Moisture barrier assembly

A microphone unit for a telephone handset comprises a microphone mount, a microphone mounted in the microphone mount, a mouthpiece detachably fixed to the handset, and a moisture barrier assembly for preventing ingress of moisture into the microphone. The moisture barrier assembly is constituted by a moisture barrier, an annular seal and a support plate. The support plate has a perforated portion covered by, but spaced from, the moisture barrier. The moisture barrier assembly is positioned between the mouthpiece and the microphone mount. Venting means vents a space between the moisture barrier and the microphone. The moisture barrier is constituted by a moisture impermeable film whose peripheral edge portion is fixed to the annular seal. The seal is fixed to the support plate and seals against the front face of the microphone.

BACKGROUND OF THE INVENTION 
This invention relates to a moisture barrier assembly for the microphone 
unit of a telephone handset, particularly that of a payphone. 
During wet weather, the handset of a payphone may be subject to partial or 
total loss of forward transmission resulting from the freezing of moisture 
(probably from a user's breath) which has entered into the transmitter. 
In order to alleviate this problem, it is known to fit a moisture barrier 
into the transmission housing of a payphone handset. A known moisture 
barrier is a simple disc made of a thin plastics film (for example 
melinex). Typically, such a disc is held between the parts of a two-piece 
crimped transmission housing, with the disc overlying the microphone. 
Unfortunately, the disc tends to separate from the housing, probably due 
to thermal shock and/or an inaccurate disc fitting process. This disc 
seperation permits moisture to creep round the disc edge and into the 
transmitter. Consequently, transmission loss occurs in freezing 
conditions. The sound holes in the mouthpiece of the handset may also be 
blocked, which increases the transmission loss. 
In an attempt to overcome this problem of moisture creep round the disc 
edge, another form of known moisture barrier has a thin film plastics disc 
provided with a surrounding annular support ring. The support ring is 
self-adhesive so that it can seal against the front of the transmission 
housing of a payphone handset. The main disadvantage of this type of 
moisture barrier is that the disc is too close to the mouthpiece, so that 
any moisture collecting between the disc and the mouthpiece tends to 
adhere the disc to the mouthpiece, and this leads to loss of transmission. 
There is also the possibility of the disc being sucked back onto the 
microphone, which would also lead to transmission losses. 
The aim of the invention is to provide an improved moisture barrier 
assembly for a payphone handset. 
SUMMARY OF THE INVENTION 
The present invention provides a moisture barrier assembly for the 
microphone unit of a telephone handset, the microphone unit being 
constituted by a microphone housing, a microphone mount positioned within 
the microphone housing, a microphone mounted in the microphone mount, and 
a mouthpiece detachably fixed to the microphone housing, the moisture 
barrier assembly being constituted by a support plate, a moisture barrier 
and a generally annular seal, the support plate having a perforated 
portion covered by the moisture barrier, the annular seal being fixed to 
the peripheral portion of the moisture barrier and to the support plate so 
as to seal, in use, against the front face of the microphone, wherein the 
moisture barrier is fixed to the support plate so as to be spaced by a 
predetermined distance from the perforated portion, and venting means are 
provided for venting the space between the moisture barrier and the 
microphone. 
The invention also provides a microphone unit for a telephone handset, the 
microphone unit comprising a microphone mount, a microphone mounted in the 
microphone mount, and a moisture barrier assembly for preventing ingress 
of moisture into the microphone, the moisture barrier assembly being 
constituted by a support plate, a moisture barrier, and a generally 
annular seal, the support plate having a perforated portion covered by the 
moisture barrier, the annular seal being fixed to the peripheral portion 
of the moisture barrier and to the support plate so as to seal, in use, 
against the front face of the microphone, characterised in that the 
moisture barrier is fixed to the support plate so as to be spaced by a 
predetermined distance from the perforated portion, and venting means are 
provided for venting the space between the moisture barrier and the 
microphone. 
In a preferred embodiment, the moisture barrier is a disc of polyester film 
which has a thickness of substantially 23 microns. The seal may be an 
annular member made of a resilient material such as polyvinylchloride 
foam, and may have a thickness of substantially 1.1 mm. 
Preferably, one annular surface of the seal is covered with adhesive 
transfer tape, whereby the seal can be adhered to the support plate, the 
peripheral edge portion of the film being fixed to the seal by said 
adhesive transfer tape. 
