APPARATUS AND METHOD FOR ENHANCING THE FREQUENCY RESPONSE OF A LOUDSPEAKER

Abstract: As shown in Fig. 2A the back surface (110B) of the diaphragm (110) of a loudspeaker (100) exhibiting relatively small excursions at midrange frequencies is substantially surrounded by a housing (140) including an acoustic filter (150, 152) therein for attenuating high frequency components of the acoustic signal radiating from the back surface (110B) of the diaphragm (110). The remaining midrange components of the acoustic signal radiating from the back surface (110B) of the diaphragm (110) are phase-delayed and redirected through at least one port (145) in the housing (140) such that the remaining midrange components of the acoustic signal radiating from the back surface (110B) of the diaphragm (110) join the midrange components of the acoustic signal radiating from the front surface (110A) of the diaphragm (110) substantially in phase therewith. The response of the loudspeaker (100) to midrange signals is thus enhanced.

Description: APPARATUS AND METHOD FOR ENHANCING THE FREQUENCY RESPONSE OF A LOUDSPEAKER Background of the Invention This invention relates to loudspeakers and, more particularly, to loudspeakers having frequency responses extended lower than the normal operating frequencies of conventional loudspeakers in free air. Description of the Prior Art I One structure conventionally employed to enhance the low frequency response of a woofer loudspeaker is the bass reflex enclosure. Typically, in the bass reflex structure, a woofer loudspeaker including diaphragm, driver and housing is mounted in one opening in an enclosure having suitable dimensions to form a resonant cavity therein. More specifically, the dimensions of the resonant cavity formed by the enclosure are selected such that the enclosure resonates at a frequency below the normal operating range of the woofer loudspeaker to extend tne response of the woofer to frequencies lower than the typical woofer in free air, as subsequently explained. The woofer loudspeaker thus mounted produces two acoustic signals, that is, a front radiating acoustic signal projecting from the front surface of the diaphragm through the opening in which the woofer is mounted and a back radiating acoustic signal projecting from the back surface of the woofer diaphragm inwardly through the ϋ . resonant cavity of the bass reflex enclosure. Although the components of the back radiating acoustic signal having a frequency less than tne normal operating range of the woofer exhibit a relatively small amplitude, the amplitude of such back radiating acoustic signal compon¬ ents is sufficiently large to excite the cavity into resonance. Acoustic energy of substantial amplitude is thus produced in the cavity at a frequency less than the normal operating range of the woofer loudspeaker. This acoustic energy is typically radiated through a port or opening in the enclosure to add to the low frequency or bass response of the woofer loudspeaker. The size and location of the port is known to affect the resonant frequency of the cavity. Although the bass reflex enclosure performs well in enhancing the response of a woofer loudspeaker over the relatively narrow band of low frequencies for which it is designed, a reflex enclosure for midrange and high frequencies is impractical due to the relatively short wavelengths of the acoustic signals involved, the relatively narrow band of frequencies which the reflex enclosure is capable of enhancing and the phase relationship problems incurred at midrange and high frequencies. Primary audio components are defined to be those audio components generated by a loudspeaker which fall within the normal operating frequency range of the loudspeaker, wnether the range is high frequency or high frequency-midrange. Diaphragm area and the amount of diaphragm excursion determine the normal operating fre- quency range of a particular loudspeaker. At frequencies less than the normal operating range of a loudspeaker, the response of the loudspeaker is degraded such that secondary audio components having smaller amplitudes than the primary components ar.e generated by the loudspeaker at these frequencies. This degradation of response OMH occurs when the diaphragm area and amount of diapnragm excursion are too small to allow the loudspeaker to generate comparatively low frequency signals (that is, secondary components) of substantial amplitude. This proolem is typical in high frequency and high frequency- midrange loudspeakers. Negligible audio response is observed from such loudspeakers at frequencies less than the frequency range of the secondary components. It is one object of the present invention to provide a loudspeaker which overcomes the foregoing deficiencies. It is another object of the present invention to provide a loudspeaker with enhanced response to secondary components despite the relatively small excursions relative to diaphragm area which the speaker diaphragm may exhibit at secondary component frequencies. These and other objects of the invention will become apparent to those skilled in the art upon consideration of the following description of the invention. Summary of the Invention The present invention is directed to providing a loudspeaker with substantial primary component response and enhanced secondary component response even though the speaker diaphragm may exhibit small excursions relative to diaphragm area for secondary component