Arrangement to prevent the intrusion of foreign matter into an electro-acoustical transducer

In an arrangement to prevent the intrusion of foreign matter into a transducer which extends with a projection provided with an opening into a sound channel of a hearing aid, a reduced need for space as well as simpler construction, compared with the state of the art, are achieved by means of the following characterizing features: A membrane made of pore-free material is provided, which has at least one bore for the passage of sound of from a few hundredths to a few tenths of a millimeter diameter, and in which the wall thickness of the membrane is small with respect to the diameter of the bore.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention concerns an arrangement to prevent the intrusion of foreign 
matter, in particular cerumen, into an electro-acoustical transducer. 
2. Technical Backoround 
From DE-AS 12 63 849, an arrangement is known for an in-the-ear hearing aid 
to prevent the intrusion of cerumen (ear wax) into a transducer, which is 
designed as a hollow member in cap form (ear insert) of elastic material 
that can be attached to the sound exit nozzle (projection) of a hearing 
aid, that in its interior presents a chamber-like expansion of a bore for 
the passage of the sound at its hemispherically-shaped end, in which a 
layer of porous, sound-permeable material is arranged. Given pore sizes of 
about 0.2 to 0.5 mm and partitions that are thin by comparison, in order 
to achieve a great density of pores per surface unit, the porous layer is 
of relatively great thickness approximately corresponding to the inside 
cylindrical radius of the hollow member. The wall thickness of the hollow 
member amounts to about one seventh of the outside cylindrical diameter. 
It is thus relatively thick. Altogether, this arrangement results in much 
space being required, especially in the direction of travel of the sound. 
Besides, two separate components are required to close the sound exit 
nozzle against the infiltration of cerumen, wherein the hollow member acts 
as a mounting support (added arrangement) for the porous layer, which in 
turn prevents the intrusion of cerumen and must be replaced regularly 
since a cleaning of this porous and hence soft material is not possible. 
From DE-GM 84 36 783 and 85 04 765, sieve-like perforated caps are known 
that can be screwed or snapped into or onto projections that extend from 
the hearing aid housing as an extension of the sound channel only for this 
purpose. 
The penetration of cerumen into the sound channel is largely prevented by 
the introduction of recesses (grooves) on the housing outside the sound 
channel. The outer projection needed only for the mounting of the 
perforated caps is an added arrangement and necessitates a relatively 
great length in construction in the direction of the sound propagation, 
even more and markedly increased by the thickness of the perforated caps 
projecting from the extensions, especially since the thickness of their 
walls is on the order of the sieve-like openings. Beyond this, the large 
number of openings arranged in a sieve-like manner increases the danger of 
penetration by cerumen. 
The object of the present invention is to design an arrangement of the type 
initially indicated, in such a manner that it will occupy less space and 
be of simpler construction compared with the state of the art. 
SUMMARY OF THE INVENTION 
This object is achieved by a hearing aid comprising a housing containing a 
sound-conducting channel, an electro-acoustic transducer having a 
projection which extends into said channel, and a nonporous membrane 
substantially covering said projection. This membrane has at least one 
bore communicating with said channel, and said bore has a diameter which 
is at least a few hundredths of a millimeter and is at most a few tenths 
of a millimeter, and a thickness which is small with respect to said 
diameter. 
An added arrangement -- i.e., a separate, cap-shaped hollow member as mount 
for an inner porous layer (membrane) or an added piece that projects as an 
extension of the sound channel from the hearing aid housing -- becomes 
superfluous, since the arrangement in accordance with the invention shares 
in the use of a projection on a transducer (e.g., earphone) that is 
necessary for other purposes, anyway, by using it as a mounting support 
for a membrane. Reduced need for space results, especially in the 
direction of sound propagation (short structural length), from the 
arrangement of the membrane in accordance with the invention in the sound 
channel that is necessary anyway. The arrangement on a projection of a 
hearing aid component extending into the sound channel was made possible 
by using a non-porous material (greater strength) -- metallic materials 
are especially suitable for this purpose -- with extremely thin 
(foil-like) walls. Measurements in the course of the invention showed, 
surprisingly, that given a negligible, foil-like thickness of the membrane 
in accordance with the invention, a bore (produced with a laser beam, for 
instance) having diameter on the order of the pores in the known thick 
membranes, preferably up to about 0.6 mm, already suffices to ensure 
largely undisturbed passage of sound (negligible linear attenuation) 
through the membrane. 
