Piezoelectric Headphones (May 1975)

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by Martin Clifford

A TRANSDUCER is a device for changing one form of energy to another; a headphone is a transducer, and so is a loudspeaker, since both change electrical energy to sound energy. A motor is also a transducer and so is an electric light bulb. Crystalline substances, such as quartz, are transducers and quartz crystals have been used for decades in radio transmitting stations to produce the fundamental frequency of the carrier wave.

Because of the way the crystal behaves, quartz is piezoelectric. Piezo (pressure) is from the Greek piezein, meaning "to squeeze." When quartz is subjected to pressure by compression, stretching, or twisting, an electric charge appears between surfaces, generally the two largest ones. The reverse effect, a change in the crystal's shape, can be obtained by putting the crystal between a pair of flat metal plates and applying a voltage to them. This crystal is comparable in size and shape to a postage stamp. Thus, when voltage is applied to a quartz crystal, a mechanical force is produced via the crystal deformation, and a voltage is produced when the crystal resumes its normal shape. This voltage is a.c. of constant frequency and far above the audible range.

In addition to quartz, various other crystalline substances are used as piezoelectric transducers, including Rochelle salts, tourmaline (a semiprecious mineral), and barium titan ate. The Rochelle salts crystal has been used as a piezoelectric transducer in speakers, microphones, and phono cartridges. Barium titanate is a crystalline ceramic with both ferroelectric and piezoelectric properties and is used in the manufacture of compact capacitors and transducers, including the ceramic photocell.

These substances, quartz, Rochelle salts, and the others, are inorganic, that is, they were never part of any living tissue. However, some organic substances, such as wood, bone, and silk, have been found to have piezoelectric properties. These are interesting but thus far impractical, for audio applications.

Recently other electroacoustic transducers have been made for applications in headphones, speakers, microphones, and also in phono cartridges. Unlike the slab-like quartz crystals, these use extremely thin piezoelectric polymer films. The polymer can be either a natural or synthetic compound and has high molecular weight, which is produced through the repeated linkage of millions of monomers. A monomer is any molecule which can be chemically bound as a unit of a polymer.

One of the earliest of the synthetic piezoelectric high polymers was gamma methyl L-glutamate. Its piezoelectric properties, however, were too small to make this substance of practical value and thus, it was said to have a small strain constant. Strain is the deformation resulting from a physical stress and can be compared to the excursion of a voice coil resulting from the current flowing through it. If the voice coil moves only slightly or not at all, the sound reproduced by the attached cone is either negligible or nonexistent.

A New Material

Quite recently however, a new piezoelectric substance, vinyldene flouride, was developed by Pioneer in Japan. This has properties allowing it to produce frequencies in the audible range with relatively low distortion. These properties include a strain constant about 10 times higher than quartz, which remains stable at temperatures up to 212° F (100° C), and its elastic stiffness is about five percent that of quartz. This lightweight substance can also be formed into an extremely thin film, which, together with its relatively low mechanical stiffness, makes it suitable as an audio transducer. The use we will consider here is in headphones, the first product in which this polymer has been used.

The production of piezoelectric vinyldene flouride as a high polymer essentially follows the techniques used in making piezoelectric ceramics. The material is stretched along a single axis to about four times its original length.

Then aluminum is deposited on both sides by evaporation to form the electrodes. Finally a high d.c. voltage is applied to the electrodes to polarize the dielectric material.

The effect of the polarization is analogous to charging a capacitor, with the potential voltage difference remaining across the capacitor after the charging voltage is removed.

In the case of this aluminum-coated polymer, we get residual piezoelectric effect after the polarization voltage is removed. This acquired property is stable and is not affected by moisture or dust.

The magnitude of the displacement of a piezoelectric body depend on its applied charge. A greater bending effect can be had by joining a pair of piezoelectric units so that the application of voltage to one of them will expand while the other contracts. This arrangement is called a bi morph element, or more casually, a bimorph. The technique is similar to that used in a bimetallic thermostat. The effect of this arrangement is to reduce the mechanical impedance.

