You will now carry out some simple experiments in pd . This will eliminate the need for specialized laboratory equipment and make it possible to ask some ``What if?'' questions. The simple mass-spring-damper model used in the previous sections was sufficient for understanding the roles of the PID controller coefficients and It turns out that the PID controller also works in a similar way when applied to a real vibrating string. In the following sections, we use a much more detailed digital waveguide vibrating string model, which is from the field of digital waveguide synthesis [3]. The model more accurately captures the physical behavior of a vibrating string, and so it also produces much more realistic simulated string sounds.