Tympanometers are among the most mechanically complex instruments in an audiology clinic. The probe assembly — containing a miniature loudspeaker, microphone, and pressure pump — must maintain precise acoustic and pneumatic performance across thousands of patient tests. Understanding calibration requirements helps clinicians recognize when results may be unreliable.
Governing Standards
Tympanometer calibration in the United States is governed by ANSI S3.39 (Specifications for Instruments to Measure Aural Acoustic Impedance and Admittance) and the international equivalent IEC 60645-5. Both standards specify minimum performance requirements for probe tone frequency accuracy, static admittance measurement, ear canal volume estimation, and pressure system accuracy.
Probe Tone Frequency Accuracy
The standard 226 Hz probe tone must be accurate to within ±3 Hz. High-frequency probe tones (678 Hz, 1000 Hz) used for infant tympanometry must meet ±2% frequency accuracy. Any deviation beyond these tolerances alters the acoustic admittance reference value and renders tympanogram classification unreliable.
Probe Tone Standards
226 Hz: ±3 Hz. 678 Hz and 1000 Hz: ±2% (i.e., ±13.6 Hz and ±20 Hz respectively). Output level: within ±3 dB SPL of nominal.
Static Acoustic Admittance Tolerances
Static admittance (Peak Y) must be accurate to within ±10% or ±0.1 mmho, whichever is greater, across the full measurement range. This tolerance is tighter than most clinicians assume — a 0.15 mmho reading could represent anything from 0.135 to 0.165 mmho in a perfectly calibrated system.
Ear canal volume estimation — critical for identifying perforations and patent tympanostomy tubes — must be accurate to ±10% or ±0.1 ml, whichever is greater.
Calibration Cavity Check
Many tympanometers include a supplied calibration cavity — a hard-walled cavity of known volume — that allows a quick in-clinic check of the pressure system and volume estimation accuracy. This cavity check is not a substitute for laboratory calibration but serves as a daily or weekly check between formal service visits.
If the cavity check reading falls outside the cavity's nominal value by more than ±10%, the instrument should be removed from use and sent for service.
Daily Cavity Check Protocol
Record the cavity volume reading each morning before patient testing. Log the value in the instrument record. If three consecutive readings trend more than 5% from the nominal cavity value, schedule a service check.
What to Expect During a Calibration Service Visit
A full tympanometer calibration visit includes: probe tone frequency and SPL verification; static admittance accuracy across multiple cavity sizes; ear canal volume accuracy; pressure system calibration (range, linearity, and rate); sweep rate verification; and multi-frequency probe tone accuracy if the instrument supports high-frequency tympanometry.
The technician will use a set of hard-walled cavities traceable to NIST volume standards, a calibrated sound level meter with a HA-1 coupler, and a precision pressure gauge. Total service time is typically 1.5 to 2.5 hours per instrument.
Frequently Asked Questions
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