CA1149928A

CA1149928A - Electrostatic loudspeaker

Info

Publication number: CA1149928A
Application number: CA000340338A
Authority: CA
Inventors: Meiro Kawasaki
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1978-11-28
Filing date: 1979-11-21
Publication date: 1983-07-12
Also published as: NL7908511A; GB2037537B; JPS5573200A; FR2443181A1; GB2037537A; JPS6117440B2; FR2443181B1; DE2947973A1

Abstract

ABSTRACT OF THE DISCLOSURE
An electrostatic loudspeaker having a diaphragm secured to a supporting frame, a field electrode secured to the-supporting frame and spaced apart from the diaphragm by a given distance, a flexible sheet secured to the supporting frame and hermetically shielding the diaphragm and the field electrode, and a metal thin film coated on the overall surface of at least one side surface of the flexible sheet, the loud-speaker being operative to suppress discharge from being in-duced between the diaphragm and the field electrode.

Description

1~49928 BACKGROUi~D OF THE INVENTION

Field of the Invention:
This invention relates to electrostatic type loud-speakers and more particularly an electrostatic type loud-speaker which is provided with a shield film for hermetically shielding a field electrode and a diaphragm of a speaker unit.

Descri tion of the Prior Art P
A conventional condenser type loudspeaker comprises a diaphragm arranged between two parallel and opposed field electrodes and is operative to supply a step-up sound signal voltage across the two field electrodes so as to oscillate the diaphragm and hence obtain a sound pressure.
In this case,-the force for driving the diaphragm increases in proportion to l/d , where d is a distance across a space formed between the field electrode and the diaphragm.
As a result, in order to improve the sound pressure sensi-tivity, it is preferable to make the distance d as small as possible.
However, if the distance d becomes small, the distance between the two field electrodes becomes also small and hence a voltage Vc for initiating discharge between the two field electrodes is lowered down to the Paschen 1 8 Law.
In the case o using a step-up transformer having a transformation ratio of 1:70, a spark discharge occurs when d = 0.3 mm and the output voltage from a power amplifier is 26 Vrms (rms meaning the root mean square). In practice, however, the discharge initiating voltage is lower than 26 Vrms due to the influence of the edge effect of apertures provided in the field electrode, contamination and humidity in air.

` 114~928 Particularly, in the c~se of reproducing a program 60urce obtained by a recent recording having a large dynamic range, it has been well known that a voltage of several tens of volts is produced at the output terminal of the power ampli-fier at instantaneous peak values. As a result, there is the risk of ~ spark discharge being induced.
If an input voltage exceeding the above mentioned discharge initiating voltage Vc causes the spark discharge to be induced between the field electrodes, the diaphragm is sub-~ected to a local temperature rise. As a result, pin holes are formed in the diaphragm and hence vibration mode are changed thereby changing the sound quality.
In an extreme case, the pin holes grow into large holes which render reproduction of sound in the low sound range imposs~ble.
In order to obviate the above mentioned detrimental ~park discharge, it has heretofore been proposed to insert a limiter circuit such as a posistor into the step-up transformer port~on of the condenser loudspeaker. Such measure, however, is not effective for instantaneous pulse inputs. A posistor ls a thermistor having a large positive resistance-temperature char-aeteri~tic, one version of which, for example, uses a barium titanate ceramic which has been dopet to reduce its resistance.
Heretofore,-it has also been propo6ed to provide a 80-called gas filled loudspeaker in which a speaker unit composed of a field electrode and a diaphragm is arranged in an atmosphere of teflon carbon fluoride gas or the like, such as sulphur hexa-fluoride gas or freon C C12F2(dichloro difluro methane or one of the other freon light gases) or the like, wh$ch has a high dis-charge insuiation breakdown voltage and excellent arc extinguish-ing effect. The use of such a gas filled loudspeaker ensures an . .;
.. ~ .,L 3 `" li4~928 increase of the input voltage which i8 about four times higher than that of the conventional loudspeaker. Tn such a ~as filled loudspeaker, use is nade of a shield film which can hermetically ~eal the ~as therein and derive the ~ound pressure with good efficiency. The shield film is coated on that part of the loud-speaker which is opposed to the speaker unit. The shield film must be formed of material which exhibits such shield effect of not only preventing diffusion of the discharge preventive gas filled in the loudspeaker, but also preventin~ penetration of gas such as water vapor or the like from the outside into the loudspeaker and which does not prevent radiation of ~ound. Such gas shield film is usually formed of polyethylene terephthalate (a salt or ester of terephthalic acid; esp. a dimethyl-es~er) (that is a ma~or startin~ material for polyester fibers and coat~s) and isrequired to have a relatively large thickness of several tens ~m for example more than 50 ~m. As a result, the shield film becomes large in mass and particularly, considerably attenuation occurs at high sound outputs and hence could not exhibit the characteri~ ~ desirable for the condenser type loud~Xer.

