Quadratic difference tones become audible only above roughly 70 dB primary levels.

Evidence

Difference tones can be cancelled only for levels of one of the two primaries above about 70 dB. For lower values of the primaries, the difference tone remains inaudible. For L1 ;::::; L2 = 70 dB, the cancellation level remains at about 10 dB. This means that it is 60 dB below the level of the primaries, corresponding to an amplitude of the sound pressure of the cancellation tone of about one thousandth that of the primaries. This difference becomes smaller for increasing level of the primaries and reaches about 40 dB (corresponding to 1%) for L1 ;::::; L2 = 90 dB. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1542

Categorization

• corroborated: 2 independent passages support this claim — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1542