Shelving filters are used to preserve the original neutral monitor frequency balance.
When low frequency room resonances are equalized the monitor sound can become brighter. AutoCal2 looks for any change in the monitor frequency balance between the mid and high frequencies after applying notch filters to remove room colorations. Room equalization can change the midrange to high-frequency balance, and this is compensated by adjusting the shelving filters. The shelving filters are set in a frequency range typically well above the Schroeder frequency, where the room modes are no longer the dominating factor the response. The shelving filters are adjusted to preserve the original midrange to high-frequency ratio, maintaining the monitor sound character after the room equalization. The direct monitor sound has neutral response.
The Schroeder frequency is the highest frequency where individual room resonances can be detected. Above the Schroeder frequency, the density of the room in frequency has become high enough so that they overlap, and individual resonance can no longer be detected. High above the Schroeder frequency, the room contributes more as a reflector and diffractor for audio and we start to see the direct sound from the monitor apart from this room contribution.