A Practical Assembly

Fig. 1-When voltage is applied across the plates (Z axis), the piezoelectric element moves along the direction of the X axis.

Fig. 2-When an a.c. voltage is applied to the aluminized surfaces, the piezoelectric material expands and contracts. Since the edges are clamped, the element is forced to move as shown.

Figure 1 shows a high polymer piezoelectric assembly. When an a.c. voltage is applied along the Z axis (3), the unit compresses and expands at right angles to the applied potential. In the example shown in Fig. 1, the physical motion would be in the horizontal plane (or X axis) as shown by the arrows. The compression and rarefaction of the surrounding air molecules is produced by the ends of the assembly. But since these ends have a relatively small surface area, the sound resulting from this limited motion would be equally limited.

If the voltage applied along the Z axis is a.c., the high polymer element will expand and contract along the horizontal plane in step with the alternating pulses of the a.c. voltage. Conversely, when an external mechanical forte is applied from the amplitude direction (the X axis, also called the polymerization direction), voltage is generated toward the Z axis (voltage application direction). However, if the piezoelectric assembly is curved, as shown in Fig. 2, and if the edges are clamped in position, both the upper and lower surface areas become involved.

With the edges fastened, the assembly is forced to move in the direction shown by the arrow in Fig. 2. Because of the greater area in motion, there is now more movement of the nearby air molecules, and thus we now hear sound. (If you move a fan so its motion is parallel to its surface, there is little air motion. But move the fan perpendicularly to its surface and the volume of air moved is considerably greater.) It is in its curved and edge-clamped form that the high polymer piezoelectric is used as a direct radiator in headphones, microphones, and loudspeakers.

The a.c. voltage indicated in Fig. 2 could be that supplied at the headphone output terminals of a receiver. Note that the action of the diaphragm is completely unlike the piston motion of the conventional cone-shaped diaphragm of a dynamic unit. Instead, the high polymer film works as a driver by expansion and extraction.

What's Inside

Figure 3 illustrates the inner construction of a production piezoelectric headphone. The vinyldene fluoride diaphragm is curved and is backed with urethane foam padding, which is in turn backed by a perforated suspension board. The edges of the circular diaphragm fit into a supporting ring which is part of the framework of the headpiece. The open-back construction helps avoid a feeling of isolation. Headphones of this construction weigh from 10 to 13 ounces.

Fig. 3--Inner construction of a piezoelectric headphone.

The construction of a piezoelectric headphone also invites comparison with a capacitor, as it is built like the basic flat two-plate capacitor with its separating dielectric. But while the aluminum electrodes are quite close to each other, a condition necessary for capacitance, the static capacitance is low, actually only about 0.1µF-small enough to have substantial capacitive reactance even at the higher audio frequencies. The result is that the high-frequency audio loss is negligible, though a steep rolloff occurs just above 20 kHz.

Practical Advantages

While the tonal characteristics of the piezoelectric headphone are comparable to those of good electrostatics, the piezoelectric film can be driven by a relatively small input voltage as compared with the high input voltages required for electrostatics. A signal drive of only 3 volts can produce a sound pressure level of 100 dB with piezoelectric headphones. Also, these units do not require the biasing power supply which electrostatics need. Piezoelectric headphones can be plugged directly in place of conventional dynamic types in standard receivers or amplifiers. They can also be connected to a tape deck or tuner, though generally these components will not have enough signal output to drive piezoelectric headphones satisfactorily.

Frequency response of the first model produced, Pioneer's SE-700, was measured at 20 to 20,000 Hz, using a B & K model 4153 ear. Harmonic distortion was less than 1% at output sound pressure levels of up to 110 dB. They operate at any output impedance from 4 to 16 ohms.

It is apparent that Pioneer has made interesting application of this new polymer in the design, engineering, and production of these stereo headphones. Pioneer has also shown prototypes of speakers, using this new material to produce both tweeters and mid-range units.

(adapted from Audio magazine, May 1975)

Also see:

Headphones around the house (May 1974)

Why Electrostatics (May 1974)

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Updated: Thursday, 2018-11-29 7:59 PST