charge disappeared and was replaced by a permanent charge in the opposite direction. Eguchi proved that this was a volume effect by slicing the wax into separate polarized layers and showed that this was similar to breaking a permanent magnet into sections.
The first U.S. patent for an electret microphone was granted in 1935, but the first practical electret microphones were made in Japan during World War II. The chief enemies of the stability of an electret are humidity and high temperature, and until recently, the shelf life was limited to two or three years. The most promising of the new materials are the fluorocarbons, and accelerated life tests at high temperatures indicate that under normal conditions, an electret can last about 500 years before the sensitivity will decrease by 1 dB. Most electret microphones and headphones use a permanently polarized diaphragm, but the mechanical characteristics of the material and the amount of charge available do not always give optimum results. Toshiba's solution to the problem is a very low-mass, metallized polyester diaphragm, which is stretched between two perforated electret elements.
Fig. 4--Construction of the Toshiba HR-71 0/8101910 series of electret headphones.
Unit Assembly Case Structure (for HR-710)
Diaphragm Unit Structure (2.5mm thick)
1. Diaphragm Membrane (HR-710; 4 microns, HR-810(910; 2.5 microns thick)
The outer frame is the diaphragm membrane ring (0.6mm thick)
2. Electrets (Back Electret Assembly)
3. Anti-Condensation Membrane (2.5 microns thick)
The outer frame is the anti-condensation membrane ring (0.3mm thick)
4. Ring (0.3mm thick)
5. Pressure Plate (ABS plastic 1mm thick)
6. Unit Assembly Case
The system operates in push-pull mode, and by separating the functions of electret and diaphragm, Toshiba has been able to maximize the efficiency of the design and reduce the matching requirements to a small transformer incorporated in the plug (Fig. 4).
Piezo Headphones
Fig. 5--Above, when a voltage is applied across the plates (Z axis), the piezo-electric
element moves along the direction of the X axis. Below, when an a.c. voltage
is applied to the aluminized surfaces, the piezo-electric material expands
and contracts.
Since the edges are clamped, the element is forced to move.
Fig. 6--Cut-away drawing of the Pioneer SE-500 high-polymer piezo-electric
headphone.
There is a whole range of substances which produce electricity when subjected to strain. The earliest known substance was quartz, which generated a voltage across the faces of the crystal when the crystal lattice was deformed. The reverse effect also holds, and a quartz crystal expands and contracts when a voltage is applied across certain faces of the crystal.
The effect is called piezo-electricity and is not confined to crystalline substances. For the sake of those who are new to the word, piezo is pronounced with three syllables-pi as in apple pie, ez as in easy-piezo.
Recently, a new piezo-electric substance was developed by Pioneer, and it has a natural application for headphones, microphones, and loudspeakers. The substance, vinyldene flouride, is a high-molecular-weight polymer which has 10 times the strain constant and a twentieth of the stiffness of quartz. The production of piezo-electric vinyldene chloride is similar to that of producing piezo-electric ceramics. The material is stretched to about four times its original length, and aluminum is vapor deposited on both sides. A high d.c.voltage is applied to the faces of the film, in a similar manner to charging a capacitor, and a residual piezo-electric property remains after the d.c. is removed. If two films are bonded together back to back, one will expand as the other contracts and the entire structure will bend. This arrangement is called a bimorph element.
The contraction of the film takes place at right angles to the direction of the voltage, so that if the signal is applied between the aluminized faces, the contraction will take place along its length in the case of a strip or along the radius of a disc.
In the case of the Pioneer headphones, the diaphragm is bowed slightly and clamped at the edges. When the material changes dimensions under the influence of the signal, the length of the arc changes, and the center is forced to move backwards and forwards. Figure 5 illustrates the movement of the diaphragm; the complete assembly consists of a perforated suspension board supporting the diaphragm and a urethane damping pad (Fig. 6).
Why They're Not "Flat"
The private world of headphones presents an entirely different acoustic environment from a normal listening room.
