Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1
Acoustic Detector
FIELD
Embodiments described herein concern acoustic detectors.
BACKGROUND
Acoustic detectors are used in a variety of environments. One particular
approach is to use an
omnidirectional detector, mounted on a pole, which can then be itself attached
either to a
vehicle or set in the ground, for stability. However, the form factor of most
such designs is badly
suited to environments where rugged use may be required. Existing technologies
have resulted
in large and fragile solutions that do not combine sufficient sensitivity and
reliability with an
acceptable shape.
SUMMARY OF THE INVENTION
According to an aspect of the present invention there is provided an acoustic
detector
comprising a mounting plate having a generally planar face in which are
mounted a plurality of
acoustic sensors, a windshield enclosing the mounting plate and forming a
substantially dome
shaped void between an interior surface of the windshield and the plate, and a
supporting cage
surrounding the windshield and engaged with the mounting plate to provide
structural support to
the detector.
DESCRIPTION OF DRAWINGS
Figure 1 comprises a side elevation of an acoustic detector in accordance with
an embodiment
described herein;
Figure 2 comprises a cross-sectional view of the acoustic detector illustrated
in figure 1;
Figure 3 comprises a plan view of the acoustic detector illustrated in figure
1;
Figure 4 comprises a plan view of a sensor plate of the acoustic detector
illustrated in figure 1;
Date Recue/Date Received 2021-07-09
la
Figure 5 comprises the cross-sectional view shown in figure 2, to demonstrate
use of the
detector.
Date Recue/Date Received 2021-07-09
CA 02878745 2015-01-20
=
2
DETAILED DESCRIPTION OF EMBODIMENTS
An acoustic detector 10 is illustrated in figure 1. The components of the
detector 10
visible in figure 1 will now be described. The detector 10 comprises a
protective cage
20, enclosing a windshield 30 of a corresponding shape. The shape of the
protective
cage 20 is described in detail below. A base plate 40 attaches to four
mounting feet
50. The mounting feet 50 can themselves attach to another unit, as required,
to
provide a stable mounting for the acoustic detector 10.
The protective cage 20, which in this example is constructed of steel wire,
defines a
generally cylindrical shape, enclosed at one end of the cylinder by a domed
end
portion. The profile of the domed end portion is, as illustrated, ellipsoidal.
The reader
will appreciate that other forms of curvature of the domed end portion may be
useful,
such as paraboloidal, or hemispherical shapes.
Thus, the protective cage 20 comprises radial and annular wires 22, 24. The
radial
wires 22 define the outline of the domed shape, and cross at a crossing point
coincident with the rotational axis of the protective cage 20. The annular
wires 24
surround the radial wires 22. At crossing points of the radial and annular
wires 22, 24,
and at the crossing point of the radial wires 22, wires are welded to provide
a sound
structure.
The cross-sectional diameter of the wires is selected, along with the steel
material, to
provide sufficient structural strength for the protective cage 20 dependent on
the
required application.
The structure and material of the protective cage is merely exemplary, and a
suitable
component could equally be provided from other materials, such as plastics or
ceramics materials.
A fixing lug 26 is affixed to the end of each radial wire 22, remote from the
aforementioned domed portion. As is illustrated in figure 3, each lug 26
comprises a
rectangular plate, oriented in a plane perpendicular to the axis of the radial
wire 22 to
which it is affixed. Each lug 26 defines a circular fixing through-hole 28.
CA 02878745 2015-01-20
3
The aforementioned windshield 30, as previously noted, is of a shape which
cooperates with the interior space defined by the form of the protective cage
20. The
windshield 30 comprises a layer of acoustic foam. The foam of the windshield
30 has
acoustic properties such that the windshield 30 allows the transmission of
sound, while
slowing the velocity of incident wind to zero, without very fast spatial
velocity gradients
which would generate turbulence and thus noise. Such foams are generally in
use and
are well known in the field. A typical foam is of an open cell form. The
windshield 30
has a substantially consistent thickness throughout its form, thereby defining
an interior
space which is similar to that defined by the protective cage 20. The
thickness of the
windshield 30 will depend upon the application to which the acoustic detector
10 is to
be put, the acoustic properties of the windshield foam, and any environmental
factors
to be taken into account. For instance, if it is known that the acoustic
detector 10 is to
be placed in an environment susceptible to high winds, different design
decisions may
need to be taken than if the detector 10 is to be used in more benign
conditions.
Formation of the shape of the windshield 30 may be achieved either by "sewing"
a flat
sheet of acoustic foam material into the desired shape or, alternatively,
milling the
shape from a solid block of the acoustic foam material. The latter
may have
advantages, in not introducing seams or other imperfections into the
windshield 30,
which might have an impact on the acoustic properties of the windshield 30. In
certain
embodiments, the windshield 30 could be moulded into the required shape from,
for
instance, liquid components. Various techniques now exist to form foam
components
Additionally, the acoustic foam of the windshield 30 is treated to impart
hydrophobic
properties. This is achieved by adulterating the acoustic foam with a suitable
material.
