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I'm not sure. I've seen plenty of deaf young ones mind you :P

Higher the number lower the tone. Higher pitches favoured for closer working dogs, lower tone travels further.

Yep, the old HTFD whistle (Hit The F*ng Deck) :D

My wife uses the 640 for her setter. That ref whistle on the other end is downright offensive! Eric Douglas threatened to throw her out of a trial if she blew that thing near him again :D

I use a 212 to really get the message out there. Only works at one pressure too which is great if you've just run up a hill and you're huffing and puffing. That and I don't know anyone else that uses...