The Probert Encyclopaedia of Science & Technology

SELECTIVITY

In radio terms, selectivity is the ability of a radio to reject signals on frequencies adjacent to the desired station. It is usually expressed as a bandwidth measured at 6 dB rejection points ('6 dB down' or '-6 dB'). For example, a selectivity specification of '6 kHz at -6 dB' means any signal outside the 6 kHz bandwidth will be reduced in strength by at least 6 dB (in other words, the interfering signal is only one-fourth as strong as it would be otherwise). Typical good selectivity measurements at 6 dB points are 6 kHz for AM, 2.5 kHz for SSB, and 0.5 kHz for CW.
Research Selectivity