signals. In accordance with one embodiment of the invention, a loudspeaker includes a diaphragm having front and back opposed surfaces and a periphery. The loudspeaker fur¬ ther includes a driving element or transducer coupled to the diaphragm for vibrating the diaphragm in accordance with an applied electrical signal. The diaphragm produces a front radiating acoustic signal and a back radiating acoustic signal when so excited, each of the acoustic signals including primary components and OMPI secondary components. The louαspeaker includes a nousing for enclosing the diaphragm and forming a chamber there¬ with including at least one port for channeling the back radiating acoustic signal outwardly therefrom in approxi- mately the same direction as the front radiating acoustic signal. A filter is situated within the housing for attenuating the primary components of the back radiating acoustic signal. The filter has substantially no effect on the secondary components of the back radiating acoustic signal. The housing of the loudspeaker is dimensioned to provide a signal path for the back radiat¬ ing acoustic signal such that the secondary components thereof propagate through the port and combine substan- _» tially in phase with the secondary components of the front radiating acoustic signal. The secondary component frequency response characteristics of the resultant loudspeaker are thus reinforced. The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by refer¬ ence to the following description taken in conjunction with the accompanying drawings. Description of the Drawings Fig. 1A is a cross-section of a conventional bass reflex enclosure known in the art and which includes a woofer loudspeaker. Fig. IB is a frequency response graph of the structure of Fig. 1A. Fig . 2A is a cross-section of one embod iment of the loudspeaker of the present invention . OMPI Fig. 2B is a frequency response graph of a prior art loudspeaker. Fig. 2C is a frequency response graph of a loudspeaker constructed in accordance with the present invention. Fig. 3 is a perspective view of the loudspeaker of Fig. 2 incluαing a cutaway portion of the interior of the loudspeaker. Fig. 4 is a cross-section of another embodiment of the loudspeaker of the present invention. Description of the Preferred Embodiment Fig. 1A illustrates a conventional bass reflex-type loudspeaker structure 10 including an enclosure or cabinet 20. Enclosure 20 includes a resonant cavity 30 therein having characteristics _already discussed in the Description of the Prior Art. ≤ass reflex structure 10 includes a woofer loudspeaker 40 having a driving element 42, a diaphragm 44 and a housing 46. Woofer loudspeaker 40 is mounted facing a hole 50 in enclosure 20 having a diameter sufficiently large to accommodate loudspeaker 40. A front radiating low frequency acoustic signal radiates from the front surface of diaphragm 44 and is designated by a plurality of solid lines with arrows indicating the approximate direction of propagation. A back radiating acoustic signal is radiated from the back surface of diaphragm 44 inwardly to resonant cavity 30, The back radiating acoustic signal is designated by a plurality of dashed lines with arrows indicating the approximate direction of propagation. Enclosure 20 includes a port or opening 50 through which the back , radiating signal travels. Port 60 is so situated in enclosure 20 that the front radiating acoustic signal and the back radiating acoustic signal ultimately propagate OMPI in approximately the same direction and join with each other after leaving enclosure 30. Port 50 is shaped and positioned to tune resonant cavity 30 to a frequency less than the normal operating range of the woofer 40 in free air. For example,, if the lower end of the normal response range of woofer 40 is 50 Hertz, resonant cavity 30 may be tuned to a frequency of 40 Hertz to extend the low frequency response of woofer 40. Fig. 13 graphically shows how the front radiating acoustic signal from woofer 40 and the components of the back radiating acoustic sig¬ nal from woofer 40 at or near the resonant frequency of cavity 30 combine to extend the low frequency response of woofer 40. It is to be noted that port 60 may be tuned only to * a relatively narrow band of frequencies, and thus the amount of additional bass response attainable in bass reflex structure 10 is limited. Fig. 2 shows one embodiment of the loudspeaker of the present invention as .loudspeaker 100. Loudspeaker 100 includes a diaphragm 110 having an upper surface 110A and a lower surface HOB. Diaphragm 110 is typic¬ ally conically snaped and exhibits an apex 112 and a periphery 114 at the outermost edge of diaphragm 110 as illustrated in Fig. 2A. Apex 112 is mechanically con¬ nected or otherwise coupled to a driving element or transducer 120 such as a piezoelectric transducer, for example. One transducer which may be employed as driving element 120 is the piezoelectric bimorph bender described in the United States Patent No. 3,548,116, issued to Schafft on December 15, 1970, and assigned to the instant assignee. Piezoelectric transducers typically exhibit relatively large diaphragm excursions when producing high frequency acoustic signals, as compared to the short wavelength of such signals, but produce a relatively low amount of transducer excursion for midrange acoustic signals. More specifically, the diaphragm excursions of a piezoelectric transducer for midrange