Note: Descriptions are shown in the official language in which they were submitted.
3~73
LOUDSPEAKER COUPLER
BACKGROUND OF THE INVENTION
This invention relates to loudspeakers, and more particu-
larly to novel means by which to couple the driven component of
a loudspeaker to the atmosphere.
Loudspeakers and other types of sound reproducers hereto-
foxe have been coupled directly to the atmosphere using such
devices as Helmholtz resonators, acoustic suspensions, infinite
baffles, tuned ports and others ~o alter the out-of-phase sound
emanating from the rear of the driving unit. These devices pro-
vide a "system resonance" intended to reinforce low frequency
10 drivers so as to achieve a "flat" response curve. Such devices
introduce sounds that are not present in the original music and
they also cause distortions created by their sharply defined
boundaries. These artifîcial sounds and distortions are ~urther
amplified when they are fed through exponential hornsO
Exponential horns are recognized as very effective devices
for coupling sound reproducers to the air which is to carry the
sound to the human ear. The primary disadvantage of an exponen-
tial horn resides in the excessive length, from its inlet to its
outlet, required to transport without distortion, those sound
20 frequencies at the low end of the audible spectrum.
Historically, it has been recognized that an exponential
horn designed to produce an undistorted audible note of 30 Hertz
from a 15 inch diameter woofex, requires that the loudspeaker be
confined in a cham~er of no less than 5200 cubic inches and hav-
ing an outlet of 75 square inches matching the inlet opening
(throat) of the horn, and that the horn must exceed 16 feet in
length and terminate in a mouth opening equivalent to 127 square
feet, or about 11 feet squareO Further~ if the inlet end of the
horn is provided with a larger cross sectional area~ for example
3~ to at least match the effective cross sectional area o~ a larger
loudspeaker diaphragm, the outlet end of the exponen~ial horn is
even more unreasonably large. In any event, the folding of such
a path requires an unacceptably large cabinet, at least for
residential usage.
In order to utilize at least some of the benefits of an
exponential horn, it has been the practice heretofore to couple
the disphragm of a loudspeaker to the inlet of the horn by means
of a "slot" forLned by a chamber which communicates with the loud-
speaker and which also has an outlet "slot" or passageway, of
smaller dimensions than the loudspeaker disphra,m but matching
10 the inlet end of the horn. The cross sectional area of the cham-
ber changes at random from the diaphragm to the slot. The cross
sectional area of this outlet passageway generally is greater
than about one-fourth the cross sectional area of the loud-
speaker diaphragm. Neverthelessg such a reduction in cross
sectional area of such a "slot" relative to the loudspeaker
diaphragm, allows the dimensions of the exponential horn to be
reduced to a degree that allows the horn to be folded within a
cabinet of overall dimensions which render it practicable for
use at least in large rooms. On the other hand, the size o~ ~ch
20 a cabinet is unsuitable for use in the average home, and further
size reduction, through further reduction in the dimensions of
the coupling slot diminishes the quality of sound reproduction
to an unusable level~
SUMMA~Y OF THE INVENTION
In its basic concept, this invention provides for the coup-
ling of an electrically driven sound xeproducer to the atmosphere
by means of a hollow coupler the cross sectional area of which
decreases progressively from its inlet end to its outlet end.
It is by virtue of the foregoing basic concept that the
principal objective of this invention is achieved; namely, to
overcome the distortions resulting from the resonances which
characterize the coupling devices of the prior art described
hereinbefore.
~ 3
Another ~mportant objective is to provide a coupler of
the class described which maybe associated with an exponential
horn and which permits the olding of such a horn within a sound
reproducer cabinet of such minimum volume as to render it pract-
icable for use in conventionally sized residential rooms.
Another important objective of this invention is the pro-
vision of a sound reproducer coupler of the class described
which, in association with an e~ponential horn, allows the in-
let end of the horn to be reduced in cross sectional area many
times smaller than has been possible heretofore, while maintain-
ing maximum efficiency of sound transferO
Still another important objective of this invention is the
provision of a sound reproducer coupler of the class described
which, in association with an exponential hoxn, provides for
matching the acoustic re~istance of the inlet of the exponen-
tial horn and the acoustic impedance of ~e sound reproducer as-
sociated therewith.