SUMMARY OF THE INVENTION

An o~ect of the invention is to provide an electro-static loudspeaker which will eliminate the above mentioned drawbacks which ,have been encountered w~th the prior art techniques.
Another ob~ect of the invention i6 to provide an electrostatic loudspeaker which does not require the lnsestion of a limiter circuit such as a posistor into a step-up trans-former portion.
A furthes o~3ect oi the invention is to provide an 11499'~8 electrostatic loudspeaker having a small humid transmittivity ~nd a small gas transmittivity.
A still further object of the invention is to pro-v~de an electrostatic loudspeaker which can firmly hold a shield film.
Another object of the invention is to provide an electrostatic loudspeaker which is provided with a relatively th~n shield film which is small in mass and hence attenuation at high sound outputs is not conspicuous.
A feature of the invention is the provision in an electrostatic loudspeaker comprising 8 speaker unit including a fielt electrode snd a diaphragm and arranged in a space hermetically shielded by a 6hield film, of the improvement in which said sh~eld film is composed of a flexible sheet ant a met~l thin layer coated on the overall surface o~ at le~t one eide ~urface of the flexible ~heet.
More particularly there is provided:
In a condenser type loudspeaker comprising, a speaker unit in-cluding a field electrode and a diaphragm mounted in a space hermetically sealed by a means comprising a supporting frame, a pair of flexible sheets attached to the supporting frame on either side of the field electrode and diaphragm and thin metal layers coated on the overall surfaces of at least one side surface of each of said flexible sheets, said thin metal layers having a thickness less than 1000 A, and wherein the space be-tween said pair of flexible sheets is filled with a discharge preventing gas.
There is also provided:
In a condenser type loudspeaker comprising a speaker unit in-cluding a field electrode spaced apart from a diaphragm and r _~ .

` 114~928 arranged in a space hermetically sealed by a pair of shield films, said shield films being composed of flexible sheets formed of polyethylene terephthalate and thin layers of aluminum coated on at least one surface of each of said flexible sheets and said layers of aluminum being less than 1000 A.

~ Other ob;ects, features and advantages of th~ inven-tion will be understood from the following detailed description with reference to the accompanying trawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure l i8 a longitudinal 6ectional view of one embodiment of an electrostatic loudspeaker according to the invention;
Figure 2 is its partial cross sectional view in an enlarged scsle;
Figure 3 is a partial cross sectional view of another embodiment of an electrostatic loudspeaker according to the ~nvention in an enlarged scale; ant Figures 4 and 5 are graphs ~howing experimental test re~ults.

