Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Background of the Invention
This invention relates to a loudspeaker system and more par-
ticularly to such a system which shall reproduce sound with a
high degree of faithfulness and with a minimum of distortion.
,
Heretofore, dynamic loudspeakers of the cone type, capable of
reproducing sound with ample volume and with acceptable dis-
tortion, in most cases, have the speaker cone or diaphragm
' 25 facing the listeners. These cone type loudspeakers may be-
classified into woofers which reproduce the lowest tones,
mid-range speakers which reproduce the middle tones and twee-
ters-which reproduce the highest tones. The cone diaphragms
of all three types suffer structional distortion when heavy
forces are exerted on them by the voice coils. Structural
distortion is greatest around the voice coil and is at an
objectionable level for about 15 per cent
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of the radial distance from the outer edge of the voice coil to
the outer edge of the cone diaphragm, which terminates at the
inner edge of the flexible edge or hinge, if one is used. While
this distortion is slight, when measured against the entire output
of the speaker system, much of it is of a shrill character and
occurs within the range of frequencies which the human ear hears
the loudest. This distortion, which I shall hereinafter refer to
as center cone distortion, has a hashy, raspy sound which intrudes
particularly on the mid-range frequencies of the human voice and
of such instruments as pianos, pipe organs and violins.
Heretofore, there have been many attempts to overcome this
center cone distortion. In the case of the woofer, one means has
, been to provide an electrical crossover network which blanks out
frequencies higher than 250 to 500 cps from the woofer and then to
add a larger mid-range speaker capable of reproducing sound down to
250 to 500 cps. However, even with these measures, a noticeable
` ghost of center cone distortion will usually be present in the
woofer and the larger, more expensive mid-range speaker will have `
`~ center cone distortion of its own at the upper end of the band of
frequencies which the human ear hears the loudest. Depending upon
the specific design of the larger mid-range speaker, it would be
expected to produce center cone distortion in the range of from
4,000 to 6,000 cps or possibly as low as 2,500 cps.
Efforts have also been made in some speaker systems to
.,:
eliminate center cone distortion by adding dampening masses to
the center area of the woofer cones. This addition of dampening
mass to the woofer cone defeats the very basic principle of a low
distortion woofer, which is to keep the inertia of the cone
assembly as low as possible. Accordingly, such added mass will
cause the woofer to distort greatly in the very low frequencies.
rsany efforts to reduce distortion in cone-type mid-range and
tweeter speakers have been tried with little in the way of positive
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results. While metal cones have been used by some, the same center
cone distortion problem exists. The result of the combined e'fect
of center cone distortion at all three levels is a high degree of
listener fatigue and an artificial sound to the loudspeaker system.
Another problem with existing loudspeaker systems is poor
distribution of the sound at the upper end of the frequency range
covered by the woofer since higher frequencies radiate in a more
linear manner and must be reflected to make them turn corners.
This effect becomes a factor of importance above about 250 cps.
Heretofore, many loudspeaker systems have employed woofers designed
to cut off at from 1,000 to 1,500 cps. The linear nature of the
frequencies which such woofers emit between 250 and 1,500 cps,
causes such frequencies to form a narrow band along the axis of
~ the speaker. These frequencies are thus poorly distributed while
; 15 the frequencies below 250 cps are well distributed because of their
non-linear nature. As the frequencies increase, the problem becomes
progressively more severe in the mid-range and tweeter speakers.
A still further problem encountered with conventional loud-
speaker systems relates to the shape of the response curve OL the
` 20 woofers. Ideally, the highest volume of reproduction should
occur at the lowest audible frequencies. But in practice most
good woofers, because of the strain in the center of the cones,
have a peak or "knee" on the response curve with the highest volume
of sound occurring at about 1,000 cps. This knee or peak, which is
usually near the cross-over point between the woofer and the mid-
lj
range speaker, causes a shrillness which is almost impossible todefeat with the aforementioned electrical cross-over network. Since
;; the loudest sound which the human ear hears ranges between about
2,000 and 5,000 cps, it is preferable to have the woofer cut off at no
more than about 1,000 cps so that the mid-range speaker can take over
at above 1,000 cps and with its very small, low inertia cone, repro-
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duce the loudest frequencies the ear hears with the lowestpossible distortion.