A further specific objecti~e of this invention is the pro-
vision of a loudspeaker couplex of the class described in which
minimiæation of the cross sectional area of the outlet end of
the coupler serves beneficially to reduce subs~an~ially the
loudspeaker diaphragm excursions required to produce a given
sound level in the air, thereby correspondingly reducing dis-
tortions of the reproduced sounds.
~ still further important objective of ~his invention is
the provision of a sound reproducer coupler of the class des-
c~ibed which is free of sound absorbing, throa~ choking material,
wkereby all of the sound frDm the loudspeaker is heard in sub-
stantially unda~ped condition throughout ~he audible spectrum,
thereby contributing beneficially to an output characterized
by crisp, lifelike sounds.
~ further important objective of this invention is the pro-
vision of a loudspeaker coupler of the class described which is
~$ ~ ~ ~
capable of utilizing the backwave sounds with minimum distortion
and which may incorporate therewith a plurality of tweeters
and other speakers arrayed in any desired manner.
The foregoing and other objectives and advantages of this
învention will appear from the ~ollowing detailed descrip~ion,
taken in connection with the accomanying drawings of preferred
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic representation, in longitudinal sect-
10 ion~ of a loudspeaker having associated therewith a coupler em-
bodying the features of this inventionO
Fig. 2 is a schematic representation, in plan view, illus-
trating the manner in which the diaphragm of a loudspeaker is
coupled to the inlet of an exponential horn in accordance with
this invention.
Figs. 3, ~ and 5 are schematic representations, in plan
view, illustrating various structural arrangements accommoda~ing
the coupling concept of this invention, and Figs. 3a, 4a and 5a
are electrical diagrams illustrating the equivalent electrical
20 circuits representing the structural configura~ions of ~igs. 4,
4 and 5, respectively.
Fig. 6 is a vertical cross section through a loudspeaker
enclosure embodying the features of this invention.
Fig. 7 is a sectional view taken on the line 7-7 in Fig.6.
Fig. 8 is a sec~ional view taken on the line 8-8 in Fig.7.
Fig. 9 is a vertieal cross section through a loudspeaker
enclosure illustrating a second embodiment of this inventionO
Fig. 10 is a sectional view taken on the line 10-10 in
Fig. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 illustrates a loudspeaker 10 supported in a housing
12, prefera~ly perforated for exposure to atmospheric pressure.
The ele~trically driven diaphragm 14 of the loudspeaker is coup-
led to the atmosphere through a hollow coupler 16 which~ inaccordance with this invention, provides an acousti-¢ally mat-
ched coupling between the loudspeaker and the environment,
thereby minimizing distortions as are created by the resonant
chambers and pipes which characterize the coupling devices of
the prior art. As illustrated, the inlet end 18 of the coupler
has a cross sectional area comparable to the effective cross
sectional area of the diaphragm 14. Further, it is to be noted
that the inlet end 18 is closed by the diaphragm and is sealed
thereby against the passage of air or other fluid pressure.
~Q The outlet end 20 of the coupler, sometimes hereinafter refer-
red to as the "bore" is of substantially smaller cross section-
al area, as discussed mo~ fully hereinafter.
The coupler may take a variety of shapes. Thus, the coup-
ler in Fig. 1 may be generated symmetrically about a common
longitudinal axis as is the case in Fig. 2 described hereinafter.
It may be rectangular in cross section, with the sides 22 ccn-
verging progressively from the inlet end 18 to the outlet end
20 as illustrated, and~ith the opposite sides 24 being disposed
either parallel to each other or converging progressively from
20 the inlet end 18 to the outlet end 20~
The sides of the coupler may be constructed of a variety
of materials. They may be made of materials such as Firtex~, or
other particle boards and constructed so that ~hey flex and
thereby respond to the internal air pressures generated by the
loudspeaker or other form of sound reproducer. The responsive
surfaces of such materials radiate the sounds to the atmosphere
devoid of the usual resonances described hereinbefore, and the
progressive change of cross sectional area eliminates the dis-
tortions also referred to hereinbefore. When made o these types
30 of materials, the coupler may serve as the only medium by which
the sound reproducer is coupled to the atmosphere, by providing
a larger sur~ace for radiating the sound than the spea~er dia-
~ c~ r ~
phragm itself.
