This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The trend in portable devices is towards providing larger displays for viewing content without straining the viewer's eyes. It is anticipated that there will be a demand for ultra-thin loudspeakers with a large footprint to deliver high quality sound. These could be electrostatic or electret types that slide out of a lid from behind the display or that fold. In use cases such as teleconferencing, or when several people listen to the same music, it is desirable to have a wide dispersion angle. A dipole or figure-of-eight directivity pattern (also known as polar response) is ideal because it provides a fairly wide angle of dispersion but also has a dead axis which provides a degree of privacy in public spaces.
As the size of a loudspeaker is increased certain factors such as the maximum loudness and bass extension are generally increased, but the increased radiating area results in narrow beam-forming at higher frequencies. This is the primary reason why high fidelity loudspeakers use multiple drive units, with each drive unit devoted to a separate frequency range. However, the use of multiple drive units complicates the design of the loudspeaker, and the multiple drive units are difficult to integrate into a small form factor.
Electrostatic loudspeakers have conventionally used a single diaphragm with electrodes partitioned into multiple concentric rings, where an attenuated and delayed signal is progressively applied to the electrodes starting from the center and working towards the outer electrodes. Reference in this regard can be made, for example, to P. J. Walker, “New Developments in Electrostatic Loudspeakers,” J Audio Eng. Soc. 28(11), 795-799 (1980). However, the attenuation reduces efficiency and the resulting directivity pattern is still too narrow at high frequencies.
It is known that with an infinitely large diaphragm it is possible to reproduce the sound field of an omni-directional point source behind it, so long as every part of the diaphragm moves with the same velocity magnitude and phase as the original sound in the plane of the diaphragm. Because the loudspeaker diaphragm is finite, the conventional wisdom is that the motion towards the outer edges of the diaphragm should be attenuated in order to avoid edge diffraction effects that would otherwise produce ripples in the on-axis response and directivity pattern.