High accuracy bass reproducer device

The present invention is directed to an apparatus for reproducing the bass components of an audio signal with a high degree of accuracy and minimal distortion, utilizing an enclosure of relatively small size. The apparatus comprises a substantially sealed enclosure in combination with a radiating diaphragm driven by an electromagnetic transducer, the principal diaphragm restoring forces being the elastic forces exerted against the diaphragm surfaces by the gaseous medium engaging such surfaces. The device is characterized by the elimination of the roll or surround joining the cone to the frame as heretofore always employed in reproducers of the air suspension type as the means for effecting a sealing of the enclosure, and the utilization of a rigid peripheral portion of the diaphragm immediately adjacent a parallel interior surface portion of the transducer mechanism to define a gap of minimal dimension, said gap being elongated in the direction of movement of the diaphragm, the clearance provided by said gap being maintained at minimal value consonant with movements of the diaphragm free from mechanical interference.

The present invention is in the field of sound reproduction and pertains 
more particularly to an apparatus for reproducing in an enclosure of 
relatively small size the bass frequencies of sound with a degree of 
accuracy or freedom from distortion not heretofore obtainable in 
enclosures of any size. 
The art of sound reproduction has progressed substantially over the past 
several decades. Changes in the electronic components providing the 
amplification of audio signals have reduced electronic distortion 
components to virtually immeasurable values. Transduction components for 
translating the magnetic information contained on a tape or the mechanical 
information contained on a record have likewise undergone significant 
improvement to the point where only minimal colorations are introduced at 
such stages of the sound reproduction process. The principal source of 
coloration and distortion of the reproduced audio signal remains the 
loudspeaker which converts the electrical impulses impressed thereon into 
acoustical energy. 
While numerous advances of an evolutionary nature have reduced the sonic 
coloration introduced by loudspeaker devices, and particularly sonic 
information in the so-called midrange or treble frequencies to tolerable, 
if not inaudible levels, progress in reducing bass frequency colorations 
has not kept pace. By way of example, although it is possible through the 
use of currently available sound reproducing equipment of moderate price 
to maintain total electronic distortion products below 0.05%, conventional 
bass reproducing loudspeaker devices are unable to approximate distortion 
levels even ten times this amount when they are driven to realistic 
listening levels. 
It is not here practicable to review in detail the many expedients 
attempted in an effort to minimize bass distortion of loudspeakers. In 
general, an area of experimentation has involved the chemical treatment of 
radiating diaphragms to reduce nodes and resonances in the diaphragm 
itself and the use of a variety of cone substances. 
Further efforts have involved the provision of complex loading or coupling 
systems including enclosures defining horns, ported enclosures of various 
types, and so-called air suspension reproducers, such as shown in U.S. 
Pat. No. 2,775,309. This patent represented a significant advance in the 
art in that bass reproduction at realistically high levels with distortion 
products as low at 2 or 3% was achieved in enclosures of relatively small 
size. 
In the so-called air suspension system of bass reproduction, the transducer 
diaphragm constitutes a boundary of a sealed enclosure within which is 
entrapped an elastic medium (air). In such system, movement of the 
diaphragm inwardly of the enclosure produces a compression of the 
entrapped elastic medium, such compressed medium providing a relatively 
linear restoring force acting equally over the surface of the diaphragm. 
Such linear acting restoring force avoided, to a degree, the non-linear 
distortions inherent in loudspeaker devices theretofore used wherein an 
elastic connection is effected between the periphery of the diaphragm and 
a frame surrounding the periphery. 
Such elastic surround mechanisms obviously involve non-linear force 
components by virtue of the fact that the restoring force urging the 
diaphragm to its "at rest" or neutral position increases progressively as 
the diaphragm is displaced from its neutral position. 
U.S. Pat. No. 2,775,309 employed a suspension system having greatly reduced 
reliance on mechanical restoring force and increased reliance on 
compressed (or rarefied) medium to effect control of the radiating 
diaphragm. The subject patent employed at the interface between the 
diaphragm periphery and the enclosure a highly compliant roll or surround 
defining a seal at the interface of the diaphragm and enclosure but 
nonetheless permitting relative movement between the diaphragm and the 
enclosure without introducing significant restoring forces to the 
diaphragm. 
