Making a deafening piezo buzzer is harder than it seems

Since I was a kid, when dissecting toys, I would find these disc shaped components called piezo transducers or speakers, which were responsible for producing the beeps, melodies and what not, emitted by the toys.

I would often play with these, and try to connect to DC supplies or audio outputs from other devices, but was always frustrated with the sound never being nearly as loud as that produced in the original device.