Subwoofer speaker system

A method and apparatus for producing low frequency sound (20 to 100 Hz) at levels of intensity above the minimum threshold of human hearing. A mechanical-electrical drive having high power capacity is coupled via a linkage to a sound radiator and is responsive to the electrical input from a sound program source. The drive is capable of large excursions at high forces which are substantially constant irrespective of the extent of excursion.

BACKGROUND OF THE INVENTION 
This invention relates to loudspeakers and more particularly to sound 
reproducing devices capable of operating at below 100 Hz and commonly 
referred to as subwoofers, which attempt to reproduce sounds in the 
lowermost range of human hearing. 
With few exceptions, all loudspeakers available today utilize a cone 
diaphragm driven by a movable voice coil, which is suspended between the 
pole pieces of a permanent magnet. Electrical energy conveyed to the voice 
coil causes the coil to reciprocate in a linear path and move the 
diaphragm. This type of speaker is commonly known as the permanent magnet 
dynamic type and generally has an efficiency of less than 5 percent and 
even less at low frequencies. 
A long standing objective in high fidelity sound systems is to provide a 
speaker that will accurately reproduce low frequency sounds down to the 
lowermost limits of human hearing. Several problems have prevented the 
attainment of this objective. It has not been possible to produce sounds 
down to 20 Hz at sufficiently intense sound levels using conventional 
speaker design. Also, attempts to reproduce low frequency sounds typically 
result in excessive distortion, due to the non-linearity of the drive at 
low frequencies. 
The maximum sound pressure level available at low frequencies is dependent 
upon the acoustic source strength, which is specified by the available 
area of the vibrating surface and the peak amplitude of vibration. Thus, 
available acoustic power is dependent upon the volume of air that is 
"pumped" by the diaphragm. To maintain a constant sound pressure level, 
each halving of the frequency requires a quadrupling of the peak to peak 
excursion. 
In attempt to achieve accurate low frequency reproduction, conventional 
speakers have been provided with long voice coils and large magnets, 
diaphragms and enclosures, etc. There are, however, several limits in the 
design of such a speaker. First, there is a practical limit on magnet 
size, design and weight. Also, activation of the longer voice coil results 
in large power losses in the form of heat (I.sup.2 R losses). Possible 
thermal destruction of the coil imposes a limit on the power handling 
capacity of the speaker. Moreover, at low frequencies, a point is soon 
reached at which the driver ceases to operate in a linear fashion because 
the voice coil is driven out of the region of constant magnetic flux. All 
of such drives have a limited degree of excursion, which limits the 
available displacement of the diaphragm. 
Another consideration is the relative insensitivity of the human ear to low 
frequencies, which in turn, requires such low frequencies to be produced 
at more intense levels to be heard. As illustrated in the Fletcher-Munson 
hearing sensitivity curves, the threshold of hearing is zero dB at 1000 
Hz, but is 40 dB at 100 Hz and about 100 dB at 20 Hz. Since a change of 40 
dB involves a corresponding power multiplication of 10,000, attainment of 
non-distorted sound frequencies in the region of 20 to 60 Hz and at high 
sound levels has not been practical using conventional apparatus and 
techniques. No other satisfactory solutions to the foregoing problems have 
been forthcoming, and low frequency response has been sacrificed with the 
use of small enclosures and the desire to produce a reasonable spectrum of 
wavelengths at an affordable price. 
In the early stages of speaker development, several proposals were made to 
utilize a galvanometer-type drive having a rotary output to drive one or 
several sound radiating panels. Such devices are described in British Pat. 
Nos. 271,021, 270,421, 212,857, Austrian Pat. No. 126,717, and Japanese 
Pat. No. 11,384. The drives of all these devices, however, are all in the 
form of a single coil immersed between two poles of a permanent magnet, 
which seriously limits available excursion. Also, the available force 
decreases as the coil departs the field. There are also limits on 
magnitude of available peak force and power handling capacity, since the 
drives in these devices have the same or similar limitations as are found 
in conventional, permanent magnet speakers. 
