reference equivalent threshold force level

4.56  reference equivalent threshold force level. At a specified frequency, the mean value of the equivalent force levels, in decibels relative to 1 μN derived from a sufficiently large number of ears of otologically normal persons of both sexes aged between 18 and 30 years inclusive, expressing the threshold of hearing in a specified mechanical coupler for a specified configuration of bone vibrator. Abbreviation, RETFL; unit, decibel (dB); symbol, L_F.