The state of the art to the present, and particularly the state of the art 
as pertinent to electromagnetic transducers, has thus embodied one or the 
other of the two principles hereinabove briefly discussed, namely, the use 
of a mechanical surround interposed between the diaphragm and a structural 
support therefor, with its inherent non-linearity characteristics, or a 
highly compliant, non-force exerting roll or surround functioning as an 
air seal but not introducing significant restoring forces to the cone 
periphery. 
I have discovered that the highly compliant roll or surround employed in 
air suspension sound reproducing devices as exemplified in U.S. Pat. No. 
2,775,309 and conceived as a means for avoiding the non-linear distortions 
present in transducers theretofore known is itself a source of subsidiary 
distortion products, phase interference and cancellation effects. 
Specifically, the highly compliant sealing roll or surround used in air 
suspension systems heretofore known has been found to act as a sound 
generating and radiating medium or mechanism acting, in large part, in 
opposition to the sound generated by the main radiating diaphragm. 
More particularly, I have discovered that as the radiating diaphragm of an 
acoustic suspension system is drawn inwardly into the enclosure, with 
concomitant increased compression of the enclosed elastic gaseous medium, 
the compressed medium tends to force the highly compliant sealing roll or 
surround in an outward direction exactly opposite to the direction which 
the diaphragm is being drawn by the magnet-voice coil assembly. Thus, 
while the diaphragm, as perceived from outside the enclosure, is producing 
a rarefaction on its inward stroke, the surround is forced outwardly by 
the enclosed gas and produces a compression of an unpredictable nature in 
opposition to the rarefaction. 
While the total area of the surround is substantially less than that of the 
diaphragm and thus the sonic effects produced thereby are smaller than the 
sonic effects produced by movements of the diaphragm, the sound output of 
the surround is sufficiently great to introduce substantial distortion 
products to the overall signal. The distortion products are observed as 
boominess, hangover, and particularly blurring of bass transients. The 
sudden transient attack, for instance, of a bass drum which is struck is 
perceived as a muddled or muddied sound due to the spurious effects 
introduced by the oppositely moving roll or surround. 
It is important to note that the roll generated sounds are of substantially 
opposite phase from the phase of the sounds generated by the diaphragm and 
that cancellation effects and time delay distortion among other types of 
distortion, including break-up of the extremely flexible roll material, 
inevitably result from the use of conventional air suspension woofers. 
I have discovered that although it had heretofore been considered that a 
completely sealed enclosure is necessary for a satisfactory air suspension 
system, satisfactory air restoring forces on the diaphragm can be 
generated through the use of an enclosure sealed except for the front 
aperture occupied by the driven diaphragm, provided that the diaphragm 
periphery and the adjacent static surfaces of the enclosure or speaker 
frame component adjacent the periphery of the diaphragm are elongated in 
the direction of movement of the diaphragm and if the clearance or gap 
between the diaphragm periphery and adjacent edge of the opening is 
sufficiently small. 
I have found that by maintaining the total cross-sectional area of the gap 
within selected low limits, and elongating said gap in the direction of 
movement of the diaphragm to a length of about 1.5 cm or more, the 
turbulence effects about the periphery of the diaphragm in the gap area 
are such as to retain sufficient of the reactive stiffness of the elastic 
enclosed medium to obtain the essentially linear resistance and restoring 
forces noted in the aforesaid U.S. Pat. No. 2,775,309 without the use of a 
flexible surround or roll with its attendant colorations and distortions. 
I have further discovered that by properly forming or configurating the 
interfitting surfaces of the periphery of the diaphragm, the problem of 
rubbing or scraping of the diaphragm periphery against static components 
is eliminated by the flow of air through the gap, which functions to 
center the diaphragm. Numerous variations in the shape or configuration of 
the diaphragm periphery and/or adjacent static surfaces in order to obtain 
modified flow patterns have unexpectedly been found significantly to 
modify the tolerable gap clearances. 
