Hey there!
I'm new here and I like to know where to find the Piezo speaker mention here:
-ikr It's and old project and the writter won't response to any question 
-
as I'm not sure which to buy in AliExpress and whenever I ask the seller is this a Piezo speaker? most of them not responding and the other don't know
Also, If you have any suggestion or improvements to the project up there I'd be happy to hear them...
[u]This one[/u] will probably work.
They work "both ways" and of course, you're not using it as a speaker. You're using it as a microphone or vibration sensor.
The important thing is to get a "transducer" (AKA "speaker", or Adafruit calls it an "element"). You do not want a "buzzer" (AKA "beeper" or "sounder").
A transducer converts sound/vibration into an electrical signal or vice-versa. A beeper has an added built-in sound generating circuit and it makes a noise when you apply a DC voltage, so it can't be used as a microphone or a speaker to play music, etc.
as I'm not sure which to buy in AliExpress
Be careful with "cheap suppliers". Sometimes they mix-up buzzers & transducers and often they have incomplete specs, not manufacturer's information, and no datasheet, etc. Plus, you might be buying junk, rejects, or counterfeit parts.
Thanks for the reply
and I think Ill go with this one here