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We claim:
1. An arrangement for altering spatial response in a directional microphone
mounted in a desktop computer display monitor, said arrangement
comprising:
a first acoustically opaque surface having a first edge, said first surface
being a housing surface of said computer display monitor and having
dimensions longer than wavelengths of sounds in human speech;
a second acoustically opaque surface oriented substantially orthogonally to
said first surface, said second surface being a housing surface of said
computer display monitor and having a second edge coincident with said
first edge, and further having dimensions longer than wavelengths of
sounds in human speech; and
a directional microphone having a first spatial response, being mounted in
said computer display monitor and disposed substantially along said first
and second edges, and receiving primary sounds from locations in front of
said directional microphone and secondary sounds from locations not in
front of said directional microphone, such that said first spatial
response is dependent principally on a pressure value and a pressure
gradient value associated with said primary and secondary sounds;
whereby said first and second acoustically opaque surfaces alter said
pressure value and said pressure gradient value, thereby altering said
first spatial response to a more directional second spatial response.
2. The arrangement according to claim 1, wherein said directional
microphone comprises:
a microphone housing having a top surface and a bottom surface;
a frontal opening disposed between said top and bottom surfaces;
a plurality of nonfrontal openings disposed in said top surface; and
acoustical resistance means surrounding and positioning said directional
microphone within said microphone housing for further adjusting said first
spatial response of said directional microphone.
3. The arangement according to claim 2, wherein said nonfrontal openings
substantially preserve said pressure gradient values across said
directional microphone and wherein said secondary sounds are maximally
preserved across said directional microphone.
4. The arrangement according to claim 1, wherein said acoustically opaque
surfaces, comprise a vertical front panel and horizontal top panel of a
computer visual display monitor.
5. The arrangement according to claim 1, wherein said directional
microphone comprises a cardioid pressure gradient microphone capsule.
6. The arrangement according to claim 1, wherein said first spatial
response comprises a cardioid pattern.
7. The arrangement according to claim 1, wherein said spatial response
comprises a hypercadioid pattern.
8. A directional microphone arrangement mounted in a computer display
monitor comprising:
a microphone housing located substantially in the center of, and coincident
with, the abutting edges of a first vertical surface of an enclosure of
said computer display monitor and a second horizontal surface of said
enclosure, said first vertical surface substantially orthogonal to said
second horizontal surface;
a cardioid pressure gradient microphone capsule disposed within said
microphone housing and having a cardioid spatial response, said cardioid
pressure gradient microphone capsule creating electrical output signals
corresponding to a plurality of primary sounds arriving from locations in
front of said directional microphone and a plurality of secondary sounds
arriving from locations not in front of said directional microphone;
at least one frontal opening disposed in said microphone housing for
substantilly conducting said primary sounds to said directional microphone
capsule to maximally preserve a sound pressure value;
a plurality of nonfrontal openings disposed in said microphone housing for
substantially conducting said secondary sounds to said directional
microphone capsule to maximally preserve a sound pressure gradient value
across said directional microphone capsule;
an electrostatic shielding disposed between said microphone capsule and the
rest of said computer display monitor, and providing shielding of said
microphone capsule from electrostatic noise emanating from said computer
display monitor; and
acoustical resistance means surrounding and positioning said microphone
capsule within said microphone housing for further adjusting said first
spatial response of said cardioid pressure gradient microphone capsule;
whereby said vertical and horizontal surfaces of said enclosure alter said
sound pressure value and said sound pressure gradient value associated
with said primary and secondary sounds to alter said cardioid spatial
response to an increased directionality hypercardioid spatial response.
9. The microphone arrangement according to claim 8, wherein said primary
sounds received by said cardioid pressure gradient microphone capsule
principally comprise unreflected sound waves.
10. The microphone arrangement according to claim 8, wherein said secondary
sounds received by said cardioid pressure gradient microphone capsule
principally comprise sound waves arriving from behind and from each side
of said microphone housing.
11. The microphone arrangement according to claim 8, wherein said surfaces
of said enclosure comprise a front panel and a top panel of a computer
visual display monitor.
12. The microphone arrangement according to claim 10, wherein said
hypercardioid spatial response includes at least two off-axis nulls in an
azimuth plane.
