The advent of new, high-resolution audio media, such as Super Audio CD (SACD; see “Tech Page: CD — The Next Generation” in the March 1998 EM) and DVD-Audio (see “Tech Page: CD? No, DVD!” in the July 1998 issue and “World of Options” in the August 2001 issue), requires enhanced playback equipment to fully exploit their potential. For example, a DVD-Audio disc can store digital audio at sampling rates as high as 192 kHz, which translates to a practical frequency response beyond 90 kHz. Humans can't distinguish anything higher than 20 kHz, but many audio professionals believe overtones above that frequency contribute to an overall sense of spaciousness and fine detail, perhaps by producing low-level combination tones within the audible range.
If that's true, the next problem is reproducing frequencies above 20 kHz. The Multimedia Development Center at Matsushita (Panasonic's parent company) is working on a piezoelectric “supertweeter” with a flat frequency response from 10 to 100 kHz. With appropriate crossover circuitry, such a driver could be combined with standard tweeters, midrange drivers, and woofers to greatly extend the frequency response of full-range speaker systems, revealing all the detail that DVD-Audio and SACD offer.
A supertweeter can be built using more conventional diaphragms, such as domes or ribbons, but it would be prohibitively expensive and not easily mass-produced. On the other hand, piezoelectric transducers have been used in various ultrasonic applications for many years, and they are relatively easy and inexpensive to make.
Pierre Curie discovered the piezoelectric effect in 1883. He noted that certain materials, such as quartz crystals, produce a voltage when they are mechanically stressed. Conversely, those materials' shapes are deformed when a voltage is applied to them. As a result, they can be used as transducers, converting mechanical vibration into an electrical signal (for example, in phonograph cartridges and mics) or vice versa (as with speaker drivers).
Quartz is the most common mineral on Earth, but it is not easy to fabricate into useful forms for piezoelectric applications. Fortunately, polycrystalline ceramics, such as barium titanate and lead zirconate titanate, also exhibit the piezoelectric effect after being heated and subjected to a strong DC electric field, which aligns the molecular dipoles within the material.
The Matsushita team started by affixing disks of a piezoelectric ceramic called PCM5 to either side of a larger nickel-iron disk held in a frame (see Fig. 1). When an audio voltage was applied to the electrodes (one connected to the ceramic disks and the other attached to the metal disk), the frequency response was markedly uneven. Piezoelectric materials tend to have sharp, narrow resonances at high frequencies, which are unacceptable in speaker drivers.
The team found that attaching rubber dampers to the ceramic disks effectively controlled the resonances. Mathematical models and subsequent experiments determined that conical rubber dampers worked the best because the amplitude of the resonances is greatest at the center and decreases toward the outer perimeter. Another factor is the cavity formed by the tweeter frame and the damper, which can affect the frequency response of the driver. Conical dampers yielded the best response, possibly because they form a short hornlike structure with the frame.
The end result is a supertweeter with a flat frequency response from 10 to 100 kHz. Now all that's needed are amplifiers, preamps, and source devices that can support the extended frequency range made possible by DVD-Audio and SACD. I'll keep you posted on developments in those areas as the brave new world of audio reproduction develops.