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July 14, 1953
2,645,301
M. DEVRIES
SOUND FILTER FOR MICROPHONES AND TELEPBONES
Flled July 19, 1950
I.
\ INVENTOR.
mnu?ek DE wanes
Bywmimh, H'nob a PM
FhTORnJ 51s
2,645,301
Patented July 14, 1953
UNITED I
STATES PATENT OFFICET
2,645,301
SOUND FILTER FOR MIGROPHONES '
AND ‘TELEPHONES
- '
>
Machiel de Vries, Amsterdam, Netherlands 7
Application July 19, 1950, Serial No. 174,734
- In the Netherlands May 11, 1950‘
1 Claim. (Cl. 181-33)
.
‘
1
2
trolled, so that the latter can be correctly posi
tioned whereby the characteristic curve of the
microphone becomes remarkably ?at without the
lower frequencies being weakened. The space
between the ?lter plates can be simply adjusted
according to the invention, for example by means
of anaxially arranged screw bolt. It is also
This invention relates to a ?lter for sound vi
brations and to a microphone and telephone pro
vided therewith.
Many microphones, for example crystal micro
phones, have a characteristic curve which at a
given frequency shows a peak in the audibility
range. Attempts have been made to attain a
possible to change the staggered’ relation be
tween the perforations by turning two successive
peak to the range of the highest frequencies,
which generally, can be achieved by a lighter 10 perforated plates relative to each other. The
?rst expedient, however, usually suffices. The
construction, smaller dimensions and a greater
desired effect can be satisfactorily attained with
resilience of the moving parts. This, however,
more'?at characteristic curve by displacing the
two . plates.
' I
reduces the output which mustbe compensated
. If the ?lter is arranged in front of the dia
for by a more expensive‘ ampli?er.
Attempts have also been made to remove the 15 phragm of the microphone, it isdesirable for the
?lter to form an accoustic dynamic whole with
peak by the use of damping material, such, for
the microphone and the‘ ?lter therefore should
example, as a ?lter comprising plates of ?brous
be su?iciently coupled to said diaphragm in
material or textile supported by metal gauze. The
order
to damp the combination of diaphragm
characteristic curve of the damping of such a
?lter is rather flat, however, so that if the peak 20 and‘ actuating member correctly. In that case
the frequencies in the peak range are damped to
is clamped out there is also a considerable
the greatest extent.
' damping out of the other frequencies simul
To this end it is necessary for the distance
taneously whereby the effective output is ad
versely affected.
A ?lter is required, therefore, which only re
between diaphragm and adjacent ?lter plate to
be small, so that only a very slight mass of air is
enclosed between said diaphragm and said plate.
moves the peak and the characteristic curve of
Good results were also obtained with a diaphragm
whose damping corresponds with the character
istic curve of the microphone without ?lter. Its
construction must be simple, suitable for mass
production, while slight differences between the
which was not ?at but slightly conical and which
had its edge bearing direct or via a thin inter
mediate ring against the edge of the said ?lter
being rapidly removed by a simple adjusting de
plate. Also in this case a thinner layer of air
between diaphragm and ?lter caused a stronger
two or a plurality of perforated plates e. g. two
to a microphone can generally also be used con
in which'the perforations of successive plates are
staggered, and especially by adjusting the space
between successive plates, the con?guration of
bodiment.
microphones of a series must be susceptible of
damping.
vice provided on the ?lter, after said ?lter has
The combination of an energy converting mem
been arranged in the microphone casing.
According to the invention the ?lter comprises 35 ber and a ?lter as described here with regard
versely for converting electric energy into sound
or a plurality of metal plates, disposed parallel
energy. The invention, therefore, can be ap
to one another and spaced by interposed rings
plied
to all microphones, telephones, loudspeak
of small thickness, so that a thin layer of air is
40 ers etc. The ?lter plates can be mass-produced
left between said plates.
and assembled cheaply by simple mechanical
If the perforations are staggered relatively to
treatments.
one another the soundwaves penetrating through
The invention will be illustrated by the accom
the perforations of a plate must spread in the
panying drawing relating to a crystal microphone
thin layer of air between said plate and the ad
jacent one before they can penetrate through the 45 which was chosen as an embodiment.
openings of said adjacent plate.
