I think the "quack" describes exactly this "nasal" , brittle and harsh sound a piezo can produce. I wouldn´t mind defining it more precisely, ´cause it´s just a phrase used for the drawbacks of piezo sound in general.
Nasal, brittle, and harsh are all good descriptors. It's definitely a defect in the high-end of the sound spectrum, as piezos do a decent job on lows. Quack also seems to be related to how hard you're playing, becoming more obvious when you pick harder.
It's almost like an obnoxious compression of sound frequencies in the treble range, killing all the airy stuff which makes sound natural.
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It's almost like an obnoxious compression of sound frequencies in the treble range, killing all the airy stuff which makes sound natural.