frequency signals are relatively small when compared to the wavelength of midrange frequency acoustic signals. A somewhat reduced midrange frequency or secondary component response is typical in prior loudspeakers employing piezoelectric transducers especially in the lower midrange frequencies. For such loudspeakers, high frequencies are the frequen¬ cies at which the primary components of the loudspeaker are typically generated. The present invention solves the problem of secondary component response degradation. Electrically conductive leads 122 and 124 are operatively coupled to driving element 120 to facilitate connection of driving element 120 to external electrical circuitry. Loudspeaker 100 includes a diaphragm support struc¬ ture 130 having a rim 132. Diaphragm support structure 130 is shaped to substantially surround diaphragm back surface HOB and includes relatively large openings or perforations therein (not shown) to allow the back radi- ating acoustic signals (shown as dashed lines) generated by excitation of diaphragm 110 to pass therethrough. Support structure rim 132 is connected by a layer of cement or other adhesive material (not shown) to the periphery 114 of diaphragm 110 to provide a termination or support for diaphragm 110. A housing 140, conveniently cup-shaped, includes a rim 142 and a chamber 144 therein. Housing 140 includes a plurality of pedestals 146 projecting upwardly from selected locations around rim 142 to support diaphragm support structure 130 which is situated on and attached to pedestals 146. Alternatively, pedestals 146 may be included as a part of support structure 130. Pedestals 146 have a sufficient height as viewed vertically in Fig. 2A to support the combined structure of diaphragm support structure 130 - diaphragm 110 above and separated from housing 140. Thus, an opening or port 145 is formed between rim 142 and diaphragm support structure 130 through which the back radiating acoustic signals pass and are redirected in approximately the same direction as the front radiating acoustic signals emitted from front diaphragm surface 110A. Port 145 comprises a number of smaller ports according to the number of pedestals 146 employed in a particular embodiment of the invention. For example, in the embodiment of the invention, loud- speaker 100, shown in Fig. 2A and more clearly in the perspective view of Fig. 3, four pedestals 146 are employed, thus dividing port 145 into four ports or sub- ports 145A, 145B, 145C and 145D. Fewer or more pedestals 146 than four may be employed providing structural integrity is maintained in the attachment of diaphragm support 130 to rim 142. Of course, the size of port 145 varies according to the height of pedestals 146. Referring again to Fig. 2A, "path length" with respect to the loudspeaker of the present invention is defined to be the distance from back diaphragm surface HOB through chamber 144 to ports 145A through 145C along which the back radiating acoustic signal propagates, that is, the distance along the back radiating acoustic signal path. A filter 150 of acoustic dampening material, for example, fiberglass, is situated within housing 140 at a location intercepting this back radiating acoustic signal path such that the primary components of the back radiat¬ ing acoustic signal are substantially attenuated while the remaining secondary components of the back radiating acoustic signal are passed without substantial attenua¬ tion to port 145. For convenience, filter 150 is shown in Fig. 2A as being situated between driving element 120 and diaphragm support member 130 although other locations within chamber 144 are equally acceptable as alternative or supplementary locations providing the above filtering --^. ' J ^∑, . MPI criteria are met. For example, filter material 152 may be situated within housing 140 at the bottom of chamber 144 also as shown providing the back radiating acoustic signal path is substantially intercepted by filter 152. Clearly, the geometry of housing 140 and chamber 144 affects the "path length" described above. The geometry of housing 140 and chamber 144 is selected such that the "path length" is sufficiently large to cause the back radiating acoustic signal to be delayed in time before exiting ports 145A through 145C by an amount sufficiently large to result in the joining of the remaining secondary component of the back radiating acoustic signal with the secondary components of the front radiating acoustic sig¬ nal substantially in phase with each other at locations external to loudspeaker 100. (The front radiating acous¬ tic signal is designated in Fig. 2 by solid lines with arrows indicating the approximate direction of propaga¬ tion while dashed lines with arrows are used to designate the approximate propagation path of the back radiating acoustic signals.) To facilitate connection of loudspeaker 100 to external electrical circuitry, air-tight electrically conductive terminals 162 and 164 are conveniently situated in the bottom of housing 140 extending from within chamber 144 through housing 140 to the outside of housing 140. Terminals 162 and 164 are respectively connected to leads 122 and 124. To -understand the functioning of loudspeaker 100, it is noted that the front radiating acoustic signal is 180° out of phase with the back radiating acoustic signal at diaphragm 110. Both the front and back radiating acous¬ tic signals have primary and secondary components, the secondary