Until recently, there was very little need to reproduce intense levels of 
sound in the range of 20 to 60 Hz because available programming sources 
were incapable of recording such frequencies. With the advent of more 
dynamic recording techniques, however, the ability to produce such sounds 
without distortion has become a highly desirable objective in the 
industry. 
SUMMARY OF THE INVENTION 
The present invention provides a method and apparatus for producing sounds 
in the frequency range of 100 Hz and down to and below 25 Hz at high 
intensities and at low distortion levels in a compact cabinet or 
enclosure. We have discovered that low frequency sound can be produced 
through the use of a separate driver having a degree of available 
reciprocal movement that is unlimited or substantially in excess of the 
largest radiator excursion required at ultra low frequencies, and one that 
is capable of handling large amounts of current in an efficient manner. In 
addition, the efficiency of the driver is not dependent on excursion, that 
is, a substantially constant driving force per unit current is exerted on 
the sound radiator, irrespective of the extent of movement of the radiator 
or driver. The ability to maintain a constant force per unit current at 
large excursions allows for accurate production of high intensity, ultra 
low frequency sounds, which is an objective never heretofore attained in 
the art. 
In the preferred embodiment described herein, the driver is in the form of 
a DC commutated servomotor having a rotary output shaft. In such a motor, 
the current is transferred or switched in the coil as the coil moves in 
the magnetic flux and causes a constant force per unit current to be 
maintained on the shaft. In addition and most importantly, there is no 
inherent limitation on the power handling capability such as exists in a 
conventional voice coil drive using a permanent magnet. 
The constant drive force described above is energized by an amplified 
signal corresponding to the sound to be reproduced, which causes the shaft 
to oscillate. The rotary output of the shaft is converted to linear 
reciprocating motion via a suitable mechanical linkage that is, in turn, 
connected to the sound radiator, all of which may be arranged in a very 
compact and versatile system of high efficiency. 
Since the force produced by the motor shaft is much greater, i.e., at least 
ten times greater than that available from conventional speaker drives, 
and excursion is virtually unlimited, the volume of the enclosure is less 
a critical factor, and sound levels in excess of 120 dB can be produced at 
25 Hz and below, an accomplishment never heretofore attained by 
conventional speaker system of comparable size, i.e. less than four cubic 
feet. More than one radiator may be driven from a single driver, and the 
driver may be geared to produce either a mechanical advantage or amplified 
linear motion. 
Since the drive is much more efficient than a conventional voice coil, 
there are no limitations that are normally associated with conventional 
speakers, such as power handling capacity. In fact, the power handling 
capability of the subwoofer of the present invention is in excess of ten 
times as much as that of the best available speakers of today. 
The electro-mechanical drive arrangement of the present invention is 
particularly and uniquely suitable for production of low frequency sound, 
which requires large masses to be moved over relatively long distances. 
Such drives offer no particular advantages in production of sound above 
125 Hz, i.e., in the range where conventional speakers become efficient 
and linear due to the shorter required excursions and lower power 
requirements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to FIGS. 1 and 2, the subwoofer of the present invention 
may generally comprise an enclosure 10 having solid or non-movable top 12 
and bottom 14 panels interconnected by a plurality of upright posts, such 
as 16. A sound radiating means 18 is resiliently suspended or connected 
along its upright edges between each pair of adjacent posts 16 to form the 
enclosure. The connection between the edges of the sound panels 18 and 
posts 16 may take the form of flexible, shape retaining strips 20, 
although other suitable connection means may be employed. 
Although the present invention will be described in connection with four 
rectangular or square panels as shown, it will be understood that a system 
may include only one radiator or panel or any number of a plurality of 
radiators or horn loading may be employed. Also, while the panels are 
shown as rigid, flat and square, other shapes such as conical, may be 
employed, and the radiators may be flexible. The final enclosure is, 
preferably reasonably air tight, and the radiators and their support 
structures are preferably of substantially the same size and weight. 
