Note: Descriptions are shown in the official language in which they were submitted.
Field of the ~nvention
The present invention relates to microphones in
general and to directional microphones in particular.
More particularly, the invention rela-tes -to a microphone
unit, which is capable of being combined with o-ther such
units, and with acous-tic radiators, for conference appli~
cations.
Background and Prior Art
An electro-acoustic microphone having a single
planar diaphragm in a free field exhibits a nuli-response
in the plane of its diaphragm, while having its maximum
response along its perpendicular axis of symmetry. Such
an ideal device is called a "cosine microphone". At any
point between the axis and the plane of the diaphragm,
the response is proportional to the cosine of the angle,
say ~, that the point is at w.r.t. the axis-centre. The
reason for a response null in the plane of the diaphragm,
of course, is that sound pressure impinges equally on
either side there~f in opposite directions~ thereby not
~o causing movement of the diaphragm, given perfect symmetry
in a free field. Practical microphones have relatively
small diaphragms. Even sound pressure emanating from a
point of maximum response far along the axis is bound to
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also reach the other side of the diaphragm with some phase
shift and thereby cause some cancellation in the response.
It is desirable in many applications -to have
good cancella~tion in the plane of the diaphragm, yet to
maintain high sensitivity on either side thereof, parti-
cularl~ in the vicinity of the axis. Such desirable char-
acteris-tics would result in a conference apparatus having
a reduced degree of voice switching, thereby enabling more
natural two-way communication between two or more parties
of conferees. It is also desirable to have a frequency
response that is relatively independant of source position
(this means quality of response is independent of the tal-
ker's position).
:` ~
United States patent No. 4,237,339 issued
December 2, 1980 to Bunting et al and assigned to The Post
Office, London~ England is an example of *he use to which
such bidirectional microphones are put for purposes of
audio teleconferencing. The patent discloses an electro-
acoustic terminal unit for use in an audio teleconferencing
- 20 system comprising a loudspeaker and one or more microphones
each having a sens;tiv;ty which is directionally depen-
dent and exhibits at least one null or substantially null
position. The loudspeaker and microphones are rigidly
mounte~ on a boom and the microphones are so located and
orientated relative to the loudspeaker that the null pos-
ition is directed towards the loudspeaker.
ZJ
In figure 3 of -the above patent to Bunting et
al, two "shallow" voice switches (35 and 44 ) are used in
order to eliminate undesirable feedbac~ between the loud-
speaker 10 and the microphones 12 and 14. The permitted
degree of "shallowness" of the voice swi-tching is clearly
dependent on the degree of isolat;on provided by the micro-
phone.
It is possible to utilize complicated microphones
to provide acceptable isolation. For instance, United
States patent No. 3,573,399 issued April ~, 1971 to
Schroeder et al and assigned to Bell Telephone Laboratories,
Incorpora-ted, N.J., U.S.A. discloses the structure of a mic-
rophone having toroidal characteristics. The microphone
is constructed from a plurality of concentric transducer
elements, the outputs of which are combined in accordance
with a predetermined formula.
It is, therefore, generally recognized in the
field of teleconferencing as desirable to have microphones
of sufficient sensitivity to pick-up distance talkers and
conferees, while simultaneously providing good direction-
ality to lessen pick-up of background noise and rever-
beratîon and to prevent feedback with a minimum of
voice switching.
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Su~mar~ o~ the Invention
_
The object of the present invention is -to pro-
vide a simple, inexpensive microphone unit which exhibits
good sensi-tivity and directionality~
The acousto-electric transducer necessary ~or
the present unit can be a simple bidirectional transducer
having a single planar diaphragm, such as the now ubi-
quitous electret transducers.
The transducer is placed at the junction of two
dish-like sound collectors, which are back-to-back with
their convex sides. An opening in each dish exposes the
diaphragm to its concave side. The total s-tructure exhibits
rotational symmetry along the axis perpendicular to the
centre of the transducer. And whether the two collectors
actually touch or not, is not of primary importance. In-
deed, the exact shape of the collectors does not appear
to greatly alter the unit characteris-tics. For ins-tance, a
collector may be part of a sphere. Or it may be para-
bolic.
Due to the symmetry o~ the total unit, the two
identical back-to-back collectors do not in-terfere with
the signal cancellation effect inherent in the cosine
transducer in the plane of its diaphragm. But the
collectors p~rform an important function along the axis
of maximum response. ~or they reduce the cancellation
effect for a sound source at a far point on the axis:
the collector facing the source enhances -the sound pres-
sure on its side, while the other collec-tor provides a
sound "shadow" -to the other side, thereby improving the
transducer output. This improv~ment directly means in-
creased acoustic isolation or directionality, achieved
with a single structure and a single element transducer.
