U.S. patent number 3,666,041 [Application Number 05/054,203] was granted by the patent office on 1972-05-30 for speaker enclosure.
Invention is credited to Alan A. Engelhardt.
United States Patent |
3,666,041 |
Engelhardt |
May 30, 1972 |
SPEAKER ENCLOSURE
Abstract
An improved speaker enclosure provides a close approximation to
a true exponentially expanded horn for a bass-responsive speaker in
a relatively small-volume cabinet that is disposed at the corner of
a room. The cabinet also includes a separate section for a
mid-range responsive speaker which can be properly phased with
respect to the acoustic waves at the output of the bass-responsive
section.
Inventors: |
Engelhardt; Alan A. (Santa
Clara, CA) |
Family
ID: |
21989428 |
Appl.
No.: |
05/054,203 |
Filed: |
July 13, 1970 |
Current U.S.
Class: |
181/145 |
Current CPC
Class: |
H04R
1/30 (20130101) |
Current International
Class: |
H04R
1/30 (20060101); H04R 1/22 (20060101); G10k
013/00 (); H04r 001/28 () |
Field of
Search: |
;181/31B,31A,31R,27 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tomsky; Stephen J.
Claims
I claim:
1. Acoustic apparatus comprising:
an enclosure having a front face and containing an acoustic channel
therein having an inlet and an outlet for the passage therethrough
of acoustic waves from the inlet to the outlet thereof, the channel
having a mean acoustic path from the inlet to the outlet and having
a cross section which increases with length along the mean acoustic
path from inlet to outlet;
said channel being substantially symmetrically disposed about a
vertical plane and being oriented with the outlet thereof disposed
in the front face of the enclosure near the base thereof and with
the inlet disposed above the outlet;
said mean acoustic path includes a vertically oriented portion
which is disposed intermediate the inlet and outlet and which is
located rearwardly of the front face of the enclosure; and
a bounded chamber within said enclosure which is disposed above the
outlet and which communicates substantially only with said inlet of
the channel in a boundary of said chamber that is disposed
rearwardly of the front face of said enclosure, said inlet of the
channel being disposed intermediate said front face and the
rearmost portion of said enclosure and being aligned along an inlet
axis which is directed generally rearwardly.
2. Acoustic apparatus as in claim 1 wherein the mean acoustic path
of said channel spirals in said vertical plane approximately one
half revolution from inlet to outlet, and the cross-sectional area
of said channel disposed substantially normal to the mean acoustic
path increases substantially exponentially with length along said
mean acoustic path.
3. Acoustic apparatus as in claim 1 comprising an electroacoustic
transducer for producing acoustic waves in response to electrical
signals applied thereto, said transducer being disposed within said
bounded chamber at the inlet to said channel for producing in
response to applied electrical signals acoustic waves which emanate
from the enclosure substantially only through said channel.
4. Acoustic apparatus as in claim 3 wherein:
said electroacoustic transducer is responsive to electrical signals
applied thereto over a selected range of signal frequencies;
said enclosure includes an additional acoustic channel positioned
above the chamber and inlet, said additional acoustic channel
having a cross-sectional area which increases substantially
exponentially with length along the mean path from the inlet to the
outlet thereof, with the mean path of the additional acoustic
channel and the inlet and outlet thereof being disposed in
substantially horizontal alignment;
said outlets of the acoustic and additional acoustic channels have
areas which are in the ratio of at least approximately 3:2 and are
positioned in the front face of said enclosure for proper phasing
of acoustic waves emanating therefrom; and
an additional electroacoustic transducer communicating with said
inlet of the additional acoustic channel, the additional transducer
producing acoustic waves in response to electrical signals applied
thereto over a range of signal frequencies higher than said
selected range.
Description
BACKGROUND OF THE INVENTION
It has long been known that proper acoustic matching between a
bass-responsive speaker operating down to about 30 cycles per
second and the volume of air in a given region or room usually
requires an exponentially expanded horn which is approximately
one-half wavelength or 16 feet long and which has an opening into
the region that is approximately 8 feet in diameter. These
requirements severely limit the practical use of such equipment for
true low-frequency acoustic coupling to a typical size room. Also,
where the low frequency and mid-range frequency speakers are
closely disposed, proper phasing of the acoustic waves from the
speakers is required in order to prevent objectionable
acoustic-wave delays that give rise to undesirable echo effects in
the range of signal frequencies for which both the low frequency
and mid-range frequency speakers show comparable response.
