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Flat panel speakers for PDAs.

Since 1925, when Chester Rice and Edward Kellogg from General Electric published their paper on the basic principles of coil-driven, mass-controlled diaphragm speakers, their design has been the standard for electric speakers. There have been many attempts at designing other systems for audio reproduction, and in the late 1990's, the distributed mode loudspeaker (DML) was introduced as an alternative flat-panel speaker design, with a distributed mode actuator (DMA) as the key component in the design.

In developing miniature hand-held DML applications, NXT, Cambridge, England, encountered problems with actuation when using their traditional transducer, based on a moving coil exciter. Size, weight and current drain are critical issues in designing such devices. Research indicated that actuation could be miniaturized by switching from an electromagnetic to a piezoelectric drive element. Piezoelectric polycrystalline ceramics are shape-changing materials, and when a voltage is applied across them, their length is altered, allowing them to be used as actuators.

However, the difficulty with using a piezoelectric transducer to drive a loudspeaker diaphragm is that it acts as a displacement device, whereas what is required for flat-panel speakers is a velocity device (traditional coil-driven speakers use an acceleration device). This disparity can be overcome using equalization, resulting in large required voltages at low frequencies. By using the piezoelectric material in an unconventional manner--within its modal, or resonant, region--it becomes a modal velocity transducer, suitable for driving the modal panel of the company's flat speakers.

The DMA comprises one or more small piezo beams, connected at a common stub mounted directly onto the underside of the lens or display screen. The drive is inertial, as with many of the moving coil exciters used in the company's panels, yet many times more efficient. This design offers significant benefits for power consumption, particularly for use with Class D amplifiers. The system efficiency (battery to acoustic power) can be as much as quintuple that of conventional speaker systems within mobile devices. Because the DMA is much shallower than a moving coil exciter, valuable space is conserved. In single beam form, a DMA is typically only 1.5 mm thick, and weighs less than 2g.

In mobile phones, the company's SoundVu technology uses the protective lens (i.e. display screen) as a speaker, eliminating need for the old-style speaker grille. As a result, new designs are possible, but more importantly, better quality sound can be produced--an important design consideration for the cell phone market. The whole lens becomes the loudspeaker, so listening to a mobile phone becomes much easier as the problematic 'hot spot' effect caused by micro speakers no longer exists. Nor does the loudspeaker suffer from a beaming effect, which means the audio in hands-free operation is heard just as well when the handset is turned away from the user.

Improved bandwidth capacity is available at low frequencies where the stiffness of the panel material, or of the narrow air gap behind a SoundVu panel, would result in premature roll-off with a moving coil exciter. Because a DMA's three and velocity can be varied independently across a wide range (by altering the length, width and thickness of the piezo crystal beam, and by adding any necessary beams), the exciter's mechanical impedance can be adjusted in a manner unachievable with conventional moving coil or piezoelectric exciters. Thus, the impedance of the panel and exciter can be dimensioned to offset roll-off in the panel's stiffness controlled region. Tests show that an additional octave of low frequency extension can be achieved in comparison to the equivalent moving coil design. SurfaceSound and SoundVu speaker panels are suited for use in a wide range of hand-held products. However, in applications where size, weight and robustness are critical, this DMA offers a smaller, lighter, more-efficient solution.

A second generation DMA has been introduced at more than 20% smaller size than the previous version, with a high quality sound and tenfold efficiency over conventional speakers. The new transducers offer a smaller footprint (22 x 7 x 3.5 mm as compared to 34 x 5 x 4 mm) to laptops, PDAs, cellular phones and other small handheld devices. In addition, the second generation DMAs have been made more robust for packaging and handling.

Circle 155--NXT plc, or connect directly at www.rsleads.com/410df-155
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Title Annotation:Five Star Product Of The Month
Publication:Designfax
Date:Oct 1, 2004
Words:707
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