A 40-dB level rise increases fluctuation strength by a factor of roughly 2.5

Evidence

With increasing sound pressure level and for all sounds considered, the fluctuation strength increases. For amplitude-modulated sounds (Fig. 1O.2a and b), the increase is somewhat more prominent than for the frequency-modulated pure tone (Fig. 1O.2c). With an increase of 40 dB in sound pressure level, fluctuation strength of modulated sounds increases on average by a factor of about 2.5 (1.7 to 3). — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1388