components being of smaller amplitude than the primary components due to the nature of the piezoelectric driving element employed in this embodiment of the invention. As mentioned, acoustic filter 150 filters out the primary component portion of the back radiating acoustic signal while passing substantially unatteπuated the secondary component portion thereof through chamber 144 to ports 145. The "path length" of the back radiat¬ ing acoustic signal path along which the back radiating acoustic signal travels is selected such that the remain¬ ing secondary portion of the back radiating acoustic signal emitted through port 145 is substantially in phase with the secondary component portion of the front radiating acoustic signal at the locations external to loudspeaker 100 where the respective wave fronts of the secondary components of the two acoustic signals meet. The combined secondary component acoustic signal emitted from loudspeaker 100 is thus a reinforced signal which exhibits an amplitude commensurate with that of the pri¬ mary components of the front radiating acoustic signal. Thtis combination of the front radiating acoustic signal and the secondary component portion of the back radiating acoustic signal is seen graphically by a com¬ parison of the frequency response curves of Figs. 23 and 2C. Fig. 2B shows a typical frequency response curve for a prior art loudspeaker. By way of example, such loud¬ speaker generates primary components (indicated by a solid line) from 2500-20,000 Hertz, but exhibits a degraded, although significant, response from 1500-2500 Hertz. Consistent with the definitions already made above, the signals produced by such loudspeaker at the latter frequencies are designated secondary components (indicated in Fig. 2B as a dashed line) . At frequencies less than the frequency of the secondary components (i.e., less than 1500 Hertz) the response of the prior art loudspeaker is negligible, as shown. However, if a speaker which has primary and secondary components the same as those discussed above is constructed in accord¬ ance with the present invention, a substantially uniform frequency response curve results and is depicted in Fig. 2C as a solid line. It is seen by comparing Fig. 2C and Fig. 2B that the response of the loudspeaker of the invention to secondary components is enhanced. The combined secondary components of the front and back radiating acoustic signals are shown between 1500 and 2500 Hertz in Fig. 2C. It should be noted that the invention is not limited to the frequencies of the pri¬ mary and secondary components shown in Fig. 2C and dis- cussed above, but rather the frequencies included in the primary and secondary components of a loudspeaker con¬ structed in accordance with the invention will vary with the particular diaphragm area and the type of transducer selected for such loudspeaker. In some embodiments the primary components may include high frequency components while the secondary components comprise midrange compon¬ ents. In other .embodiments, the primary components may comprise both high frequency components and some midrange components while the secondary components comprise some midrange components. In other embodiments of the inven¬ tion in which the diaphragm area of the loudspeaker is relatively small, the primary and secondary components may be entirely within the high frequency audio range. Fig. 4 shows a simplified embodiment of the loud- speaker of the invention as loudspeaker 200. Loudspeaker 200 is substantially similar to loudspeaker 100 of Fig. 2A with like numbers designating like elements, except for the following modifications. Housing 140 includes a plurality of pedestals 146 situated at a suf- ficient number of locations around rim 142 to support a circular diaphragm support member or ring 210. The peri¬ phery 114 of diaphragm 110 is cemented to or otherwise adhesively attached to ring 210 to provide support to diaphragm 110. Pedestals 146 exhibit a sufficient height (that is, distance measured vertically in Fig. 4) to form OMPI a port 220 between diaphragm periphery 114 and rim 142. Port 220 may be visualized as a plurality of ports or subports 220A, 220B and so forth according to the number of pedestals 146 selected for the particular embodiment of loudspeaker 200. Loudspeaker 200 includes an acoustic filter 230 meeting the same criteria as filter 150 described above in the discussion of the loudspeaker of Fig. 2A. Filter 230 is comprised of an acoustic dampen¬ ing material, such as fiberglass, which occupies a suffi- cient portion of chamber 144 to substantially attenuate the primary components of the back radiating acoustic signal projected into chamber 144 by vibration of diaphragm 110 before such back radiating acoustic signal reaches ports 220. Consistent with the structures described above for enhancing the secondary component frequency response of a loudspeaker, a method for accomplishing this purpose is briefly described hereinbelow. Referring again to Fig. 4, a diaphragm 110 is operatively driven with a varying electrical signal applied to a transducer 120 coupled to diaphragm 110. Diaphragm 110 is thus caused to vibrate in accordance with the varying electrical signal such that diaphragm 110 generates a front radiat¬ ing acoustic signal emitted from the front diaphragm surface 110A and further generates a back radiating acoustic signal emitting from the back diaphragm surface HOB. Tne front and back radiating