the level of a pure tone of a given frequency that is just detectable by a person with normal hearing. For example, the audiometric zero for a 1 kHz pure tone is 7.5 dB SPL (decibels sound-pressure level) for TDH49 headphones, according to U.S. standards (ANSI S3.6-1996 Specification for Audiometers). Audiometric zero is used to calibrate audiometers and, by definition, is 0 dB HL (hearing level). See audiogram.