13. A method for altering spatial response in a directional microphone
arrangement mounted in a computer display monitor comprising the steps of:
providing a first acoustically opaque housing surface of said computer
display monitor having a first edge, and having dimensions longer than
wavelengths of sounds in human speech;
positioning a second acoustically opaque housing surface of said computer
display monitor substantially orthogonal to said first surface, said
second surface having a second edge coincident with said first edge, and
having dimensions longer than wavelengths of sounds in human speech; and
locating a directional microphone having a first spatial response
substantially along, and in the center of, said first and second edges,
said directional microphone receiving primary sounds from locations in
front of said directional microphone and secondary sounds from locations
not in front of said directional microphone, said first spatial response
dependent principally on a pressure value and a pressure gradient value
associated with said primary and secondary sounds;
whereby said first and second acoustically opaque surfaces alter said
pressure value and said pressure gradient value, thereby altering said
first spatial response to a more directional second spatial response.
14. The method according to claim 13, wherein said directional microphone
comprises a cardioid pressure gradient microphone capsule.
15. The method according to claim 13, wherein said first spatial response
comprises a cardioid pattern.
16. The method according to claim 13, wherein said second spatial response
comprises a hypercardioid pattern.
17. The method according to claim 13, wherein said primary sounds received
by said directional microphone principally comprise unreflected sound
waves.
18. The method according to claim 13, wherein said secondary sounds
received by said directional microphone comprise sound waves principally
originating from behind and from each side of said directional microphone.
19. The method according to claim 13, wherin said acoustically opaque
surfaces comprise a vertical front panel and a horizontal top panel of a
computer visual display monitor.
20. The method according to claim 13 further comprising the steps of:
providing a microphone housing having a top surface and a bottom surface;
placing at least one frontal opening disposed between said top and bottom
surfaces;
providing a plurality of nonfrontal openings disposed in said top surface
to substantially preserve said secondary sounds whereby said pressure
gradient values are maximally preserved across said directional
microphone;
and
surrounding said directional microphone with acoustical resistance means
for further adjusting said first spatial response of said directional
microphone. |
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Description |
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RELATED APPLICATIONS
The present invention is related to the following specifications filed
concurrently with the application on which this patent is based: "Computer
Visual Display Monitor with Stereo Speaker and Directional Microphone and
Method for Construction", Ser. No. 08/082,682 filed Jun. 23, 1993; and,
"High Performance Stereo Sound Enclosure for Computer Visual Display
Monitor and Method for Construction", Ser. No. 08/082,217, filed Jun. 23,
1993, now U.S. Pat. No. 5,400,408.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to visual display devices. More
particularly, the present invention relates to a computer visual display
monitor having an integral stereo sound reproduction facility and an
integral microphone.
2. Art Background
Video displays are pervasively used as visual output devices for all types
of computer systems. Most commonly, a cathode ray tube (CRT) is enclosed
within a supporting enclosure together with associated subsystems such as
CRT control circuits and power supplies to form a computer visual display
unit commonly known as a monitor. The monitor is then coupled to receive
video signals from a main processor unit including video control logic.
Prior art monitors have not heretofore incorporated high quality audio
capability. In fact, prior art computer systems as a whole have only
included minimal, low quality sound generation capabilities for limited
purposes such as generating user prompt signals, system warning signals,
and others. For example, most personal computer systems incorporate a
small relatively low performance speaker to produce the above prompt and
warning signals, usually comprising one or more low fidelity "beep" tones,
or very simple synthesized speech phrases. In low performance audio
applications, construction and placement of the speaker system usually is
immaterial. In fact, in most personal computer systems the speaker is
typically mounted to a bottom-, side-, or rear-facing panel of the
computer enclosure, thus leaving the front panel of the enclosure free for
user access to mass storage devices contained therein, or other user
controls. Remote speaker placement is usually adequate for the
above-mentioned prompt and warning signals because such sounds generated
by computer systems typically comprise relatively narrow bands of sound
without any significant low frequency (bass) components, and are played at
low volume levels. When complex sounds such as voice and music are
generated or played back through a low quality, arbitrarily positioned
speaker, the resulting uneven frequency response severely restricts the
utility of the sound system, which is otherwise adequate for simple
warning tones.
More recently, with the advent of digital sound recording and processing
techniques, the increased use of sound by computers within computing
applications, as well as the need for high quality recording and
reproduction of sound within personal computing systems, is desirable.
