Field Audiometer Daily Biological Calibration & Monthly Functional Check Log

ContextPlacement technique directly affects results. The center of each TDH earphone cushion must align with the ear canal opening — shifting it even 5–10 mm off-center attenuates high frequencies by 10–15 dB due to poor acoustic coupling, making the audiometer appear under-powered at 4000–8000 Hz. The headband should hold both phones firmly in position without being so tight that it causes discomfort or distraction. Instruct the listener clearly: 'Press the button each time you hear a tone, no matter how faint.' Experienced biological check personnel respond reliably to near-threshold tones; a first-time listener may wait for a louder, more certain signal and produce thresholds that appear 10–15 dB worse than their baseline. Rushing this setup step is the single most common cause of false-positive biological check failures.

ContextUse the same clinical threshold-search method used for patient testing: present tones 5 dB above expected threshold, descend in 5 dB steps until inaudible, then ascend in 5 dB steps accepting the lowest level with at least 50% responses across a minimum of four ascending runs. Do not use supra-threshold screening levels — the entire point of the biological check is to find threshold, because small audiometer drifts are only detectable at threshold, not at 40 dB HL. Record each result to the nearest 5 dB as you go. Including 6000 and 8000 Hz is worthwhile even if your program only tests employees through 4000 Hz, because high-frequency transducer drift is disproportionately common in audiometers experiencing thermal cycling or mechanical wear. Testing both ears doubles data points and helps localize faults: a unilateral deviation implicates the transducer; bilateral deviation implicates internal circuitry or power supply.

ContextA ±5 dB deviation at any single frequency is the threshold for concern per NIOSH and CAOHC recommendations. A single frequency at exactly 5 dB off is a yellow flag — remove the earphones, reposition them carefully, and retest that frequency before escalating. Consistent deviations of 10 dB or more at one frequency, or deviations of ≥5 dB at three or more frequencies simultaneously, should be treated as instrument failure until electroacoustic verification proves otherwise. Pay close attention to direction: deviations in the positive direction (the listener needs more intensity to reach threshold) are more clinically dangerous than negative deviations. A +5 dB audiometer error means every patient tested that day has thresholds recorded 5 dB better than they truly are, which could mask a standard threshold shift and expose the employer to regulatory and legal liability.

ContextBefore pulling the audiometer from service, rule out the two most common causes of false-positive failures: earphone placement error and transient listener inattention. Remove both earphones completely, wait 30 seconds, replace them with confirmed correct positioning, and conduct a fresh threshold search on the failing frequency. If the result returns within 5 dB of baseline, mark it as a placement error and document accordingly. If it fails again, proceed to transducer inspection or instrument removal. When a second biological listener is available, testing that person is the fastest way to distinguish a listener ear change from an equipment problem — particularly useful when the deviation is unilateral and you suspect the listener may have had recent noise exposure or is experiencing early-morning temporary threshold shift.

ContextThe audiometer must not be used for employee or patient testing if it fails the biological calibration check after verified correct positioning. Affix a printed 'OUT OF SERVICE — DO NOT USE' label over the power switch to prevent accidental use by other staff. Notify your occupational health supervisor or overseeing audiologist within 24 hours. Any audiograms obtained since the most recent passing biological check are suspect and may need to be repeated once a calibrated unit is returned to service. Document the date and time the unit was removed, the specific frequencies and deviation amounts, and the name and credentials of the person who made the out-of-service decision — this record protects both the program and the employees if a compliance inspection occurs.

ContextEarphone cushions made from MX-41AR synthetic rubber or foam degrade with exposure to skin oils, cleaning agents, heat, and UV light. A cushion that has hardened, cracked, or permanently compressed no longer seals against the pinna, causing a predictable high-frequency roll-off above 2000 Hz that makes the audiometer appear under-powered at those frequencies. Replacement cushions for TDH-39 and TDH-50P earphones typically cost $15–$40 per pair and should be treated as consumables — replace at the first sign of visible degradation or every 1–2 years for heavily used units regardless of appearance. Do not clean cushions with alcohol wipes more than once daily; repeated alcohol exposure accelerates surface cracking and leaves residue that can irritate the skin of patients with sensitive ears.

ContextHeadband tension determines how firmly the cushion presses against the pinna. ANSI S3.6-2018 specifies earphone contact force within a defined range (typically 400–500 g for TDH-39P and TDH-50P). Without a calibrated force gauge in the field, a practical test is that the phones must require noticeable manual force to compress and must return immediately to full extension without lagging or stopping partway. A headband overstretched by storage draped over a thick object or left open in a bag generates insufficient contact force on smaller or medium-sized heads, leaving a gap at the inferior cushion edge that especially affects 4000 Hz and above — mimicking a high-frequency sensorineural loss. If the headband shows visible permanent deformation or plastic set, contact the manufacturer or calibration lab; replacement headbands for most models cost $20–$60.

