Segmented stator
There are also designs of electrostatic loudspeaker that use a segmented stator [2,10].
Rice. 9. Electrostatic acoustics with a segmented stator. Patent US4122302.
In this case, different sounds fed through the filter have their own segment. A wide radiation pattern is ensured by supplying the full range of the audio signal to the middle part of the structure, which has small geometric dimensions.
For mid frequencies, segments located at the edges of the structure are intended. High-frequency sound in this design is emitted by a very small segment. In this case, the recoil suffers significantly, since the sound pressure of the electrostatic emitter depends on the area of the film surface. As a result of this design flaw, a decrease in the graph of the amplitude-frequency characteristic is observed.
Plates with curved surface
Another design solution for a flat electrostatic panel is a complex of plates with a curved surface, through which sound waves are emitted. This set of plates forms a group of waveguides, which open at the output in the form of a lens at a given angle. This approach provides the desired radiation pattern in the high frequency range[7].
Rice. 7. Electrostatic acoustics with a curved surface. Patent US3668335.
However, due to the complicated design, lens production is quite labor-intensive. For high-quality execution, it is necessary to have complex technological equipment, as well as complex preliminary mathematical calculations.
↑ Background
This story didn’t start with headphones at all.
About 3 years ago I lived in a high-rise building in Dnepropetrovsk and listened to my laptop + S70 speakers with an EMOS system. The only thing I liked about them (and my neighbors didn't like it at all) was the bass. The middle and top merged into an indistinguishable mess, and this upset me incredibly. But I wanted something without hyperbass, but with a beautiful midrange and transparent highs. The built-in transistor amplifier was blamed for all the sins and, after reading articles about the delights of tube sound, I called my father and asked his opinion. He told me something like “Nothing difficult! Try 6P6S+6N9S. Do it, and then you’ll decide for yourself.” And he sent me some lamps and a couple of output transformers from old TVs.
In just a day, albeit on a piece of wood, I assembled this amplifier. But the miracle did not happen. The sensitivity of the S-70 in passive mode was so low that there was almost no sound. I remembered the 10GDSh speakers that had been in the closet for a long time. I connected them and... the miracle did not happen again. It became louder, and even cleaner, but it was far from the ideal that I had been waiting for.
In heavy thoughts, I wandered to the radio market. And there, one guy who had been selling lamps from time immemorial gave me the contacts of his friend, who invited me to visit to listen to the results of his work.
He assembled an amazing single-ended circuit using 300V tubes with huge trans outputs and speakers with full-range speakers. The amplifier, as he told me, produced 6 watts per channel. When he turned THIS up to a third of the volume, I just saw the light! Nothing has made such a strong impression on me. It was cool. The scene was not just panoramic, but THREE-DIMENSIONAL! And I asked him what the secret is?! To which he answered me: “The amplifier is of course good... But the sensitivity of these speakers is 95 dB/W and the price is about $500.” And in the end he explained that the most noticeable role in the sound is played not by the amplifier, but by the acoustics. I left him even more thoughtful. I didn’t have five hundred dollars for speakers, and even weekend trances...
Since then I started looking for alternatives and after some time I came across electrostatic sound emitters. This was what was needed. But I couldn’t make a full-size system right away, because I didn’t know many aspects, and I didn’t want to sculpt large stats at random.
Perforated sheets are also not cheap. Therefore, I decided to practice on a scaled-down prototype and assembled my first electrostatic headphones. At that time I did not know any dimensions, proportions, what to make it from, and in general I knew almost nothing except that it should be a stretched conductive film between the plates of a flat-plate capacitor.
I made my first headphones using 4 HDD disks with holes drilled in them and metallized mylar for wrapping gifts on double-sided tape. I won’t say anything about the amplifier at all. In short, due to lack of information, I did everything from a flashlight. The headphones played very quietly. You could only listen to them at night, but the sound was amazing.
With the first musical composition I listened to on them, I realized that I needed to continue doing this. And I've been doing it for a couple of years now. A little over a year ago I moved to Moscow to someone else’s apartment and therefore now I don’t have to think about full-size acoustics. I'm developing the topic of headphones. Now I’m already doing the fifth option. The sound is loud, clear and smooth (well, except for the very bottom).
But until recently, I had never heard other electrostats, and just recently I went to the store to listen to the famous STAX. And I was surprised to find that my headphones sound almost identical. Only the Staxes have a membrane that is 2 times larger and 2 times thinner, so the bass is deeper. In the latest version of my headphones I want to repeat the proprietary membrane.
conclusions
A wide polar pattern in the high-frequency range and an extended reproducible range in the low-frequency range is achieved by the following electrostatic loudspeaker design:
- the vertical dimensions of the product are significantly larger than the width;
- the design contains two bases;
- two rigid, smooth reflectors with a curved surface, providing reflection in different directions;
- an electrostatic emitter (see Fig. 2, item 1) converts the electrical signal into an acoustic one.
