def play_bar (pitches, durations, dynamic_start, dynamic_end)
volumes = (line dynamic_start, dynamic_end, steps: (pitches.length), inclusive: true)
pitches.each_with_index do |pitch, index|
play_pattern_timed [pitch], [durations[index]], amp: volumes[index]
end
end
(see https://github.com/cewillis001/sonic-pi-compos/blob/master/4partSongTemplate.rb#L20-L25 )