The support plate may be made of a hard plastics material such as 
polycarbonate, and the perforated portion of the support plate may be a 
central, stepped portion. Preferably, the annular seal is such that it 
adheres to the support plate in the region of the support plate 
surrounding the central stepped portion thereof, whereby the moisture 
barrier is spaced from the perforated portion by said predetermined 
distance. Conveniently, that surface of the seal remote from the 
perforated portion of the support plate is provided with a groove 
extending from its inner peripheral edge to its outer peripheral edge, 
said groove constituting venting means. Preferably, the venting means 
further comprises an annular space formed between the support plate and 
the microphone, a radial groove formed in the support plate, and an axial 
groove formed in the internal wall of the microphone mount, the annular 
space surrounding the annular seal and being contiguous with the groove in 
the annular seal and with said radial groove, and the radial groove being 
contiguous with the axial groove in the internal wall of the microphone 
mount, whereby the venting means extends, in use, from the space between 
the moisture barrier and the microphone to a space within the handset on 
that side of the microphone remote from the moisture barrier. 
Advantageously, the perforated portion of the support plate is defined by a 
plurality of circular apertures, the minimum diameter of each of the 
apertures being 2 mm, and the minimum total cross-sectional area of all 
the apertures being 100 mm.sup.2. 
In a preferred embodiment, the support plate is provided with three 
peripheral equispaced lugs which mate with three complementary recesses 
formed in the upper edge portion of the microphone mount. Preferably, 
there are three equispaced axial grooves formed in the internal wall of 
the microphone mount, whereby one of said axial grooves is aligned with 
the radial groove in the support plate no matter which way the support 
plate is mounted on the microphone mount by engagement of their 
complementary lugs and recesses.

DESCRIPTION OF PREFERRED EMBODIMENT 
Referring to the drawings, FIG. 1 shows the microphone unit 1 of a payphone 
handset 2. The unit 1 has a microphone (transmitter) 3 supported within a 
microphone mount 4. The microphone mount 4 is made of compliant rubber, 
either injection or compression moulded, and is positioned within the 
handset 2. A moisture barrier assembly 5 is positioned on the microphone 
mount 4 above the microphone 3. A mouthpiece 6 is screwed onto the handset 
2 to trap the moisture barrier assembly 5 between the mouthpiece and the 
microphone mount 4. The handset 2 and the mouthpiece 6 are made of a 
plastics material such as polycarbonate. 
The moisture barrier assembly 5 is constituted by a disc 7 of coloured 
polyester, an annular seal 8 made of polyvinylchloride foam, and a support 
plate 9. The disc 7 has a thickness of 23 microns, and the seal 8 has a 
thickness of 1.1 mm. The support plate 9 is generally disc-shaped, and is 
made of a hard plastics material such as polycarbonate. The central 
portion 9a of the support plate 9 is provided with a plurality of sound 
holes 9b (see FIG. 4). The holes 9b have a minimum diameter of 2 mm, and 
their minimum total cross-sectional area is 100 mm.sup.2. Alternate holes 
9b at the outer edge of the perforated portion 9a (see FIG. 4) preferably 
have a larger diameter of 3 mm. Alternatively, all the sound holes 9b 
would have a diameter of 2 mm, in which case there could be six central 
sound holes equispaced on a circle of radius 6.5 mm and sixteen outer 
sound holes equispaced on a circle radius 19 mm. This central portion 9a 
is inwardly stepped from the rear surface 9c of the plate 9 by means of 
two concentric shoulders 9d and 9e. Three equispaced lugs 9f project 
outwardly from the rear surface 9c at the circumferential edge thereof. 
These lugs 9f mate with complementary recesses 4a formed in the top edge 
surface of the microphone mount 4, thereby locating the support plate 9 on 
the mount. The support plate 9 locates and protects the actual moisture 
barrier 7. It also minimises the flow of air across the barrier 7 by 
acting as a diffuser. 
One surface 8a of the seal 8 is provided with a self-adhesive transfer 
tape, and the peripheral edge portion of the disc 7 is fixed to the seal 
by this adhesive. The seal 8 extends outwardly beyond the peripheral edge 
of the disc 7, so that an annular layer of adhesive remains exposed after 
the disc has been fixed to the seal. This remaining layer of adhesive is 
used to fix the seal/disc 8,7 to the plate 9. As can best be seen from 
FIG. 3, the outer diameter of the seal 8 is such that the seal fits snugly 
against the shoulder 9d. In this position, the disc 7 forms a moisture 
impermeable barrier which completely covers the central portion 9a of the 
support plate 9. 