On the other hand, the coupler may function to couple aloudspeaker or other sound reproducer to the inlet of an expon-
ential horn. In such instances~ it is preferred that the sides
of the coupler be made so that they are substantially inflexible
and thereby do not respond efficiently to the internal pres-
sures generated by the reproducer. Materials for this type of
coupler include a variety of synthetic thermosetting and ther-
moplastic resins, wood, ceramic, and others.
Referring now to Fig. 2 of the drawings, there is illus-
trated schematically an exponential horn 26 the inlet end of
which is generally referred to as the "throat" and is coupled
to the electrically driven diaphragm 14 of the loudspeaker 10
through a hollow coupler 16. As explained hereinbefore, the
inlet end 18 of the coupler preferably has a cross sectional
area comparable to the effective cross sectional area of the
diaphragm 14 and is closed and sealed by the latter. The out-
let end, or "bore" 20 of the coupler and the throat 20'of the
horn are connected at the same ~ransverse plane and therefore
obviously have the same cross-sectional area and shape.
The arrangement illustrated in Figo 2 achieves a funda-
mental objective of this invention; namely, the acoustic re-
sistance of the bore 20 matches the reflective impedances of
both the coupler 16 and horn 26~ whereby to effect cancelling
of acoustic resonances in either. This is analogous to the
termination of an electrical transmission li~ by a resistor
equal in value to the characteristic impedance of the trans-
mission line, whereby to effect cancelling of reflected waves.
Thus, referring to Figs. 3 and 3a of the drawings, wherein
Fig. 3 is the same as Fig. 2, the transformer 26 in Fig. 3a
30 represents the electrical equivalence of the exponential horn
25 and the transformer 16 represents the electrical equivalance
of the coupler 16. The resistor 28 represents the electrical
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equivalance of the throat resistance and the resistor 30 rep-
resents the electrical equivalence of the room resistance.
This arrangement substantially improves the acoustic impedance
matching of ~he speaker 10 through ~he exponential compression
transformer 16 to a resistive load 28 at the junc~ion 20 be-
tween the coupler 16 and the exponential horn 26. This re-
sistive load 28 matches the impedance of the input end of the
exponential horn 26 with the open end of the horn matching the
very high impedance oE the sound-carrying air within a room.
Accordingly, it will be apparent tha~ it i9 primarily
the ratio of the cross sectional areas of the closed inlet end
18 of the coupler 16 and the outlet end 20 thereof that deter-
mines whether the magnitude of acoustic resistance is suffi-
cient to render the hollow coupler non-resonant. The effective
cross ~ectional area of the outlet end 20 is reduced somewhat
by forming the coupler walls 22 and 24 of such sound absorb-
`-- ing materials as Firtex and others, as previously mentioned.
Thus, the condition of non-resonance is established by measure-
ments of sound levels at given frequencies attributable to coup-
20 lers of different materials having different inlet and outletratios, the condition of non-resonance being exhibited by the
lack of variations in sound levels at given frequencies.
Fig. 4 illustrates a modification of Fig. 3 by the incor~
poration of one or more resonant chambers, two such chambers
32 and 34 of difEexen~ ~ol~unes being illustrated, each communi-
cating with ~he throat 20 through a radial opening 32', 34'
therein.
In Fig. 5~ one or more tuned pipes 26 are interposed
between the bore 20 and throat 20'. Although only one such
30 tuned pipe i9 illustrated, it will be understood that a plur-
ality of differently tuned pipes mayke disposed side-by-side
within the transverse dimensions o:E a loudspeaker enclosure,
as will become appaxent hereinafter.