Briefly, as more fully set forth hereinafter, the invention may be 
summarized as directed to an enclosure sealed except for an opening, which 
opening is filled with a diaphragm which is relatively stiff, at least at 
its periphery, which diaphragm is driven by a conventional linear acting 
electromagnetic motor, e. g. a voice coil riding in an annular magnetic 
gap. The diaphragm includes centering or guide means within the enclosure 
which enable reciprocal movements in a direction parallel with the axis of 
the voice coil, the centering means preferably engaging the diaphragm 
assembly at points longitudinally spaced in the direction of the voice 
coil axis so as to assure precise axial movement of the diaphragm. 
By so configurating and arranging the elongate gap between the periphery 
and the internal surface of a static portion surrounding the periphery, 
there is provided a low frequency reproducer apparatus having exceedingly 
low distortion products of an order comparable to the distortion products 
of electronic components of a sound reproducing system. 
It is accordingly an object of the invention to provide an improved low 
frequency reproducer assembly. 
A further object of the invention is the provision of a low frequency 
reproducer assembly which includes a sealed enclosure and a diaphragm 
relatively rigid at least at its periphery portions, the periphery of the 
diaphragm and the internal surface of the enclosure surrounding the 
diaphragm defining an elongate gap therebetween, said periphery and 
surface being free from any surround, roll or like constraint which will 
interfere with the linear action of the enclosed elastic medium on the 
movements of the diaphragm. 
Still a further object of the invention is the provision of a low frequency 
reproducer of the type described wherein the sole flow passage from the 
interior to the exterior of the enclosure is defined by a gap surrounding 
the moving diaphragm, the dimensions of such gap being such as not to 
reduce the acoustical stiffness reactance of the fluid medium within said 
enclosure to a value below about 95% of the theoretical stiffness 
reactance of said medium if said container were completely sealed. In 
determining the stiffness reactance, it should be recognized that the 
turbulence effects generated between the relatively moving parts are such 
that the stiffness reactance is higher than might be theoretically 
calculated, considering the area of the gap in a static condition. 
A further object of the invention is the provision of a low frequency 
reproducing device of the type described wherein extremely long excursions 
of the diaphragm relative to the enclosure may be accommodated, making 
practical the use of diaphragms which are relatively small in area without 
sacrifice of sound intensity and without the introduction of distortion 
components.

Turning now to the drawings, there is diagrammatically illustrated in FIG. 
1 a bass reproducer device in accordance with the invention. It will be 
readily appreciated by those skilled in the art that the reproducer device 
10 may comprise a separate unit capable of reproducing only the bass 
frequency range of the musical spectrum or, more usually, may comprise an 
element of a unitary loudspeaker device capable of reproducing the entire 
sonic spectrum. 
The bass reproducer device 10 includes an enclosure 11, the enclosure being 
completely sealed except for a circular opening 12 in the front face 13 
thereof. A transducer assembly 14 is mounted within the enclosure 11. The 
transducer assembly includes a frame 15 having an annular flange 16 
rigidly secured thereto. The flange 16 is securely mounted in the opening 
12, for example by screw members or like fasteners 16' extending radially 
through the flange and into the front face 13 of the enclosure at 
angularly spaced-apart positions. In the mounting of the flange 16 within 
the opening 12, care must be taken to assure an airtight interfit between 
the noted parts, for which purpose a glue or caulking compound is 
desirably interposed between the interface of the flange and the opening. 
The frame 15 may include a plurality of angularly spaced-apart, rearwardly 
extending ribs or struts 18, the rearwardmost ends 19 of which carry an 
essentially conventional magnet assembly 20. The magnet assembly 20 
defines an annular magnetic gap 21 between the central pole piece 22 and 
the annular pole element 23, which gap is axially elongated and preferably 
of an essentially constant flux density through the length thereof. 
The frame member 15 carries a diaphragm assembly 24, the diaphragm assembly 
including a sound radiating component 25 of substantial diameter, normally 
in the order of from a minimum of about 6" upwardly to about 12" or more. 
