This refers to the classic diagram originally sourced from Bolt, Beranek and Newman portraying the 'controllers of steady state room acoustic response' for LARGE acoustical spaces.
This describes the behavior about the critical frequency, fc , as well as the large room frequency as defined by Manfred Schroeder, FsubL. The critical frequency, fc, is synonymous with FsubL, and you will see both notations used. While fc refers to the characteristic as it exists in any acoustical space, the FsubL nomenclature refers to when a space effectively becomes “large” in its acoustical behavior.
For wide range frequency response to 30 Hz, this corresponds to a space greater than ~250,000 cubic feet.
Since FsubL is dependent upon the low frequency response one desired it to accommodate (meaning the lower the effective LF reproduction cutoff, the larger the room), it is possible to divide the audio spectrum into 3 decades on a linear frequency scale - with the first decade as a characterized by modes, while approaching the upper two decades consisted of diffusion, and also of absorption of specular reflections.
FsubL = K {Sqrt (RT60/V)}
where
FsubL = large room frequency
K = 2000 in SI and 11,885 in US metrics (Bolt, Beranek & Newman have used a value of 1,893 in SI and 11,250 for US)
V = volume of room
Note, that a small acoustical space lacks a substantial reverberant sound field that rises above the ambient noise floor. Hence this equation in this form describes the large acoustical space where a statistically reverberant space exists.
The LEDE standard (what some have called the ‘Davis frequency’ after Don Davis) has used the following which yields a closely corresponding value:
FsubL = {3(Velocity of Sound)}/( Room’s smallest dimension)
Note: FsubL moves to ~250-500Hz for a small acoustical space.
FsubL is a transitional area. Where the behavior shifts from that of a small acoustical space to a large acoustical space (per Schroeder). This critical frequency, fc, varies as the signal wavelengths progressively become equal to, and shorter than the various room dimensions and as their behavior shifts from the pressure model that characterizes the modal region into the ray/particle model characterized by the dominance of specular reflections.
What is most important is to recognize the shift in behavior and methods used to characterize and also to measure said responses based upon the predominant determinant behavior in each region.
Controllers of steady state room acoustic response
