March 01, 2013
| by Igor Levitsky
Electrostatic tweeters and speakers have some important advantages that allow them to reach extremely high levels of performance and realism. The diaphragm is extremely light and responsive, it uses voltage instead of current and it does not produce heat. They also don’t have distortion related to cone and dome break-up resonance, voice-coil inductance, and non-linearity and magnetic field modulation that are detrimental to sound quality in electro-dynamic drivers.
However, there are also attributes that limit them to mostly high-end stereo. First, they need external high voltage supplies that plug into wall outlets. Second, their diaphragms have to be large to get a reasonable sensitivity. That size works against wide and constant dispersion, and this is a serious problem especially at high frequencies. Electrostatic technology’s sensitivity to dust accumulation, humidity and other factors is also a consideration.
Quad, MartinLogan and Final Sound are among the few electrostatic speaker brands.
Tweeters that employ piezo technologies have only been used on a limited basis. The main advantage is that they are very inexpensive and they do not require a crossover. They are resonant transducers.
The most widely used piezo tweeter has a thin (0.1 mm) piezo ceramic disc of about 1 to 2 inches (25-50 mm) in diameter, bonded to an aluminum or bronze disk of slightly larger diameter. When voltage is applied, the piezo disc expands and contracts (changing its diameter on a very small scale).
A metal substrate rigidly bonded to the disc transforms tensile and compressive deformation into bending deformation, and the bending disc vibrates and radiates sound. A small-diameter (1.5-2 inch) paper or polymer bonded by its apex to the center of the piezo transducer can be added to increase sensitivity. A cone can also be added by loading it with a horn to further increase sensitivity.
Piezo tweeters are not high fidelity tweeters and are mostly used in inexpensive combo and car stereo systems (mostly with just piezo disc) and some commercial speakers. Motorola has been the most frequent supplier of piezo tweeters to the commercial market.
As with electrostatic tweeters, the absence of any magnetic motor and voice coil-induced distortion and very light diaphragm allow for transparency and resolution. A disadvantage of piezo film tweeters could be related to complexity of the manufacturing process, including matching the transformer, the cost, the limited sensitivity of the design and limitations related to dispersion.
This exotic transducer is based on the principle of modulating ionized plasma by using the audio signal.
Plasma can be generated either by gas ionization by high-voltage electric fields (corona discharge) or by burning gas at a very high temperature. Plasma speakers have the shortest path from electric signal to oscillating air - it happens directly. There isn’t even a small diaphragm like in electrostatic or ribbon speakers, the response is immediate and there is no distortion related to mechanical or electromagnetic components.
One could argue that plasma tweeters are the closest design to ideal tweeter. The sound of plasma tweeters is exceptionally transparent and free from any apparent distortion. However, the price of an Acapella plasma tweeter, for example, is about $9,000 each [Editor’s note—price corrected from original story], and they are not environmentally friendly.
Thin Film/Foil Diaphragm
There are distinctive types of electro-dynamic tweeters that are all based on the use of thin diaphragms made of foil or combination of polymer film and foil that carry signal current and immersed in magnetic fields.
Unlike dome tweeters these do not have a voice coil wound on a bobbin, and they do not have inductance that adversely affects sound quality.
These types of tweeters are known as ribbon tweeters, air motion transformer (AMT) and planar ribbon (planar magnetic) tweeters. Although all of them share similarities, especially in sound quality, they have quite different design concepts and applications: