Note: Descriptions are shown in the official language in which they were submitted.
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.
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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
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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
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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
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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.
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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-
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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.
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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.
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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
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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
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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
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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.
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