Sound like U2's guitarist by using Euler's number to set delay effect (in ms)

gistfile1.rb

require 'singleton'

module U2
  class Edge
    include Singleton

    attr_accessor :delay

    def set_delay_for_tempo(bpm)
      # http://5cense.com/Edge_Delay.htm
      @delay = ((60.0 / bpm / Math::E) * 1000).ceil
    end
  end
end