Several designs of speaker enclosures are known in the prior art
for overcoming these problems associated with the proper matching
or loading of the acoustic impedance of a speaker with the volume
of air in a given region or room. A 16-foot horn having a straight
mean path is one design which has proven to be useful in coupling
low frequency acoustic waves into theaters and large auditoriums.
However, this horn design usually presents phasing or acoustic
delay problems in connection with a mid-range speaker disposed near
the opening of the horn. Also, this horn design is not practical
for typical room-sized installations. In these installations,
folded horns have been used in which the mean path for acoustic
waves has little or no exponential expansion with length and is a
tortuous or serpentine path through a series of chambers and
baffles. Several variations of this horn design are described in
the literature but most of them commonly include a mean path length
for acoustic waves that is less than one-half wavelength at 30
cycles per second. This introduces a significant internal loss of
acoustic wave energy at the lowest audible frequencies with the
result that higher harmonics of the lowest audible frequency are
radiated into a room at acoustic power levels that are higher for
the harmonics than for the lowest audible frequency. Also, several
folded horn designs known in the prior art attempt to introduce
exponential expansion with length along the mean path by widening
out the acoustic passage along its length, but only using
side-to-side expansion or top-to-bottom expansion with length, and
not both. Folded-horn speaker enclosures of these designs usually
introduce large internal acoustic mismatches and losses with
concomitant poor response down to the lowest audible frequencies
and also usually introduce undesirable standing wave resonances
between the parallel surfaces that form the acoustic passage.
SUMMARY OF THE INVENTION
In the speaker enclosure of the present invention, the mean
acoustic path spirals outwardly in a vertical plane from a central
speaker to an opening of the horn disposed at floor level below the
speaker. The present design utilizes the floor and intersecting
walls of a room (as at a corner) as a portion of the exponential
expansion of the horn with length along the mean acoustic path.
Also, the speaker enclosure and associated apparatus of the present
invention obviates the usual phasing problems of mid-range and
low-range responsive speakers that are disposed in the same
enclosure. In the present apparatus, the horn opening for the
mid-range responsive speaker is positioned above the horn opening
for the low-frequency responsive speaker so that phasing between
the two sections can be accomplished.
DESCRIPTION OF THE DRAWING
FIG. 1 is a perspective view of the speaker enclosure of the
present invention;
FIG. 2 is a sectional view of the enclosure of FIG. 1 along the
vertical plane of symmetry;
FIGS. 3a, b and c are graphic presentations of the logarithmic
expansion utilized by the present invention;
FIGS. 4a and b are graphic diagrams of the shape and location of
the sections of acoustic passage according to the present
invention; and
FIG. 5 is a top view of the enclosure of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the perspective view of FIG. 1, there is shown the
speaker enclosure 9 of the present invention disposed in the corner
of a room. The enclosure 9 includes three vertically oriented
principal sections, namely, the opening 11 of the low frequency
horn at the base immediately adjacent the floor 13, the low
frequency speaker enclosure section 15 disposed above the horn
opening 11 and the top section 17 which houses the mid-range and
high frequency responsive speakers. Each of these principal
sections are acoustically isolated from each other by horizontally
disposed rigid baffles 19 and 21. The front of central section 15
is enclosed to prevent acoustic energy from radiating outwardly
into the room from the back side of the low frequency responsive
speaker 23 (shown in broken-line view for clarity). This speaker 23
is so mounted in the central section 15 of the enclosure 9 that
acoustic energy emanating from the front of the speaker is radiated
in a direction that is generally upwardly and rearwardly within the
enclosure 9. The mean path 29 of acoustic waves from the speaker 23
then diverts downwardly near the back corner of the enclosure and
then diverts forwardly out through the lower opening 11 of the
enclosure 9, as shown in FIG. 2. The mean path for acoustic waves
thus spirals smoothly in a vertical plane from the speaker 23 to
the lower opening 11 of the enclosure 9 without foldbacks or
serpentine transitions. More importantly, the cross-sectional area
of the acoustic passage from the speaker 23 to the lower opening 11
is substantially exponentially expanded in two orthogonal
directions with length along the mean path. This more closely
approximates the true form of an exponential horn (e.g. a tuba) for
improved acoustic coupling characteristics and concomitant
efficiency of the over-all design.