Another attempt to solve the problem of center cone dis-
tortion has been to strengthen the cone sufficiently to eliminate
such distortion, such as by providing a sandwich-like cone having
inner and outer metal members with a non-metallic layer there-
between. The Hitachi metal cone loudspeaker shown in the October
1977 issue of Stereo Review r~aga~ine, page 12, is such a speaker.
he Tannoy speaker shown in the June 1977 issue of Stereo Review,
page 107, shows a speaker cone in which ribbing is employed to
prevent cone break-up or distortion under stress. ~nother attempt
to prevent center cone distortion has been to use high strength
- carbon filaments in the cone paper.
There are at least three factors tending to cause the radial
lS stresses in a loudspeaker cone to increase as we progress from
the outer edge of the cone toward the voice coil at the center
of the cone. These factors are:
(1) The area of a cone increases as the second power
of the slant height of the cone. Accordingly, the resistance of
the air to the movement of the cone would increase as the second
power of the slant height of the cone.
(2) Since the outer portions of the cone cause compression
or tension forces along the slant height, depending on which half
of the sound wave is being induced at the time, such that the
inner portions of the cone must bear not only the force caused
by its pushing of the air but must also bear the forces from these
outer portions as well, another first power must be added.
(3) Since the cross section of the paper cone through which
this force must be transmitted to the voice coil diminishes to the
first power of the reduction in slant height, the unit stress will
increase as the first power as we progress along the slant height
of the cone toward the voice coil.
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From the above it can be seen that the unit stress along
the slant height of the woofer cone increases at least as the
fourth power as we progress from the outside of the cone toward
the voice coil. The combined total of the tensile or compressive
forces in a paper cone about .010 inch thickness may exceed one
pound near the center of the cone. This can induce a radial unit
stress of as much as 20 pounds per square inch near the voice
coil. While this may seem like a very low unit stress, it is
enough to compress the inner one inch of slope distance by about
.00025 inch. But because of the fourth power function causing
the stresses to lessen drastically with each inch we move away from
the voice coil, we would compress the portion of the cone one inch
farther out from the voice coil by only .000,015,6 inch under the
same conditions. From this it can be seen that the inner 3.5
inches of a typical woofer cone would distort radially by about
16 times as much as the portion of paper cone between diameters of
; 3.5 inches and 5.5 inches.
It will be understood that the distortion with which we are
concerned can be produced in sufficient quantity to offend the
human ear by compression or tension forces so minute as to be
difficult to measure.
The Wolff patent No. 1,786,279 and the Leon patent No.
2,643,727 both show reflectors for speaker systems. However, both
patents show the reflectors as extending all the way across the
center portion of the speaker whereby the reflectors reflect sub-
stantially all of the sound output of the speaker. Accordingly,
with such reflectors, there could be no discrimination of sounds
emitted from the speaker. While the Thuras patent No. 2,037,185
and the Buchmann patent No. 2,714,047 both disclose a loudspeaker
system having a damper-like member mounted adjacent the center of
the speaker cone, these patents do not teach the use of such a
damper member in combination with a reflector member mounted in
position to reflect only sound from the outer portion of a speaker
` ~27~
cone aimed at least 90 away from the listeners with the inner-
most edge of the reflector being located at least 15 per cent
of the radial distance measured outwardly from the outer edge of
the loudspeaker voice coil to the outer edge of the cone diaphragm.
Summary of the Invention
In accordance with my invention I overcome the aforementioned
difficulties and provide superior quality and distribution of the
sound by mounting the speakers whereby they are turned at least
90 away from the listeners so that direct projection of the
center cone distorted sound toward the listeners is minimized.
