Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to loudspeakers, and more
particularly to improved loudspeakers having apertur~d
impedance frontal loading elements.
Electrodynamic loudspeakers, especially those
intended tv be of low cost as for utilization in automobiles
and the like, typically use small-volume or low-weight
magnets for the diaphragm motor, resulting in a low damping
factor on the moving system, quantitatively defined by "Q",
resulting in a Q in excess of approximately 1.2. This low
damping factor has a deleterious effect on the acoustical
performance in the vicinity of the principal ~lowest)
resonant frequency of the moving system characterized by a
peak in the steady state acoustical output, with a
concomitant increase in harmonic and intermodulation
distortion and impaired transient pexformance resulting in
"ringing" of the system. The same low-cost speakers with
small magnets also typically utilize sound-radiating
diaphragms, commonly called cones, having a low mass
characterized by a weight-to-area ratio typically in the
.04-.15 gm/in. range, in order to maximize the efficiency.
The low-mass cone also tends to increase the amplitude of
distributed mode resonances in the cone, which results in an
increased sound output in the upper frequency range, i.e.,
above about 2, noo Hz, and which may not be desirable. A
further performance problem in low-cost, small-size,
low-cone mass loudspeakers is that the principal resonant
frequency, fO, which establishes the low frequency limit of
performance in many applicationsg cannot be made as low as
desired due to cone manufacturing limitations involved in
felting the outer cone suspension areas sufficiently thin.
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The principal resonant frequency Eor a given mass cone is a
function of the cone's suspension compliance which is in turn
a function of the thickness of -the suspension area of the cone.
The increased levels o~ sound outpu-t in the vicinity
of the principal resonance frequency and in the upper frequency
range of the loudspeaker are no-t always desirable performance
attributes. A uniform or "Elat" amplitude vs. frequency
characteristics is often desired but difficult to achieve.
Various techniques for increasing the attenuation
of the peak in the sound output in the vicinity of the
principal resonant frequency and/or in the range of upper
frequencies will be found in an article entitled "Acoustic
Resistance Damping for I.oudspeakers" by John L. Grauer in
AUDIO, Vol. 49, No. 43, p. 22, March 1965; in United States
Patent 2,840,178 entitled "Devi.ce Eor the Reproduction of
Sound"; and in United States Patent 4,012,605 entitled
31 Input/Output Transducer with Damping Arrangement".
United States Patent No. 4,387,787, which issued on
June 14, 1983 to ~larman International Industries, assignees
of John A. King, discloses an improved acoustic impedance
arrangement for such relatively low-cost loudspeakers,
particularly for use in automobiles and other locations where
they may experlence high moisture and/or dust conditions. The
acoustic impedance of such loudspeakers comprises a planar
sheet of air-permeable fibrous felt which covers the projected
frontal area of the speaker diaphragm and is supported only at
the peripheral region thereof. The felt
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material has particular air flow resist~nce, density, and
thickness characteristics such that it provides attenuation
of the acoustical output, both at the principal resonant
requency and in the upper frequency range. Moreover, it
reduces harmonic and intermodulation distortion and lowers
the principal resonance frequency so as to extend the
response range.
The acoustical impedance of the loudspeaker
discussed in the immediately preceding paragraph provides
desired damping o the loudspeaker cone motion in the vicinity
of the principal resonant frequency. For the 12 ounces per
square yard felt im~edance element, significant attenuation
oE the acoustic output exists in the 2-5 kHz frequency
range; however, the attenuation at ~requencies above 5 kHz is
particularly pronounced, and with some cones having
inherently diminished high-frequency output, the degree of
high-frequency attenuation may be undesirable if faithful
reproduction of music and other sounds in those upper
Erequency ranges is required~ On the other hand r for the
8 ounces per square yard felt impedance element, the output
in the higher frequencies above 5-8 kHz is as desired, but
the output exhibits excessive peaking in the 2-4 kHz rangeO
The aforemen~ioned U.S. Patent 2,840,178 refers to
improving the high-frequency response of a speaker by a
centered apertur~ acting as an acoustic mass; however, no
parameters are set forth and the acoustic resistance material
is either a rigid disc of wire-net or an acoustic resistance
material cemented to a perforated metal sheet. U.S Patent
2,646,851 also discusses the use of an aperture or slot in
the area in front of the diaphragm, but such slot is for the
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purpose of defocusing the emanating sound waves and is formed
in a surface which is presumably non-permeable to air.
