>Does anyone have a source for a spec sheet for a 40KHz ultrasonic
>transducer?

Murata has some, here's their web site : http://www.iijnet.or.jp/murata/
You'll find there a search engine and data sheets.

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                          Luc Martin
                  luc@greco2.polytechnique.fr
        http://www.geocities.com/SiliconValley/Bay/2752