As with all other tweeters, piezoceramic tweeters come in a variety of sizes and power-handling capabilities. Often, piezos are used without a crossover because the piezo element simply does not respond below its rated frequency range. Trouble is, this presents a purely capacitive load to amplifiers. And at their lowest response frequencies, piezos tend to get harsh and trashy-sounding. To use a volume control, or L-pad (constant impedance attenuator), you have to give piezos a resistive element so that the amplifier "sees" something approximating an impedance across its terminals. Doing this considerably improves the sound and power handling characteristics.
Determine --- according to the specifications packed with the tweeter --- what frequency range and type of piezo you have. Most direct-diaphragm automotive or small-speaker piezos have a bottom-end limit of about 4 or 5 kilohertz (kHz). The ubiquitous black plastic horn piezos used in hundreds of low-end PA speakers have a usual cut-off of 3 kHz, and the larger compression drivers used in some concert speakers can theoretically produce sound lower than 3 kHz, but can benefit greatly from a proper crossover network.
Use a 1.5 microfarad (µF) capacitor with a small piezo, a 2.2 µF cap with a small plastic horn piezo, and a 5 µF cap with a large compression horn driver. Capacitors need to be non-polarised polypropylene. Use a 22-ohm, 100-watt resistor with all of them. Solder the resistor across the positive and negative terminals of the tweeter. Solder the capacitor to the positive tweeter terminal.
Lay the L-pad on its back, shaft up and terminals toward you. Strip a 1/2-inch of insulation off and solder single speaker wire leads to all three L-pad terminals. Connect Terminal 1, or left, to tweeter negative along with the negative speaker lead from the amplifier. Connect Terminal 2, or centre, to the amplifier side of the capacitor you just connected to the tweeter. Connect Terminal 3, or right, to the positive speaker amplifier lead.
Mount the L-pad with its indicator face where you can get to it easily. Turn the shaft fully counterclockwise and slide the knob over the fluted shaft with the knob marker at the "Off" or 0-dB position. Test with music, turning the knob up gradually until you have the desired result.