It is a principal object of the present invention
to provide an improved loudspeclker having acoustic impedance.
Included within this object is the proYislon of acoustic
impedance which results in a relatively flat response across
a relative wide-frequency range.
It is another object of the present invention to
provide an improved loudspeaker having acoustic impedance
especially suited for use with loudspeakers in automotive
applications requiring relative immunity to moisture and/or
solid particulates.
It is a further object to provide an improved
loudspeaker having acoustic impedance of relatively low-cost
manufacture~
In accordance with the invention, there is
provided in a direct radiator dynamic loudspeaker having a
natural Q greater than about 1.2 improved acoustic impedance
in the form of an apertured acoustic impedance element.
The acoustic impedance element is formed princlpally of an
apertured sheet of felt. A second sheet o acoustically
transparent non-woven cloth is bonded to the felt so as to
cover the aperture(s) therein. The felt sheet includes at
least one aperture to facilitate the transmission of
high-Erequency signals. Both sheets are preferably
thermoplastics which are bonded to one another by limited
heat staking around the at least one aperture in the ~elt
sheet. The apertured sheet of felt has particular
airflow resistance, density, and thickness characteristicsO
The impedance element is supported only about its
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peripheral reglon in planar form~ The resulting Q is less than
about 1.2.
According to a broad aspect, -the present invention
provides a direct radiating dynamic loudspeaker o~ the type
having low mass diaphragm and magnet means, the improvement
comprising
a substantially planar acoustic impedance means
positioned in front of and covering the projected frontal area
of said diaphragm, said impedance means being a non-rigid,
fibrous air-permeable material supported only at the periphery
thereoE;
said impedance means having an opening therein
providing an acoustic mass; and
a second sheet o:E acoustically transparent material
covering said opening and being bonded to said impedance means~
The invention may best be understood by referring to
the following description and accompanying drawings which
illustrate the inventlon. In the drawings:
Figu:re 1 is a front elevational view o:E a loudspeaker
including the acoustic impedance of the inventi.on;
Figure 2 is a side elevational view, partly in section,
of the loudspeakerof Figure l; and
Figure 3 is a graph illustrating the acoustic output
vs. fre~uency response characteristics of similar loudspeakers
with no acoustic damping, with a single continous sheet of felt
to provide acoustical impedance, and with the acoustic impedance
element of the invention, respectively.
Referring to Figures 1 and 2, there is illustrated a
3~ inch loudspeaker 10 which includes the improved acoustic
impedance element 12 of the invention. A loudspeaker 1.0 is
of the direct radiator type and includes a moving voice coil
14 and a diaphragm in the form of cone 16. A frame or basket
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18, generally of metal, serves as the principal structural
member of the loudspeaker. The motor for cone 16 is formed
by annular magnet 20 disposed about center pole 22 and
rearwardly of front pole 24. The voice coil 14 concentrically
encircles center pole 22 and is mounted on a cylindrical form
25 in annular air gap 26 between the annular front pole 24 and
the center pole ~2. The cone 16 is affixed at its apex end to
the moving coil Eorm 25 for axial displacement in response to
the interactlon of the variable electrical current through the
coil 14 with the field of
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~r-~
magnet 20. A voice coil centering suspension element 30,
commonly called a spider, is secured both to the Eront pole
24 and to the coil form 25 for posi~ioning and facilitating
operation oE the voice coil and the cone. A dust cap 32
covers the coil form 25. The base of cone 16 is suitably
suspended from the annular rim of basket 18 by an annular
compliance suspension 34 which may be unitary with the cone
or separate therefrom.
