CA1193357A - Loudspeaker having apertured acoustic impedance frontal loading element - Google Patents

Abstract

Abstract of the Disclosure A direct radiating dynamic loudspeaker has an apertured impedance frontal loading element for improving its acoustic impedance. A planar sheet of non-rigid, fibrous air-permeable material has an apertured area and is positioned in front of the diaphragm of the loudspeaker. The sheet of air-permeable material is supported only at its periphery. A sheet of acoustically transparent material is bonded to the sheet of air-permeable material to cover the apertured area.

Description

~33~i7 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 ~ ~93;3~7 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 --~I

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 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 33~'7 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 -6a-~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 -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 ~33S7 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 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 v~

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.

Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A direct radiating dynamic loudspeaker of 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 thereof;
said impedance means having an opening therein providing an acoustic mass; and a second sheet of acoustically transparent material covering said opening and being bonded to said impedance means.

2. The loudspeaker of claim 1 wherein said impedance means comprises a sheet of felt.

3. The loudspeaker of claim 1 wherein said second sheet is composed of non-woven cloth.

4. The loudspeaker of claim 2 wherein said sheet of felt and said second sheet are synthetic materials.

5. The loudspeaker of claim 4 wherein said synthetic materials of both said sheet of felt and said second sheet are thermoplastics.

6. The loudspeaker of claim 5 wherein said first and second sheets of thermoplastic material are bonded together by heat staking.

7. The loudspeaker of claim 6 wherein heat stake bonding is accomplished around and near the opening in said impedance means.

8. The loudspeaker of claim 7 wherein said heat stake bonding is accomplished with a substantially continuous closed perimeter bond around the opening of said impedance means.

9. The loudspeaker of claim 1 wherein said second sheet is positioned intermediate said impedance means and said diaphragm to prevent the passage of solid particles through the opening in said impedance means.

10. The loudspeaker of claim 1 wherein the opening through said impedance means comprises a circular aperture axially aligned with the center of said diaphragm.

11. The loudspeaker of claim 2 wherein said diaphragm means is of low mass and said motor therefor is a low-weight magnet means such that the normal Q of said loudspeaker is greater than about 1.2 and wherein said first sheet of felt has an airflow resistance of about 100 cubic feet per minute, a density in the range of 10-15 ounces per square yard, and a thickness in the range of 0.06-0.09 inch, thereby resulting in a Q factor less than about 1.2.

12. The loudspeaker of claim 11 wherein said density of said felt sheet is about 11.5 ounces per square yard.

13. The loudspeaker of claim 5 wherein the thermoplastic material of the felt impedance means is composed of polyester and wherein the second sheet of acoustically transparent material is composed of cellulose acetate.