The diaphragm assembly includes a reduced neck portion 26, to the 
rearwardmost end of which is secured a voice coil 27. The leads (not 
shown) from the extremities of the voice coil 27 are secured either to the 
output terminals of the loudspeaker where the device is to be used as a 
bass reproducer only, or alternatively, to the bass frequency output taps 
of a crossover network, where the reproducer constitutes an element of a 
full range loudspeaker assembly. 
The voice coil 27 preferably is elongated in the direction of the axis of 
the neck portion 26 and is preferably so constructed and arranged that an 
essentially constant number of turns of the voice coil is disposed within 
the constant flux magnetic gap throughout the anticipated range of 
excursions of the diaphragm. Preferably the diaphragm is of the long 
excursion type wherein deflections of up to about 1/2" inwardly or 
outwardly from the null or center position may be achieved. 
Suspension means 28, 29 are interposed between the neck portion 26 of the 
diaphragm assembly and the struts 18 of the frame. The suspension means 
permit substantial movement of the neck portion inwardly and outwardly 
while supporting said neck portion against deviation from the desired 
axial or linear orientation in which it is mounted, i.e. the neck portion 
is maintained precisely perpendicular to the front surface 13 of the 
enclosure. 
Numerous suspension means have been found suitable for the intended 
purpose, including specifically a series of corrugated, radially extending 
resilient or elastic strips, such as the strips 28, 29. Alternatively, the 
suspension means 28, 29 may comprise annular corrugated, highly compliant 
disks. 
In order to preserve the linearity of movement of the diaphragm assembly, 
the forces exerted by the suspension in the direction of the axis of the 
neck portion 26 of the diaphragm are maintained at a minimum. Since, in 
contrast to conventional loudspeaker assemblies, the suspension means 28, 
29 constitute the principal mechanism for maintaining the axial alignment 
of the diaphragm, the suspension components are preferably spaced apart a 
substantial distance along the axis of the neck portion 26. 
While there has been illustrated in FIG. 1 a pair of suspension means 
engaging the neck portion at axially displaced positions, it is 
contemplated that a single suspension means may be employed, provided that 
alternative means which are, in fact, spaced axially relative to the neck 
portions are provided for preventing angular deflection of said neck 
portion. 
By way of example and without limitation, the neck portion may be fitted 
with a cylindrical Teflon sleeve, precision interfitted with a 
complemental Teflon sleeve made fast to the frame assembly 15 of the 
transducer, whereby an exceedingly low friction bearing is defined between 
the parts, with the diaphragm being supported for axial movement. 
The enlarged or radiating portion 25 of the diaphragm assembly terminates 
in a peripheral edge 30, which edge is disposed in intimately spaced 
relation to the interior surface 17 of the flange 16 to define between the 
adjacent surfaces 30 and 17 a continuous gap of minimal cross-sectional 
area. The edge 30 is elongated in the axial direction, as is surface 17, 
to assure that the gap is of a substantial longitudinal extent, for 
purposes which will be more fully explained hereinafter. 
The diaphragm assembly 26 is preferably fabricated of rigid and lightweight 
material such as, by way of example, a closed cell Styrofoam material. The 
mass of the diaphragm assembly should be retained at the lowest possible 
value consonant with the achievement of structural rigidity. At least the 
peripheral portion adjacent the surface 30 of the radiating portion 25 of 
the diaphragm should likewise be comprised of rigid material, it being 
appreciated that whereas the diaphragm assembly 24 in the illustrated 
embodiment is disclosed as constituting an integral unit, it is 
contemplate that for purposes of reducing mass there may be formed 
internal voids and the interior of the radiating component may be hollow 
and a treated paper diaphragm component supported in position within the 
rigid peripheral portion. 
It is critical to the satisfactory operation of the instant device, since 
the same depends largely on the elastic medium disposed within the 
enclosure as the diaphragm restoring medium, that the total 
cross-sectional area defined by the gap G between the inner flange surface 
17 and the peripheral portion 30 be maintained at the lowest feasible 
value. 