If the diameter of the bore in the thin, foil-like membrane in accordance 
with the invention is reduced to a few tenths of a millimeter, preferably 
to around 0.15 mm, surprisingly a clearly non-linear effect on the 
acoustics occurs, with the result that with linearly increasing output 
level of the sound source, an increasing attenuation, non-linear, by 
contrast, occurs in the sound transmissivity of the membrane in accordance 
with the invention. 
Measurements and trials in connection with the invention have shown that 
with this additional, novel effect of the arrangement in accordance with 
the invention, it is possible to largely replicate the effect of an 
electric output-level-limiting circuit (peak-clipping or PC) and/or an 
automatic gain control (AGC). The arrangement in accordance with the 
invention therefore also achieves the substantial advantage that an 
electrical arrangement for non-linear sound level attenuation, hitherto 
necessary, becomes superfluous, simultaneously reducing the need for space 
in the hearing aid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The in-the-ear hearIng aid according to FIG. 1 shows a housing 1, in which 
parts essential to the invention are illustrated, inter alia. Between an 
outer surfaCe 2 and an inner surface 3 of the housing 1, there extends a 
sound channel 4, projecting into the sound channel 4 is an earphone 5 with 
a proJection 7 provided with an opening to the projection 7 a membrane 8 
made of a pOre-free, metallic material has been arranged, which has a bore 
9 for the passage of sound. The diameter of the bore 9 is only a few 
tenths of a millimeter. A decisive factor is that the wall thickness of 
the membrane 8 must be much less than the diameter of the bore 9 of about 
1:10 results in a good sound transmission effect given a wall thickness of 
about 2/100 millimeter. 
The membrane 8 is designed in the form of a cap, and made of a 
deep-drawable material: e.g., mu-metal, as a deep drawn . This will 
produce good mechanical stability, the negligible wall thickness 
notwithstanding, especially during mechanical cleaning (scraping off) of 
cerumen with, for instance, a wire loop. The membrane 8 has a cylindrical 
area and a convex area 11 pointing away from the former. The convex area 
11 increases the mechanical stability of the membrane 8 and the 
cylindrical area 10 enables a simple clamping attachment to the 
customarily cylindrical projection 7 of the transducer, here shown as an 
earphone 5. The projection 7 has a diameter of about 1.4 mm and a length 
of about 1 mm. The membrane 8 is arranged completely within the sound 
channel 4, whereby, in conjunction with the convex area 11, a ring-shaped 
depression 12 results within the sound channel 4. Cerumen can collect in 
this ring-shaped depression 12, making a special groove to be molded into 
the housing 1 for the collection of cerumen superfluous. 
The projection 7 on the earphone 5, together with the membrane 8 
clamp-fastened thereto, is held in a sleeve 13 of sound-damping material, 
e.g., a silicon tube, in the housing 1 by means of a snap or catch 
connection 14 and is at the same time designed as the sound channel. The 
inside diameter of the sound channel 4 corresponds more or less to the 
diameter of the projection 7. Given a negligible elasticity of the sound 
channel 4, the inner diameter of the sound channel 4 can be made greater 
by up to double the wall thickness of the membrane 8. A recessed 
arrangement of the sleeve 13 (the length of construction is made shorter 
than the distance between the outer surface 2 and the inner surface 3 of 
the housing 1), results in a further ring-shaped depression 15 in the 
outer surface 2 of the housing, in which cerumen can also collect. The 
result is a very long path for the cerumen, before it can reach the bore 9 
arranged in the center of the convex area 11 of the membrane 8. This 
results in great time intervals between cleanings of the apparatus by a 
mechanical and therefore inexpensive and simple method. 
Because of the negligible wall thickness of the membrane 8, it is also 
possible to arrange the same (not illustrated herein) within the opening 6 
of the projection 7 of the transducer here shown as an earphone 5, whereby 
the penetration of foreign matter, especially cerumen, into the respective 
transducer is also prevented. 