We expect to see amplifiers with an absolutely flat response from d.c. to light and demand that our speakers produce straight lines across the graph with no phase deviations. We acknowledge the effects of the room and are doing something about it, yet although the direct coupling between the linear movement of the headphone driver and the ear canal should remove most of the problems associated with listening to speakers, when we hear a set of headphones with a flat response, they sound terrible.
Headphones present problems of their own, and to see why, let us look at the "hearing system" of Fig. 7. The loudspeaker is on one side of the room, and the sound produced takes an infinite number of routes to reach the ear. It travels directly, and it will reach the ear from almost any direction via multiple reflections from the surrounding walls, and is thus both delayed and changed in frequency response.
Fig. 7--Block diagram of the "hearing system" designed by Mother
Nature.
Fig. 8--Transformation of sound pressure level from free field to eardrum
(after Shaw).
The external ear, or pinna, also has an effect on the sound.
Those complicated flaps and folds have distinct resonances which are excited to a greater or lesser degree, depending on the direction of the sound and alter the frequency characteristic before the sound reaches the eardrum. Figure 8 shows the effect of the external ear on the sound pressure at the eardrum compared with the free field. The acoustic gain increases rapidly from about 1500 Hz and reaches a maximum of 17 dB when the sound is coming from the front. Sounds coming from 45° are increased by 21 dB and towards the rear the gain falls, but even sounds from directly behind you do not go below +6 dB. This huge increase is due to the primary resonance of the pinna at 2600 Hz and is maintained up to about 5 kHz by the resonance of the concha, which is the funnel-shaped area just in front of the ear canal. The curve shows a dip in the response to sounds coming from the 135° direction at 4.5 kHz, caused by diffraction at the edge of the external ear.
The lack of symmetry in the directional pattern plays an important part in sound localization, particularly in distinguishing between front and back.
Our ears are all different, and it is reasonable to expect wide variations between individuals. This being so, it would seem futile to have perfectly linear sound equipment, since we all hear differently. This, of course, is wrong. We listen with the same pair of ears all the time, and all our judgments regarding the accuracy of sound are done through them.
A loudspeaker is only required to produce an accurate sound field at the listener's ears. The rest of the "equipment" will be constant.
Figure 9 shows the situation when headphones are worn, and note that two important elements are missing-the room and the external ear. Suitable electronics could simulate the reflections caused by the room, but we are still left with the task of compensating for the response of the ear. The question is "What should the frequency response of the ideal headphone be like?" There have been two ways to measure headphones. The simple approach for the circumaural types is to use a flat plate coupler. As its name suggests, the microphone is mounted flush with the surface of a board and the headphone is placed over it. No account is taken of the volume taken up by the ear or of the texture of the board. It is simply a method of comparison and superimposes its own series of resonances on the correct response.
The second method is a bit more scientific. There is in the audiometric branch of the art a standard 6-cc coupler. The microphone sits at the bottom of a cavity of this capacity and the earphones, usually supra-aural, sit over it. Audio-metric earphones are standardized, and an enormous amount of data has been collected regarding the complete performance at discrete frequencies. In spite of the dignity imparted by the work done by distinguished scientists, the 6-cc coupler is not a really good tool for measuring hi-fi phones. It gives repeatable measurements, but the best we can say is "These sound right to me, and this is the response obtained with the coupler. Other headphones with the same response should therefore sound the same." The coupler itself does not confirm the accuracy of the headphones, all it does is to act as a transfer standard. What we require is a measuring system which has the same physical shape and acoustical performance of a real human ear. It would be sufficient to have a replica of a human ear which is essentially complete, except for the eardrum and ear canal.
Fig. 9--Block diagram of the "hearing system" designed by John Koss,
et al.
Fig. 10--Envelope of transformation of sound pressure at entrance of blocked
ear canal for various angles, average of 10 subjects (after Shaw).