However, despite substantial improvements in personal computing system
performance in terms of numeric processing speed and visual display
clarity, the recording and reproduction of high quality sound within
computer systems have not enjoyed similar advancements. In other words,
although modern digital recording techniques produce very high quality
source material, the recreation of high quality sound from such recorded
media within computing environments generally, and personal computing
environments specifically, has been extremely poor in the prior art. The
foregoing is principally due to the inability to generate high quality
full frequency sound from a small panel-mounted speaker, as previously
described in connection with personal computer systems. The audio
reproduction problem is compounded even further when reproduction of high
quality stereophonic sound is desired.
Although relatively high quality full frequency stereophonic sound has been
integrated into television environments, such integration is fundamentally
different from the application of high quality sound to computer visual
display monitors. First, television images are generated by relatively low
resolution analog signals, which images can be created on large dot pitch
CRTs, i.e., where the illuminating phosphor dots are spaced further apart.
By contrast, computer generated images, whether animated or sampled video
signals, are significantly higher resolution digital images requiring much
finer dot pitch CRTs. Typically, television CRTs employ what is known as a
shadowmask to help direct the phosphor-exciting electrons to the
appropriate phosphors. Shadowmasks are opaque sheets having as many holes
as there are phosphors, which holes are usually grouped in threes with
space provided between the groups. Although completely adequate for most
television viewing, shadowmasks are inherently performance limited: the
hole spacing limits the ultimate resolution, and the mask itself blocks a
significant percentage of total electron flux, thereby limiting the CRT's
brightness. A few CRTs employ an alternative screening device known as an
aperture grill, such as the Trinitron.RTM. CRT manufactured by Sony
Corporation. Trinitron.RTM. CRTs employing aperture grills can be
constructed in normal resolution as well as high resolution variants, and
consist essentially of an arrangement of finely spaced wires which can be
caused to vibrate at some resonant frequency. Unlike the larger, normal
resolution aperture grills which resonate at a very low frequency or not
all, the smaller, high resolution aperture grills used in computer display
monitors can have their resonant frequencies well within musical and
ordinary listening frequencies. Accordingly, if a high quality
stereophonic sound system is brought into sonic communication with a high
resolution aperture grill having its natural frequency within the musical
spectrum, the grill will vibrate and severely degrade visual image quality
displayed on the CRT, usually manifesting itself as shimmering waves of
darkness and brightness similar to Moire interference. The foregoing image
degradation usually is not encountered in lower resolution displays
intended for television use for several reasons, including the fact that
television images typically comprise non-static, non-uniform backgrounds.
In contrast to typical television images, computer system images
frequently include stationary, homogeneous shapes and colors, against
which the image degradation due to aperture grill vibration is easily and
noticeably seen. However, the aperture grill problem will have to be
addressed when designing high resolution CRT display devices incorporating
high quality sound reproduction systems, whether for computer environments
or for the anticipated High Definition Television (HDTV) systems presently
under design.
Finally, prior art computer systems are generally not constructed to
facilitate recording of voice signals, in particular for purposes of
speech recognition. Where facility for voice recording is provided in
prior art systems, typically an external microphone must be maintained in
close proximity to the speaker's mouth, for example by gooseneck extension
or by a headset arrangement. Such microphone attachment effectively ties
the user to the computer and restricts the user's movement. Alternatively
in the prior art a microphone may be attached to an exterior surface of a
computer system component such as the processor enclosure or the monitor.
However, such external attachments typically result in non-optimal
microphone performance, where directional characteristics of the
microphone, if any are provided by its design, are arbitrarily modified by
nearby reflecting surfaces and refracting edges. The result is that the
ratio of the desired speech signal to undesired room noise and reflections
is unacceptably degraded.
As will be described in the following detailed description, the present
invention overcomes many of the limitations associated with prior art
computer systems and visual displays by providing an integrated, high
resolution, video display monitor incorporating high quality stereophonic
sound generation facility as well as speech recognition differential
microphone. A user of the present invention can use the integrated monitor
of the present invention to display high quality graphics or video data
accompanied by full frequency stereophonic sound without compromising the
quality of the projected image. Furthermore, the integral directional
microphone is positioned and optimized to receive and resolve human speech
generated by the computer user, and to provide maximum cancellation of
unwanted sounds. The signal from the microphone may then be used for many
applications where clear pickup of speech is required, such as voice
recognition, teleconferencing, and voice annotation.
