Sound System Engineering by Don and Carolyn Davis
Manfred Schroeder has defined a large room frequency (FL) as the frequency above which a large number of room modes will be excited to vibrate at the source frequency.
FL is the large room frequency in Hz,
K is 2000 in the SI and 11,885 in the U.S.,
RT60 is the apparent reverberation time for 60 dB of decay in seconds,
V is the volume of the room in m3 or ft3.
If we assume for sound systems: