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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Head phones of spatial sounding. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers The recent development of the German engineer F.M. Keninga - head phones of the HFI series - so interested our reader that he decided to test the effectiveness of the new design by prototyping. This is what is discussed in the article. Today's attitude towards headsets (HT) does not seem to give cause for concern to either manufacturers or consumers. Every year, along with "unsinkable" models like the "Sennheiser HD-580"1, new designs appear with a lighter and more detailed sound. Meanwhile, since the 60s, that is, since the beginning of the widespread use of stereophonic headphones, the effect of sound localization inside the head has been noticed. However, if the sound transmission is a kind of convention, then the GTs only aggravate it somewhat, with which they simply got used to for 30 years and almost stopped noticing, just as they did not notice the hiss of shellac records before. Another little-known fact is closely connected with this effect - the very unimportant real frequency response of the GT. The existing technique for monitoring GT parameters is based on measuring the sound pressure generated by a telephone (or just a capsule) loaded on the so-called "artificial ear", which has nothing to do with the natural ear, except for the name2. When a phone tuned in this way is loaded on the natural auditory canal, little remains of the adjusted frequency response - peaks and dips are formed with a swing of up to 20 dB. This is not at all like the usual picture - the perception of natural sound or listening to music through loudspeakers: due to the directional properties of the auricle, the outer ear is a fairly linear system. The final correction takes place in the brain, "tuned" just to the individual characteristics of the listener. It is not surprising that the exclusion of any link from the chain of hearing organs leads to a mismatch of a complex and precise system. - production of recordings using an "artificial head" that imitates a spatial panorama well, or the creation of a complex digital model of the functions of the outer ear, but with mandatory frequency correction in both cases (and individual!). It is clear why these methods have not found serious application: they are expensive and inconvenient to use; recording on an "artificial head" is incompatible with the dominant multitrack and only limitedly compatible with the two-microphone recording technique, which is rare in practice. The new HR series headphones are unrivaled in terms of simplicity and efficiency of the solution [1]. This, of course, is not the first attempt to "include" the auricle in the tract - GTs with "spacious" ear pads and capsules shifted forward are known (Koss Pro4, etc.). However, it turned out that only an additional displacement of the capsule downwards makes it possible to uniformly "sound" the entire surface of the auricle, bringing the listening conditions closer to natural ones [2].
On fig. Figure 1 shows the results of sound pressure measurements directly in the auditory channel, obtained when testing HFI-100 stereo phones with high quality capsules, typical of good GT models. Capsules in all cases are located below the auditory canal, and different curves correspond to different angles of transducer displacement forward relative to the vertical axis [1]. Several conclusions immediately arise:
Two models of stereo phones of the HFI series were presented by the developer at a conference of sound engineers (Karlsruhe, 1992), and the results of a survey of hundreds of experts are quite remarkable. A convincing expansion of space was noted by 81% of experts; a distinct frontal panorama - 39%; a feeling of remoteness of apparent sound sources (SIS) - 62% of respondents. In general, these results should be approached with caution, if only because it is difficult to conduct a pure "blind" testing of GT, nevertheless the result is impressive: 98% of professionals noted positive differences between phones. The offset position of the pair of capsules in the GT makes it easy to modify them for a four-channel Dolby Surround Sound system. The corresponding design of HFI-3D phones (3 Dimensionen - three dimensions) is considered in [1] in fig. 1. The idea of such a scheme seems to lie on the surface, but the creation of a surround sound panorama in the limited space of ear cushions is far from obvious, as evidenced by the rise and fall of the popularity of quad phones in the 70s. The peculiarity of this design is that additional capsules are also installed taking into account the acoustic features of the auricle - not only behind, but also above, approximately symmetrically to the main ones with respect to the auditory canal, which makes it possible to obtain a much more convincing picture, moreover, three-dimensional, in the entire upper hemisphere.