Particularly if thin, low mass sound panels 18 are employed, the interior 
sides thereof may be and are preferably reinforced with a bracing network 
or framework, shown generally at 22. Such bracing or reinforcing network 
is preferably coextensive with the interior surface an uniformly supports 
the panel to prevent bending from the mechanical actuator hereinafter 
described. A suitable pivot support 24 is secured centrally at the 
innermost side of each of the frameworks 22. 
In summary, one or a plurality of sound radiating means are resiliently 
mounted around their peripheries and are capable of substantial 
reciprocating movement along an axis against the surrounding atmosphere. 
Reciprocation of the radiators for a given distance and rate causes sound 
to be produced at a given frequency and intensity. 
Drive means are provided for reciprocating the sound radiators to produce 
low frequency sounds, i.e., below 100 Hz, at high intensities, or at 
intensities that are audible to the human ear in the desired frequency 
range. The drive means is also capable of producing a drive or output 
force that is substantially constant at a given excitation level, i.e., 
per unit of current used to activate the drive means. 
In the preferred embodiment, the drive means preferably is a high speed DC 
commutated servomotor. Such motor has a coil immersed in a magnetic field. 
In addition, the motor includes commutation means, i.e., means to transfer 
or switch the current in the active portions of the coil as the coil is 
rotated, such that the active portion of the coil is always immersed, and 
driven by, the region of constant magnetic flux. The shaft of the motor, 
which is capable of unlimited rotation, therefore produces a force that is 
substantially constant per unit of current carried by the coil, regardless 
of the extent or degree of rotation of the shaft. One type of suitable 
motor is sold under the name Electro-Craft as Model No. M-1450/M-1460. 
It will be appreciated that in conventional speaker drives, the extent of 
the region of constant magnetic flux is limited, and linear excursions in 
excess of 0.5 inch are difficult to attain. In the drive of the present 
invention, DC resistance and I.sup.2 R losses are reduced and the coil 
inductance is lowered, and very large or unlimited excursions without 
positional dependance are easily achieved. Whereas a voice coil drive has 
a typical maximum efficiency of about 10%, the drive of the present 
invention has an efficiency in excess of 75% and typically 85%. Moreover, 
for a given frequency range and acoustic output, the subwoofer of the 
present invention will require approximately one-eighth of the volume 
required by a conventional speaker system. 
It will be apparent that other types of drives, both linear or rotary, may 
be employed, provided that they meet the foregoing criteria, namely: (i) 
efficient high power handling capability; (ii) with an unlimited amount of 
excursion, or an amount equal to or in excess of that required to produce 
low frequency sound at required intensities; and (iii) having an output 
that is substantially constant per unit of excitation, irrespective of the 
degree of excursion. Other suitable drives, for example, might be based on 
switchable linear electrical drives, or pneumatic or hydraulic drives. 
In the preferred embodiment, the electric motor 26 having an upright shaft 
28 is mounted centrally within the enclosure 10 on a support 30 rigidly 
affixed to the base 14 or other suitable support. The motor shaft 28 is 
positioned so as to be substantially equi-distant from the vertical 
centerline of each of the sound radiators. 
Means are provided for translating the rotary output of the motor shaft 28 
into suitable motion for driving one or more of the radiators 18, or the 
rigid framework 22 associated therewith. Such means, for example, may 
include rods 32 pivotally connected at one end to each of the supports and 
pivotally connected by vertical pin pivots 34 to a disc 36 secured to and 
mounted for rotation with the motor shaft 28. The pivot points of pivots 
34 are preferably equi-spaced from the axis of shaft 28 such that 
substantially an equal driving force will be imparted to each of the rods 
32 and their associated frameworks 22 and sound panels 18. Also, in the 
embodiments shown, the pivots 34 of opposite panels fall on a common 
centerline through the panels, such that the entire arrangement is highly 
symmetrical and balanced. 
