Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a telephone
conferencing apparatus and particularly to a unitary loudspeaker/
microphone unit which gives adequate sound power output of
good quality, is omnidirectional in both transmit and receive
modes, and is aesthetically and practically acceptable in
telephone conferences or public address uses.
Oonventional telephone conferencing devices
have certain disadvantages, depending upon the ~orm of the
device. Many have the speaker more obvious than the m~crophone
which creates a human behaviour problem in that users speak to
the speaker (which represen~s the remote party) instead of to
the microphone. This can reduce the level and quality of the
speech signal. In some devices the microphone is not omni-
directional and differing signal levels and quality arise as
different participants in a room talk. The speaker and/or
; microphone may be very obvious and create some reaction by
talkers.
The present invention provides an apparatus
which is omnidirectional but still has adequate acoustic
separation between speaker and microphone and is aesthetically
; pleasing, not being intrusive in a conference.
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; The speaker and microphone are on a common
vertical axis, the speaker facing upward and the microphone
downward. The microphone is at the centre of an inclined surface
which avoids standing wave and other distortions. The speaker
and microphone are omnidirectional, while at the same time good
acoustic separation is provided. An existing electronic circuit,
as used in an existing speakerphone, can be used, mounted in
the housing of the apparatus, for connection to a telephone
system, and a telephone dial, and other controls, can also be
mounted in or on the housing. It is possible to have several
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of the speaker/microphone units interconnected. With voice
switching - which is part of the normal electronic circuit - the
unit be;ng addressed will act as the microphone while the other
units will be in a loudspeaker condition.
The invention will be readily understood by the
following description of certain embodiments by way of example,
in conjunction with the accompanying drawings, in which:-
Flgure 1 is a vertical cross-section through one
form of apparatus;
Figure 2 is a view in the direction of arrow A
in Figure 1, with the base plate removed;
Figure 3 is a plan view on the loudspeaker end;
Flgure 4 is a side view, illustrating positioning
of contacts and a telephone dial;
Figure 5 is a perspective view of an alternative
form of housing.
The apparatus as illustrated in Figures 1 to 4
has a housing 10 forming an enclosure elongated in the vertical
direction, as seen in Figures 1 and 4, and having a square cross-
section normal to the~vertical axes, as seen in Figures 2 and 3.The dimensions of the square cross-sections, in the example
illustrated, is sufficient to house the speaker 11. The top
surface 12 of the housing 10 has an aperture 13 over the speaker
; 11. The speaker works in an enclosed chamber 14, the chamber
being closed at the base by a diaphragm 15.
Attached to the diaphragm 15 is a microphone
housing 16. The microphone housing has an aperture 17 facing
: downwards and a microphone 18 is mounted in the aperture, facingdownwards. The microphone housing has an inclined surface 19,
extending outwardly and upwardly to the housing 10. The angle of
the sur~ace 19, relati~ve to the horizontal, is important.
.~; The apparatus can be placed on a horizontal
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surface, with the microphone spaced close to the surface. Thus
legs 20 can be used to support the apparatus with the desired
clearance between microphone and supporting surface. However,
the type of the supporting surface can affect the microphone.
For example, if placed on a hard surface, such as a wood surface
as in a table, the microphone will be effective. However if
; placed on a carpetèd floor, or on a cloth surface of a table,
the sensitivity of the microphone will be reduced especially
at high frequencies. To make the apparatus independent of
supporting surface, a hard base member 21 can be provided,
attached by the legs 20, for example. The provision of the
base 21 makes the unit suitable for any kind of mounting, for
example on the floor, on a table, on a wall, or on the ceiling.
The sensitivity will not be affected, nor will the omnidirectional
feature.
The size of speaker can be varied, but it has
been found that a 6" speaker gives adequate sound power output
and acceptable quality. With a smaller speaker, base tones are
- lost and output sound power is less.
There are various parameters which should be
taken into account when the apparatus is designed, There must be
sufficient acoustic separation between the speaker 11 and the
microphone 18. It has been found that this can be obtained by
making the minimal peripheral distance between the speaker centre
line and the microphone about 15" - that is the distance as
indicated by the chain dotted line and arrows 25 in Figure 1.
This distance can be reduced slightly, but the desired level of
acoustic separation startsto be reduced, if the distance is much
less than about 15". The distance between the front surface,or
input, of the microphone and the support surface - the distance
; "a" in Figure 1 - should be of the order of 1/2~s and the angle
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of the inclined surface 19 relative to the suppor~ surface -
in Figure 1 - should be of the order of 30.
The physical separation - the distance from the
speaker centre line to the microphone centre line around the
casing, is of course a function of the width oF the housing
and thè height. It is possible to make the housing tall and thin
or short and fat. There is no objection to increasing this
distance but it is a matter of aesthetic appeal and also of
being unobtrusive that provides some determinat;on. For example,
if stood on a table in a conference, with participants sitting
around the table, it is desirable that the housing does not
interfere with eye contact between participants. Also if made
too thin then some local enlargement for the speaker will be
necessary.