I have unexpectedly discovered that, notwithstanding the existence of the 
gap G, the speaker assembly will act in the manner of an air suspension 
speaker, the operating principles of which are fully elaborated on in the 
above referenced U.S. Pat. No. 2,775,309. More specifically, the noted 
patent teaches that improved bass reproducing performance may be achieved 
in an enclosure of small size through the use of an entirely sealed 
enclosure of which the radiating diaphragm defines a boundary, and further 
teaches that a complete sealing must be effected in order for satisfactory 
performance to be achieved. 
It is known, for instance, in the use of air suspension speakers 
commercially available that even unsealed apertures in the enclosure of 
the magnitude of a screw hole, seriously degrade the sound produced by the 
device by compromising the elastic restoring effects set forth in the 
above mentioned patent. 
I have discovered that the elastic restoring forces necessary to 
satisfactory operation of the device as an air suspension speaker are not 
compromised, notwithstanding the absence of a completely sealed enclosure, 
provided the air passage which compromises the sealed nature of the 
enclosure is disposed in surrounding relationship of the radiating surface 
of the diaphragm, i.e. at the interface between very closely spaced, 
relatively moving parts. The clearance at the gap must be maintained at a 
minimal value consonant with efficient production methods and the gap is 
elongated in the direction of movement of the diaphragm. Gap spacings in 
the order of as little as 0.01" or less can be commercially attained and 
provide satisfactorily small total gap areas to retain the desired elastic 
restoring force. Bass performance of the loudspeaker is not degraded by 
the diaphragm surrounding gap although the total cross-sectional area of 
the gap may be many times greater than that of an aperture which, if 
otherwise located in the enclosure, would significantly compromise bass 
reproduction. 
I have further discovered that modification of the configuration of the 
surfaces of the relatively moving components at the interface between the 
diaphragm periphery and flange has a significant effect upon the allowable 
gap tolerances. Obviously, a configuration which enables satisfactory 
operation with greater tolerances provides a more commercially desirable 
structure from the manufacturing standpoint. 
More particularly, I have determined that satisfactory restoring forces 
and, hence, proper acoustical suspension operation, may be achieved where 
the acoustical stiffness reactance of the fluid medium within the 
enclosure is maintained at a value not below about 90 to 95% of the 
theoretical stiffness reactance of said medium if the container were 
completely sealed. 
It will be appreciated that the stiffness reactance will vary in accordance 
with the mass of the enclosed medium and, thus, in many instances, to 
avoid standing waves and to reduce the necessary volume of enclosed air 
for a given desired acoustical compliance, it may be desirable to 
incorporate quantities of "fiberglass" filler material F within the 
enclosure surrounding the transducer, and preferably separated therefrom 
by a layer of fine mesh material. The fiberglass reduces the velocity of 
propagation of sound in the enclosed medium, and insures isothermal 
compression of the enclosed fluid, and also acts as an acoustical 
dampening element whereby, as noted above, standing wave formation is 
reduced. Other expedients for reducing the formation of standing waves may 
desirably be employed, such as baffles, etc. 
The acoustical stiffness reactants St in dyne/centimeters may be calculated 
in accordance with the following formula for a static condition of the 
device: 
EQU St=(R C.sup.2 A.sup.2)/V 
where R equals the density of the enclosed medium in grams per cm.sup.3 ; C 
is the velocity of sound in the enclosed medium in cm per second; A is the 
area of the diaphragm in cm.sup.2, and V is the volume of the enclosure in 
a cm.sup.3. 
The above formula is appropriate to calculation of the stiffness compliance 
of a completely sealed enclosure. In a static condition, the provision of 
an aperture of the size of the total area defined between the gap and the 
flange would be expected to have a profound effect upon the said 
compliance. 
Surprisingly, and apparently by virtue of the fact that the gap is defined 
between relatively moving surfaces, the reduction of the elastic stiffness 
is not nearly of the magnitude which might theoretically be expected. 
