People with above-normal hearing exist.
Normal hearing was defined as the average of a group of young healthy individuals. These normal hearing levels are currently used to express acoustic sensitivities. One commonly used way is to use decibels relative to this normal hearing level (dB NH). This scale is used in audiograms (Schnupp et al., 2012). When hearing senstivities are reduced by 20 dB or more it is regarded as diagnostic for hearing impairment (Fig. 1).
Fig. 1. Audiogram. Source: (Schnupp et al., 2012)
Statistical outliers always exist. In Fig. 2 is an audiogram showing above-normal hearing sensitivities in the low frequencies in the left ear and in the high frequencies in the right ear (Fig. 2).
Fig. 2. Audiograms showing normal hearing (solid lines) and noise-induced hearing loss (dotted lines) Source: Fraser University, CA.
Schnupp et al., Auditory Neuroscience, MIT Press 2012