At least one reflector is mounted in position to reflect only the
undistorted sound from an outer portion of the speaker cone toward
the listeners and to distribute this sound over a broad area
toward the listeners. The sound being reflected by the reflector
lS is directed outwardly of an enclosure defining a sound chamber
for the loudspeaker system at least 90 from a line extending
forwardly along the axis of the loudspeaker cone. The innermost
edge of the reflector is located at least 15 per cent of the
radial distance as measured from the outer edge of the loudspeaker
voice coil to the outer edge of the cone diaphragm. A damper pad
is provided adjacent and along the axis of the speaker cone in
position to intercept the center cone distortion and damp it out.
The proper arrangement of these elements results in a loudspeaker
system which produces sound of very low distortion, well distributed
at all frequencies and with a nearly ideal response curve and
with the utmost in economy and simplicity.
Description of the Drawings
Loudspeaker systems embodying features of my invention are
shown in the accompanying drawings, forming a part of this
application, in which:
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FIG. 1 is a sectional view showing my loudspeaker system
with the speaker facing away from the listeners and the reflector
and damper pad in operating position;
FIGS. 2, 3 and 4 show three modified forms of reflectors
which may be employed;
FIG. 5 is a sectional view showing a loudspeaker system
having two speakers with the reflectors and damper pads in
operating position;
FIG. 6 is a sectional'view showing a loudspeaker system
having two reflectors applied to a single speaker for better
distribution of the mid-range frequencies;
FIG. 7 is a graph showing the approximate response curve
of a ten inch acoustic suspension woofer aimed toward the
listeners, aimed away from the listeners and'aimed away from
the listeners with my improved reflector associated therewith;
FIG. 8 iS a sectional view showing a loudspeaker system
having two reflectors applied to a single speaker with the inner-
most edge of each reflector being shown in dotted lines as located
at approximately 15% of the radial distance as measured away from
a line extending forwardly from the outer edge of the loudspeaker
voice coil to the outer edge of the cone diaphragm and shown in
solid lines as being'located approximately 85% of such radial
distance;
FIG. 9 iS a sectional view showing a loudspeaker system
having a reflector in the general shape of a torus;
FIG. 10 is a perspective, partly broken away, view of the
loudspeaker system shown in FIG. 9; and
FIG. 11 is a graph showing the approximate amounts of
harmonic distortion which are objectionable and tolerable, to the
human ear as related to locations along the radial distance from
the outer edge of the loudspeaker voice coil to the outer edge of
the cone diaphragm when a 40 cps tone is being reproduced on a
10 inch acoustic suspension speaker at 3.66 electrical watts RMS
(root mean square).
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Referring now in detail to the clrawings for a better under-
standing of my invention, I show in FI5. 1 an enclosure 10, formed
of wood or other suitable material. Mounted within the enclosure
10 is a low frequency speaker or woofer 11. It will be understood
that my improved sys~em is also adapted for use with a medium
frequency or mid-range speaker and a high frequency speaker or
tweeter as indicated at 12 in FIG. 8.
The woofer 11 is provided with a paper cone 14 which is
moved by a current of electricity which is induced in a voice coil
16 acting against the magnetic field in a magnet 17. The voice
coil 16 produces a remarkably heavy force on the relatively weak
paper cone and at the same time the cone 14 must be light in
weight and therefore thin in order for it to have a minimum of
inertia.
Since the force on the cone 14 is great and its strength is
low, the cone 14 will distort or strain slightly under the stress
induced by the voice coil 16. This distortion of the center
portion of the cone 14 causes the center portion of the cone to
radiate distorted sound in the area indicated at "X" in FIG. 1.
A damper pad 18 is supported by a bracket 19 in position to absorb
most of the center cone distortion. What little of this distorted
sound radiated past the pad will be expended for the most part
since it is quite linear in nature and it is projected away from
the listeners.