ContextCord failure is the single most common hardware fault in portable audiometers. Stress concentrates at two points: where the cord exits the earphone housing (strain relief area) and where the plug enters the audiometer's input jack. Flex the cord slowly through a 90-degree bend at both ends while either listening to a tone or watching the audiometer's output indicator for dropouts. A cord that crackles, intermittently cuts out, or produces asymmetric output at any hearing level must be replaced before the next test session. Replacement cords for TDH earphones typically cost $30–$70 and are field-swappable without tools on most models. Do not tape a damaged cord — tape masks the fault and the failure is unpredictable, potentially passing through hundreds of tests before dropping 15 dB at the critical moment.

ContextThe bone oscillator is substantially more fragile than air-conduction earphones because its transducer element can be damaged by a drop from desk height — the small piezoelectric or electromagnetic element inside is not mechanically reinforced for impact. Run a fingertip across the vibrating surface: it should feel smooth and intact with no chips or cracks. The headband spring must hold the oscillator firmly against the mastoid without slipping during test; a weakened spring reduces coupling force and elevates bone conduction thresholds, which could falsely indicate a conductive overlay. Apply the same cord flex test used for air-conduction earphones. If bone conduction thresholds during the biological check are consistently elevated without a clear cause, suspect oscillator damage before suspecting the listener — these transducers are less mechanically durable than earphones and typically require factory servicing rather than field repair.

ContextField audiometers experience conditions stationary clinic units never face. Check every control: the frequency selector should click firmly and positively into each position without play or skipping; the HL attenuator should produce clear discrete steps with no sticking or mushiness; the tone presentation button must actuate cleanly with a definite tactile break. Gently wiggle each knob — looseness in the shaft indicates the retaining nut has backed off and the potentiometer may produce non-repeatable output levels under normal operation. Debris in rotary attenuator channels is common in dusty industrial settings and can cause intermittent contact or sticking at specific dB levels; compressed air through the channel slots clears most debris safely. Knob looseness requires internal access and must be flagged for service.

ContextA pure tone at 1000 Hz, 60 dB HL should sound like a clean, perfectly steady sine wave with no buzzing, crackling, or pitch variation. Distortion that is audible at suprathreshold levels is almost always worse at threshold, making this a useful upstream screen for transducer and amplifier problems. If distortion is present, repeat at 500 Hz and 4000 Hz — distortion at only one frequency suggests a transducer fault, while distortion across all frequencies suggests an internal oscillator or amplifier issue. Test for cross-talk separately: present a 60 dB HL tone to one earphone while holding the other to your own ear with the channel level set to -10 dB — any audible signal leaking into the non-stimulated channel constitutes cross-talk, which will artificially elevate thresholds in patients with asymmetric hearing, and must be referred for electroacoustic service.

ContextThe HL attenuator must produce perceptually equal loudness decrements at each 10 dB step. Start at 80 dB HL and step down every two seconds; each step should produce a clearly audible reduction in loudness — no two adjacent steps should sound the same. Attenuator dropout, where a step sounds identical to the one above or below it, is a common aging fault caused by worn resistive contacts. A malfunctioning step in the 20–40 dB HL range is particularly dangerous in occupational audiometry because most occupational hearing thresholds are recorded in exactly this range; a sticky or skipping attenuator here generates threshold records shifted by 10–20 dB. Note the specific step or steps where irregularities occur in the monthly log so the calibration lab can target those contact points.

ContextTone presentation clicks — broadband transient bursts at the moment of tone onset or offset — are a well-documented source of false responses in audiometric testing. They occur when the tone is switched at a phase other than zero-crossing, generating a click that is far easier to hear than the test tone at near-threshold levels. To check: present a 1000 Hz tone at 10 dB HL, switching it on and off at one-second intervals. Any click that is audible at this output level will elicit conditioned responses from patients at frequencies where their true threshold is well above 10 dB, collapsing apparent thresholds toward 0. Clicks that appear only at specific frequencies indicate frequency-dependent switching problems. This fault cannot be field-repaired and requires factory or certified lab service.

ContextRotate or step through every position — 250, 500, 1000, 2000, 3000, 4000, 6000, 8000 Hz, and extended range if equipped — listening for correct pitch at each stop. The frequency should change clearly and abruptly at each position; a selector that sticks between positions produces two simultaneous frequencies that are perceived as a roughened, unstable tone. On digital audiometers, confirm the displayed frequency matches what you hear. A 1000 Hz position that actually outputs near 800 Hz, or a 4000 Hz position that outputs near 3800 Hz, will generate audiometric configurations in patients that appear frequency-specific but actually reflect instrument error. Any frequency mismatch, however subtle, invalidates all thresholds recorded on that channel for that position and requires immediate service.