In FIG. 2, the enclosure 9 is shown sectioned along a vertical
plane that is aligned with the bisector 24 of the intersecting
angle of the rear walls 25, 27 of the enclosure 9 (typically, the
rear walls are commonly disposed at right angles but it should be
understood that other intersection angles may also be used). The
enclosure 9 is substantially symmetrical about this vertical
bisector plane and the present design of folded horn may thus be
viewed conveniently from this plane. As was previously mentioned, a
half wavelength horn which expands exponentially with length
(approximately doubling the rate of expansion every 24 inches) is
the ideal horn for properly coupling the lowest audible frequencies
from a speaker to the volume of air in a given region or room.
Accordingly, the acoustic passage in the present enclosure design
has an approximate exponential expansion in two orthogonally
lateral directions (within a few percent error) over a mean path
length to the opening 11 of the horn of approximately 4 feet. The
acoustic passage of the enclosure 9 may be viewed at regular
intervals of, say, 24 inches along the mean path 29 for acoustic
waves. Viewed in this way, it should be noted that the corner
volume occupied by the present enclosure 9 is utilized to a
substantially optimum degree to achieve an approximate bilaterally
exponential expanded horn. Specifically, the speaker 23, say, of 15
inch circular design is properly back loaded by the confining
chamber 31 at the front of the enclosure 9. The volume in cubic
feet of the confining chamber is computed in a conventional manner
to properly load speaker 23 on its rear side. Also, the speaker 23
is matched to the horn at its front radiating face by partially
blocking the upper and lower portions of the speaker face using an
inlet port 33 to the horn that is circular at the sides with upper
and lower chords that define an oblong inlet port 33, as discussed
in detail hereinafter. From this port, the acoustic passage is
defined by upper and lower walls 35 and 37 and by the sidewalls 39,
41, as shown in FIG. 1. Thus, the horn inlet port 33 is
approximately 8 inches by 15 inches, or less, and expands laterally
with length along mean path 29 beyond location 46 to the first
transition region that includes corner baffle 43. There, the mean
path 29 for acoustic waves in the passage is diverted downward as
the lateral dimensions expand and also undergoes a transition, say,
at location 47 from the rectangular passage near inlet port 33 and
at location 46 to a polygonal shape including the walls 25 and 27
of the enclosure 9 and the corner baffle 43, as shown in the top
sectional view of FIG. 3 and in FIG. 4b. Further along the mean
path 29 at location 49 the acoustic passage is triangular as formed
by the rear baffle board 45 and the walls 25 and 27 of the
enclosure 9. The cross-sectional area of the horn thus expands
about orthogonally lateral axes at each location along the mean
path 29. This passage again becomes polygonal in shape at the lower
transition region including the lower corner baffle 51. Thus, at
location 53, the rear baffle board 45, the walls 25 and 27 of the
enclosure 9 and the lower corner baffle 51 divert the mean path 29
outwardly toward the opening 11 of the horn. Similarly, the
acoustic passage retains polygonal shape of larger cross-sectional
area further along the mean path at location 55 where the
horizontal baffle 19, the walls 25 and 27 and the corner baffle 51
define the passage. The acoustic passage widens out with length
along the mean path 29 toward the opening 11 which is defined by
the base 57, the horizontal baffle 19 and the walls 25 and 27 and
which thus approximates the curved shape of a wave front emanating
from the horn. These aforementioned sections and transitions along
the mean path are shown in FIGS. 4aand b. The vertical walls,
horizontal baffles and transition baffles are all rigidly
interengaged in the angular relationships shown in FIG. 1 using
such suitable fastening means as glue and screws. The planar
surfaces that form the acoustic passage and surroundings are thus
inherently braced against undesirable mechanical resonances.
It should be noted that further along the mean path 29 beyond the
enclosure 9, say, at location 59, the floor and walls of a room at
a corner thereof, together with the front walls of the enclosure 9
define the acoustic passage which continues to expand approximately
exponentially with length. Also, for a distance of about 8 feet
from the present enclosure, unobstructed walls and floor of the
room may constitute the exponentially expanding boundaries of the
acoustic passage of the horn associated with speaker 23 which thus
permits production of fundamental acoustic waves down to about 31
Hertz. The present horn design contained within the lower portions
of the enclosure 9 which is approximately 3 feet high thus provides
acoustic coupling of the speaker 23 to the volume of air within the
room which is approximately 75 percent as efficient as the coupling
that is possible using the ideal 16-foot long exponential horn
having an 8-foot diameter opening. Where it is not essential that
acoustic waves be produced down to the lowest design frequency of
the present horn, the enclosure may be disposed in a room away from
a corner and, since its acoustic passage is substantially complete
within itself, may still provide comparatively efficient coupling
down to about 35 to 40 Hertz.
