Rubber doesn't conduct energy well; worse, it conducts energy at some frequencies, absorbs at others, and stores and releases energy at other frequencies. In other words, a rubber or Sorbothane foot acts as a kind of passive equalizer/phase shifter. Also, because rubber has non-linear storage and release characteristics, it can interfere with and distort important time information by introducing spurious group-delay characteristics back into your system. ‑from Symposium USA.