Without limitation to any specific theory, it is presently believed that 
the expected stiffness loss is not experienced by virtue of the 
formulation, due to turbulence effects at the interface of the moving 
parts, of a de-facto fluid seal. The existence of such turbulent seal 
enables the advantages of an acoustical suspension system to be realized 
without the attendant disadvantages of a sealing acoustical surround, as 
heretofore considered indispensable. Elimination of the surround has 
permitted the elimination of the previously described attendant 
distortions. 
Turning now to the embodiment of FIG. 2, in which like parts have been 
given like reference numerals, the sound reproducer and the assembly 
thereof is substantially identical to that of FIG. 1, with the exception 
that the radiating portion 25 of the diaphragm includes a forward 
extension 31. A suspension member 29' is interposed in this instance 
between the extension 31 and surrounding flange 16 or struts extending 
therefrom, the suspension 29' being preferably comprised of a series of 
radially directed, angularly spaced-apart, resilient corrugated strips 
having minimal area. The advantage of such disposition of the forward 
suspension 29' in greater spaced relation from the rear suspension 28 is 
in the more efficient application of the supporting forces which offset 
tilting or departure of the diaphragm from the desired axis of movement. 
The peripheral edge 30' of the diaphragm of the embodiment of FIG. 2 
includes inclined or part-spherical outer and inner surface portions 32, 
33. The utilization of such inclined edges provides a degree of 
self-centering of the diaphragm in the course of axial movement. Such 
self-centering is believed to result from the air passing through the gap 
reacting against the diaphragm in a direction which moves the diaphragm 
away with progressively greater force as the extent of the gap becomes 
progressively less. 
If, for example, the diaphragm should sag to a slight degree from its 
optimal position, with the result that one peripheral portion should 
approach the flange more closely, the air passing through the reduced 
clearance portion will be preferentially effective to center the diaphragm 
due to the closer proximity of the flange portion against which air has 
been deflected by the beveled portions 32, 33. 
Additionally, the semi-spherical configuration of the periphery of the 
diaphragm will permit a degree of axial misalignment without rubbing. 
In FIG. 3 there is disclosed a still further embodiment of the invention 
wherein the peripheral edge 30" has been formed with a series of annular 
corrugations. The existence of such corrugations appears to create 
augmented turbulent flow patterns at the interface between the inner 
surface 17 of the flange 16 and the peripheral portion 30", forming a more 
effective fluid seal and consequently better preserving the elastic 
stiffness of the enclosed medium. 
In accordance with the embodiment of FIG. 4, the flange member 16" is 
provided with an outwardly facing annular member 34 which is L-shaped in 
section, thus defining with the flange 16" an outwardly directed, 
forwardly open annular chamber 35. 
The diaphragm 25 includes at its peripheral portion a cylindrical formation 
36 which enters into the chamber 35. It will thus be seen that there is 
defined a labyrinth for air entering or leaving the enclosure in the gap 
between the diaphragm and the flange. The provision of such labyrinth has 
been found to reduce the loss of acoustical elasticity occasioned by the 
presence of the gap. 
It will be readily recognized from the foregoing that the present invention 
is directed to an improved low frequency sound reproducing apparatus 
characterized principally in that a diaphragm surrounded by an air gap of 
minimal cross-sectional extent is mounted for movement in an opening in an 
otherwise sealed enclosure, the periphery of the diaphragm being free of 
the usual roll or surround, whereby there is achieved bass reproduction 
free from the coloration and distortion effects inherent in bass 
reproduction loudspeakers heretofore known. 
It is understood in the light of the instant disclosure that variations may 
be made in details of construction without departing from the spirit of 
the invention. Particularly, further experimentation may suggest 
modifications of the details of the suspension as well as of the shape of 
the components at the interface defining the gap. By way of example and in 
order to avoid canting of the diaphragm assembly under the influence of 
gravity, the diaphragm may be arranged so as to move in a vertical plane 
and the opening in the enclosure may be located in a horizontally disposed 
bottom wall. Such arrangement will not interfere with the realism of the 
sounds produced if the bass frequencies fed to the horizontally arrayed 
transducer are kept below about 100 Hz, due to the non-directionality of 
audio signals in this frequency range. 
Accordingly, the invention is to be broadly construed within the scope of 
the appended claims.