The impedance element 12 of the invention i5
~upported only at its periphery by being secured to the front
surface of the annular rim of basket 18 through an
intermediate gasket or pad ring 36. Pad ring 36 may be made
of hot melt type material and is adhered to both the basket
18 and the impedance element 12 in a manner described in U.S.
Patent 4,191,865, also by the inventor of the present
application.
The impedance element 12 is comprised o~ an
apertured sheet of air-permeable fibrous felt material 60.
A ~heet of acoustically t~ansparent non-woven cloth 70 covers
2Q at least the apertured area of felt material 60 in bonded
facing relationship therewith. Both the fibrous felt
material 60 and the non-woven cloth 70 are of thermoplastic
materials to facilitate that heat stake bonding, which will
be hereinafter discussed in greater detail.
The apertured sheet of felt 60 is particularly
selected to provide frequency selective acoustic damping
and/or attenuation when placed over the front oE cone 16,
Although a plurality of apertures might be employed to
advantage, the felt sheet 60 in the preferred embodiment
includes a single aperture 62 having a diameter of about
7~
3;3~7
1~ inches and being centrally positioned on khe 3~ inch
diameter of ~heet 60. The non woven cloth sheet 70 is
typically thinner than felt sheet 60 and comprises a
continuous, acou~tically permeable sheet, or chemotextile,
o~ non-woven thermoplastic material~ The felt sheet 60 may
suitably be polyester and the non-woven cloth 70 may be a
viscose fiber such as cellulose acetate with an acrylic
binder.
The felt sheet 60 preferably has an airflow
resistance of about 100 cubic feet per minute with a pressure
drop of about 0.5 psi and a density in the range of
10-15 ounces per square yard, i.e., 11.5 ounces per s~uare
yard, and a nominal thickness in the range of 0.06 0.09 inch,
i.e., 0.075 inch.
The thermoplastic cloth sheet 70 covering the
aperture 62 in the felt sheet 60 is relatively thin, being
in the range of about 0.01 ~ 0.002 :Lnch and having a weight
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of about 1.0~1.4 ounces per square yard, to minimlze its
effect on the acoustic impedance o~ the element 12. Although
acoustically permeable, the composition of non-woven sheet 70
is such as to be substant~ally water and dust impermeable,
thereby isolating cone 16 from the deleterious effects of
dirt and moisture. Covering sheet 70 is here illustrated as
being located between the felt sheet 60 and cone 16, but it
will be understood that it might be placed on the side of
sheet 60 remote from cone 16u
In accordance with another aspect of the invention,
the aperture-covering sheet 70 is bonded to the felt element
60 by heat staking at limited locations or regions of the
element 12 It is preferable that the bonded area between
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sheets 60 and 70 be minimized to avoid excessive occlusion of
the felt; however, bonding is required at least about the
periphery of the aperture 62 in felt sheet 60. Accordingly,
in the illustrated embodinent in which only a single,
centered aperture 62 is in sheèt 60, the covering sheet 70
extends only a small distance radially beyond the ap~rturer
and an annular heat stake bond 64 exists about the peripheral
region of aperture 62. It is preferred to minimize the area
which sheet 70 covers to minimiæe any interfering interaction
between sheets 60 and 70 and the need for additional bonds,
as well as to minimize the quantity of fabric required;
however, it will be understood that sheet 70 may cover
additional apertures in sheet 60 and might cover the entire
sheet if required. The heat stake bonding of the two
thermoplastic sheets 60 and 70 may be accomplished in a
conventional manner, as by applying a contact tool heated to
about 450F to the outer surfaces of the composlte 12 for
about l second. While the bonding may be e~fected with a
plurality of spot bonds, lt is generally faster and less
costly and therefore preferable to make peripheral bond 64
with a single continuous narrow bonding tool, thereby forming
a CGntinuOuS annular bond having a width of about 0.06 inch~
A speaker 10 employing the impedance element 12
having the above-described geometry and characteristics has
been seen to noticeably increase the damping in the vicinity
of the principal resonant frequency~ f0, and to also
significantly attenuate the amplitude of the acoustic
response above 2,000 Hz relative to an undamped speak~r~
Additionally~ this damping element lowers the principal
resonant frequency~ f0, and both the harmonic and the
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intermodula-tion distortion in the sound output rela-tive to an
undamped speaker. Moreover, relative to a loudspeaker having
acoustic impedance of the type described in the aforementioned
United States Patent No. 4,387,787, the present impedance
element 12 with apertured felt sheet 60 maintains a relatively
flat response in the 2-4 kHz range and above 5 kHz to
frequencies in the range of 10-15 kHz. Thus, a relatively
flat response is provided across a relatively wide-frequency
range.
