ANSI S3.6 Explained: Everything Audiologists Need to Know About Audiometer Calibration

ANSI S3.6, published by the American National Standards Institute, defines the electroacoustic performance specifications that audiometers must meet to produce valid hearing threshold data. The current edition is ANSI S3.6:2018.

The standard specifies reference equivalent threshold sound pressure levels (RETSPLs) for each transducer type — insert earphones, supra-aural earphones, circumaural headphones, and bone conduction vibrators — at each test frequency. It also sets tolerances for pure-tone frequency accuracy, harmonic distortion, attenuator linearity, and masking noise levels.

ANSI S3.6 defines two levels of calibration. An exhaustive calibration — also called a full electroacoustic calibration — is a comprehensive measurement of all audiometer output parameters against the full set of standard tolerances. It must be performed by a calibrated laboratory using NIST-traceable equipment.

An annual calibration (sometimes called a periodic or limited calibration) verifies a subset of critical parameters — primarily output levels at key frequencies, attenuator linearity, and frequency accuracy. This is performed annually at minimum, and additionally any time the audiometer is dropped, repaired, or produces questionable results.

The allowable deviation from the reference output level varies by frequency. At 1000 Hz, the tolerance is ±3 dB. At frequencies above 4000 Hz, the tolerance widens to ±5 dB due to the increased variability in coupler and transducer behavior at higher frequencies.

It is important to note that these tolerances apply to the audiometer output level as measured at the transducer using the specified coupler — not to the patient's perceived loudness. A properly calibrated audiometer ensures that a 0 dB HL stimulus represents normal hearing level as defined by the standard.

ANSI S3.6 requires that harmonic distortion in the pure-tone output does not exceed 3% at output levels at or below 75 dB HL, and does not exceed 5% at levels above 75 dB HL. Distortion above these limits introduces spectral splatter that can produce artifactual thresholds, particularly at adjacent frequencies.

The one-year interval is a minimum, not a target. High-volume clinics testing 20 or more patients per day should consider six-month calibration cycles given the cumulative mechanical wear on transducers.

Calibration should be performed immediately — regardless of the scheduled interval — whenever an audiometer is dropped or physically impacted, when test results are inconsistent with patient history, when the clinic fails an external audit, or when insert earphone tubing is replaced.

A valid calibration certificate must include the instrument make, model, and serial number; the date of calibration; the measured values at each frequency and transducer; the reference values from the standard; the pass/fail result for each parameter; the measurement uncertainty; and the name and accreditation status of the performing laboratory.

Certificates should be retained for a minimum of three years and made available on request during accreditation surveys. Digital storage with versioned backups is strongly recommended.

Not all calibration services are equal. A laboratory performing audiometer calibration should hold ISO 17025 accreditation from a recognized national accreditation body and should use NIST-traceable reference standards. Ask any prospective laboratory for their scope of accreditation, which specifies the exact measurement parameters and uncertainty values they are accredited to perform.