So I came across this ‘fun fact’: the tempo of music can affect one’s heart rate. In other words, our heart rate matches the tempo of the song we’re listening to.
I wonder if Cain ever let Cass listen to any music. He probably used it only as a training tool. It was when Cass joined the batfam that she was truly able to listen to music for her own enjoyment. Tim was the one to discover this, and made a list of song recommendations for her. Bruce was the one who bought Cass her first set of headphones and a new ipod. He told her that fact I mentioned, and encouraged her to sometimes use music as an alternative to meditation, when she’s stressed.
Ok I made up that headcannon just so I have an excuse to draw Cass feeling her heart beat while listening to this song.
Full resolution here.