Are Piezo Tweeters frowned upon?

I salvaged a pair of mini piezos and was thinking of adding them as super-tweeters.
Do they need a cap? They were originally installed behind 2.2uF?
Do they need a resistor? What does the resistor do?

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Since they present a pure capacitance as an impedance, the resistor actually acts like a cap would for a dynamic (magnet - coil) speaker. A resistor forms a high-pass filter when in series. It's sorta counterintuitive.
 
Using a transformer with zobel, and hopefully rolling before its resonance, they are amazing.

Otherwise, actively crossed outside of their resonance perhaps with a resistor, some are amazing.

Search djk on audio asylum under the high efficiency forum.
But, true Motorola piezo haven't been made in what, 20 years ?
 
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Try making a frequency response measurement of the piezo. The cheap ones with plastic horn which I tried, had a terrible resonance at 5 kHz.

Sometimes a small piezo is added to a full-range driver. Technically, this makes it a two-way speaker, which it will say on the box. 😉

Since they present a pure capacitance as an impedance, the resistor actually acts like a cap would for a dynamic (magnet - coil) speaker. A resistor forms a high-pass filter when in series. It's sorta counterintuitive.
No, the resistor would be a low pass filter. Usually the cut-off is at around 30 kHz, so it does not really do much acoustically. It is added for the reason stated by @Jeffh01.
 
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Just to reiterate >
Adding series resistance to a Piezo does create a low-pass filter. How much you 'hear' depends on the R value.
Adding series capacitance to a Piezo creates volume level reduction. And once again, the value determines what you hear.
The common practice of adding 10 ohms series is to protect BOTH amplifier and driver.
 
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Thank you for all your responses. Whilst the theory seemed like a good idea a practical test proved the obvious. I'm sure the inclusion of the super-tweeter extended the frequency range of the cabinet, I'm no spring chicken - I couldn't hear any difference. @Mister Audio was right - any improvements were purely cosmetic.