Piezoelectric Loudspeaker Units

If you look inside most loudspeaker enclosures that use a piezo high frequency unit you will see no crossover but simply a resistor in series with the piezo. This resistor is to prevent the amplifier from oscillating due to the capacitance of a piezo driver. Adding such a resistance in series with a piezo will roll off the high frequencies. Increasing the resistor value will cause the high frequencies to roll off to a greater extent. To attenuate a piezo a series capacitor should be added (see right), which creates a voltage divider with the capacitance of the piezo drive element. Most piezo elements are about 0.1 to 0.26 uF so a capacitor of a similar value as the piezo element will attenuate it 6 dB.

Crossing over Piezo's can improve their sound significantly and I believe that some of the bad reputation that piezos have is due to the fact that you can use them without a crossover. However, just because you can get away with it, does not mean it is optimal. As most piezo's are used in inexpensive systems, the cost of adding in unnecessary components is often never even considered.

To crossover a piezo, add a resistor in parallel. This makes the driver look like a current driven device to any outside components, such as a crossover capacitor. The impedance of most piezo's is quite high compared with conventional drive units, so using a 22 ohm resistor, which will dissipate less power, works just as well as an 8 ohm one and makes any series crossover capacitor smaller and less expensive. Alternatively it is possible to use piezos in systems that use an active crossover

For most piezos, use of a 22 ohm resistor, and a 4-4.7 uF cap will allow the response to be close to what it was originally, but rolls off the lows at 6 dB/oct below approximately 1 kHz. This has the benefit of increasing the power handling of the piezo, which is voltage limited. Exceed the voltage that polarized the piezo element during manufacture and it will loose sensitivity and eventually burn out. Most of the better grade piezos will handle 35 volt transients, and 28 volts continuous as they come, which corresponds to 150 watts and 100 watts into 8 ohms respectively. As stated above adding a capacitor and 22 ohm resistor increases the power handling significantly, just as it does with ordinary drive units. As well as sounding less harsh and gritty, Piezos crossed over in this manner tend to be more reliable.

Many of the piezo units have a mild peak just before they roll off in the low frequency, so making the series capacitor a little smaller can flatten the response, and provide even more protection and smoother sound. For the smaller piezo units that cut off at 4-5 kHz, a series capacitor of 1.5 uF will do the trick, larger units that go down to 3 kHz can use a 2.2 uF, and the large compression driver units meant to be mounted on a horn need about 5 uF.

Attenuation, high frequency roll-off and crossing over can all be done at the same time. To attenuate, place a capacitor in between the piezo and the 22 ohm resistor that is shunting across the unit, additionally if high frequency roll-off is desired, use a series resistor in this location too. Then the series crossover capacitor should be in front of the 22 ohm resistor. Such a circuit is shown above.>

For more information visit CTS who manufacture piezo units. From the home page go to products > piezoelectric > speakers. Application notes can be found here