As current is applied to the motor 26, the shaft 28 and disc 36 rotate, 
displacing the pivots 34 toward their respective panels and causing each 
of the panels 18 to be displaced outward. To achieve this effect, it will 
be apparent that the pivots at zero power are located on the disc 36 to 
one side of the center line through its associated panel in order to 
provide necessary leverage for movement. The mechanical arrangement is in 
effect a series of compound levers or toggles, which are capable of 
directly imparting linear motion to the panels. 
Another form of mechanical linkage that may be used is shown in FIG. 3. 
This embodiment is similar in operation to that shown in FIG. 2, and 
comprises a disc-like member 40 mounted on a shaft 42 and having a 
plurality of ears 44 equally spaced around the perimeter of the disc. The 
ears 44 are connected to rods 46 by means of a relatively thin web 48, 
rather than the mechanical joint shown in FIG. 2. Thus, the FIG. 3 
embodiment may be a one piece construction made from a tough, flexible 
polymer, which would minimize development of sloppiness in the mechanical 
system. 
It will be appreciated that many other means may be used to translate the 
rotary motion of motor 26 into a motion suitable to drive the radiators 
18, as, for example, illustrated in FIGS. 5, 6 and 7. 
As shown in FIGS. 5 and 6, the shaft 28' may be provided with a geared or 
toothed surface at 60 as shown. The rods 32 shown in the previous 
embodiment are replaced by rigid elongated beams 62 and 64 which may have 
bifurcated ends that overlap on opposite sides of the shaft 28' as shown. 
The beams 62 and 64 are wide in a direction parallel to the shaft for 
added stiffness in a direction perpendicular to their length. 
As shown in FIGS. 5 and 6, a flexible toothed belt 66 is secured at one end 
at 68 near the end of one beam 62, wrapped around one side of the shaft 
28' and secured at the other end at 70 near the end of the other beam 64. 
A second belt 72 is disposed around the other side of the shaft above the 
first belt and has its respective ends secured at locations 74 and 76 
inwardly of the ends of the respective beams 62 and 64. The teeth of the 
belts engage the teeth of the shaft 28' to prevent any slippage 
therebetween. The belts in effect define opposing loops around the shaft, 
and the belts are tightly secured relative to each other to eliminate any 
free play. As shown in FIG. 6, a second set of belts 78 and 80 may be 
employed around the shaft for added integrity in the arrangement. 
A similar mechanical arrangement is shown in FIG. 7 wherein a pair of 
bendable but otherwise substantially rigid strips 82 and 84 are disposed 
around opposite sides of the shaft 28' and secured as aforesaid to the 
respective beams 62' and 64'. The strips 82 and 84 may be composed of a 
suitable material such as spring steel. In this embodiment, positive 
engagement between the shaft 28' is achieved by means of features 85 or 
other attachment means extending between the strips and the shaft. 
Preferably, the fasteners 85 are located approximately in the center of 
each strip to allow maximum rotation of the shaft in either direction. 
In operation, it may be seen that the belts 66 and 72 and the strips 82 and 
84 are operatively connected to the shaft, and upon rotation of the shaft, 
serve to push or pull both beams simultaneously in opposite directions. 
The embodiments of FIGS. 5-7 have several advantages in that there is 
little or no opportunity for slack to develop in the linkage that might 
adversely affect performance of the speaker. Also, it may be seen that the 
beams reciprocate in a direction substantially perpendicular to the plane 
of the speaker panels rather than at a slight angle required in the 
previously described embodiment. This in turn allows the speaker panels to 
reciprocate more exactly in parallel and eliminates the tendency for any 
movement away from an axis normal to opposed panels. 