On fig. Figure 2 shows the timing diagrams of the passage of an impulse signal in all four capsules of phones connected to Dolby Surround Pro Logic amplifiers. The signal goes directly to the capsules of the front channels without delay, and then (with a delay of up to tens of milliseconds) "echoes" appear in the rear channels, well simulating reflections from the walls and ceiling of some conditional room. Sounds seem to come, depending on the amplitude-time relationship, from behind, then from above, etc., and the overall effect is very close in perception to a Home Theater system with loudspeakers. The fundamental simplicity of the considered designs allows a wide range of radio amateurs to manufacture such telephones on their own and opens up scope for further experiments, for example, with isodynamic telephones, with various types of diaphragms to obtain a point or directional source, etc. The model of stereophonic GTs with displaced radiators made by the author consists of mini-phones "H-23S-1" (included in the package of the tape recorder-prefix "Vega MP-122S") and large ear cushions that do not deform the auricle. From the inside, they are pasted over with a layer of felt, which serves as a damper and sound absorber. The optimal amount of additional sound absorber is determined experimentally. In the cases, two conical holes are cut out with an outer diameter of 30 and an inner diameter of 27 mm, to which foam rubber ear pads from mini-phones are pressed. The centers of the holes are shifted forward and downward at an angle of 40° to the vertical by 2 cm; the amount of displacement has been found to be proportional to the depth of the ear cups. "Tuning" is reduced to finding the optimal position of the emitters, which is easy to do by ear, since the main effects are expressed very clearly. Of course, one should not expect a miracle from such simple converters: the real illusory panorama is absent or unstable. However, the sound really "leaves the head" beyond the base of the GT and forward, and the well-known effect in acoustics of the rise up of the KIZ located in the middle is significantly weakened. Instead of the trivial "right - left", a clear azimuth of the CIZ appears (and even some echoing spatiality), but the depth of the location of the CIZ, unfortunately, is difficult to determine and the space is rather compressed. Apparently, this is due to the low quality of capsules, "binding" the sound. The more unexpected is the metamorphosis of sound quality. First of all, the expansion of the frequency range is noticeable, especially in the low-frequency region, as well as the lack of sharpness. The sound for such cheap capsules is surprisingly soft, light and deep. The most promising is the following. As it turned out, listening to recordings through such GTs for a long time does not "deafen" and does not tire at all. Aesthetically, this means the absence of monotony, and in addition, the possibilities of using such GTs are expanding, say, in the field of communications or with constant monitoring of recordings on tape recorders with a through channel, and in general wherever you have to sit in headphones for hours. In sound engineering, in addition, mixing is greatly facilitated, since the picture created by such GTs is much closer to the "monitor" one. However, the manufacture of professional audio equipment is beyond the capabilities of most radio amateurs. The author has already received an unfavorable response to the idea of refining mini-phones according to fig. 2 in [1], and a few words need to be said about this. With such refinement, one should proceed from the design features. So, in the same model "N-23S-1" sound is emitted through a series of holes located along the circumference, and it is pointless to simply close part of them. Therefore, for the plug to work effectively, it must be placed on top of the foam ear pads. Blanks with a diameter of 38 mm are cut out of felt and glued to the foam rubber along the outer semicircular edge. In another, simpler and more successful (in the opinion of the author) version, the same semicircular blanks, but without a central cutout, are bent into a cone and tucked under the ear pads. When converting mini-phones with solid foam ear cushions, a central hole with a diameter of about 2 cm is cut out in the latter. In both versions, the radiating holes are open and the conditions for sound propagation from bottom to top are preserved, which ensures the operation of the structure. The author did not have the opportunity to optimize the GT, so the above effects are less pronounced, but even in this form GTs provide a very pleasant, thin and soft sound and relatively safe operation. And the real surprise was presented by the "combined" experiment: telephones with plugs, inserted into large ear pads, finally led to the desired effect of natural-sized space. At the same time, the panorama is rather deep than wide (without clear boundaries), but very stable and lively, with excellent aerial perspective. Undoubtedly, felt diaphragms have weakened the "binding" of QIZ to phones. The sound quality is very good, considering that the layout is assembled from improvised components. In all cases, we are talking, of course, about a relative improvement in sound; high results can only be obtained with high quality capsules. It is especially valuable that the greatest effect is achieved at very moderate volume levels, corresponding to a significant remoteness of the ICD, while intelligibility and tonal balance do not suffer. Therefore, such GTs simply force the consumer to protect hearing organs. Recall, however, that even modified GTs should not be used in transport, especially in the subway, where the noise level reaches 90 dB (it is better to wear headphones without a player there). As for the "gag" phones, in their current form, in my opinion, they should be recognized as unacceptable both aesthetically and, especially, medically, and banned from import and sale. Hi-Fi Music expert E-Gould, in his review of the latest GTs, remarked: "...You've probably already seen from your own experience that there are practically no good ... headphones." The thought is true, but is it for all time? We hope that radio amateurs will soon prove the opposite. Notes
Literature
Author: R. Kunafin, Moscow
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