  We give a method for designing a mechanical impedance to suppress the
propagation of disturbances along a chain of masses. The key feature of our
method is that it is scale free. This means that it can be used to give a
single, fixed, design, with provable performance guarantees in mass chains of
any length. We illustrate the approach by designing a bidirectional control law
in a vehicle platoon in a manner that is independent of the number of vehicles
in the platoon.