It may also be seen in connection with the embodiments of FIGS. 5 through 7 
that rotary motion of the shaft can be easily geared up or down to produce 
a mechanical advantage or to provide additional excursion per unit of the 
shaft, depending on the specific requirement of the system. 
The present invention provides several advantages that have never before 
been available for sound production because of theoretical and practical 
limitations. The primary advantage is the ability to produce high 
intensity, undistorted musical or other sounds from a loudspeaker within 
the frequency range of 20 to 100 Hz, which is enabled because of the 
linear, high power motion available to the radiators and the ability to 
move the radiators through large excursions. Unlike sound reproduction at 
middle or upper frequencies, a subwoofer is more akin to an air pump, and 
performance is directly dependent upon the volume of air that can be 
moved, i.e., excursion limits and area of the radiator. Thus, the system 
of the present invention is very uniquely and specifically adapted to 
production of high intensity, low frequency sound. 
Another problem that is overcome by the present invention is the ability to 
produce undistorted sounds at low frequency. A conventional voice coil 
speaker can easily produce middle and upper frequencies because the 
required coil-cone excursion is very small. When the frequency decreases, 
not only do the power demands become greater, but the required radiator 
excursion causes the voice coil to move outside of the region of constant 
flux of the permanent magnet, and the available drive force decreases 
rapidly, causing gross distortions. Such distortions are eliminated in the 
present system because the drive force per unit current remains constant, 
regardless of the amount of excursion. 
The preferred circuitry and components for driving the speaker system are 
shown in FIG. 4. Inasmuch as only well known conventional components are 
being employed, they will be described by function for the sake of 
brevity. 
As shown, an audio signal from any source is fed into a cross-over network 
50, which is an electrical filter that separates the output signal into 
two or more separate frequency bands. In the present example, the higher 
frequencies, e.g., above 100 Hz are separated and routed to other 
speakers, and the frequencies below 100 Hz are fed into the present 
system. 
The incoming signal is preferably amplified to the desired degree by an 
amplifier 52, since the incoming signal from conventional sources would 
usually be insufficient to drive the motor 26 at the desired output. 
In addition, a negative feedback system may be provided around the motor 26 
and amplifier 52, which serves as a corrective means to improve 
performance. As shown, a position sensor 54 is responsive to motion of a 
sound panel, and the output of the sensor is fed back into a differential 
amplifier 56 connected between the cross-over 50 and the amplifier 52. The 
sensed voltage is proportional to the degree of oscillatory motion of the 
sound panel. 
As shown, the position sensor 54 is of the variable reluctance type having 
an arm 58 connected directly to one of the sound panel bracings 22 whereby 
the relative position of the panel is sensed and fed back to the 
differential amplifier 56. Other electromechanical sensing devices may be 
employed, as well as others, including optical and air pressure means. 
The differential amplifier 56 is in effect an amplifier having two similar 
input circuits so connected that they respond to the difference between 
two voltages or currents but effectively suppress like voltages or 
currents. The differential amplifier therefore creates an error signal 
which is converted to an output signal and has a transient response which 
decays with time. The negative feedback therefore effectively controls the 
movement of the sound panels 18 and tends to correct such movement to the 
incoming signal and improves distortion characteristics. 
In operation, the incoming signal is amplified and fed into the motor, 
causing the shaft 28 first to move counterclockwise and then oscillate 
rapidly in response to the input frequencies. The sound panels, in turn, 
move in and out together in phase to reproduce the low frequency sound 
waves. 
As an example of the present invention, a subwoofer having the following 
performance characteristics was prepared in an enclosure of less than 3 
cubic feet: 
Power Capacity: 300 watts RMS, 3000 watts peak 
Response: .+-.3db 25-100 Hz at 300 watts RMS 
Excursion limit: 1.5 inches 
BL: 27N/amp 
Effective intertia: 2.5 lbs. 
Max force at full avg. power: 48 lbs. 
Peak force: 180 lbs.