2Pure-tone audiometry — technique & calibration
The audiogram is the most familiar picture in audiology, and the most easily taken for granted. Behind each little × lies a careful procedure — the down-10/up-5 bracketing that defines threshold — and a chain of calibration that converts the audiometer's dial reading into the sound pressure the cochlea actually receives. Get the technique right and the audiogram anchors the whole battery; get the calibration wrong, especially in a baby's small ear, and every later target is off. This module covers how a pure-tone threshold is obtained, why insert earphones have quietly become the transducer of choice, what the deceptively simple unit 'dB HL' really references, and how the real-ear-to-coupler difference rescues the measurement in the ears where it matters most.
FThe audiogram
Pure-tone audiometry plots the softest level a listener reliably detects at each frequency — air-conduction thresholds from 125 to 8000 Hz, including inter-octave frequencies, found by the down-10/up-5 bracketing method. Its shape — flat, sloping, profound — carries diagnostic and surgical meaning.
TTechnique & transducers
Insert earphones are preferred over supra-aural cushions for three reasons: greater ambient-noise attenuation, greater interaural attenuation (so less cross-hearing and less need to mask), and 2-cc-coupler calibration that lets the dial level be converted to SPL at the eardrum.[2020]
TWhat dB HL means
The audiometer reads dB HL — hearing level — referenced so that 0 dB HL equals the average threshold of otologically normal young adults at each frequency. That reference (the RETSPL) is built into the machine, which is why a 30 dB HL threshold means the same thing at 250 Hz and 4000 Hz even though the underlying sound pressures differ.
CThe calibration chain & the infant ear
The full conversion is SPL at the eardrum = dial (dB HL) + RETSPL + RECD. The real-ear-to-coupler difference corrects for the individual ear, and it is decisive in children: by Boyle's law a smaller ear canal develops more SPLfor the same dial level, so a baby's RECD is large and must be measured individually — otherwise thresholds and amplification targets are wrong. Calibration is verified against ANSI standards on a schedule.[1999]
CThe low frequencies matter
The low-frequency thresholds (125 and 750 Hz) deserve special attention. They establish the baseline residual acoustic hearing that determines suitability for hearing preservation and electric-acoustic stimulation, and give the reference against which post-operative residual hearing is tracked — a thread that runs into the candidacy and device chapters.
What was the methodological error?
Why are insert earphones preferred for pure-tone audiometry?
Why must the RECD be measured individually in infants?