We have heard very often about piezo tweeters and how important it is to your birdhouse. You need to give more attention to it's selection and please choose the right manufacturers.
Since many of us have no idea what are they and how they function, I have this write up that might be of special interest to all my blog readers:
This article can be reached at: http://www.pulsardevelopments.com/products/detail/piezoan.html
Introduction
Piezo tweeters, in use for 30 years now, offer a quality cost-effective, high-frequency sound source in a rugged, high-efficiency package when used properly. Used improperly, however, they fail to meet their potential. It is the purpose of this article to help the user attain the maximum benefits from pie-zohs, pizzas, pee-zoids, or whatever else they have been called.
Background
Piezoelectricity was discovered by Jacque and Pierre Curie in the late 1880s. They found that certain natural crystals generate an electric field under the influence of a mechanical force. They named the phenomenon piezoelectricity, from the Greek meaning "pressure" electricity. The correct pronunciation is pi e' zo; however, Pe a' zo (the Latin pronunciation) has become as common. Shortly thereafter, it was discovered that this phenomenon is a reversible one. That is, when an electrical field is impressed across the crystal, it undergoes a physical deformation. Since the actual displacements are very small (measured in millionths of an inch), the practical applications for piezoelectricity were slow in coming. The various natural occurring materials were found to be piezoelectric, among them quartz, tourmaline, Rochelle salt, and even wood. The advent of radio resulted in the need for a frequency-stable circuit component. quartz crystals, vibrating at resonance, were found to operate consistently and are still the state of the art in frequency-stable components. This was the first high-volume major application of piezoelectricity.
Underwater warfare in W.W.II generated a need for detection equipment analogous to radar used for planes. It was known that acoustical signals travel extremely well in water, and the first acoustical application for piezoelectricity emerged. A piezoelectric crystal was acoustically coupled to the water through a metal diaphragm. A short burst of energy (ping) caused the crystal to vibrate, setting up and acoustical wave in the ocean. When the wave encountered a hard object, a reflected signal was returned to the sender. Since the piezoelectric device also worked as a receiver, after the initial transmit "ping," it was switched to a receive mode and listened for the returning signal. The time lapse between the transmit and receive was translated directly into distance. Further, by adding multiple receivers aimed in different directions, a direction (bearing) could be determined. Rochelle salt was first used for this application because of its extremely high sensitivity. Unfortunately, it exhibited several temperature and moisture problems that made its use impractical.
A better material was needed. Independent research on both sides of the ocean resulted in a family of synthetic materials that offer high electro-mechanical conversion efficiency with greatly improved temperature and humidity stability characteristics. This synthetic material is actually a ceramic and is processed using methods similar to conventional ceramic sintering techniques. The material is called PZT because it is a polycrystalline lattice structure of the oxides of Lead (P for Pb), Zirconium (Z), and Titanium (T). Since it can be formed using conventional ceramic processes, it offers more design latitude to the transducer engineer than do crystals.
A major difference between PZT and piezo materials found in nature (crystals) is that PZT must be processed further to make it piezoelectric. The microscopic crystallites, known as domains, are in random orientation in the PZT and must be aligned if the material is to be useful. This is done in a process called "poling." A high potential D.C. field is momentarily imposed across the material causing the domains to align themselves with the field. Upon removal of the field, the domains remain aligned (see Figure 1). The poled PZT is now truly piezoelectric and will stay that way unless an excessively high voltage is imposed upon it, or unless it is heated to a very high temperature (Curie point). If either of these conditions is reached, the energy input to the domains exceeds the internal binding force holding the domains in alignment, and the material once again becomes unpoled. This entire process is very much like the magnetizing of a magnet except that we deal with electric fields instead of magnetic fields. It should be noted that the ability of the PZT to retain its polarity is a function of the quality of the material. There are available low quality materials which will de-pole under normal use causing the speaker to gradually lose efficiency (sensitivity). CTS manufactures only the highest grades of PZT.
Theory of Operation
In operation, the domains within a poled PZT wafer (as shown in Figure 2) alter their position slightly when an external field is applied. This causes a slight deformation in the physical geometry of the wafer. When the field is removed, the wafer returns to its original size. These displacements are very small (measured in millionths of an inch) but high in force, and when coupled directly to a liquid or solid medium, are very useful for generating discrete motions. When coupled to air, however, motions of these dimensions are useful only in the ultrasonic region where the acoustic impedance of the air is higher, and provides a better match to the PZT. To provide useful motion in the audio region, a "mechanical lever", or transformer, is required to convert the high-force, low-displacement motion to low-force, high-displacement.
This is done by coupling two wafers face-to-face (Figure 3). The wafers are connected such that as one expands, the other contracts. When coupled at their faces with a metal member (centervane), the resulting stress causes the sandwich to dish in and out depending on the amplitude and polarity of the applied signal. This "sandwich" is called a bimorph, as it consists of two active piezo elements.
By affixing a cone to the center of the bimorph and anchoring the cone at its periphery, the bimorph vibrates in synchronism with an applied audio signal and pumps the cone for and aft, while pushing against its own mass (Figure 4). This concept, called the "Momentum Drive Principle" was developed and patented by Motorola in 1970. It is the fundamental principle behind a broad family of speakers introduced in the ensuing 20 years through many technical developments and dozens of patents.
Piezo Tweeter Construction
Let's consider, in detail, the construction of the CTS Super Horn piezo tweeter. Although developed and patented in the early 1970s, it is still a workhorse in commercial sound installations. The circular PZT bimorph in this case consists of two wafers, 0.89" in diameter and 0.0055" thick. The ultra-thin wafer is required to achieve the desired acoustical performance. The bimorph is coupled at it's center to the apex of a specially impregnated diaphragm which then works into a compression volume. Slots in the compression the compression space direct the sound into the throat of the horn. The radial slots are transformed into a 3" circular mouth through the unique shape in the throat of the horn. The actual horn contour is a hybrid design between a pure exponential contour and a hyperbolic one. Again, this computer-generated geometry is optimized for the best acoustical output.
No comments:
Post a Comment