Below the microwave regime, conductivity spectra of crystalline ionic conductors like RbAg4I5 and others display remarkably uniform characteristics. Depending on the ranges of temperature and frequency, these may be approximated in terms of UDR (universal dynamic response) and NCL (nearly constant loss) behaviour, while a plateau is observed at millimetre wave frequencies. All these features are consistently described by a new non-power-law, non-KWW master curve. Its formal structure is shown to be equivalent to a proportionality of the rates of relaxation via the single- and many-particle routes. This is the essence of the concept of mismatch and relaxation (CMR).