The mathematical relationships between the various shapes and
dimensions of sections of the acoustic passage are determined by
the exponential rate of expansion of the cross-sectional area with
length along the passage from the inlet port 33 for a selected low
frequency note. Thus, from certain design considerations, a low
frequency note of, say, 30.8 Hertz (i.e. B below low C) may be
selected as the design standard for the exponential flare of the
acoustic passage. It can be shown that the cross-sectional area of
the acoustic passage must double approximately every 24 inches to
provide the desired exponential flare for operation above cutoff at
this design standard frequency of approximately 31 Hertz. Referring
to FIG. 3a, there is shown a graphic presentation of the increments
of cross-sectional area alone, the length of an exponential horn.
In particular, FIG. 3ashows the ideal expansion of the
cross-sectional area at 24-inch increments along the mean length of
an exponential horn over approximately one-quarter wavelength of an
acoustic wave in air at approximately the design standard frequency
of 31 Hertz. Thus, FIG. 3agraphically illustrates the orthogonal
two-axis expansion of cross-sectional area of an ideal acoustic
passage over area limits that may be broader than the actual areas
involved in the present invention.
In practice, a speaker or driver unit is selected which has a
natural resonant frequency at about the design standard frequency
(i.e. about 31 Hertz) and such a unit 23 of commercially available
variety may be found to have approximately 15-inch diameter face.
Thus, by backloading the speaker or driver unit 23 in the enclosure
31, as shown in FIG. 2, and by decreasing the inlet part area 33
immediately adjacent the face of the driver unit to an area of
approximately 8 inches by 15 inches, the acoustic impedance of the
driver unit 23 may be properly matched to the horn mouth with a
natural resonance frequency down to or below the design standard
frequency. The area of the inlet port 33 thus becomes the initial
area from which the exponential flare of the associated acoustic
passage is established, as shown in FIGS. 3b and c . At a location
along the length of the ideal exponential horn represented by FIG.
3a, there is an area of approximately 120 square inches (i.e. about
11 inches square) and, within a 24-inch increment of length from
there, another area that is twice the initial area (i.e. about 15
1/2 inches square, or about 240 square inches), and so on at each
24-inch increment of length thereafter.
In the present enclosure, the initial area at the inlet port 33 is
about 120 square inches in approximately rectangular form of about
8 inches by 15 inches. And at section A--A which is about 24 inches
away from the inlet port 33 along the mean path 29, as shown in
FIG. 2, the cross-sectional area is about 240 square inches in
trapezoidal form, as shown in FIG. 3c. Further, at section X.sub.1
--X.sub.1, X.sub.2 --X.sub.2 (i.e. the curved mouth of the horn)
which is about 24 inches away from the plane of section A--A along
the mean path 29, the cross-sectional area is about 480 square
inches in rectangular form as shown in FIG. 3c. Thus, the acoustic
passage within the present enenclosure provides substantially true
two-axis exponential expansion over approximately one-eighth
wavelength at about the design standard frequency of 31 Hertz. This
exponential expansion is also maintained at the intermediate
intervals, as shown in FIGS. 4a and b, to assure a reasonably
smooth expansion over the mean path length 29 of the acoustic
passage. This one-eighth wavelength of the acoustic passage at the
design standard frequency supports fundamental acoustic waves at
the design frequency for high-efficiency acoustic coupling to the
volume of air in a room. However, the unobstructed portion of a
room corner (i.e. walls and floor in the region about 8 feet in
front of the present enclosure) contributes to the formation of a
substantially true exponentially-expanded horn of one-third
wavelength for acoustic waves at the design standard frequency of
31 Hertz. The present enclosure and the resulting acoustic horn
thus provide a relatively compact design which is conveniently
adaptable to a room as an unobtrusive piece of furniture and which
provides approximately 75 percent over-all coupling efficiency at
the design value of low frequency acoustic waves. Also, the
continuously and reasonably smoothly expanding horn of the present
invention substantially eliminates plane-parallel surfaces (except,
perhaps, at the mouth where the acoustic wave energy is
considerably less concentrated) which can support undesirable
lateral acoustic reflections or standing waves.