acoustic signals each include primary and secondary audio components. The back radiating acoustic signal is acoustically filtered to suostantially attenuate the primary compon¬ ents thereof while passing without substantial atten¬ uation the remaining secondary components thereof. The remaining secondary components of the back radiating acoustic signal are pnase-delayed by an amount of time sufficiently large for these remaining secondary P components of the back radiating acoustic signal and the secondary components of the front radiating acoustic signal to be joined together substantially in phase with each other and thus reinforce each other. The step of phase-delaying the remaining midrange components of the back radiating acoustic signal may be accomplished by passing the remaining secondary components of the back radiating acoustic signal through an acoustic signal path of appropriate length, as already discussed. The remain- ing secondary components of the back radiating acoustic signal are directed to join together with the front radiating acoustic signal thus resulting in secondary component enhancement. The foregoing describes a loudspeaker apparatus and a method capable of providing a relatively uniform frequency response throughout the midrange and high frequency audio spectrum. The loudspeaker structure and method of the invention provides enhanced secondary component frequency response in spite of the relatively small diaphragm excursions at secondary component fre¬ quencies characteristic of the driving elements or trans¬ ducers which may be employed consistent with practice of the invention. While only certain preferred features of the invention nave been shown by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the present claims are intended to cover all such modifica¬ tions and changes as fall within the true spirit of the invention. OMPI

Claims: Claims 1. An improved loudspeaker with enhanced response, comprising: a diaphragm having front and back opposed surfaces and a peripnery; driving means coupled to said diaphragm for vibrating the same in accordance with an applied elec¬ trical signal and producing front and back radiating acoustic signals in response thereto, each of said acous¬ tic signals having primary and secondary components; housing means for enclosing said diaphragm and forming a chamber therewith including at least one port for channeling said back radiating acoustic signal outwardly therefrom in approximately the same direction as said front radiating acoustic signal; filter means. ithin said housing for attenuating the primary components of said back radiating acoustic signal and having substantially no effect on the second¬ ary components of said back radiating acoustic signal, and said housing being dimensioned to provide a signal path for said back radiating acoustic signal such that the secondary components thereof propagate through said at least one port and combine substantially in phase with the secondary components of said front radiating acoustic signal resulting in enhanced loudspeaker response. BAD ORIGINAL 2. The loudspeaker of claim 1 wherein said driving means comprises a piezoelectric transducer. 3. The loudspeaker of claim 1 wherein said diaphragm is situated spaced apart from an opening in said housing by a distance sufficiently large to form said at least one port between said housing and the periphery of said diaphragm. 4. The loudspeaker of claim 3 wherein said housing comprises a cup-like shaped member having a rim situated spaced apart from said diaphragm periphery by a distance sufficiently large to form said at least one port between said housing and the periphery of said diaphragm. 5. The loudspeaker of claims 1, 2 or 4 wherein said filtering means is comprised of fiberglass. 6. A method for enhancing the frequency response of a loudspeaker which generates primary audio components of substantial amplitude over a predetermined frequency range and generates secondary audio components of smaller amplitude and lesser frequency than said primary compon¬ ents, said method comprising: operatively driving a diaphragm with a varying electrical signal via a transducer coupled to said diaphragm such that said diaphragm vibrates in accordance with said varying electrical signal, thus generating front and back radiating acoustic signals each having primary and secondary audio components; filtering said back radiating acoustic signal to substantially attenuate the primary components thereof while passing substantially unattenuated the remaining secondary components thereof; phase delaying , he remaining secondary compon- ents of said back radiating acoustic signal and directing the same through at le ^ ast one port such that the ramain- ing secondary components of said back radiating acoustic signal and the secondary components of said front radiat¬ ing acoustic signal join together substantially in phase with each other. D 0WG lNAL 7. The method of claim 6 wherein the step of phase delaying the remaining secondary components of said back radiating acoustic signal is accomplished by passing said remaining secondary components through an acoustic signal path having a sufficient length to cause the secondary components of said front and back radiating acoustic signals to join together in phase with each other. OMPI

Inventors: BAITCHER N
Applicants: MOTOROLA INC
Application year: 1981
Main classification: H01L41
IPC codes: H04R1, H04R17