Note: Descriptions are shown in the official language in which they were submitted.
35~
This invention relates to electret microphones
particularly for use in telephones.
Telephone electret microphones need to be shielded
from electromagnetic interference to which the microphone is
subjected in normal use. Such fields existing in the home, for
example, are radiated from nearby television and radio transmitters
and from electric motors. In a known telephone electret microphone
design, the microphone components are surrounded by, and the
electret element shielded by, an aluminum casing. One or more
circular holes in the surface of the casing allows transmission of
acoustic waves to the electret element. Unfortunately, as a
telephone user brings his face towards the hole, the electret element
is subjected to a distorted electric field due to body capacitance.
Normally, the output from the electret element is taken to an amplifier
and then to a balanced line, balanced line transmission being used to
compensate for interference occurring in the transmission path.
However, unbalance produced by a distorted field at the electret
element will not be compensated and is seen as a component of the
acoustic signal. Consequently, shielding of all interference at the
microphone, including that produced by body capacitance, is necessary.
A known electret microphone used in a telephone
consists of the following components. The top component, which, in
use, is located nearest the speaker's mouth, is one part of a two-part
aluminum casing. The casing has a hole through its center to allow
passage of acoustic waves. Beneath this casing part is a moisture
barrier which is r,ormally a thin film of plastic material such as
Mylar (Registered Trade Mark) which is pressed into sealing
~ , ~ 1
.
,
~ 1~585~
engagement with the top part of the casing. Below the moisture
barrier and a compressible mounting ring for the moisture barrier,
lies the electret element which, together with associated electrical
components, seats within a second part of the aluminum casing.
By the invention, it is proposed that the thin,
acoustically transparent moisture barrier be a conducting component
and that said component be situated such that it electrically contacts
the electret microphone casing or other grounded or fixed potential
body.
Preferably the component comprises a substrate plastic
film, the film having a conductive coating deposited thereon.
. An embodiment of the invention will now be described
by way of example with reference to the accompanying exploded view
of an electret microphone.
Referring in detail to the drawing, the microphone
illustrated has a top ferrule or casing part 10. The ferrule is
made of aluminum. It has an upper ridge 12 which can engage an
internal threaded part of a telephone handset housing (not shown).
An upwardly pressed annular portion 14 accommodates and centers a
sealing washer 16. On assembly, the sealing washer 16 presses a
combined moisture barrier and shielding element 18 into the
recessed portion 14. The element 18 has an upper conducting surface
20. The element is manufactured by vacuum-depositing a thin layer
of aluminum onto a plastic film 21, such as Mylar of a thickness of
10 ~m. The thickness and flexibility of the element 18 is such that
it is rendered transparent to ~coustic vibrations of between 10 Hz
and 4 KHz. The sealing washer 16 acts to tension the element 18.
8 ~ ~
If the element is improperly mounted, then there is a risk of its
affecting the voice frequency vibration transmitted by it.
Below the flexible sealing washer 18 and tight against
it is a transducer element 22~ Basically the transducer element
comprises a top frame 24 which clamps a piece of electret foil 26
against a bottom plate 28 by means of chips 29. The foil has a
metallic top surface and a bottom layer which has the property of
being able to store a charge for extended periods. The structure
of the electret is well-known. The charge storage face is separated
from a conducting layer 30 on the back plate 28 by 50 microns thick
strips of dielectric film 32. The plate 28 in the region of the
conducting layer is formed with holes 34 to permit the electret to
vibrate in response to acoustic waves passing into the microphone.
Because the charge stored in the bottom layer of the electret foil
is invariable~ then as it vibrates, the potential difference between
the conducting layer 30 and the conducting surface of the electret
varies to give an electric analog of the voice frequency vibration.
By means of a printed conductor on board 36 which has wire leads 37
bonded to the electrical surfaces of the electret element 22, the
varying electret voltage is taken to a field effect transistor (not
shown) mounted on the reverse surface of the board. The field effect
transistor projects into a chamber 40 which is formed in a bottom
part 42 of the aluminum casing. The chamber size is chosen to
optimize vibration of the electret foil 26. Contacts tnot shown)
are also formed on the reverse surface of the board 36 and
communicate electrically with the circuit formed on the board upper
face. The contacts project through a passage 46 in the casing
. . ' ~
.
~ ~658S~
part 42. A second seal ~8 which surrounds the contaçts 44 protects
the inside of the microphone from adverse environmental conditions.
In use, the casing comprising ferrule 1a and bottom
part 42 is grounded via one of the conductors on board 36, and so,
consequently, is the top surface of the element 18. The element 18
thus functions to seal the microphone from moisture and gaseous
contaminants and acts also to make the electromagnetic shield around
the electret element complete. As previously indicated, the primary
affect of this additional shielding part is in reducing the
interference of that electric field produced by body capacitance
which would otherwise affect the electret microphone output transmitted
to a balanced line.
The combination of a plastic substrate and a thin
deposited coating for the sealing element 18 is viewed as being an
optimal but non-limiting construction. Thus the element 18 could,
instead, be a single layer of conducting foil. However, it would
be difficult to manufacture such a thin foil of, say, aluminum having
the required transparency to voice frequency vibration, while
retaining sufficient strength to mechanically protect the electret.
Aluminum is particularly preferred as a conducting coating for the
element 18 firstly, since it is easily vacuum-deposited on a plastic
substrate, secondly, since the ferrule is also made of aluminum and
therefore would not form an electric cell with the coating when
damp, and lastly, since aluminum is a good conductor. However, it is
appreciated that in other circumstances other conductors such as
copper or zinc may be preferred, or the element may alternatively
be composed of a carbon loaded plastic.
5~5~
In the embodiment shown, the top surface of the
element 18 is made conducting and that surface contacts the
grounded ferrule 10. In other arrangements it may be preferred
not to ground the casing around the electret in which case the
conductive coating on the element 18 can be deposited on
whichever surface of the element is made to contact a grounded or
other fixed potential body.
In the embodiment described5 the conductive coating
extends over the full surface area of the element 18 so as to
completely surround the transducer element 22 with an electro-
magnetic shield. However, in other circumstances, it may be
preferred to limit the extent of the conducting coating on the
element 18 to a central or marginal region vertically aligned with
the central hole through the ferrule 10,
A conductive coating can be deposited on both sides
of the element 18 in order to facilitate assembly.
'
.