'~ Tr~7GJ~ rk
In Fig. 2, the combination of loudspeaker diaphragm 14,
coupler 16 and exponential horn 26 is shown to be generated
symmetrically about a common longitudinal axis 38. In practice,
however, loudspeaker cabinets generally arerectangular in shape,
as illustrated in Figs. 6-10. Accordingly, since loudspeaker
diaphragms are usual~y circular in cross section, it is nec-
essary that the transition from the circular cross section of
the diaphragm to the rectangular cross section of the cabinet
be made in such a manner that the rectangular cross sectional
area immediately adjacent the diaphragm is compara~le to the
effective cross sectional area of the diaphragm. Ideally, it
is desirable that the cabinet be provided with a transition
volume by which the circular cross section of the cabinet open~
ing registering with and matching the effective cross section-
al area of the loudspeaker diaphragm, be converted to an out-
let opening of rectangular cross section of equal area. This
ideal arrangement is provided in the embodiment illustrated
in Figso 9 and 10 and described in detail hereinafterO
Referring first to the embodiment illustrated in Figs.
20 6, 7 and 8, the cabinet is formed of side walls 40, back wall
42, bottom wall 44, top wall ~6 which supports the loudspeaker
10, and cover 48, which conceals the loudspeakPr.
The top wall 46 is provlded with a circular opening 50
having a cross sectional area equal to the effective cross
sectional area of the diaphragm of the loudspeaker. It is
through this circular opening that the loudspeaker communicates
with and closes the inlet end 18 of thP coupler 16, the outlet
end, or bore 20 of which communicates with the inlet end, or
throat, of the exponential horn 26.
The coupler 16 and exponential horn 26 may be provided
within the cabinet by various construction techniques. In the
embodiment illustrated in Figs 6, 7 and 8, the interior of t~
~Z~
cabinet is fitted with preformed structural sections 52, 54
and 56 which cooperate with the ~a~inet walls to provide be-
tween them the hollow coupler 16 and exponential horn 26. The
sections may be formed of any desired material, sucha, wood,
rubber, paper, fiberboard, synthe~ic thermoplastic or thermo-
setting resins, or other suitable material, and by any well
known procedure, such as molding, stamping, fabricating, or other
desired techniques.
In the emb~diment illustrated, the sections 52 and 54
10 are formed of a mixture of rubber and synthetic thermoplastic
resin and supported in properly spaced-apart relation by means
of the end support plates 58 and the laterally eætending dowel
pins 60.
The integrated assembly of sections 52 and 54 with
structural plates 58 may be secured withi~ the cabinet by any
suitable means, such as adhesive, screws, etc.
It is to be noted, particularly from Fig. 8 of the draw-
ings, that the inlet end 18 (Fig4 6) of the coupler 16 is of
rectangular cross section and somewhat larger in area than the
~ cross sectional area of the circular opening 50 in the top wall
; 46. Although some loss of fidelity of sound transfer xesults
from this less than ideal arrangement~ it has been found that
the loss is more technical than apparent in the human ear, and
therefoxe the axrangement, in which the cross sectional areas
of the opening 18 and diaphragm 14 are comparable, is qute ade-
quate for all practicable purposes.
It is to be noted from Fig. 6 that the coupler 16 ex-
tends from its inlet end 18 downward to its outlet at point
20 of minimum cross sectional dimension. This '~ore" also is
30 the inlet or "throat" of the exponential horn 26 which, in its
folded condition, extends upward toward the rear of the cabinet
between the forming sections 52 and 54 and thence downward betw-
een the back wall 40 of the cabinet and forming section 52,
_9.
8~
~and 5~ and thence downward between the back wall 40 of the cab-
~inet and forming section 52, thence forward to the open front of
the cabinet. Typical dimensions for this cabinet are 38" high,
17" wide and 18" deep, accommodating a 12" diameter cone speaker.
Experiments conducted with the illustrated arrangement
have shown that the size of the bore 20 has been reduced to as
small as 0.2" in height by 15" in width and has performed succes-
sfully with a 12" diameter speaker. This cross sectional area is
1/38 that of the speaker. Of particular interest is the observa-
tion that speakers of smaller diameter but of comparable qualitymay be utilized in this cabinet, with no noticeable loss in
performance.
Further, such experiments have also shown that frequencies
well below the theoretical cut-off limit of the horn itself are
passed without large attenuation, as expected by cut-off calcu-
lations. For example, a horn and coupler assembly calculated to
provide undistorted transmission of a minimum 65 Hertz note,
actually passed an undistorted 12 Hertz note.