SUMMARY OF THE INVENTION
A cathode ray tube (CRT) visual display monitor intended for personal
computer systems and integrating a high quality stereophonic speaker
system and a directional speech recognition microphone, and methods for
construction of same, are disclosed. The CRT is contemplated to be of a
high resolution type incorporating a comb-like vertical aperture grill
having a natural frequency within the musical scale.
The stereo speaker system comprises a ported speaker enclosure within which
are mounted two conventional speaker assemblies. A stereo audio amplifier
and control unit for modifying the audio signals delivered to the speakers
is mounted within the speaker enclosure. The speaker enclosure is
internally mounted within the monitor such that the speakers mounted
therein project sound in a direction towards the user facing the monitor.
Further, the speaker enclosure is mounted with a minimum number of
attachment members, which members include vibration isolation material to
attenuate transmission of vibration components near the resonant frequency
of the aperture grill when the speaker system is operating. The material,
shape, and assembly method of the speaker enclosure are selected to reduce
structural deformations contributing to airborne transmission of vibration
to the aperture grill.
The integral directional microphone assembly comprises a pressure gradient
electret microphone capsule located within a directional
sensitivity-modifying microphone housing. The housing further comprises a
microphone preamplifier, and is mounted internally within the monitor
housing facing the user and adjacent to the top surface of the monitor
housing. The microphone housing has a large frontal opening and a
plurality of rearwardly facing openings which together enable differential
sound pressures to be applied to the directional microphone capsule
retained therein by acoustically transparent foam. The directional
characteristics of the microphone assembly, when installed within the
monitor housing, are optimized for maximum rejection of noise and
undesired sounds.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood more fully from the detailed
description given below and from the accompanying drawings of the
preferred embodiment of the invention in which:
FIG. 1A is a perspective illustration of a cathode ray tube (CRT) high
resolution video display monitor integrating a stereo speaker arrangement
and a directional microphone arrangement according to the present
invention being used in a typical personal computing environment.
FIG. 1B is an exploded view illustration of the high resolution CRT monitor
shown in FIG. 1A, including a CRT having an aperture grill.
FIG. 2A is a front perspective view of the stereo speaker arrangement and a
first vibration attenuating grommet.
FIG. 2B is a rear perspective view of the stereo speaker arrangement and a
second vibration attenuating grommet.
FIG. 3A is a perspective view of the first vibration attenuating grommet.
FIG. 3B is a perspective view of the second vibration attenuating grommet.
FIG. 4 is an exploded view illustration of the stereo speaker enclosure of
the present invention.
FIG. 5A illustrates the theoretical frequency response of structural and
airborne vibrations transmitted from the speaker enclosure to the
CRT/aperture grill.
FIG. 5B is an empirical plot of the audio frequency response of the speaker
enclosure.
FIG. 6A is an exploded view perspective illustration of the directional
microphone arrangement of the present invention.
FIGS. 6B-C are alternative top plan and front elevation views of the voice
recognition microphone shown in FIG. 6A.
FIG. 7 is a polar plot of directional microphone spatial response relative
to on-axis response at 400 Hz and 1 kHz.
FIG. 8 is a cross-sectional side view of the interior of the integral
microphone illustrating the electrostatic shielding arrangement.
DETAILED DESCRIPTION OF THE INVENTION
The present invention discloses an integration arrangement for a cathode
ray tube (CRT) high resolution visual display monitor intended for
personal computer systems and integrating a high quality stereophonic
speaker system and a directional microphone, and methods for construction
of same.
In the following description, for purposes of explanation, specific
numbers, times, dimensions, and configurations are set forth in order to
provide a thorough understanding of the present invention. However, it
will be apparent to one skilled in the art that the present invention may
be practiced without these specific details. In other instances, well
known systems are shown in diagrammatic or block diagram form in order not
to obscure the present invention unnecessarily.