The opposite surface 8b of the seal 8 is provided with a radial groove 8c 
having a width of 1.0 mm and a depth of 0.5 mm. This groove 8c leads to an 
annular space 9g (see FIG. 1) formed between the microphone 3 and the 
plate 9, this space being delimited internally by the outer 
circumferential edge of the seal 8 and externally by the shoulder 9e. The 
space 9g, which has a width of 1.0 mm and a depth of 0.5 mm, leads to an 
axial groove 4b formed in the internal cylindrical wall of the microphone 
mount 4 via a radial groove 9h formed in the rear surface 9c of the plate 
9. The grooves 4b and 9h each have a width of 1.0 mm and a depth of 0.5 
mm. The groove 8c, the annular space 9g, and the grooves 9h and 4b 
constitute an air path for venting the space between the moisture barrier 
assembly 5 and the microphone 3. In practice, since groove 9h is aligned 
with one of the lugs 9f (see FIG. 4) which lugs are received in 
complementary recesses 4a (see FIG. 3) and the internal wall of the 
microphone mount 4 is provided with three equispaced axial groves 4b, one 
of the axial grooves will align with the radial groove 9h, thereby 
allowing the support plate 9 to be positioned on the microphone mount in 
any of its three possible postions without the risk of obstructing the 
small airway constituting the venting means. 
The moisture barrier assembly 5 described above does not suffer from the 
disadvantages of known assemblies. In particular, the assembly 5 
completely covers the access region from the mouthpiece 6 to the 
microphone 3, thereby preventing ingress of any moisture into the 
microphone. Moreover, the stepping of the central portion 9a of the 
support plate 9 is effective to space the disc 7 sufficiently far away 
from the sound holes 9b to prevent any moisture gathering in the space 
between the disc and the portion 9a tending to adhere the disc to that 
portion. The venting of this space also reduces the possibility of 
transmission losses arising from barometric equalisation problems. Thus, 
if the barrier were completely sealed to the front of the microphone, the 
volume and pressure of air trapped between the microphone and the barrier, 
and inside the microphone itself, would change with changes in 
temperature. If the handset 2 was subjected to rapid changes of 
temperature, this change in pressure would distort the barrier 
sufficiently to impair transmission before equalisation could occur 
through air leakage. (As a guide, reducing the temperature from 25.degree. 
C. to -10.degree. C. would result in a differential force on the barrier 
of about 1 Kg (F)). By venting the space between the microphone 3 and the 
barrier 7 to the main body of the handset 2, there is sufficient leakage 
from this area via the earpiece etc., to prevent transmission losses due 
to such thermal shock. Furthermore, the grooves 8c, 9h and 4b and the 
annular space 9g are effective to vent the enclosed sealed volume between 
the disc 7 and the front face of the microphone 3, thereby avoiding the 
risk of the disc being sucked onto the front face of the microphone. Thus, 
the barrier assembly 5 prevents transmission losses which would arise from 
the barrier 7 engaging either the support plate 9 or the microphone 3. It 
should also be noted that, by locating the support plate 9 in the 
microphone mount 4 by means of the three protruding lugs 9f which fit into 
the recesses 4a in the microphone mount, the handset 2 can be assembled 
with the greatest freedom. Moreover, the airways constituting the venting 
means are sufficiently large to accommodate any distortion (and hence 
misalignment) as the mouthpiece 6 is tightened. 
Acoustically, the moisture barrier is quite complicated, as it comprises a 
series of tuned cavities. However, all the resonances are tuned out of the 
speech band, by a careful choice of dimensions, so that the net effect on 
the frequency response is minimal. It is important to note that any change 
to these dimensions will alter the response and involve further acoustic 
testing to ensure performance is adequate. 
Another advantage of the moisture barrier assembly 5 of the invention in 
that the holes 9b in the support plate 9 can be positioned so as to be out 
of alignment with the sound holes in the mouthpiece 6, thereby preventing 
access to the relatively fragile moisture barrier disc 7 from outside the 
handset. Earlier moisture assemblies did not have this feature, and were 
susceptible to vandalism. 
It will be apparent that a number of modifications could be made to the 
moisture barrier assembly. For example, the shape of the moisture barrier 
could be modified to suit the shape of different telephone handset 
mouthpieces.