A mechanical filter is a signal processing filter usually used in place of an electronic filter at radio frequencies to allow a range of signal frequencies to pass, but to block others. The filter acts on mechanical vibrations. Transducers at the input and output of the filter convert the electrical signal into, and then back from, these mechanical vibrations. The mechanical elements obey mathematical functions which are identical to their corresponding electrical elements. Electrical theory has developed a large library of mathematical forms that produce useful filter frequency responses for use in the design of mechanical filters. Steel and nickel–iron alloys are common materials for mechanical filter components. Resonators in the filter made from these materials need to be machined to precisely adjust their resonance frequency prior to final assembly. The high "quality factor", Q, that mechanical resonators can attain, far higher than that of an all-electrical LC circuit, made possible the construction of mechanical filters with excellent selectivity. Good selectivity, being important in radio receivers, made such filters highly attractive. Contemporary researchers are working on microelectromechanical filters, the mechanical devices corresponding to electronic integrated circuits.