Sonder is a solo bass piece with a single performer who reads and plays 2 staves simultaneously.  While the upper staff displays traditional bass parts (to be performed pizzicato or arco as designated) in traditional bass notation, the lower staff displays unique percussive notation made specially for this piece.  Each of the 4 notes found on this lower staff denotes a specific way of striking some part of the bass to create distinct percussive sounds.  Check out the perusal score for more information.