Volume level designated to be used as the standard in THX certified theaters and home THX installations setting volume at 85 decibels when fed a continuous test tone and achieving 105 decibel peaks when the volume control is at the reference 0 decibel setting (0 decibels is reference on the volume control with negative numbers being the number of decibels below reference and positive number being the number of decibels above reference). The reference level is set using a test tone generated by a surround sound preamplifier and a sound pressure level meter (SPL meter) to measure the sound output or volume. hile providing a realistic and enlivening auditory experience, the reference level is very loud and is often turned down somewhat by listeners. However, surround sound audio systems should be set to the THX reference level and all channels properly equalized (each channel putting out the same sound pressure levels with a reference signal) even if the system will not be listened to at the true reference level. It is an interesting exercise to demonstrate a home theater system for friends when it is set at the true THX reference level though.