A function that efficiently converts bpm to rate (in Hz) with the equation

The example uses bpm2dur to create a 130BPM clock signal with metro, fed into trand, which is used to control the frequency of a sine wave.

 # Create 130bpm clock signal with metro
 130 bpm2rate metro 
 # feed clock into trand, generating frequency values between 200 and 400. 
 200 400 trand 
 # the rest of sine
 0.3 sine