Thus, the sens~tivity o~ the microphone of the
present invention is proportional to collector size,
whereas its directionality is practically independent
thereof, meaning that the frequency response, i.e. the
variation of output with frequency or wavelength, is in-
dependent of collector s;ze. Such independence of direc-
tionality from frequency response translates into better
quality independent of talk~r position. This is of some
importance in conferencing and conference telephony.
Accordingly, the present invention provides a
microphone unit comprising a bidirectional acousto-elec-
tric transducer disposed in proximity to, and between,
two dish-like back-to-back sound collectors each having
an aperture therein exposing one of two active, opposite
surfaces of said acousto-electric transducer, whereby
said microphone unit exhibits substantially rotational
symmetry around a cen-tral axis of bid;rectionality.
More narrowly, the acousto-electric transducer
is of the cosine response type.
More narrowly s-till, each of -the -two opposite
surfaces of the transducer being within a substan~ially
coex~ensive aperture in the respective collector.
In a preferred embodiment the transducer is an
electret microphone.
In another preferred embodiment, two micro-
phone units are positioned one above the other, having their
axes of bidirectionality at a right angle and both having a
common null-axis in the third spatial dimension, thereby
yielding a quasi-toroidal directionality pattern.
It is particularly advantageous for conference
applications to combine the microphone unit of the present
invention with an electro-acoustic transducer placed to
substantially symmetrically intersect with the symmetry
plane of the microphone unit that is perpendicular to
its central axis o~ bidirectionality.
By a variant thereof, two microphone units, one
having its axis of bidirectionality perpendicular to -that
of the other, and both having a common null-axis in the
-third spa-tial dimension, are combined with an elec-tro-
acoustic -transducer placed -to substantially symmetrically
intersect with the symmetPy plane of each of the two
microphone units, each symmetry plane being perpendicular
to the central axis of bidirectionality of the respective
microphone unit.
Brief Description of the Drawings
The present invention will be better understood
in describing the preferred embodimen-ts in conjunction
with attached drawings, in which:
Fig. 1 schematically illustrates a microphone
unit according to the present invention;
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Pig. 2 schematically illu~trates a microphone
unit acco~ding to the present invention wherein truncated
parapelic reflec-tors are utiliæed;
Fig. 3 shows a combined microphone unit and a
loudspeaker for conference applica-tions;
Fig. 4 shows a combined microphone unit and
two loudspeakers for conference applications;
Fig. 5 shows an alternative arrangement using
a microphone unit and two loudspeakers for con~erence
lQ applications;
Fig. 6 shows an alternative arrangement for
a microphone unit and a loudspeaker for conference pur-
poses;
Fig. 7 shows an arrangement utilizing -~wo
orthogonal microphone units and a loudspeaker for con-
ference purposes;
Fig. 8 is a plan view of the general direc-
tionality pattern of the two orthogonal microphone units
shown in figure 7; and
Fig. 9 shows a more detailed palar direction-
ality pattern of a microphone unit in the plane of its
axes.
Detailecl Description of -the Preferred Embodiments
Figure 1 of the drawings shows a basic mi.cro-
phone unit 10 of the present invention. The unit 10 com-
prises two dish-like collectors 11 and 12, each having a
surface of an electret transducer 13 exposed through a
coopera-ting aperture therein. Ideally, the outside sur-
faces of the transducer 13 are each coplanar with the in-
side surface of therespective collector 11 and 12. Shown
schematically is the diaphragm 14 of the transducer 13,
t~e plane o~ ~hich is perpendicular to the plane of the
drawing and is the null-plane, or plane of minimum sen-
sitivity, of the total unit 10. Axis A is the axis of
maxi~um sensitivity, the sensitivity or response of the
unit 10 declining with the decrease in the angle with the
plane of the diaphragm 14, Thus, for practical purposes
a rotational angleo~defines a dead-zone of the microphone
unit 10. The angle ~ is in the vicinity of thirty deg-
rees, and the response on the surface of the dead-zone
is an average of - 14dB from the maximum response along
the axis A by a collector width W of five inches. The
average response of - 14dB does not vary appreciably with
frequency, and remains within ~ldB from 300 Hz ~o 3,000 Hz.
