The present invention relates to audio frequency speakers and, more particularly, to an improved audio frequency speaker for extending the frequency range of a voice coil speaker (woofer or mid range) by using one or more piezoelectric actuators mounted to one or more components of the speaker.
Sound system designers have historically used electromagnetic voice coil transducers or speakers to generate air displacement.
When operating at low frequencies, the cone 12 of such prior art transducers tends to maintain its shape and move in and out much like a rigid piston. As the driving frequency increases, the cone 12 eventually reaches a frequency at which it begins to breakup or resonate in various structural modes. Such speakers which are driven within the breakup region have poor quality sound because the resulting resonances create audible peaks and dips in the frequency response. One method of dealing with such breakup problems is to mechanically damp out the resonant modes to minimize their influence. Another method is to stop driving the transducer 10 within its breakup region and instead cross over to another, smaller transducer (not shown) which is operating below its own breakup frequency. This approach creates multi-transducer speaker assemblies that may include woofers (for low frequencies), mid range transducers (for mid frequencies) and tweeters (for high frequencies). In a premium sound system, a woofer and a tweeter are two physically separate components. The woofer covers the low frequency range (generally up to 5 KHz) and the tweeter covers the high frequency range (typically 5KHz and above). When space is a concern, the tweeter cone may be located on the center of the woofer cone, typically where the dust cap 18 is located. Although this method permits multi transducer assemblies to fit within a smaller housing, the speaker system is made up of two independent devices. Such speaker assemblies typically require additional electronic crossover components to direct the drive signal at various frequencies to the specific transducers that handle them. Such speaker assemblies also are more prone to peaks or dips in the frequency response in the crossover region where two transducers may be operating at the same output level.
Piezoelectric materials are materials which are capable of converting electrical energy into mechanical energy and vice versa. Such materials can be used as sensors (mechanical input-electrical output), as actuators (electrical input-mechanical output) or as vibration control devices (active when power is supplied to the system and passive when the energy generated is dissipated by an electrical component). If a piezoelectric actuator is affixed to a structure and the piezoelectric actuator is driven by an audio frequency drive signal, the structure moves according to the input signal and the dynamic response of the structure itself. The sound quality of a piezoelectric actuator speaker is typically limited by the shape and size of the structure to which the piezoelectric material is attached. For this reason, piezoelectric materials have been used in the past as buzzers or as single frequency alert devices. In such devices the piezoelectric actuator excites the structure at its resonance frequency generating a tone or limited frequency band.
There have been some attempts at using piezoelectric electric material as a full range speaker but the size and rigidity of the structure have presented significant hurdles. In order to achieve a low frequency range the structure itself has to be large and very soft so that its first response is below the subwoofer range (20 Hz or so).
Unlike conventional loud speakers, piezoelectric speakers operate entirely within the breakup mode region. The fundamental frequency of the driven structure defines the lowest frequency at which the appreciable displacement is generated by the piezoelectric speaker. The objective in designing piezoelectric speakers is to make sure that there are enough structural vibration modes which are closely spaced to result in a smooth frequency response across the entire operating frequency (i.e. no large peaks or dips in the frequency response where resonances either are not present or are not over abundant). The complex surface displacement of the driven structure resulting from the superposition of several vibration modes at a given frequency causes the pressure wave to radiate away from the structure in very complex, multi-beam patterns. While some listeners notice no appreciable difference in this type of radiation, other say that this complex radiation sounds more ambient or spacious than traditional speakers.
The present invention comprises an improved audio frequency speaker extending the range of a typical voice coil speaker by using one or more piezoelectric actuators. The piezoelectric actuators are located on the speaker cone 12 or a panel-like component of the speaker, such as the dust cover 18. The piezoelectric actuators are driven by the same electrical drive signal that is delivered to the voice coil 14. The piezoelectric material converts the electrical input of the drive signal to strain which actuates the cone 12 of the speaker. The piezoelectric actuators excite the cone 12 at the high frequency range since they are more efficient at such high frequencies and generate sound by moving the cone 12 at the higher order frequencies. There is no need for special electrical circuitry or for a crossover network in order to achieve such a driving condition because the present invention uses the main feature of each driving actuator (low frequency for the voice coil and high frequency for the piezoelectric actuator). Additionally, the piezoelectric actuators can be integrated into the cone material either as fibers or as piezoelectric plates. This technique not only allows for an integrated, low cost design but it also provides great flexibility in the frequency ranges to be covered. Additionally, speakers using the disclosed technique may be consistency manufactured so as to provide a repeatable frequency response.