Typically, neoprene can be used for this purpose. As the reader will
appreciate, it is
important not to impart so much neoprene that the windshield 30 forms an
acoustic
suspended mass, which would affect the acoustic performance of the detector
10. The
exact level of neoprene adulteration to be imparted will depend on the exact
design
employed, and thus can be determined by experimentation.
Alternative hydrophobic treatments could also be used, such as nano-coatings.
Nano-
coatings, suitable for acoustic foams, already exist in the market, such as
AridionTM
produced by P2i Limited of Abingdon, Oxfordshire, UK.
4
The interior surface of the windshield 30 is treated further with a
waterproofing layer of
neoprene. This can be sprayed onto the interior surface to form a thin layer
(at least, thin
relative to the thickness of the windshield 30 itself), thereby preventing
ingress of water, incident
on the exterior of the windshield 30, into the interior space defined by the
windshield 30. This
waterproofing treatment is useful, particularly for implementations of the
acoustic detector 10
which are intended for use in environments where precipitation can be
expected. Of course, for
an acoustic detector 10 only for use in interior situations, or in extremely
dry environments, this
waterproofing treatment may not be necessary.
Note that the waterproofing treatment of the windshield is advantageously on
the interior surface
thereof.
Other materials could be used for the windshield 30, such as acoustic fur.
Acoustic fur is
already used to shield microphones for use in outside broadcasting
environments.
The base plate 40 is generally circular, and is dimensioned so as to encompass
the four lugs 26
at its circular edge. Four circumferentially spaced and threaded through
holes 42,
corresponding with the positions of the through-holes 28 allow connection of
the cage 20 with
the base plate 40, using fixing screws 44.
The mounting feet 50 are affixed to the base plate 40, in a manner to
correspond with the
positions of the lugs 26. There is no particular significance to this
correspondence, and, in other
embodiments, the number of mounting feet and the number of lugs need not be
equal. The
mounting feet 50 are of moulded silicone gel mounts, to act as mounting
springs, so as to
reduce the possibility of vibration of the acoustic detector 10, with
reference to the unit to which
it is affixed, having an operational effect on the acoustic detector 10.
Through holes 28 are
formed in the mounting feet 50, as illustrated in figure 2, to enable
installation of the device on a
platform. For instance, the device could be installed on a motor vehicle, in
us, though other
installation configurations could readily be contemplated by the reader.
Figure 2 illustrates the interior construction of the acoustic detector 10,
with particular reference
to the thickness of the windshield 30 and the space it defines with the base
plate 40. The space
can be considered to comprises two parts. A cylindrical part of
Date Recue/Date Received 2021-07-09
CA 02878745 2015-01-20
the space is taken up by a sensor enclosure 60, which is itself a cylindrical
construction
with one closed end. The closed end is substantially adjacent the base plate
40, but
spaced therefrom to define a thin cylindrical cavity, within which anti-
vibration
mountings 45 support the sensor enclosure 60 on the base plate 40. The
opposite end
5 of the enclosure 60 is closed by a sensor plate 62. The sensor plate 62
has five
through holes 64 defined therein. As further illustrated in figure 4, the
through holes 64
are arranged in a cruciform formation, centred with the disk. Each through-
hole 64 has
a microphone 70 mounted therein, oriented towards the domed portion of the
interior
space of the acoustic detector 10, that is, upwards towards the windshield 30
and the
cage 20 as illustrated in figure 2.
The anti-vibration mountings 45 are, in this embodiment, made of silicone gel,
tuned,
together with the mass of the acoustic sensor enclosure 60 to as low a
frequency as
possible, commensurate with adequate strength to keep the senor enclosure 60
restrained under shocks delivered through any structure or vehicle to which
the
detector 10 may be attached.
As will be appreciated by the reader, the five microphones 70 are themselves
connected to electronic processing components, designed, for a particular
application,
to detect and process electronic signals arising from acoustic energy incident
on the
microphones 70. The particular detail of the electronic processing components
is not
necessary for an understanding of the present embodiment. While the particular
electronic processing capabilities of the device is not part of this
disclosure, an
accelerometer 80 is provided, mounted on the base plate 40. The accelerometer
80
can be used to provide a phase reference for interfering vibrations,
permitting their
cancellation from acoustic sensor outputs.
It will be appreciated by the reader that various numbers and arrangements of
microphones may be used. In most cases, the microphones 70 will be arranged in
a
coplanar configuration, such as arranged in a circle, concentric circles or as
a cruciform
pattern. The present embodiment includes five microphones as an example only.
In some cases, a configuration may include at least one microphone positioned
out of a
plane defined by the other microphones. This may aid in three dimensional wave
CA 02878745 2015-01-20
6
detection. For instance, the illustrated embodiment could be modified such
that one of
the illustrated microphones is positioned above the plane of the sensor plate
62.
The shape of the void created between the sensor plate 62 and the windshield
30 has
an operational effect on the acoustic detector 10. While various specific
profiles may
achieve the same, or similar, end results, it is useful here to describe the
intended
properties to be achieved, so that the skilled reader can select a shape
suitable to fit
the circumstances of any particular implementation.
