lib.percep_spec.roughness_c:
This should be based on Hutchinson and KNopoff 1978 Kameoka an
Kuriygawa 1969a; Viemeister 1988; Aures 1985; Plomp and Levelt
1965.
lib.percep_spec2.roughness_c:
This should be based on Hutchinson and KNopoff 1978 Kameoka an
Kuriygawa 1969a; Viemeister 1988; Aures 1985; Plomp and Levelt
1965.