The 0 dB HL audiometric zero varies across frequency by design

Evidence

The hearing level is the standardized reference that defines the lowest sound pressure level that causes an auditory event at each frequency. Since the sensitivity of hearing varies with frequency, the sound pressure level needed to produce 0 dB HL varies with frequency. The hearing level has been defined by measuring the hearing thresholds for a large number of young individuals with normal hearing. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 2634

Categorization

• DEFINE: Clarifies that 0 dB HL is frequency-dependent by construction, a subtle and often-misunderstood point about audiometric reference levels.