The internal section 56 is illustrated as ~eing formed of
a curved section o wood secured to the walls of the cabinet by
any unsuitable means. It may be formed of the same material as
the sections 52 and 5~. If desired, it may be formed in segments,
in which case the support plates 58 are extended downward to the
bottom wall 44 of the cabinet to support the segments of section
56, in the same manner as previously described.
Figs. 9 and 10 illustrate a s~ructural arrangement in
which the coupler 16 extends from its inlet downward and thence
rearward and upward to the point 20 of minimum opening. Thi~ is
the bore of the coupler 16 and the throat o~ the exponential
horn 26, as previously explai~ed. The horn progresses upward
and rearward, in surpentine fashion, and thence downward and
forward to the open front of the cabinet. The arrangement illus-
trated provides, within a cabinet 40" high, 15" wide and 22"
deep, housing a 12" diameter loudspeaker, an exponential horn
having an over-all length of 10'. This provides for the undistort-
ed transmission of sound frequencies as low as 30 Hertz.
The embodiment of Figs. 9 and 10 also illustrates means
providing a transition volume 62 by which the circular cross sect-
ion of the opening 50 in the top wall 46, registering with, closed
by and matching the effective cross sectional area of the diaph-
ragm of the loudspeaker 10, is converted to rectangular cross
section of equal area. This transition volume is provid~d by asso-
10 ciating with the upper portions of the segments 52 and 54, pre-
formed sections 64 which span the space be~ween the upper portions
of the sections 52 and 54. These sections 64 have semi-circular
inner edges at their top end and a straight edge at their bottom
end, merging wi~h the side walls 40 of the cabinetc This transu-
tion volume is an in~egral par~ of the coupler 16 and provides
the ideal structural arrangement reerred to hereinbefore, sinc~
it converts the circular cross section of opening 50 to the rect-
angular cross section of equal area.
As noted hereinbefore, the preferred coupler 16 of this in-
20 vention is characterlzed by having an inlet 18 closed and sealedby diaphragm 14 and of cross sec~ional area comparable to the
effective cross sectional area of the loudspeaker 10. I~ also has
an outlet 20, or bore, of cross sectional shape generally match-
ing the cross sectional shape of its inle~, but much smaller in
dimensions. The shape and cross sectional area of the bore mat-
ches the cross sectional shape and area of the inlet, or throat
of the exponential horn~
Additionally, the coupler of this invention has a cross
sectional area which decreases progressively from its inlet to
30 its outlet. This progressi~e decrease in cross sectional area
may be uniform, providing the coupler with the shape of a trun-
cated cone -~ig. 2~ or wedge (Fig. 6). As previously mentioned,
all four sides may decrease in cross sectional area progressively
from its inlet to its outlet. In the preferred form of coupler,
~he cross sectional area decreases exponentially.
Thus, in the embodiment illustrated in Figs. 6 and 9, al-
though the lateral sides of the coupler are parallel, being de-
fined by the lateral sides ~0 of the cabinet, the front and back
walls defining the depth dimensions of the coupler are curved ex-
ponentially. Although a variety of configurations is suitable for
this purpose, the hyperbolic curve has been found to provide opti-
mum results.
The coupler 16 of thi5 invention provides many advantages:
It functions to provide acous~c damping upon any remote tenden-
cy_toward resonance within the coupler, by virtue of the true
acoustic resistance quality of the bore 20. With loudspeakers of
the movable diaphragm type, it reduces the excursions required
to produce a given sound level in the air. By thus reducing
Hook's law forces, corresponding reduction in sound reproduction
distortions also is achieved, from both front and ~ack sides of
the speaker.
The damped horn characteristics o the coupler render it
20 useful for driving resonant chambers for special ef~ects.
-rt is by virtue of the provision of the hollow coupler that
the inlet end of the exponential horn may be reduced to minimum
cross sectional area without introducing acoustic distortions
and o~her deleterious factors. Minimization of the cross section-
al axea of the inlet end of the horn beneficially affords minimiz-
ing the size of the cabinet in which to contain the horn, by
allowing the hown to be folded in a variety of ways to minimize
the volume containing it.
