(RC) A set of contours that serves as optimum spectrum shapes for background sound in buildings. Octave band spectra that align with a single RC curve are considered neutrally balanced. Neutrally balanced spectra have the proper amounts of low-, mid- and high-frequency sound energy to cause them to be perceived as innocuous, even though audible. RC curves are straight lines set at -5 dB/octave slopes. The RC rating of a spectrum is determined through methods defined in ANSI S12.2. The rating method arrives at a single-number rating with other designators identifying neutral, rumbly or hissy characteristics.