BC-HIS cheat sheet

Sound is a longitudinal pressure wave; frequency (Hz) determines pitch and amplitude (dB) determines loudness.

dB SPL = 20 log10(P/P0), with P0 = 20 micropascals; dB IL = 10 log10(I/I0), with I0 = 10^-12 W/m^2.

dB HL is referenced to normative human thresholds (ANSI S3.6), so 0 dB HL differs in SPL at each frequency.

Wavelength in meters = 343 / f (Hz); a 1000 Hz tone has a wavelength of about 0.343 m.

Doubling distance from a point source in free field reduces intensity by 6 dB (inverse square law).

Adding two equal, uncorrelated sound sources increases total level by 3 dB; two coherent sources can add up to 6 dB.

Resonance occurs when a system is driven at its natural frequency, amplifying output near that frequency.

The speech spectrum peaks near 500 Hz for vowels; consonant energy is concentrated above 2000 Hz.

Human audibility spans roughly 20 Hz to 20,000 Hz, with greatest sensitivity between 1000 and 4000 Hz.

The outer ear (pinna and ear canal) provides roughly 10-15 dB of gain in the 2000-5000 Hz region.

The tympanic membrane area (~55 mm^2 effective) versus stapes footplate (~3.2 mm^2) yields a pressure gain of about 17:1.

The ossicular lever ratio is approximately 1.3:1; combined with the area ratio, middle ear gain totals about 25-30 dB.

The acoustic reflex is mediated by the stapedius muscle (CN VII) and typically occurs at 70-90 dB SL, with a latency near 100 ms.

The cochlea is a tonotopically organized spiral; the base responds to high frequencies and the apex to low frequencies.

Outer hair cells provide active cochlear amplification (the cochlear amplifier), sharpening tuning and producing otoacoustic emissions.

Inner hair cells are the primary afferent sensory cells, synapsing with roughly 90-95% of type I auditory nerve fibers.

The auditory nerve (CN VIII) travels via the internal auditory meatus to the cochlear nucleus; the first brainstem synapse is ipsilateral.

The stria vascularis generates the endocochlear potential of about +80 mV relative to perilymph.

Conductive loss shows air-bone gaps with normal bone conduction; sensorineural loss shows air and bone thresholds within 10 dB.

Pure-tone average (PTA) typically averages thresholds at 500, 1000, and 2000 Hz.

Degree categories: normal (<=25 dB HL), mild (26-40), moderate (41-55), moderately severe (56-70), severe (71-90), profound (>90).

The speech banana spans roughly 250-8000 Hz at 30-60 dB HL; high-frequency loss preferentially degrades consonant perception.

Word recognition scores below 50% in quiet suggest poor amplification benefit and warrant counseling on expectations.

Otoscopy precedes testing to rule out cerumen impaction, perforation, or drainage that contraindicates ear impressions.

Tympanometry: Type A peak near 0 daPa, Type B flat (effusion or perforation), Type C negative peak (<-100 daPa, Eustachian dysfunction).

Acoustic reflexes are absent in severe-to-profound cochlear loss and elevated or absent in retrocochlear pathology.

Presbycusis most commonly presents as bilateral, symmetric, high-frequency sensorineural loss with reduced speech-in-noise ability.

ANSI/ASA S3.22 specifies measurement of saturation output (OSPL90), full-on gain, frequency response, and harmonic distortion.

OSPL90 is the maximum output with 90 dB SPL input; it must not exceed the patient's uncomfortable loudness level.

Gain is the difference between output and input SPL at a specified frequency and input level.

Peak gain typically occurs near 2500-3000 Hz for a standard behind-the-ear coupling; venting reduces low-frequency gain.

Harmonic distortion should be below 3% at 500 and 800 Hz and below 5% at 1600 Hz per S3.22.

Equivalent input noise (EIN) below 30 dB SPL is considered acceptable for hearing instruments.

Feedback occurs when loop gain exceeds unity; reduced by venting changes, occlusion, and phase cancellation or gain limiting.

Directional microphones improve the signal-to-noise ratio by roughly 3-5 dB in typical diffuse noise environments.

Compression knee point is the input level at which gain reduction begins; compression ratio = input change / output change above knee.

Prescriptive targets (NAL-NL2, DSL v5) derive gain from audiometric thresholds, coupling, and patient-specific factors.

Real-ear measurement compares the aided ear canal SPL to prescriptive targets; probe-tube placement is typically 5 mm from the eardrum.

Real-ear aided response (REAR), real-ear occluded response (REOR), and real-ear insertion gain (REIG) are key verification measures.

The speech intelligibility index (SII) ranges 0-1 and estimates audibility of the speech spectrum through the aided response.

Aided speech recognition should be compared with unaided performance; no improvement warrants troubleshooting or adjustment.

The 30-day trial period is common practice; follow-up visits typically occur at 1-2 weeks, 1 month, and 3 months.

Hearing aid fitting in children requires DSL v5 targets and verification with real-ear-to-coupler difference (RECD).

Validation measures include the COSI, APHAB, and IOI-HA, which quantify self-perceived benefit and satisfaction.

Earmold or dome selection must balance venting and occlusion; excessive occlusion produces the occlusion effect below 500 Hz.

The FDA regulates hearing aids as Class I medical devices; a medical evaluation is required before fitting adults unless the patient signs a waiver.

Patients under 18 years require a physician evaluation (preferably ENT) before hearing aid fitting.

Hearing aid dispensers must be licensed per state law; licensure requirements and scope of practice vary by state.

The ASHA or AAA Code of Ethics prohibits misrepresentation of credentials and requires informed consent and confidentiality.

HIPAA governs the privacy of protected health information; audiologic records are subject to its safeguards.

The FTC Hearing Aid Rule requires a user instructional brochure and a receipt with a 30-day return policy statement.

Binaural fitting is standard of care for bilateral loss unless contraindicated by asymmetry or medical factors.

Dispensing records must be retained per state and federal requirements; documentation supports continuity of care.

Conflict-of-interest disclosure is required when the dispenser benefits financially from a product recommendation.