The acoustic detector 10 is intended to be used in circumstances presently
fulfilled, to
an extent, by omnidirectional acoustic detectors. However, it is observed here
that the
majority of acoustic signals which will be of interest to a user, will emanate
from
sources roughly at the same altitude as the observation position ¨ i.e. the
position of
the acoustic detector. Hence, omnidirectionality is not a key requirement for
such
detectors. The present arrangement, therefore, focuses on effective detection
of
acoustic waves emanating in directions roughly parallel with ground level.
This working assumption has two main impacts.
Firstly, the general direction of propagation of acoustic waves will be
roughly parallel to
the sensor plate 62. Secondly, reflected waves will also emanate from the
source of
any acoustic waves ¨ a particular mode of reflection will be a direct ground
reflection
between the source and the detector, but other modes may also exist. The time
of
arrival of these reflected waves at the detector may be a time delay after the
arrival of
the primary wave, but the time delay may be very short, especially for distant
sources
close to ground level.
The design of the acoustic detector 10 therefore very much governs how
sensitive the
detector will be to such acoustic waves, and particularly to reflected
acoustic waves.
Previous approaches have relied upon omnidirectionality as being a key
component of
the sensitivity to reflective acoustic waves, but the present device does not
enable this
approach. Instead, the internal curvature of the windshield 30 plays a key
element in
this. The dome-shaped interior void provides a reflection profile which is
suited to the
particular application. Especially, the curvature should be such that
reflections from the
internal surface of the windshield 30 do not reflect back onto the microphones
70.
CA 02878745 2015-01-20
7
Thus, the exact shape of the dome of the windshield 30 (and the consequent
shape of
other components) will be determined by experimentation. Certain properties
have
been found to have an ameliorative impact. In particular, it has been found
beneficial
for the height of the dome (i.e. the distance between the interior surface of
the
windshield 30 and the closest surface thereto of the sensor plate 62) to be
substantially
the same as the axis-to-axis distance between microphones 70.
The curvature of the dome can be ascertained by application of fundamental
theory
relating to curved reflectors. In particular, by using a substantially
paraboloidal
approach (with suitable approximation), the passage of an acoustic wave from
beneath
the plane of the sensor plate 62 can be managed such that it does not become
incident
on a microphone. Figure 5 illustrates this in simple terms.
Such an acoustic wave will propagate through the acoustic detector 10 in
several ways.
Firstly, a part of the wave will, inevitably, diffract at the edge of the
sensor plate 62, and
will then propagate substantially in the plane of that sensor plate. This will
then be
detected at the microphones 70. This is labelled "1" in figure 5. Another part
of the
wave (labelled "2") will continue onwards through the void defined above the
sensor
plate 62, to the interior surface of the windshield 30. The interior surface
of the
windshield 30 will cause a further segmentation of the propagation of the
acoustic
wave. Part of the wave (labelled "3") will, again, propagate directly through
the
windshield 30 and out of the detector 10. Another part (labelled "4") will be
reflected by
the internal surface of the windshield 30. This may, in part, be abetted by
the neoprene
waterproofing coated to the internal surface. However, the curvature of the
windshield
means that the resultant acoustic reflection will not reflect back onto the
microphones 70 ¨ it will, instead, reflect down beyond the outer extent of the
microphones 70 and out of the detector 10.
30 Thus, as set out above, the detector 10 offers an opportunity for
incident sound waves
to be presented cleanly to the microphones 70, in a manner whereby internal
reflections within the detector 10 are minimised if not eliminated. Moreover,
the spatial
properties of an incident sound wave are maintained on entry to the detector,
ensuring
that the sound waves incident on the microphones are not distorted in time or
space.
In essence, the detector's acoustic transmissive properties allow its use as
if the
CA 02878745 2015-01-20
'
,
8
shielding 30 were not in place, but with the advantageous noise reduction and
water
resistant properties of the device 10.
As a result, the sound waves are incident on the microphones 70 in a manner
such that
timing information can be determined from the moment of incidence of a sound
wave
on each microphone in turn. By this timing information, it can be seen that
electrical
signals generated at the microphones 70 can be processed straightforwardly to
deduce
the bearing of an incident sound wave, and thus the origin of the sound
emission
producing the sound wave.
The exact curvature to be employed, will therefore be subject to design
selection. It
has been found that a paraboloidal shape is effective, but other shapes may
also be
effective and so the present disclosure is not limited to that particular
shape.
While certain embodiments have been described, these embodiments have been
presented by way of example only, and are not intended to limit the scope of
the
inventions. Indeed, the novel devices described herein may be embodied in a
variety
of other forms; furthermore, various omissions, substitutions and changes in
the form of
the methods and systems described herein may be made without departing from
the
sprit of the inventions. The accompanying claims and their equivalents are
intended to
cover such forms or modifications as would fall within the scope and spirit of
the
inventions.