THE INTEGRATED MONITOR
Reference is now made to FIG. 1, wherein is shown an exploded view
illustration of an integrated, cathode ray tube (CRT) high resolution
video display monitor 5 integrating a stereo speaker arrangement and a
voice recognition microphone arrangement intended for personal computing
environments. As shown in FIG. 1, monitor 5 comprises a housing 7
enclosing a high resolution CRT 8 of the type employing a vertical
aperture grill 9 to direct an impinging electron beam (not shown) sourced
from an electron gun 12 towards a multiplicity of screen phosphors 13
formed on an interior surface of a CRT screen 14. Unlike the more commonly
employed perforation shadow mask (not shown) comprising an opaque shield
having a large number of regularly spaced clusters of circular
perforations aligned with the CRT phosphors 13, the aperture grill 9
comprises a comb-like structure having a large number of narrowly spaced
wires 16 which vertically span the interior surface of screen 14. Wires 16
are aligned to CRT 8 such that electrons in the electron beam which pass
between any two wires strike and illuminate the corresponding phosphors
13, the space between any two wires corresponding generally to the width
of a phosphor. Alternatively, electrons which strike any wire 16 will not
strike, and thus will not illuminate, a phosphor 13. In CRT 8, the wires
16 of aperture grill 9 are more closely spaced than the perforations in a
shadow mask, and extend in a direction orthogonal to the horizontal scan
direction of the electron beam. A widely known example of a CRT employing
an aperture grill is the Trinitron.RTM. CRT manufactured by Sony
Corporation. Trinitron.RTM. CRTs employing aperture grills can be
constructed in normal resolution as well as high resolution variants.
As described, wires 16 of aperture grill 9 form a tuned mechanical system
which has a resonant, or natural, frequency. The natural frequency depends
upon the length, the diameter, and the spacing of the wires 16 comprising
grill 9, as well as the size of the CRT screen 14. As practiced in the
present invention, the aperture grill 9 of the high resolution
Trinitron.RTM. CRT has its natural frequency within the audible spectrum
near 440 hertz (Hz), equivalent to the musical tone middle "A". Other
tunings are possible depending on materials and dimensions of the aperture
grill being designed. Throughout this section, and for the balance of this
detailed description, the present invention benefits from engineering
features which minimize or preclude transmission of structural or airborne
vibration from sound generation devices to CRT 8, and specifically to the
aperture grill 9 within the CRT 8.
As shown in FIG. 1, housing 7 comprises a bottom panel 7a, a front panel
7b, side panels 7c and 7d, a back panel 7e, and a top panel 7f. Housing 7
is mounted upon and supported by a base 6 which includes a tilting and
swiveling means 4 for adjusting housing 7 to any desired position for
viewing depending on the user's preference. A ported stereo speaker
enclosure 20, comprising a pair of conventional voice coil actuated cone
speakers 21a and 21b mounted at laterally opposing ends of enclosure 20,
is mounted inside housing 7 below CRT 8. As shown in FIG. 1, speaker
enclosure 20 is located inside housing 7 directly behind front panel 7b
and positioned above bottom panel 7a and between side panels 7c and 7d.
Speaker enclosure 20 is mounted to housing 7 with an absolute minimum
number of attachment points to minimize structurally transmitted vibration
from the speaker enclosure 20 to aperture grill 9 within CRT 8. Details
concerning the internal arrangement and construction of speaker enclosure
20 will be described more thoroughly below in the section entitled
"Speaker Enclosure".
With reference to FIGS. 1A and 1B, attachment points are provided on
enclosure 20 by identical first and second cylindrical bosses 22 and 23
extending outward from opposing distal ends of the enclosure 20, as well
as an ovaloid tubular member 25 extending outward from an ovaloid opening
24 in the rear portion of enclosure 20. In an alternative embodiment,
bosses 22 and 23 may be replaced with a pair of pin members or
self-tapping screws (not shown) projecting through a locating opening (not
shown) formed in a tab (not shown) extending from enclosure 20. As shown
in FIG. 1A, bosses 22 and 23 are solid members with no porting
functionality. In FIG. 1B, tubular member 25 is shown to have a flanged
portion 25a at the end of a tube shaft 25b, and in addition has an open
volume 25c disposed within the interior of shaft 25b. Flanged portion 25a
of member 25 is commensurately larger than the cylindrical bodies of
bosses 22-23, due to the presence of the open volume 25c defined within
shaft 25b. The function of tubular member 25 is two-fold. Firstly, member
25 serves as an attachment point for the posterior portion of the speaker
enclosure 20 to the monitor enclosure 7. Secondly, opening 24 fitted with
tubular member 25, specifically volume 25c within shaft 25b, comprises a
port through which air may pass as necessary in connection with motion by
the speaker cones of speakers 21a and 21b, as is generally known in the
design and construction of speaker systems. Acoustic aspects and other
associated functionality of the port provided by opening 24 and tubular
member 25 will be discussed in more detail below in connection with the
speaker enclosure itself.