Dr. A.G. Shaw, of the National Research Council in Ottawa, Canada, has done extensive research on the performance of the external ear, and Fig. 10 shows the average response of 10 subjects to sounds from various directions when the ear canal is blocked by a specially fitted plug. Since we receive sounds coming from all directions, we could say that these curves would form an envelope for an "ideal" response taken with an artificial ear with a microphone placed at the entrance to the canal. There are still variations in performance which are due to differences in sealing and different positions, but at least we now have access to an "idealized" response envelope with which to compare the performance of real headphones under normal operating conditions. Now that we are on the threshold of producing the "perfect" headphone, what next? Why not put a dummy head in the best seat in the house and place microphones where the ears should be? This is not exactly a new idea. It was first demonstrated by Alexander Graham Bell at the Columbia Exposition in 1892 in Chicago and has been resurrected every few years since then. There is an attractive simplicity about the whole concept. By using a model head, all the time delays and phase relationships between the ears would be preserved. Unfortunately, the practice falls down on several counts. The first is that no two ears are alike, so the directional clues which are associated with the world around us will not conform well with our own experience. The second is that if the ears on the dummy head were acoustically the same as real ears, then with "ideal" headphones, the modification of the frequency response would be doubled.
Either we should resort to probe-type headphones, or a relatively complicated network should be used for kundstkopf or dummy-head recordings. The kundstkopf technique does to a certain extent overcome the primary objection to headphone listening that the action is all in one's head. There is a side-to-side impression and things happen to the rear, but there is very little, if anything, happening in front. Even with the dummy head, circular space seems to be deformed into a bent figure eight, with very little front-to-back resolution. For this reason, the technique is better suited to plays, where verbal clues and more or less familiar surroundings augment the purely audible directional cues.
Work is going on to overcome the final obstacle to truly realistic sound when wearing headphones-getting the sound outside the head. Benjamin Bauer suggested his cross-feed circuit, which reduced the effects of absolute isolation between the ears and produced a delay effect at low frequencies.
Recently, workers at the Matsushita Electric Co. have proved that the ratio of direct-to-indirect sound is important when producing "out of the head" localization and have even produced the effect using mono recordings by creating delayed indirect signals electronically and mixing them with the original recording. They have produced a new system for listening to stereo, based on the idealized frequency characteristics of sounds reaching the ear from 30 degrees and using a bucket-brigade reflection generator, although this is not yet available in the United States. For those who wish to try the idea, a suitable circuit is described by Dr. M.V. Thomas in the July/August, 1977, /our. of the A.E.S. Headphones are getting better, lighter and more comfortable almost by the day. Soon, it seems, the last reasons for objecting to the sensation of listening to inner space will disappear, and we will be able to choose the aural perspective we desire.
Koss' Doc Severinson need no longer stand with his back to the orchestra!
References
1. P.R. Milton, "Interview with John Koss," Audio Market News, Canada, Vol. 3, No. 8.
2. F.V. Hunt, Electroacoustics, Harvard Univ. Press, Cambridge, Mass., 1954.
3. E.A.G. Shaw, "The External Ear: New Knowledge," invited paper, Vllth Danavox Symposium, Gl. Avernaes, Denmark, Aug. 27-31, 1975.
4. F.E. Toole, "Measuring Headphones," AudioScene Canada, Aug., 1976; Vol. 13, No. 8.
5. N. Sakamoto, T. Gotoh, and Y. Kimura, "On 'Out-of-Head Localization' in Headphone Listening," J. Audio Eng. Soc., Nov., 1976; Vol. 24, No. 9.
6. M.V. Thomas, "Improving the Stereo Headphone Image," J. Audio Eng. Soc., July/Aug., 1977; Vol. 25, Nos. 7/8.
(Adapted from: Audio magazine, May 1978)
Also see:
Build a Headphone Crossfeed Circuit (May 1980)
Headphones: As Close as You Can Get (Apr. 1991)
Headphones around the house (May 1974)
Illusions for Stereo Headphones (Mar. 1987)
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