Adding a series resistor greatly improves most uses of piezoelectric tweeters. Adding about a 47 to 50 ohm or so resistor reduces the extreme treble response by about 3 dB at 20 KHz, and by about 2 dB at 16 KHz. Lower treble frequencies are less affected, but the effect is great enough to consider the sensitivity reduced by 1 dB or maybe slightly more. If the effect is not enough to tone down the "hissy" nature of most piezo tweeters, then use a slightly higher value resistor. However, you should not have to go over 100 ohms, since this has quite an effect on upper treble frequencies.
The KSN1025 tweeter and its relatives (including Radio Shack 40-1379) have an internal resistor of 30-33 ohms. It may be good to add about 10 ohms to this. Please note that this model has about double the usual .15 or so uF of capacitance.
For general home stereo use, 1/2 watt resistors should work. For professional duty applications, you need power resistors. This even means shorting out the internal resistor in KSN1025 tweeters with a thin piece of wire and using external power resistors.
One advantage of adding resistance is an increase in power handling and reliability.
Use of Damping Material
The back of piezo tweeters is generally removable. This permits you to stuff in a bit of fiberglass between the diaphragm and the ceramic piezo element, as well as behind the ceramic piezo element. This somewhat damps down a "harsh" resonance around the lowest frequencies that piezo tweeters work at.
With most models other than the KSN1025, you may need a bit of a trick in order to put the tweeter back together. The trick is to solder thin pieces of wire to the ceramic piezo element leads. There are holes in the terminals on the back of the tweeter, although they may be filled with solder. You can unplug the holes and thread the added wires through.
Use of Crossovers
Although piezo tweeters are widely used without crossovers, you will gain some power handling and reliability if you remove frequencies out of their range. You may even slightly reduce some of their upper midrange/lower treble harshness in the process.
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