Other technologies / without moving coils
Piezoelectric transducers are frequently used as beepers in watches etc., and are sometimes used as tweeters in less-expensive speaker systems, such as computer speakers and portable radios. Piezos have several advantages over conventional loudspeakers when applied to such purposes:
There are also disadvantages:
Plasma arc loudspeakers
The most exotic speaker design is undoubtedly the plasma arc loudspeaker, using electrical plasma as a driver , once commercially sold as the Ionovac . Since plasma has minimal mass, but is charged and therefore can be manipulated by an electric field, the result is a very linear output at frequencies far higher than the audible range. As might be guessed, problems of maintenance and reliability for this design tend to make it very unsuitable for the mass market; the plasma is generated from a tank of helium which must be periodically refilled, for instance. A lower-priced variation on this theme is the use of a flame for the driver , flames being commonly electrically charged. Unfortunately, the recent marketing of plasma displays as high-end television sets and computer monitors has caused the "me-too" labeling of many speakers as "plasma" which have nothing whatsoever to do with plasma , much as the advent of digital audio caused the marketing of a large number of "digital" headphones and speakers whose drive-units were analog in nature.
Actual digital speaker driver technology not only exists, but is quite mature, having been experimented with extensively by Bell Labs as far back as the 1920s. The design of these is disarmingly simple; the least significant bit drives a tiny speaker driver, of whatever physical design seems appropriate; a value of "1" causes this driver to be driven full amplitude, a value of "0" causes it to be completely shut off. (This allows for high efficiency in the amplifier, which at any time is either passing zero current, or required to drop the voltage by zero volts, therefore theoretically dissipating zero watts at all times). The next least significant bit drives a speaker of twice the area (most often, but not necessarily, a ring around the previous driver), again to either full amplitude, or off. The next least significant bit drives a speaker of twice this area, and so on.
There are two problems with this design which led to its being abandoned as hopelessly impractical, however; firstly, a quick calculation shows that for a reasonable number of bits required for reasonable sound reproduction quality, the size of the system becomes very large. For example, a 16 bit system to be compatible with the 16 bit audio CD standard, starting with a reasonable 2 square inch (13 cm²) driver for the least significant bit, would require a total area for the drivers of over 900 square feet (85 m²). Secondly, since this system is converting digital signal to analog, the effect of aliasing is unavoidable, so that the audio output is "reflected" at equal amplitude in the frequency domain, on the other side of the sampling frequency. Even accounting for the vastly lower efficiency of speaker drivers at such high frequencies, the result was to generate an unacceptably high level of ultrasonics accompanying the desired output. In electronic digital to analog conversion, this is addressed by the use of low-pass filters to eliminate the spurious upper frequencies produced; however, this approach cannot be used to solve the problem with this digital loudspeaker, since it is the last link in the audio chain.
Electrostatic loudspeakers (ESL)
Some speakers are electrostatically driven rather than via the usual electromechanical voice coil, thereby giving a more linear response. Today, modern materials and insulative coatings have allowed engineers to design electrostatic speakers that are safe and reliable, but this wasn't always the case. For many years electrotstatic loudspeakers had a reputation as a dangerous and unreliable product; the disadvantage was that the signal must be converted to a very high voltage and low current, which was problematic for reliability and maintenance as they attracted dust, and developed a tendency to arc, particularly where the dust provided a partial path; the point where the arc occurs often became more prone to arcing, as carbon built up from the burned dust.
Electrostatic loudspeakers are large by nature. The sole objective of loudspeakers is to move the air analogue to the electric signal applied to them. The amount of air that can be moved is determened by the size of the membrane and the allowed excursion. With electrostatic loudspeakers the excursion is limited to millimeters as where dynamic loudspeakers can move centimeters. This means that the membrane of an electrostatic loudspeaker has to be larger than an equivalent dynamic loudspeaker.
Converting ultrasound to audible sound
A transducer can be made to project a narrow beam of ultrasound that is powerful enough, (100 to 110 dBSPL) to change the speed of sound in the air that it passes through. The ultrasound is modulated-- it consists of an audible signal mixed with an ultrasonic frequency. The air within the beam behaves in a nonlinear way and demodulates the ultrasound, resulting in sound that is audible only along the path of the beam, or that appears to radiate from any surface that the beam strikes. The practical effect of this technology is that a beam of sound can be projected over a long distance to be heard only in a small, well-defined area. A listener outside the beam hears nothing. This effect cannot be achieved with conventional loudspeakers, because sound at audible frequencies cannot be focused into such a narrow beam.
There are some criticisms of this approach. Anyone or anything that disrupts the path of the beam will disturb the dispersion of the signal, and there are limitations, both to the frequency response and to the dispersion pattern of such devices.
This technology was originally developed by the US (and Russian) Navy for underwater sonar in the mid-1960s, and was briefly investigated by Japanese researchers in the early 1980s, but these efforts were abandoned due to extremely poor sound quality (high distortion) and substantial system cost. These problems went unsolved until a paper published by Dr. F. Joseph Pompei of the Massachusetts Institute of Technology in 1998 (105th AES Conv, Preprint 4853, 1998) fully described a working device that reduced audible distortion essentially to that of a traditional loudspeaker.
There are currently two devices available on the market that use ultrasound to create an audible "beam" of sound: the Audio Spotlight and Hypersonic Sound.
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