B -5a-4~9Z8 DETAILED DESCRIPTION OF TH~ PREFERRE~ EMBODIMENTS

A preferred embodiment of an electrostatic loud-speaker according to the present invention will be described with reference to the accompanying drawings. Referring to Figure 1, reference numeral 1 designates a vibrating film, that is, a diaphragm. At opposite sides of the diaphragm 1 are arranged two field electrodes, that is, stationary electrodes 2, 2 each of which is provided with a number of apertures and formed of a metal sheet, for example. Reference numeral 3 shows a supporting frame for supporting the diaphragm 1 and the field electrodes 2, 2 at a predetermined position and provided with current supplying terminals (not shown) connected to the field electrodes 2, 2, respectively. On both side surfaces of the supporting frame 3 are coated shield films 4, 4 which together with the supporting frames 3, 3 define a hermetically sealed space 5. In the space 5 is arranged a speaker unit 6 composed of the diaphragm 1 and the field electrodes 2, 2. Air having the smallest possible humidity is charged into the hermetically sealed space 5.
Alternatively, in place of such air, freon carbon fluoride gas or the like such as sulphur hexafluoride SF6 or freon C C12F2, which has a discharge preventive effect, may be sealed into the hermetically sealed space 5.
The shield film 4 is composed of a flexible sheet 4a and a thin metal layer 4b coated on the overall surface of the outside surface of the 1exible sheet 4a.
The flexible sheet 4a may be formed of a high molecular film such, for example, as polyethylene terephthalate (PET) having a thickness of the order of 2 to 25 ~m, polyethylene (PE) having a thickness of the order of 10 to 25 ~m, poly-1145~928 propylene (PP)having a thickness of the order of lO to 25 ~m,polyvinylidene chloride (PVDC) having a thickness of the order of 1 to 30 ~m, Nylon 66 having a thickness of the order of 10 to 30~ m or the like.
The thin metal layer 4b may be formed of a metal element selected from the group consisting of aluminum, zinc, tin, gold, silver, titanium, copper or the like, or may be formed of an alloy such as SUS stainless steel or the like.
As a method of coating the thin metal layer 4b on the high molecular film 4a, use may be made of a vacuum vapor deposition method by a resistance heating system, vapor deposition method by an ion beam heating system or the like or in these methods an electric field is applied between a vapor deposition source and the high molecular film so as to increàse the bonding strength. In addition, a sputtering method i8 ef~ective and has the advantage that pin holes are difficult to be produced in the thin metal layer.
The flexible sheet 4a formed of polyethylene tere-phthalate (PET) having a thickness of the order of lO to 25~ m i8 relatively tough and aluminum is finely fitted thereto. In addition, aluminum is easy in handling so that a combination of PET and aluminum exhibits good workability.
Figure 4 shows a graph illustrating the results of experimental tests on the relation between the thickness of the thin metal layer measured in A and the oxygen gas trans-mittance (cc/m 24-~rs. atm.).
Figure 4 shows the experimental test results of the PET (12 ~m thickness~, PE (25~m thickness) and PP (25~m thickness) sheets each vapor deposited with the thin aluminum layer.

As seen from the experimental test results shown in Figure 4, when the thickness of the thin metal layer is on the order of 400 to 1000 A, the oxygen gas transmittance takes a value which allows the use of the loudspeaker in practice. The oxygen gas transmittance obtained when the thickness of the thin metal layer is on the order of gO0 A, for example, is reduced to about 1/100 times smaller than that when the thin metal layer is absent.
Figure 5 shows a graph illustrating the result of experimental tests on the relation between the thickness in A of the thin metal layer and the humidity transmittivity (g/m 24 Hrs) (40C). Figure 5 shows the experimental test result of the PET (12 ~m thickness) sheet vapor deposited with the aluminum thin layer-.
As seen from the experimental test result shown in Figure 5, when the thickness of the thin metal layer is on the order of 400 to 1000 A, the humidity transmittance takes a value which allows the use of the loudspeaker in practice.
The humidity transmittance obtained when the thickness of the thin metal layer is on the order of 600 A, for example, is reduced to about 1/100 times smaller than that when the thin metal layer is absent.
The humidity transmittance of each o~ PET, PE, PP, PVDC, and l~ylon 66 sheets coated with a thin metal layer having a thickness of 600 A and compared with that of the corresponding sheet not coated with the thin metal layer is shown in the following Table.