Referring to Figure 3, the acoustic output vs.
frequency response of a conventional 3~ inch speaker with no
acoustic damping element is depicted by dotted line 40.
Similarly, the amplitude vs. frequency response of an identical
speaker including a simple continuous impedance element of the
type clisclosed in the aforementioned United States Paten-t No.
4,887,787, is depicted by the phantom li.ne 45. Finally, the
acoustic output vs. frequency response of an identical speaker
10 including the acoustic impedance element 12 of the invention
is depicted by the solid line 50.
The frequency response 40 of a conventional non-damped
speaker is seen to have a significant peak in the region of the
principal resonant frequency, fO,i.e., about 150 Hz, and a
broad peak in the upper frequency range above about 1,000 Hz.
The phantom line trace 45~ on the other hand, shows about a 5 dB
decrease in acoustic sound pressure at the principal resonant
frequency, fO, which may represent an undesirable over-damped
condition. Moreover, this
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over-damping is seen to extPnd into and to be even greater in
the upper frequency ranges above about 2 kHz. Specifically,
it will be noted that the output indicated by response curve
45 is about 6~ dB below that of response curve 50 at 10 kHz.
It should be noted that the felt impedance element utilized
in the loudspeaker which provided trace 45 had a density of
12 ounces per square yard and having an airflow resistance of
100 cubic feet per minute~ A lighter felt having a density
of 8 ounces per square yard and an airflow resistance of
50 cubic feet per minute provides a response (not shown)
which is better in the upper frequency range but which
exhibits excessive peaking in the ~-4 kHz range.
Referring to trace 50 representing the response ~f
the loudspeaker 10 employing impedance element 1~ of the
present invention, it is observed that the output level at
the principal resonant frequency, ~0, of 150 ~z is
intermediate that of response curves 40 and 45, having Q's of
about 1~5 and 0.85 t respectively, and exhibits approximately
optimum damping for loudspeakers used in non-resonant
~0 enclosures, with Q = 0.97. Importantly, trace 50 is
intermediate the two response traces 40 and 45 in the range
of 5-15 kHz The response depicted by trace 50 in this upper
frequency range i5 seen to flatten the overall response
characteristic relative to the responses of the system
depicted by traces 40 and 45. Peaks in the response from
fO through 10 kHz do not e~ceed about 1.5 dB relative to the
400 kHz level This has the efect of extending the fre~uency
band over which a high Eidelity reponse may be expected.
Thus, it will be seen that utilization of the
aforedescribed impedance element 12 with the speaker 10
3~
results in a significant improvement in the measurable
performance of the speaker relative both to non-damped
speakers and to speakers having simple continuous impedance
elements. Moreover, the use of thermoplastics and
heat-bonding techniques to form the covered-aperture
impedance element l2 contribute to minimizing the
manufacturing cost o an impedance element to be utilized
with a relatively low-cost speaker. It will be appreciated,
however~ that the concepts and manufacturing techniques may
find similar applicability to speakers of a variety of
qualities and costs~
Although this invention has been shown and
described with respect to detailed emhodiments thereof, it
will be understood by those skilled in the art that various
changes in form and detail thereo may be made without
departing from the spirit and scope of the claimed invention.
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