Piezoceramic (piezofilm) speakers

Among the significant variety of electroacoustic transducers, a separate sector is occupied by piezofilm (formerly known as piezoceramic) variations of emitters that differ from classical designs.

These products are based on the piezoelectric effect discovered by the Curie brothers (Pierre and Jacques) in 1880. The essence of the effect lies in the formation of electric charges on the crystal faces. Depending on the choice of crystal (quartz, tourmaline or Rochelle salt), electric charges will provoke either displacement, bending, or torsion of the solid. The effect is also observed in the opposite direction. When a piezoelectric crystal is exposed to electricity, the deformation of the crystal is fixed: it lengthens, bends or twists.

These physical phenomena have been successfully used as the basis for the creation of a number of devices. In the 20-40s of the last century, piezofilm transducers were used in pickups, microphones and other ultra-acoustic transducers.

Investigations were carried out for the possibility of using the effect in the emitters of acoustic structures, in their sound frequency range. Investigations carried out in the USA, Japan and the Russian Federation, based on barium titanate and lead zirconate titanate with strontium and niobium, have been created piezoceramic materials used in audio equipment.

Loudspeaker design features

Piezo ceramics have been extremely interesting for the creation of a whole line of tweeters as well as public address loudspeakers. The reason for this popularity is the simplicity of the design, the high level of sensitivity against the background of the absence of magnetic circuits. However, it was necessary to take into account the resonant frequency of the piezoelectric element, which depends on the radius and thickness of the product:

where, E – Young’s modulus, density and Poisson’s ratio, t – product thickness, r – radius.

In order to achieve a significantly lower resonance frequency, it is necessary, on the one hand, to reduce the thickness

Production and application

A number of companies have successfully developed high frequency piezo speaker systems, but Motorola remains the industry leader.

The production of piezoelectric ceramics is carried out on the basis of spontaneous polarization in a crystal at certain temperatures (ferroelectricity). Titanium dioxide and barium oxide are mixed in a powdered form in appropriate proportions. This is followed by the process of chemical synthesis, which takes place in the temperature range 900–1300 ° С.

With the method of precipitation from aqueous solutions, due to the optimal mixing of the components, it is possible to reduce the heat treatment to 750-1000 ° C. As a result, the resulting synthesized material is poured under pressure into preforms of a given configuration. The pressing method is also used. At the end, the blanks are subjected to thermal firing to achieve the required properties of future piezoelectric elements.

The area of ​​application of piezoelectric materials is extremely wide. These are medical devices and instruments, equipment for semiconductor production, communication control and a whole range of measuring instruments. There are four main categories of piezoelectric applications: generators, power drives, transducers and sensors.

When creating piezo emitters intended for operation in the audio frequency range, there are three main directions for further development:

• creation of horn high-frequency loudspeakers based on piezoceramic bimorph elements. Having created a whole line of acoustic systems, Motorola is recognized as the leaders in this direction;
• creation of acoustic systems based on piezo-film tweeters (Pioneer and Audax are leading companies in this direction);
• creation of high-frequency and low-frequency speakers based on soft piezoceramic materials – thick-film PZT.

Modern achievements in the field of studying the possibilities of piezoceramic materials are allowed to assert that this groundwork will become the basis for significant progress in the creation of new designs of emitters.