Such frequency range is approxi~ately the standard band-
width of a telephone channel, The collectors 11 and 12
may be made of a wide choice of materials such as plastic,
plexiglass, me-tal and -the like, and the transducer 13 is
simply glued to the edges of the apertures in the collec-
tors 11 and 12, which themselves are glued together at
their junction by a compatible glue~ Of course, other
methods of assembly, such as riveting are possible.
The collectors in ~igure 1 are shown to be
1~ spherical, or almost spherical. In figure 2, however, the
collectors 15 and 16 are parabolic surfaces truncated some
distance from the apex in order to permit placing of the
transducer 13 at or close to the parabolic focus of both
reflectors 15 and 18. A planar insert 17 closes the opening
and accomodates the transducer 13 in a suitable aperture.
The unit shown in figure ~ ~xhibits somewhat higher direc-
tionality so that the dead-zone angle ~ is somewhat lar-
ger than the angle c~ in figure 1, given the same width
W of the collectors 11, 12 and i5, 16. Both the micro-
phone unit 10 and that of figure 2 are rotationally sym-
metrical with respect to the axis A.
Figure 3 shows a conference device comprising
the microphone unit 10 and a loudspeaker 18 placed in a
suitable enclosure 19 on a conference table ~0. The
loudspeaker 18 radiates upwardly, and the conference
participants sit along the long sides of the table 20.
In this arrangement no conference participants may sit
along the narrow sides of the table 20, which are largely
in the dead-zone~
Figure 4 shows a more preferred arrangement than
that in figure 3, because two loudspeakers 21 and 22 are
radiating one to either side of the conference table 20.
In this arrangement it is mandatory that the loudspeakers
be driven in-phase and be identical. Moreover, they must
be posi-tioned sy~metrically on either side of the null-
plane of the unit 10 within the dead-zone of the unit 10.
This arrangement is preferred over the previous one~
~ because of the higher "treblc" content of the sound reaching
the conference participants when the lol-dspeakers 21 and
22 are facing them.
' - . .
In the embodiment of figure 5, the unit 10 is
placed on the top of the conference table 20, while two
loudspeakers, or loudspeaker rows, 23 and 24 are piaced as
shown under the ta~le 20 facing the conference participants
Any feedback from the loudspeakers 23 and 24 to the unit 10.,
if the loudspeakers 23 and 24 are operating in phase, would
shown under the table 20 partially facing the conference
participants. Any feedback from the loudspeakers 23 and
24 to the unit 10, if the loudspeakers 23 and 24 are operat-
ing in phase, would cancel in the unit 10 ar,d produce minimal
net feedback, given good symmetry.
.
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Figure 6 shows a loudspeaker 25 suspended from a
point above the unit 10, which is placed on the conference
table 20. This arrangement gives good quality probably due
to the treble frequencies from the loudspeaker 25 bouncing
off the table 20 top to the participants on either side of
the -table 20.
~ n figure 7 is shown an arrangement similar -to
that in figure 6, except that two microphone units lOa and
- lOb are placed on the table 20 top orthogonal to
each other. This way, by summing the outputs from the unit`s
lOa and lOb, a quasi-toroidal pattern is obtained with its
axis of symmetry vertical to table 20. This quasi-toroidal
pa-ttern is shown in a plan view in figure 8. The two axes of
maximum sensi-tivity of the units lOa and lOb are perpendicular
and parallel to the plane of the table 20. At ~5~ from
either of these two àxes, the output of each of the units
~Oa and lOb is 3dB below maximum~ but because the outputs
of the units lOa and lOb are summed the total response of
the combined units lOa and lOb is again maximum along the
~0 45 directions. Thus, the pattern is close to being tor-
oidal, and the total response or sensitivity is almost
constant at any angle in the horizontal plane, dependent
- only on the distance from the units lOa and lOb. This is
a desirable condition for conference applications.
~h~.16~
The arrangement shown in figure 7 is particularly
suitable for a conference room with a hard ceiling and
sound-absorbing walls, whereby the sound level of the loud-
speaker is enhanced, while acous-tic feedback is reduced.
Figure 9 shows a typical response of a single
microphone unit 10 in one quadrant of -the plane of the axis
A. As may be seen3 the response declines from its maximum
(OdB~ oh the axis A to its minimum in the plane of the dia-
phragm of some - 20dB. The important feature is the rela-
tive constancy of the response irrespective of frequency.
The three plots at 300, 1000 and 3000 H~ are almost coin-
cident, indicating the aforement;oned independence of
response quality from the talker's position.
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