Dynaudio AIR 15 Reference Manual page 61

Checking and optimizing monitor placement
X-Curves
For small rooms (defined as less than 5300 cubic feet
or 150 cubic meters), ANSI/SMPTE 222M calls for a
modification of the X-curve with flat natural response to
2 kHz and then a 1.5 dB per octave roll off above 2 kHz.
This curve is useful when mixing in a small room and be
playing back in a large room.
Fig. 31: X-Curve 2 – modified magnitude response
Fig. 30: X-Curve 1
Another variation is to begin the high-end roll-off at
4 kHz and roll off 3 dB per octave instead of 1.5 per
octave.
Dynaudio Professional AIR reference manual – 2014-09-28 58