piezo transducer/speaker

Can anyone who has one of these see if you can use it to detect how hard its been hit? http://www.rapidonline.com/Electrical-Power/Miniature-Piezo-Transducer-12-5mm-35-0045

I am looking for a small one, but without casing (i.e piezo transducers) I cant find them smaller than 20mm. these ones with casing are 15mm. Crazy world.

What about this http://uk.rs-online.com/web/p/piezo-buzzer-components/7214947/ or this http://uk.rs-online.com/web/p/piezo-buzzer-components/7541983/ both are external drive so should be fine for detecting knocks.