Development of Flat Panel Speaker for Personal Computers
Masahiro Sato Katsuhiko Tsumori Tetsuya Kurosaki

Shigeki Kato

M. Sato et al.: Development of Flat Panel Speaker for Personal Computers

1. Introduction The use of the sound feature on personal computers (PCs) is becoming more common with the growing popularity of the Internet and software games. Consequently, more frequent PC speaker use is anticipated. In the meantime, demand is increasing for speakers offering a good balance in terms of sound quality, design, and cost. Currently, PC speakers are mostly box-shaped speakers housing amplifiers. In spite of the hurdles presented by constraints on size and emphasis on desktop units, numerous superior products have been developed. Moreover, with manufacturers making headway in developing space-saving PCs with even thinner display panels in recent years, demand is also growing for slimmed-down speakers to maintain a sense of balance and harmony in terms of the design and "feel" of the PC package as a whole. Conversely, it used to be difficult to develop slim speakers unsurpassed in terms of thinness, while maintaining sound quality. So far, a number of slimming techniques have been employed with the aim of creating thinner speakers. One of these techniques involves making the cone-shaped diaphragm of the speaker shallower. However, using this technique led to excessive distortion and sound pressure that was too low. Against this backdrop, the NXT speaker technology announced in 1996 involved a new approach to the vibration modes of the diaphragm. To obtain superior acoustical characteristics from a flat diaphragm, the distributed mode had to be used in the most efficient manner possible. NEC and Fujitsu have already used this technological approach to market speakers for spacesaving PCs. Recently, we developed a flat panel speaker for release as a new product by further slimming down its shape and by making improvements in terms of quality. These improvements are described in the sections that follow. 2. NXT Speaker 2.1 Features NXT technology was developed by New Transducers Limited, a UK electronics firm that holds the patents to this technology. The features of conventional speakers and those of NXT speakers are described below. To generate sound waves, conventional speakers use the piston movement of a diaphragm to cause the air to

vibrate, as shown in Fig. 1. With the outer rim of the diaphragm supported by a frame, the diaphragm center, which is supported by springs called dampers, is driven. To ensure that the diaphragm executes the piston movement while maintaining its shape, it is generally made in the shape of a cone so that it maintains stiffness in its direction of vibration. As shown in Fig. 2, however, if the directivity is to be maintained, the high frequency sound pressure falls in directions other than the direction in which the speaker center axis points. Additionally, it is difficult to make cone-shaped diaphragms slimmer.

Driving point Basket Diaphragm Damper Magnet Voice coil

Fig. 1 Conventional speaker (cross section)

Sound wave Divergence between phases Diaphragm Divergence and reduced (Cancel out pressure interfere with, each other and cancel each other out Vibration owing to Sound pressure falls reversed phases)

Fig. 2 Directivity

By eliminating all of the above-mentioned weak points associated with conventional speakers, NXT technology has made it possible to create slim speakers by using a flat panel for the diaphragm. With NXT technology, vibrations are applied to specific points on the flat-panel diaphragm to generate bending vibrations in the diaphragm. At any frequency, numerous vibrations in random phases are distributed over the entire diaphragm. In other words, multiple-point sound sources are created over the entire diaphragm.

FUJITSU TEN TECH. J., NO.14 (2000)

M. Sato et al.: Development of Flat Panel Speaker for Personal Computers

Conventional speaker