ContextNarrowband masking noise centered at each frequency should have a characteristic tonal quality: 1000 Hz masking sounds like a low, buzzing rumble; 4000 Hz masking sounds like a concentrated hiss. Both should increase clearly in loudness as the masking level dial is advanced. Silence in the masking channel when masking is engaged is an obvious fault. More subtle failures include masking noise leaking into the test channel (which would artificially elevate threshold by masking the test tone in the test ear) or masking activating in the wrong channel entirely. To confirm channel isolation, use a two-phone arrangement: test tone in right, masking in left, listening to each phone independently. If your program includes speech audiometry, briefly verify speech input produces clear, intelligible output at comfortable listening levels with no static, breakup, or level inconsistency.

ContextInsert phones such as the ER-3A or EARTone 3A introduce failure points absent in supra-aural setups: the foam tip, the plastic nozzle, the acoustic tubing, and the tubing-to-transducer coupling joint. Inspect the tubing from end to end for kinks, cracks, or hardened spots — a single kink in the tubing causes dramatic high-frequency attenuation above 3000 Hz, closely mimicking a high-frequency sensorineural loss. Check the nozzle for cerumen or debris with a penlight; even partial occlusion can reduce output by 15–20 dB at 4000–8000 Hz. Foam tips are single-use in clinical practice; if cost constraints require reuse in occupational settings, inspect each tip for reduced expansion — degraded foam that does not fully seal the canal creates a sound leak that elevates thresholds unpredictably. Run the monthly biological check with insert phones using an insert-phone-specific baseline, not the supra-aural baseline, because their calibration reference levels differ by several dB.

ContextMost portable audiometers have a low-battery warning, but this indicator typically activates below the voltage threshold where output levels first begin to drift — meaning the audiometer can produce incorrect output without triggering any alert. For battery-powered units, perform the monthly functional check on a fully charged battery. During routine testing sessions, note whether biological check thresholds worsen predictably toward the end of a test day; progressive deterioration over hours is a reliable sign of battery voltage sag and warrants a battery replacement or service evaluation. For AC-powered field units, confirm the power adapter is manufacturer-specified — generic adapters with slightly different voltage output can destabilize the oscillator and produce frequency or level errors that closely mimic equipment aging.

ContextOSHA 29 CFR 1910.95(m)(3) requires audiometric test records to be maintained for the duration of the affected employee's employment. A log with gaps — missing dates, missing serial numbers, unsigned entries, or entries showing only 'bio check OK' without numerical threshold data — can expose an employer to OSHA citations even when the underlying audiometry was performed correctly. Every entry in the log must clearly document the frequencies tested, the baseline value for each, today's measured value, the dB deviation, and whether the check passed or failed. Use a dedicated form that travels with the audiometer so the record never becomes separated from the instrument it certifies.

ContextWhen an audiometer fails the biological check, determine the exposure window — how long might it have been out of tolerance? Review the log for the most recent entry showing all frequencies within ±5 dB and flag every test session between that date and today as potentially invalid. The corrective action record must note: the date and time of discovery, the credentials of the person removing the unit, the specific deviations at each frequency, and the disposition (sent for electroacoustic calibration, cord replaced and re-checked, cushion replaced and re-checked). If a component replacement resolves the failure, document that a post-repair biological check was performed, all results returned within tolerance, and the unit was returned to service by name and date. A record that trails off with 'sent to lab' but has no return entry is incomplete.

ContextA biological calibration check confirms relative output consistency against a human reference, but it cannot measure absolute sound pressure level in dB SPL, harmonic distortion percentage, frequency accuracy in Hz, or rise/fall time — all of which are measured by an electroacoustic lab using a calibrated sound level meter, an acoustic coupler (6 cc for supra-aural phones, HA-1 coupler for insert phones), and standardized test procedures. ANSI S3.6-2018 and OSHA 29 CFR 1910.95(h) together require electroacoustic calibration at minimum annually. Laboratory calibration for a portable audiometer typically costs $150–$400 depending on the lab, the number of transducers, and geographic location, with 2–5 business days of typical turnaround. Build a backup audiometer into your program budget so calibration service does not interrupt the testing schedule.

ContextCAOHC-accredited hearing conservation programs and OSHA-regulated employers are required to operate under professional supervision — a licensed audiologist, otolaryngologist, or occupational health physician must maintain oversight of the calibration program. Quarterly countersignature review enables an experienced professional to identify subtle trends that escape notice session-to-session: a frequency drifting steadily toward the 5 dB tolerance edge, a biological listener whose thresholds have been slowly deteriorating over months, a pattern of one channel consistently underperforming. Document each quarterly review with a dated signature and any recommended corrective actions, and retain these records with the calibration log. Calibration records, including quarterly supervisor review documentation, must be made available to OSHA compliance officers on request within 24 hours.