il49928 TABLE
Humidity Transmittance ~ /m2-24Hrs)(40C) _ Kind of Sheet O O

PET Thickness 0 A Thickness 600 (12 m Thickness) 45 0.5 (25 m Thickness) 25 0.6 (25 m Thickness) 20 1.0 PVDC
(20 m Thickness) 2.3 0.5 Nylon 66 (15 m Thickness) 250 0.5 Similar to the humidity transmittance of PET sheet shown in Fig. 5, the humidity transmittivity of PE, PP, PVDC
and Nylon 66 sheets each coated with the thin metal layer having a thickness on the order larger than 600 A takes a small and ~ubstantially constant value.
As seen from the above, both the gas transmittivity and the humidity transmittance of the high molecular sheet coated with the thin metal layer having a thickness on the order of 400 to 1000 A takes a value which is far smaller than that of corresponding high molecular sheet not coated with the thin metal layer and which permits the loudspeaker to be used in practice.
It is not preferable to make the thickness of the thin metal layer more than 1000 A because the shield fllm in itself becomes large in stiffness, thereby attenuating the reproduc-tion sound in its high sound region.
Figure 3 shows another embodiment of the electro-static loudspeaker according to the invention. In the present '' ` l~g9928 embodiment, a shield film 4 i8 composed of a flexible sheet 4a formed of a kigh molecular film such as PET, PE, PP, PVDC, Nylon 66 or the like and coated on itg both surfaces, that is, on the outside and inside 6urfaces thereof with thin metal layers 4b, 4b formed of aluminum or the like, respectively.
In this case, it ~s possible to ~ake the shield film 4 small in thickness and hence small in mass. The total sum of the thickness of the thin metal layers coated on both the outside and inside surfaces of the flexible sheet 4a is selected to be O O O
400 A to 1000 A, butis selected not to have ~ value exceed~ 1000 A-The use of such measure is preferable for reproducing sound in a high sound region.
Though not shown, to both the field electrodes 2, 2 ~hown in Figure l are connected two terminals of a secondary coil of a transformer having a primary coil supplied with a sound signal. The tiaphragm l formet of a non-electret high molecular film is coated on one of its surfaces or on both surfaces thereof with a surface active agent or charge pre-ventive agent and a polarizin~ tirect current source 18 connected to the diaphragm l and a mitpoint of the secondary coil. Such measure is not required for the tiaphragm 1 formed of an electret high molecular film.
In a back-electret type condenser loudspeaker, those surfaces of the field electrodes 2, 2 which are opposed to the t$aphragm l are coated with the electret high molecular f$1m and the diaphragm l formed of the non-electret high molecular film is coated on both surfaces thereof with a ~ur-face active agent or charge preventive agent.
As stated hereinbefore, the invention is capable of providing an electrostatic loudspeaker provided with a shield ~3 -10--:

1~49928 film whose humidity and gas transmittance can be made small without deteriorating the mechanical and accoustic characteristics of the loudspeaker.
As a result, even if the distance between the two field electrodes is made small, spark discharge occurs with difficulty and hence substantially no pin holes are produced in the dia-phragm due to the spark discharge. Thus, there is substantially no risk of the loudspeaker being subjected to change in vibra-ting mode and hence in sound quality.
It will be apparent to those skilled in the art that many modifications and variations may be effected without departing from the spirit and scope of the novel concepts of the present invention.
.

. ,

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a condenser type loudspeaker comprising, a speak-er unit including a field electrode and a diaphragm mounted in a space hermetically sealed by a means comprising a supporting frame, a pair of flexible sheets attached to the supporting frame on either side of the field electrode and diaphragm and thin metal layers coated on the overall surfaces of at least one side surface of each of said flexible sheets, said thin metal layers having a thickness less than 1000 A, and wherein the space between said pair of flexible sheets is filled with a discharge prevent-ing gas.

2. The condenser type loudspeaker according to claim 1, wherein said flexible sheets have thin metal layers vapor de-posited thereon.

3. The condenser type loudspeaker according to claim 2, wherein said thin metal layers are coated onto said flexible sheets by sputtering.

4. The condenser type loudspeaker according to claim 1, wherein said thin metal layers coated on said flexible sheets has a thickeness of 400 A to 1000 A.

5. In a condenser type loudspeaker comprising a speaker unit including a field electrode spaced apart from a diaphragm and arranged in a space heremetically sealed by a pair of shield films, said shield films being composed of flexible sheets formed of poly-ethylene terephthalate and thin layers of aluminum coated on at least one surface of each of said flexible sheets and said layers of aluminum being less than 1000 A.