~ounted forwardly of and adjacent the outer portion of the
speaker cone 14 is a reflector 21 which is supported in place by
a stud 22. The reflector is so disposed and shaped that it
will reflect only sound from the outer, undistorted portion of
the woofer cone 14 as indicated at "Y". The reflector 21, for
ideal results, should have a textured surface 13 of fabric,
thin fabric or other textured material to reduce the reflection
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of the sound at the upper end of the response curve of the
woofer by just the right amount to produce an ideal response
curve. By using a properly shaped reflector, preferably a
modified parabolic reflector, the upper and more linear sound
frequencies from the woofer 11 can be dispersed into a broad
band as indicated at "~". For the mid-range speaker the reflector
will normally reach farther in toward the voice coil. Accordingly,
the reflector surfaces may or may not need texture depending on
the cross-over frequency to be achieved and the type of driver
employed. Since tweeters reproduce the upper end of the response
curve and the upper end is open-ended no texture will normally
be required. That is, tweeters will most often have a non-
textured surface so as to allow maximum emission of high frequencies.
While a modified parabolic reflector, such as is shown at
21 in FIG. 1 is ideal, it will be understood that satisfactory
results can be obtained with many other shapes of reflectors.
For example FIG. 2 illustrates a "V" shaped reflector 21a with
which good results can be obtained.
The reflector can also take the form of two separate surfaces
adapted to cooperate with each other to serve as a concave re-
flector. One such compound reflector is illustrated in FIG. 3
in which a support bracket 20 supports two flat reflectors 21b
in such a manner that they act as a concave reflector. Yet
another form of compound reflector is shown in FIG. 4 in which
a bracket 24 supports two curved reflectors 21C in position for
the reflectors to cooperate with each other to give the effect
of a concave reflector.
In certain applications, it may be desirable to use more
than one woofer 11. The advantages of my invention apply equally
well to a speaker system with more than one woofer 11 as illus-
trated in FIG. 5. The use of two reflectors 21 in the embodiment shown
4 ~
in FIG. 5 gives excellent distribution of the frequencies at
the upper end of the response curve of the woofer.
It is not necessary to have more than one woofer to obtain
the advantages of the excellent lower mid-range distribution
provided by the use of more tnan one reflector 21. FIG. 6
illustrates how one reflector 21 can be applied on either side
of the woofer 11. In FIG. 6 a bracket 26 bridges between the
reflectors 21 in order to support the damper pad 18 in the proper
position.
In FIGS. 9 and 10 I show a reflector 21d in the general
shape of a torus which surrounds the entire outer portion of a
speaker cone lla. Where the reflector covers a small segment
of the outer portion of the cone diaphragms, as shown in FIGS.
1 throuyh 6, more ambience is produced. That is, the sound
produced is similar to that produced in a concert hall. On the
other hand where there is more coverage of the diaphragm, as shown
in FIGS. 9 and 10, less ambience is obtained but a better per-
formance is obtained in puhlic address speaker systems in large
live halls. In a larger room which is fairly live, the sound
should ideally come from one sow~e and be projected in only one
general direction as is the case with the reflector shown in
FIGS. 9 and 10.
As set forth on page 34, FIG. 1.138, of "Audio Cyclopedia,
second edition, published in 1978, about 1.8 per cent distortion
is the maximum amount of harmonic distortion (music) which a
human ear can tolerate before the reproduction becomes objection-
able. In actual practice, I find that this point is reached
when the innermost edge of my reflector is located approximately
15 per cent of the radial distance from the outer edge of the
loudspeaker voice coil to the out~r edge of the cone diaphragm.
In FIG. 8, the radial distance from the outer edge of the
voice coil to the outer edge of the cone diaphragm is
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indicated at "R".
The graph shown in ~IG. 11 shows the amounts of harmonic
distortion which are objectionable and tolerable to the human ear
as related to locations along the radial distance from the outer
edge of the voice coil to the outer edge of the cone diaphragm.
I have found that sounds produced at the left of line "A" on
the graph are objectionable while sounds produced at the riqht
side of line "s" on the graph are tolerable.