Since an electrical non-linear sound level control can also be largely 
replicated by means of the invention, the arrangement is also especially 
suited for use in conjunction with a transducer designed as a microphone, 
though not illustrated herein. This would, on the one hand, keep foreign 
particles away from the microphone that are larger than the diameter of 
the opening 6 in the membrane 8, while also making other, added 
arrangements, such as electric level-limiting circuits (PC, AGC) 
superfluous, and an overload of the electrical hearing aid amplifier and 
its resulting side effects can be avoided from the start. 
The attainable degree of non-linear influence on the acoustics by the 
arrangement according to the invention, depending on the level of the 
sound source and the diameter of the sound exit opening in a membrane 
according to the invention, is shown by examples in FIGS. 2 to 4, with the 
aid of curves 16 to 22 and 16' to 22', respectively. To simplify the 
interpretation of the measurement results, the wall thickness of the 
membrane, with the indicated curves 16' and 22', was uniformly established 
at 0.02 mm and a membrane always made of deep-drawn mu-metal used in every 
case. All curves were measured on a hearing aid in which an earphone was 
operated by constant current. 
The following parameters apply to FIG. 2: 
Diameter of the bore in the membrane: 0.4 mm 
Wall thickness of the membrane: 0.02 mm 
Maximum output level at the earphone: 110 dB, given curve 16 without, and 
curve 16' with, membrane. 
FIG. 2 shows clearly that given a diameter Of the bore of 0.4 mm and a 
maximum output level of 110 dB according to curve 16, an attenuation of 
ca. dB occurs only at this high output level vis-a-vis the curve 16'. At 
the lower output levels outside the frequency range of 2 kMz the 
attenuation difference is clearly less or even non-existent. 
The following parameters apply to FIG. 3: 
Diameter of the bore in the membrane: 0.25 mm 
Wall Thickness of the membrane: 0.02 mm 
Maximum output level at the earphone: 110 dB, given curve 17 without, and 
curve 17' with, membrane 
Maximum output level at the earphone: 100 dB, given curve 18 without, and 
curve 18' with, membrane 
Maximum output level at the earphone: 90 dB, given curve 19 without, and 
curve 19' with, membrane. 
FIG. 3 records the broken-line curves 17' to 19', given a diameter of 0.25 
for the bore in the membrane. Between the two curves 17 and 17', there now 
results a maximum difference in level of ca. 7 dB, again given a maximum 
output level of 110 dB. Given a maximum output level of 100 dB, the 
resulting maximum difference between the curves 18 and 18' is only about 4 
dB. Given a maximum output level of 90 dB, the difference between curve 19 
and 19' is only 3 dB. From this it can be seen clearly that the 
attenuation through the arrangement in accordance with the invention 
increases non-linearly with an increasing output level. Beside this, the 
attenuation increases with the decreasing diameter of the bore, as is made 
clear from a comparison of curves 16 and 16' in FIG. 2 with curves 17 and 
17' in FIG. 3. 
The following parameters apply to FIG. 4: 
Diameter of the bore in the membrane: 0.15 mm 
Wall thickness of the membrane: 0.02 mm 
Maximum output level at the earphone: 110 dB, given curve 20 without, and 
curve 20' with, membrane 
Maximum output level at the earphone: 100 dB, given curve 21 without, and 
curve 21' with, membrane 
Maximum output level at the earphone: 90 dB, given Curve 22 without, and 
curve 22' with, membrane. 
In FIG. 4, the described relationships become even clearer, as the diameter 
of the bore in the membrane has been reduced to 0.15 mm. Vis-a-vis the 
curve 20 with a maximum sound output level of 110 dB, an attenuation of 
about 17 dB results with curve 20', recorded with membrane in accordance 
with the invention. In addition, this reduction of diameter in the 
membrane also achieved a nearly rectilinear frequency response curve in 
the transmission range of the earphone. With reduced output level, the 
attenuation decreases again non-linearly, as may be seen from the curves 
21 and 21', which show a maximum difference in level of 14 dB, and the 
curves 22 and 22', which show a maximum difference in level of only some 
11 dB, given a maximum output level of 90 dB. 
There has thus been shown and described a novel hearing aid which fulfills 
all the objects and advantages sought therefore. Many changes, 
modifications, variations and other uses and applications of the subject 
invention will, however, become apparent to those skilled in the art after 
considering the specification and the accompanying drawing. All such 
changes, modifications, variations and other uses and applications which 
do not depart from the spirit and scope of the invention are deemed to be 
covered by the invention which is limited only by the claims which follow.