A constant pure sine wave tone or tones played over an audio system to allow critical measurement and adjustment of an audio system both when individual components are being developed and when a complete system is being installed. Test tones frequently consist of white noise or pink noise. They are used in system set-up to help determine the optimum placement for speakers to achieve the flattest frequency response. Test tones are also used to measure sound pressure levels (SPL) from the channels in an audio system in order to achieve equal levels from all channels and to adjust equalization when an equalizer is to be used.