age of Thunderbolts

[FS T-Bolt]

can you tell how old a Thunderbolt is by listening to it? i ask that question because of sounds like these two:
http://www.airraidsirens.com/mp3/thunderbolt1000.mp3

http://www.airraidsirens.com/mp3/thunderbolt1000t.mp3

and then i hear Thunderbolts like the one Mike Nelson recorded and wonder why the same siren model sounds different. is it the age, electrical reasons, or is it because its defective.

[Gil]

Different port ratio, different chopper speed, different phase, etc.