An improved audio frequency speaker comprises a speaker cone suspended for movement to generate air displacement, a voice coil having at least one winding attached to the cone and a magnet having a magnetic field. The magnet is located such that at least a portion of the coil is within the magnetic field to thereby cause the coil and the cone to move when a current from an audio frequency drive signal flows through the coil winding. The improvement comprises at least one piezoelectric actuator secured to the cone and adapted to receive the audio frequency drive signal. The at least one piezoelectric actuator moves the cone at least at higher order frequencies of the audio frequency range to thereby enhance the performance range of the speaker at the higher order frequencies so that a single speaker covers the entire audio frequency range.
The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
In the drawings:
Referring now to the drawings, wherein the same reference numerals are used to designate the same components throughout the several figures, there is shown in
The speaker 10 further comprises a magnet 16 also of a well know type which establishes a magnetic field. At least a portion of the coil 14 is located within the magnetic field established by the magnet 16. In this manner, when current from an audio frequency drive signal flows through the windings of the coil 14 the magnetic field causes the coil 14 and the cone 12 to move and to thereby generate acoustic waves.
The speaker 10 may also include additional typical components such as a dust cap 18 which covers at least the coil 14 of the speaker 10. Front and back plates 22 and 24 may also be provided on opposite sides of the magnet 16. If desired, the speaker 10 may be placed in an acoustic baffle or housing 20 (only partially shown) in order to improve performance and the appearance of the speaker 10. Other additional components known to those of ordinary skill in the art may also be included with the speaker 10.
As discussed above, the present invention is an improvement on the prior art speaker 10 as shown in
As described above, and as shown in
An audio transformer 32 (shown in phantom on
If desired, and typically in the case of high end speakers, the piezoelectric actuators 30 can be driven out of phase by switching the positive and negative leads of the piezoelectric actuators 30. The combination of driving the piezoelectric actuators 30 out of phase and selecting the location of the individual piezoelectric actuators 30 on the speaker cone 12 may result in a smoother, less peaky, less resonant, response. The overall sound pressure level may be slightly reduced in the frequency range where the piezoelectric actuators 30 are driven out of phase.
In an alternate embodiment, the piezoelectric actuators 30 may be applied to some other structure of the speaker 10. As shown in phantom on
The cost of the piezoelectric actuators 30 is minimal with respect to the rest of the speaker system. Typically, the cost of adding one or more piezoelectric actuators 30 is less than the cost of adding a separate tweeter cone or tweeter to the speaker 10. Additionally, the weight added by the piezoelectric actuators 30 is well below the weight of an additional speaker, such as a tweeter. The integration of the piezoelectric actuators 30 directly into the manufacture of the speaker cone 12 provides substantial advantages and flexibility in speaker design.
The addition of one or more piezoelectric actuators 30 to a speaker 10 permits the use of a single speaker to cover substantially the entire audio frequency range. In this manner, the use of additional speakers, such as tweeters, to cover the higher frequencies of the audio frequency range can be avoided. If desired, the piezoelectric actuators 30 may be positioned so they are concealed from a viewer to thereby avoid detracting from the appearance of the speaker 10. The piezoelectric actuators 30 do not require complicated filtering or cross over functions which would be required if a separate tweeter speaker was employed. The capacitive nature of the piezoelectric actuators 30 is very conducive to making audio drive signals stable at the higher frequencies of the audio frequency range. Further, a speaker 10 in accordance with the present invention including one or more piezoelectric actuators 30 provides greater ambient and spacious sound due to the complex radiation patterns created by actuating the higher order modes of the audio frequency range as described above.
From the foregoing it can be seen that the present invention comprises the addition of one or more piezoelectric actuators 30 to a woofer or mid range speaker in order to substantially improve speaker performance, particularly at the higher frequencies of the audio frequency range. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.
This application claims priority from U.S. Provisional Patent Application No. 60/547,209 filed Feb. 24, 2004 and entitled “Loud Speaker Including A Piezoelectric Actuator,” the entire subject matter of which is hereby incorporated herein by reference.
Number | Date | Country | |
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60547209 | Feb 2004 | US |