In actual practice I have found that the maximum distance
the inner edge of the reflector can be positioned outwardly away
from the voice coil and yood results be achieved is approximately
85 per cent of the radial distance measured outwardly from the
; outer edge of the loudspeaker voice coil to the outer edge of
the cone diaphragm.
I have found that the specific location at which the
reflector or reflectors are mounted on my pilot speaker are
determined to a great extent on the basis of tone balance and
anlbience as well as on distortion. Accordingly, the specific
position of the reflector may vary with the inner edge of the
reflector located from 15 per cent to 85 per cent of the radial
distance measured outwardly from the outer edge of the loudspeaker
voice coil to the outer edge of the cone diaphragm.
The smaller the cone, the lower its inertia will be. ~lso,
its tendency to break up will be somewhat less and the frequency
at which the smaller cone breaks up will be higher. Accordingly,
the damper pad will not need to be quite so large in diameter
and the reflector can be placed so as to reach a little farther
inward toward the voice coil without getting too much distortion.
The larger the speaker and consequentially the lower the
frequencies handled, the more this lower pitched sound will bend.
Accordingly, a reflector cannot reach so far inward toward the
voice coil on a woofer as for a mid-range or a tweeter since a
tweeter handles more linear sound. Otherwise, the greater bending
would cause an excessive amount of lower pitched distortion to
be reflected.
Since loudness con~our curves and cross-over considerations
dictate how the roll-off or reduction in loudness must be handled
at the upper end of the woofer response curve, the reflector will
not normally reach as far inward toward the voice coil. This
reduces the amount of the high frequencies reflected. The most
ideal roll-off will also normally call for heavier texture on the
surfaces of the reflectors of a woofer to further shape the upper
end of the response curve.
Another important variable is that the deeper the cone, the
farther the reflectors must reach in toward the voice coil. A
cone tends to project its output of sound somewhat perpendicular
tothe surface of the diaphragm of the cone.
Also, the lower the frequencies to be reproduced, the farther
the damper pad must be placed from the cone. This is necessary
because the amplitude of the air movement is greater for low
frequencies and the pad will physically interfere with the air
movement on lower frequencies if any impedance is placed near
enough to the diaphragm of the cone to throttle the movement of
air. The combined impedance of the damper pads and reflectors
must be carefully considered to prevent damping of the lower
frequencies.
The sound chamber for the speaker system will vary according
to frequencies being reproduced by a particular speaker. The
woofer will have a large air space, the mid-range will have a
much smaller air space and the tweeter may have either a very
small air space inthe speaker box with holes in the speaker frame
for air movement in the frame or the frame of the tweeter may be
solid so that the space between the frame and the cone is the sound
chamber.
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Referring now to the graph shown in FIG. 7 of the drawings,
the approximate response curve of a ten inch acoustic suspension
speaker aimed toward the listeners is shown in solid lines at 27
and the approximate response curve of such a speaker turned away
from the listeners is shown in dash lines 28. ~ue to center cone
distortion in must prior art woofers, the highest volume of repro-
duction does not occur at the lowest audible frequencies but has
a peak or sharp knee in the response curve with the highest volume
of sound occurring at from 1,000 to 2,000 cps, as shown by the
solid line 27. This peak or knee in the response curve of the
woofer usually occurs near the cross-over point between the
response curve 27 of the woofer and the response curve 29 of the
mid-range speaker and causes a shrillness which is almost impossible
to eliminate with an electrical cross-over network. The response
curve of the high frequency speaker or tweeter is indicated at
31. The smooth rounded response curve produced in accordance
with my invention is indicated by the dotted line 32.
From the foregoing it will be seen that I have devised an
improved loudspeaker system which reproduces sound with a minimum
of distortion and maintains the mass of the speaker cone suffi-
ciently low to prevent the introduction of an ob~ectionable amount
of distortion. Also, my improved loudspeaker system is simple
of construction, economical of manufacture and may be assembled
with a minimum of time and effort.
While I have shown my invention in several forms, it will
be obvious to those skilled in the art that it is not so limited,
but is susceptible of various other changes and modifications
without departing from the spirit thereof.
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