Fig. 1 is a plan view of a piezo-electric micro
The waves of higher frequency are damped to
phone provided with a ?lter according to the in
a greater extent thereby than those of lower fre
vention and portions of the upper ?lter plate, of
quency, but the particular effect which is ob
the lower ?lter plate and of the diaphragm have
50
tained in this manner is that by the selection
been cut away to enable the parts located there
of. the bore of the perforations, of the degree
under to be better seen.
the characteristic curve of the ?lter can be con
Fig. 2 shows a cross section of the same em
55
Fig. 3 shows a large scale section of the ?lter
3
2,645,301
4
according to Figures 1 and 2 taken on the line
III—III in Figure 1.
Figure 4 shows a characteristic curve obtained
with the microphone according to Figures 1 and
a unit, i. e. the diaphragm can be considered as
an end-member of the ?lter. Together with the
diaphragm the mass of air between diaphragm
and ?lter plate, as Well as the mass of air be
tween the ?lter plates vibrates, for the two
2 when used without ?lter.
Figure 5 shows the characteristic curve which
masses of air are connected through the open
is obtained when the ?lter is used.
ings of the separating ?lter plate.
In Fig. 1, which shows a microphone 2-3 times
While the ?lter, regarded as an independent
enlarged, a ?anged ring I holds the upper ?lter
unit, passes the sound Waves selectively, it forms
plate 2, which is spaced from the lower ?lter 10 in combination with the diaphragm at the same
plate 4 by means of a spacing ring 3, for exam
time damping means for the frequencies to be
ple, of paper. The staggered relationship of the
removed. Upon comparing the characteristic
apertures in the two ?lter plates is shown in Fig.
curve according to Figure 4 with the one accord
3, which is a section on line III-III in Fig. l, on
ing to Fig. 5 it is seen that the peak in Fig. 4
a larger scale and with a smaller number of
disappears assoon as the ?lter according to the
apertures.
invention is mounted, without the lower frequen
The lower ?lter plate is followed by the cone
cies being damped.
diaphragm 5, the edge of which bears against
I claim:
the edge of the ?lter plate 4. The casing is desig
A filter for sound waves comprising a plurality
nated by 6 and contains a crystal plate 8 sup 20 of superposed plates having apertures, said plates
ported by a clamping plate 1, a corner of which
being coaxial, parallel to one another and slight
crystal plate is connected with the top of the
ly spaced from one another thereby providing a
cone 5 and is actuated thereby. In Fig. 3 a screw
thin layer of damping air between said plates, the
9 is indicated by means of which the space be
apertures in successive plates being arranged in
tween the ?lter plates and as a result the thick 25
staggered relation to one another, said plates be
ness of the intermediate layer or" air ‘can be
ing rotatively adjustable for adjusting the stag
varied. In Figures 1 and 2 said screw has been
gered relationship of said apertures, adjustment
omitted for clearness’ sake.
means adjustably connecting said plates for vad
It will be clear that the ?lter does not ap
justing the space between the plates, and a ring
preciably in?uence the shape, the dimensions and 30 of compressible material inserted between said
the weight of the microphone and it can be easily
plates and constituting spacing means therefors
accommodated in the box 6. Also there is an in
MACHIEL 1m VRIES.
teraction between ?lter and diaphragm, i. e. not
only the ?lter imparts energy to the diaphragm,
References Cited in the ?le of this patent
UNITED STATES PATENTS
Number
Name
Date
but the diaphragm which is made to vibrate also
imparts energy to the ?lter, that is to the air in
the ?lter, so that the undesirable frequencies are
repeatedly damped therein if the distance be
tween diaphragm and ?lter is chosen small
enough. Diaphragm and ?lter, therefore, form
410
1,804,688
1,915,358
2,022,060
Harrison _________ __ May 12, 1931
Giles ____________ __ June 27, 1933
Swickard _________ __ Nov. 26, 1935

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