Planar loudspeaker

Details Planar loudspeaker Planar loudspeaker

2002-03-07

2004-02-03

Le, Huyen (Department: 2643)

Electrical audio signal processing systems and devices

Electro-acoustic audio transducer

Electromagnetic

C381S386000, C381S425000, C381S431000

Reexamination Certificate

active

06687381

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to loudspeakers and, more particularly, to flat loudspeakers having a planar rectangular configuration.
Most speakers are configured with a cone shaped diaphragm attached to an electromagnetic driver assembly. However, conventional expedients have required massive speaker enclosures in order to increase the efficiency of the speaker, or to increase the quality and bandwidth of sound emitted by such speakers. Many alternative speaker designs have been proposed to reduce the size, and particularly the thickness, of speakers. Although the use of such expedients may permit a reduction in the thickness of a given speaker, they generally do not produce the same quality or output level of sound as do conventional cone speakers.
Recently there has become a need to produce inexpensive, thin compact speakers that are extremely resistant to harsh environmental conditions, and capable of producing a high output sound level over a wide bandwidth throughout the life span of the speaker. Such applications include the automotive industry, computer industry, and the like. The previous expedients in speaker development have generally been unable to meet this need.
Thus, there has been recognized a need to make more compact planar or flat type speakers for use in restricted areas, which also have the ability to produce sounds at a high output sound level over a wide bandwidth throughout the life span of the speaker.
2. Description of the Prior Art
The most common speaker driver assembly for conventional speaker utilizes a voice coil and permanent magnet attached to a cone diaphragm wherein the passage of a fluctuating electrical current through the voice coil causes the diaphragm to vibrate. As the diaphragm vibrates, airwaves are produced which are perceived as sound. Conventional voice coil drive units with conventional cone speakers are highly inefficient, converting less than about five percent of the applied electrical energy into sound energy. Attempts to improve the efficiency of these units have undesirably required massive speaker enclosures, and the like.
Large speakers have many disadvantages. For instance, the large mechanical inertia inherent in these speakers reduces the frequency range at which they can vibrate, which in turn reduces the bandwidth of sounds they can produce. Another disadvantage is that these speakers cannot be used in applications requiring installation in highly restricted and compact areas. Such applications, for example, in automobile door panels, and the like, typically require relatively flat and compact speaker configurations.
An alternative speaker driver to the voice coil assembly is the piezoelectric transducer. The piezoelectric transducer utilizes crystalline materials that mechanically vibrate when subjected to a supplied voltage. Although the piezoelectric type speaker has the ability to be used in more compact speaker configurations, the crystalline vibrations produced generally are unable to produce a practical level of sound output and wide bandwidth of reproducible sound. Hence, piezoelectric transducer speakers, by themselves, have generally been unable to achieve the high level of sound output and quality of sound reproduction required in many space-restricted applications.
Another alternative speaker drive assembly is the electrostatic driver, which uses a sheet or film as a sound radiator coupled with a flat plate or mesh. Generally, the film and plate act together as a capacitor. An audio signal is mixed with a high DC polarized voltage that is applied across the capacitor. When the high DC polarized voltage is varied in accordance with the audio signal; the electrostatic charge across the capacitor varies. As the charge varies, so too does the force between the plates, which in turn causes the film to vibrate. However, the electrostatic driver requires an expensive DC voltage source and transformer to operate, which, in turn, increases the production cost and size of the speaker. Hence, electrostatic speakers are inherently both costly and bulky and are generally unacceptable not only for general applications, but even more so in space-restricted applications.
One relatively compact flat speaker expedient utilizes a solid panel as a sound resonator driven by a direct connection to either a conventional voice coil or piezoelectric driver. However, it is difficult for the solid panel resonator to produce a wide sound bandwidth unless its vibration characteristics conform to a complex bending behavior. In order to configure the rigid panel to respond accordingly, the panel must be precisely manufactured and assembled to exacting tolerances. This is not only time consuming but costly, and is highly undesirable in speaker design. Thus, the use of the rigid panel flat speaker is unacceptable in space-restricted applications.
Another prior flat speaker design utilizes a single thin sheet or film membrane that is pre-stressed in tension within a frame. The single thin sheet functions as a sound resonator. Although the thin membrane eliminates the expense of the rigid panel diaphragm, it too has its drawbacks. For instance, it is difficult to obtain the proper pre-stress during assembly. In addition, the pre-stress must remain essentially constant throughout the life span of the speaker in order to produce quality audio performance over time. Maintaining this pre-stress is difficult, as aging and thermal effects on the film membrane tend to substantially reduce the amount of pre-stress over time. Another drawback with the thin membrane speaker is that it is highly vulnerable to physical damage such as punctures that can significantly reduce the sound quality of the speaker. Thus, the thin film membrane flat speaker, although useable in space restricted applications, does not satisfactorily produce high quality sound output consistently and repeatably over the life span of the speaker.
Previously proposed expedients include, for example, Yokoyama U.S. Pat. No. 5,009,281. Yokoyama proposes several embodiments of acoustic apparatus where the diaphragm of a vibrator radiates directly and also drives a resonator. The disclosed resonators are in the form of chambers, not flat panels. Yokoyama also includes a catalog like listing of prior art transducers. Polk U.S. Pat. No. 4,903,300 discloses a flat speaker for use within wall cavities, but uses the entire volume of the wall space to get the desired output. Kumada et al. U.S. Pat. No. 4,352,961 discloses a flat speaker where a piezoelectric driver is used in a watch. The driver is mounted to the transparent face of the watch, which is used as the resonator. Another thin profile audio device with a piezoelectric driver is shown in Kumada U.S. Pat. No. 4,471,258. Skaggs U.S. Pat. No. 4,714,133 discloses a speaker structure where a conventional cone speaker is acoustically coupled to a radiator. Kasai et al. U.S. Pat. No. 4,551,849 discloses a thin automotive audio system uses a vehicle panel that is directly driven by a driver. Yanagishima et al. U.S. Pat. No. 4,514,599 likewise discloses an automotive vehicle audio system in which a vehicle panel is driven by a driver of the speaker. Watters et al. U.S. Pat. No. 3,347,335 proposes the use of a honeycomb core sandwiched between two stiff sheets as a flat acoustic radiator. Matsuda et al. U.S. Pat. No. 4,122,314 discloses a loudspeaker with a plane vibrating diaphragm where the diaphragm is in the form of a sandwich structure. Guenther et al. U.S. Pat. No. 6,097,829 discloses a flat-Plane diaphragm fabricated using sandwich construction. Barlow U.S. Pat. No. 3,111,187 likewise discloses a flat panel diaphragm fabricated using sandwich construction. Pearson U.S. Pat. No. 3,861,495 discloses a loudspeaker in which a cone speaker is acoustically coupled through telescoping frusto-conical members to a flat vibrating panel. Murase U.S. Pat. No. 3,674,109 discloses a thermoplastic laminated vibration plate for a loudspeaker, which includes a centrally located cone portion and a flat port

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