Referring again to the perspective view of the enclosure 9, as
shown in FIG. 1, the upper section 17 is arranged to couple
acoustic waves of mid-range frequencies to the volume of air in a
room using a horn design which includes exponential flaring in only
one lateral direction with length, the other lateral dimension
remaining constant as the plane-parallel spacing between the
horizontal baffle 21 and the top 18. Ideally, the rate of expansion
of the horn with length for operation at its lowest frequency of
300 Hertz should double approximately every 21/2 inches over a half
wavelength path of approximately 8/10ths of 1 foot, or about 10
inches. In the enclosure of the present design, this rate of
expansion with length is accomplished, as shown in the top view of
FIG. 5, by forming the sidewalls 62, 64 of the acoustic passage in
approximately three planar sections from the inlet port 61 at the
front of mid-range speaker 63. The back side of this speaker is
also completely enclosed to form a backloading chamber 65. The
mid-range horn thus formed is believed to be of conventional design
but has its opening 67 so arranged with respect to the opening 11
of the bass-responsive horn in accordance with the present
invention that proper phasing of the acoustic waves from each such
horn can be attained at frequencies for which the acoustic outputs
of the two horns is comparable and this eliminates objectionable
acoustic delays or echo effects at the cross-over frequencies of
the low and mid-range speakers. More importantly, however, within
selected efficiency limits the ratio of the size of the mouth of
this mid-range horn to the size of the mouth of the bass horn must
be approximately 2:3, plus or minus about 25 percent, in accordance
with the present invention to ensure that the speaker system within
the present enclosure provides properly balanced or substantially
flat frequency response over the frequency range covered by the
associated speakers 23 and 63.
The cross-over frequency for the low and mid-range speakers 23 and
63, respectively, is determined by the cross-over and phasing
network 68 connected as shown in FIG. 2. This cross-over frequency
may be set in practice to be about 300 Hertz as required by the
characteristics of the low and mid-range speakers 23 and 63 and the
phasing of the two speakers is determined by the relative
connections of these speaker to their respective pairs of signal
conductors 71, 73. By "phasing" is meant the movement (i.e. forward
or rearward) of the speaker surface in response to the
instantaneously-applied signal. Thus, for the mean length 29 of the
low frequency horn of the present invention designed about
approximately 30- 32 Hertz, and for the mean length of the mid
frequency horn 17 designed about approximately 300 Hertz. The
phasing of the speakers 23 and 63 to avoid objectionable acoustic
delays at about the cross-over frequency should be that the speaker
surfaces move in the same direction for a given instantaneous
signal applied to both speakers.
The present invention may also include a high frequency speaker or
speakers 76 arranged at the mouth of the mid-range horn 17, as
shown in FIGS. 2 and 5. These speakers 76 may be driven by the
crossover and phasing network 68 in proper phase and frequency
relationship to the mid-range speaker 63. Since the human ear is
relatively insensitive to acoustic phase differences at high
frequencies, the relative phasing of the mid-range and high
frequency speakers 63 and 76 at the cross-over frequency of these
speakers is less critical than the phasing of the mid-range and low
frequency speakers 63 and 23, as discussed above. In practice,
however, the cross-over frequency for speakers 63 and 76 may be set
at about 2,000 Hertz with both speakers' surfaces phased to move in
the same direction on instantaneous signal applied to both speakers
63 and 76.
In general, the cross-over frequencies are selected in
consideration of the fact that a speaker mounted in a horn usually
cannot uniformly produce acoustic waves over more than three
octaves because of limitations of the horn. The cross-over points
in this enclosure are selected to allow each of its three sections
to reproduce about three octaves for maximum clarity and presence.
Also, this selection of cross-over frequencies eliminates harmonic
distortion due to the inability of exponential horns to produce
more than three octaves of sound above their lowest cutoff
frequency without introducing harmonic distortion. The exterior of
the enclosure may be styled and finished as desired with porous
grill cloth covering the entire front face of the enclosure.
Therefore, the compact speaker enclosure of the present invention
when properly positioned in a room corner provides an acoustic horn
having substantially true exponential expansion over approximately
1/3 wavelength at about the lowest audible frequency. Also, the
present invention obviates objectionable acoustic delays by
providing proper phasing of low and mid-range speakers in relation
to acoustic propagation delays through the exponentially-expanded
horn.
* * * * *