Over each of the cylindrical diameters of each of the bosses 22 and 23, and
over the ovaloid diameter of tubular member 25, is fitted an elastomeric
grommet 27a, 27b, and 27c, respectively, formed of an energy dissipative
material specified to absorb vibration components near the natural
frequency of aperture grill 9, and principal harmonics thereof. The
elastomeric grommets 27a-c are formed of any suitable commercially
available vibration damping material such as IsoLoss or C-1002. In one
embodiment, grommets 27a-c are fabricated of chlorobutyl rubber,
manufactured by Exxon Corporation.
Reference is now made to FIGS. 2 and 3. In FIG. 2, grommets 27a and 27b are
identical and are shown in perspective view, each grommet 27a and 27b
having an annular groove 28 extending around an external circumference 29a
centered between two planar faces 30a and 30b. Further, grommets 27a and
27b have an internal circumference 29b dimensioned to conform snugly over
either boss 22 or 23, internal circumference 29b further comprising a
first multiplicity of rib members 31. As shown, rib members 31 minimize
contact area between grommets 27a-b and bosses 22-23 and stiffen the
grommet material contacting the boss 22 or 23. Every other one of rib
members 31 extends beyond the planar faces 30a and 30b of the grommet,
thereby forming vibration absorbent cushions 31a which provide vibration
attenuation in an axial dimension. Rib members 31 minimize total contact
area between grommets 27a-b and bosses 22-23, and stiffen the grommet
material contacting the boss 22 or 23 when enclosure 20 is mounted to the
housing 7. The combined effect of contact area reduction and material
stiffening reduces the conduction path for mechanically transmitted
structural vibrations produced by speakers 21a and 21b within the speaker
enclosure 20 from reaching the aperture grill 9. In FIG. 3, grommet 27c is
shown in perspective view to comprise an ovaloid body 32 having an
internal circumference 32a conformally extending over the shaft portion
25b of tubular member 25. Moreover, grommet 27c is shown to have an
external circumference 32b, there being an annular flange-like structure
32 bounding one side of a cylindrical body 32. A second multiplicity of
rib members 33 is distributed at regular intervals on external
circumference 32b. As in the case of grommets 27a and 27b shown in FIG.
1a, the rib members 33 formed on external circumference 32b of elastomeric
grommet 27c generally serve to minimize total contact area between the
support point at the rear of speaker enclosure 20, namely opening 24, and
housing 7, thus ensuring that structural vibrations sourced from the
speaker enclosure 20 are effectively interdicted prior to adversely
affecting the aperture grill 9 within the high resolution CRT 8.
Importantly, the size and number of first and second pluralities of rib
members 31 and 33 are adjusted so that the natural frequencies of the
several grommets (27a-27c) are the same.
Referring now to FIGS. 1-3 inclusively, mounting of speaker enclosure 20
into housing 7 will be described. When speaker enclosure 20 is moved into
position above bottom panel 7a and between side panels 7c and 7d, the
bosses 22 and 23 fitted with elastomeric grommets 27a and 27b, are
respectively received into a pair of complementary slotted sockets 33a and
33b respectively formed in the side panels 7c and 7d of housing 7. At the
same time, the flanged portion 25a of tubular member 25 is received into a
socket 33c formed in rear panel 7e of housing 7. The elastomeric grommet
27c fitted to the shaft 25b of tubular member 25 is thereby seated within
opening 24 in enclosure 20. Slotted sockets 33a and 33b bear against the
annular grooves 28 of each of the respective elastomeric grommets 27a and
27b, and partially compress the compliant material so as to secure the
grommets, and thus the bosses 22 and 23, within the slotted socket region.
The speaker enclosure 20 is thus retained within housing 7 with only three
support points: one at each of the distal ends of the speaker enclosure 20
(bosses 22 and 23), and one at the rear of the enclosure 20 (opening 24).
With respect to grommets 27a, 27b, and 27c, the desired degree of vibration
isolation between | | |