This design of electrostatic acoustics is relatively simple to manufacture, does not require high-tech equipment and uses inexpensive consumables.
↑ About the amplifier for electrostats
I listened to these headphones for quite a long time with this amplifier:
Here is his diagram:
This is a Stax amp. Only I installed our 6N8S lamp in the preliminary stage, and 6N6P in the final stage. I selected the modes of the final stage, but I don’t remember the values, and I already disassembled the amplifier. Its sound was too soft for me. I listen to electronica, trance, Goa and ambient and I was missing the scope. The transition into the video, although soft, is still noticeable. Although at half the volume the amplifier sounded very pleasant. Now I have finally switched to transistors and based on preliminary results I will say that it is not in vain (op-amps and OOS rule) and I will not return to lamps in the near future.
Thank you for your attention!
This is my first article. It's good that I managed to finish it. I am glad to share my experience, and I hope that the information will be useful for many lovers of high-quality sound.
Calculation example:
F=Vsv/L
where, Vsv is the speed of sound equal to 343 m/s
We take L = 700 mm, the calculation result will correspond to 485 Hz. Consequently, if the sound wave is lower than the calculated value, then it will not be heard, since the sound waves emanating from one side of the emitter will close with the waves from the second side.
If, for example, a single panel with a width of 185 mm is used, then the path length will be 180 mm and the lower reproduced frequency will be 1890 Hz.
Triangular emitter
An electrostatic emitter that is close to a regular spherical surface can also be achieved using a set of emitters in the shape of a triangle [9].
Rice. 8. Triangular electrostatic acoustics. Patent US4289936.
This way you can also achieve acceptable radiation patterns in the high frequency region.
A significant disadvantage of this variation of the emitter is the need for a significant number of emitters in the shape of a triangle. Since the dimensions of the triangles affect the uniformity of the diagram over a wide range, a large number of them incredibly complicates the entire design and makes production quite expensive.
In particular, to ensure a uniform level of the radiation pattern up to 16 kHz, triangular emitters with side dimensions of up to 20 mm are required. And this is technically impossible.
Footnotes
- Pat. US3345469, US C1. 179-111. Electrostatic loudspeakers, Robert L. Rod., October 3, 1967;
- Pat. US3892927, MPKH04R 19/02, US C1. 179/111R, 179/180. Full Range Electrostatic loudspeaker for Audio Frequencies , Theodore Lindenberg, July 1, 1975;
- Pat. US6175636, MPKH04R 25/00, US C1. 381/398; 381/163, 381/386, 381/191. Electrostatic Speaker with Moveable Diaphragm Edges , Elwood G. Norris, James J. Croft. Jan. 16, 2001;
- Pat. US6188772, MPKH04R 25/00, US C1. 381/191; 381/116, 381/342, 381/176; 179/111 R; 179/180. Electrostatic Speaker with Foam Stator , Elwood G. Norris, James J. Croft., Feb. 13, 2001;
- Pat. US6304662, MPKH04R 25/00, US C1. 381/191; 381/174. Sonic Emitter with Foam Stator , Elwood G. Norris, James J. Croft., Oct. 16, 2001;
- Pat. US8184832, MPKH04R 25/00, US C1. 381/191; 381/162. Electrostatic Loudspeaker Capable of Dispersing Sound Both Horizontally and Vertically / Murray R. Harman. — May 22, 2012;
- Pat. US3668335, IPC H04R 19/02, US C1. 179/111 R. Electrostatic Loudspeaker, Harold N. Beveridge, June 6, 1972., Pat. US4270023, IPC H04R 5/02, US C1. 179/1 GA; 179/1 E. Cylindrical Speaker Mechanism, Harold N. Beveridge, May 26, 1981;
- Pat. US4289936, MPKH04R 19/00, US C1. 179/111 R. Electrostatic Transducers / John P. Civitello. — Sept. 15, 1981;
- Pat. US4122302, MPKH04R 1/20, US C1. 179/1 E, 181/145, 181/155. Two Way Dynamic and Electrostatic Speaker Enclosure with Side Vent for Greater High Frequency Dispersion , Lloyd J. Bobb, Oct. 24, 1978;
- Pat. US3778562, MPKH04R 19/02, US C1. 179/111 R, 181/31 V. Electrostatic Loudspeaker Having Acoustic Wavefront Modifying Device / William Michael D. Wright. — Dec. 11, 1973;
- Pat. US2009/0016552 A1, MPKH04R 19/02, US C1. 381/191. Electrostatic loudspeakers , Andrew Peter Medley, Duncan Robert Billson, Jan. 15, 2009;
Source: patent RU 2 547 897 C2