The mounting of the speaker and microphone on a
common vertical axis is advantageous in that acoustic separation
is symmetrical about the housing. Also, a very important feature,
with the speaker facing upwards it is omnidirectional, and
similarly with the microphone facing downwards it is also
~ 20 omnidirectional. This avoids the need for multiple speakers and/
; or multiple microphones, or alternatively the occurrence of
directional sensitivity variations which can occur with other forms
of conference apparatus.
With a downward facing microphone, facing a
hard surface and the microphone mounted in a flat surface,
strong resonances occur at the entrance to the microphone, due
to standing waves. By inclining the surFace surrounding the
microphone, the forma~ion of such resonances is avoided. This
means that the apparatus itself does not add to the resonances
of the room in which the apparatus is used and a substantially
uniform frequency - response of the microphone results. The
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;nclination of the surface can vary, an optimum of about 30
having been obtained. However this can be varied - 5 quite
readily. The inclined surface 19 is preferably flat but can be
slightly curved, with the introduction of some undesirable
frequency irregularities.
As stated, the vertical distance between the-
microphone entrance and the surface opposi~e the microphone, be
; it the base 21, when provided, or a table top or the like if
base 21 is not provided, is important. A distance of about .5"
- 10% gives acceptable results, the efficiency of the microphone
decreasing rapidly as the distance varies from the above.
Outside these limits the microphone frequency response will not
be uniform to an extent which is not acceptable.
The physical separation distance is preferably
; of the order of 15". Reduction of this dimension will increase
acoustic feedback from speaker and the microphone. This
distance is important because low frequencies, for example below
1000 Hz, can diffract easily around obstacles. With increase in
frequency, the increase in speaker directionality, as well as
the effect of the "shadow" of the housing in the microphone area,
make up for the increase of microphone sensitivity with frequency
as is the case with telephone transmitters. Therefore a
substantially constant value of acoustic separation is obtained
and the acoustic stability of the unit becomes independent of
frequency. The distance can be increased, but the above
mentioned aesthetic and unobtrusive features then become a
source of concern.
~ hile in Figures 2 and 3, an open aperture is
shown over the speaker this will normally be closed by a grill or
screen to avoid damage to the speaker.
The use of a microphone which is sensitive to
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airborne sound and insensitive to mechanical vibration enables
the speaker and microphone to be in a common housing. Such a
microphone is an electret microphone as the electret microphone
is insensitive to mechanical vibration. This provides a simple
and economic solution to the problem of solid-borne acoustic
feedback through the housing.
The configuration, substantially as illustrated
and with the preferred physical characteristics given above,
provides an acoustic separation in excess of 20 decibels. This
provides stable acoustic operation of the apparatus with a
switch loss ( a usual technique to avoid howling, in speaker-
phones) of no more than 50 decibels. This makes the effect o~
voice-switching almost unnoticable.
In long subscriber loops there is excessive
attenuation toward the high end of the telephone frequency range-
above 1000 Hz. A resonator 26 attached at the microphone inlet
boosts this high end of the frequency range, giving a crisper
sound, a higher intelligibility of speech signals and an
improved chance for talker recognition. The resonator 26 is
a ~lat disc with a peripheral rim, and having apertures 27 in the
disc portion. The apertures can be slots, holes or other
formations.
The apparatus can be used in various ways. Thus
it can be wired into an existing speakerphone set, using the
circuitry of the set. It can be used as a combined conferencing
and public address system in that each unit can have its own
circuitry contained within the housing and the various units
interconnected. The unit nearest to whoever is talking at a
particular time will be voice-switched to a microphone regime,
3~ while the remaining units will be in a speaker regime, ensuring
everyone can hear what is said.
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Controls can be provided and, for example
volume and/or tone controls can be provided. The unit can be
directly wired into the telephone system, without an intervening
speaker phone unit. In such a case the speakerphone circuitry
is mounted within the hous;ng, as indicated in chain-dotted
outline at 28, in Figures 1 to 4. The dial 29 can be mounted in
one side, together with various control buttons 30 and control
knobs 31.
While the apparatus has been illustrated, and
described as being of square cross-section, when viewed on the
top or bottom, other cross-sections can be provided. Thus
cylindrical housings, and multisided housings such as hexagons and
octagons can be used. Figure 5 is a perspective view of a unit
having a cylindrical housing 35. In the particular example
illustrated, the outer casing extends right the way down to
encompass the microphone housing. The microphone is surrounded
by an inclined surface in the same manner as in the previously
described example, the inclined surface being conical. A
perforated section 36 of the housing 35 surrounds the microphone
section.