The provision of the coupler functions to match the acous-
30 tic resistance of the bore with the reflected impedances of boththe coupler and exponential horn, whereby to provide for maxi-
mum transfer of energy from the speaker to the environmentO By
collecting the sound through purposeful compression in the coup-
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~ 3
ler into a high acoustical impedance and then guiding ~he
sound expansion exponentially through the horn, the ~irtual
radiating surface is enlarged many times, with a marked improve-
ment in ~he ability to reproduce low frequency sounds, as well
as the higher requency sounds throughout the audible spectrum.
Many variations of the structural arrangements illustrat-
ed in the drawings may be made. For example, the outlet end of
the coupler may communicate with the inlet end of a passageway
of uniform cross section, the outlet end of which communicates
10 with the inlet end o~ the exponential horn. This passageway may
be one or more tuned pipes, as illus~rated in Fig. 5 and des-
c~ibed hereinbefore. The outlet end of the coupler may communi-
cate both with the inlet end of the eæponential horn and with
one or more resonant chambers, as illustrated in Fig. 4, if it
is desired to alter the characteristics of the loudspeakPr such
as to alter its resonant peaks or to e~tend its frequency-ampli-
tude performance. In all instances, the coupler performs its
function of impedance matching as described hereinbe~ore~
In the embodiment illustrated in the drawings, the loud-
20 speaker is shown arranged with the front side of the diaphragmfeeding the couplerO It will be understood, of rourse~ that the
disposition of the s~eaker may be xeversed, with the back side
of the diaphragm feeding the coupler. Further, sound reproducers
other than the electromagnetically driven diaphragm type illus-
trated and de$cribed hereinbefore, are intended to be included
in the general term "loudspeaker", as utilized in the specifica-
tion and claims. Such other sound reproducers include electro-
static speakers, the Keff woofer, the Magnaplaner, and othersO
Figs. 6-10 show the cabinet provided with a cover 48 which
30encloses the loudspeaker. It is preferred that the cover be per-
forate, as illustrated by openings ~9 maintaining atmospheric
conditions within it. Such an arrangement allows for the utiliza-
tion of the back wave sounds from the sp~aker. This enhances
i
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~stereo imaging. Such a cover may serve to conceal within it a
plurality of tweeters and other speakers arrayed to provide most
effective dis~ribution of sounds in the medium and higher fre-
quency portion of the spectrum.
on the other hand, the cover may be imperforate and de-
signed to provide a resonant chamber ~or the purpose of alter-
ing speaker perormanc~ as a horn driver, also as is well known
in the art.
The cabinet illustrated and described h~reinbefore may
be formed in a variety o ways and in a variety o~ shapes and
sizes. Indeed, the assembly of intexnal sections 52 and 54, in-
tegrated by plates 58 to form the coupler and horn channels,
together with the top wall 46 supporting the loudspeaker 10,
may be associated in a variety of ways wi~h surfaces o tables
and ceiling, ~loor and wall components of a room effectively to
provide an enclosure or the assembly, in the manner of the cab-
inet described hereinbefore by which to complete the hollow
coupler and exponential horn.
The cabinets illustrated in Figs. 6 and 9 may be ~ormed
20 in sections to facilitate portability. For example, the portion
below the section 52 may be formed separately, for detachable
connection to the upper section.
It will be understood, of course, that the cabinets may
be turned upside down for use. If the cabinet is sectioned, the
section including member 56 may be omitted and replaced by sur-
faces of furniture, walls, etc D
A plurality of couplers may be utilized as a coupler sys-
tem with one or more loudspeakers as a loudspeaker system, and
a single coupler as a coupler system maybe utilized with a plur-
30 ality of loudspeakers as a loudspeaker system, provided thetotal cross sectional area of the common inlet of the coupler
system and the total ef~ective cross sectional area of the as-
sosiated loudspeaker system are comparable.
1~73
It will be apparent to those skilled in the art that
various other changes may be made in the size, shape, type, num-
ber and arrangement of parts described hereinbefore, without
departing from the spirit of this invention.
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