Clinical Audiometer Product

Overview

A clinical audiometer measures hearing sensitivity. It presents tones of known frequency and precisely calibrated level to one ear at a time and records the quietest level at which the patient responds, the hearing threshold. Plotting thresholds across the octave frequencies from 125 Hz to 8 kHz gives the audiogram, the standard chart on which hearing loss is diagnosed and quantified. A diagnostic (Type 1) instrument under IEC 60645-1 has two independent channels, so a masking noise can be played to the opposite ear while the test ear hears the stimulus, and it supports both air-conduction and bone-conduction testing along with speech audiometry.

The instrument is built around the Signal Generator Module, which synthesizes the stimuli, and a set of calibrated transducers: the Audiometric Headphone Set for air conduction and the Bone Conduction Vibrator for bone conduction. The patient sits in a sound-treated booth holding the Patient Response Unit button; the audiologist drives the test from the Operator Console and can talk to, and hear, the patient through the Speech Audiometry Channel. Everything is coordinated by the Main Electronics, and the Calibration Kit keeps the output levels traceable.

How it works

Each channel begins at a DDS Tone Synthesizer, a direct-digital synthesis source that produces a pure tone at the selected audiometric frequency. Warble (frequency-modulated) and pulsed variants are generated digitally, and the Masking Noise Generator supplies narrow-band noise centred on the test frequency for the contralateral ear. The synthesized signal passes through the 24-bit Audio Codec, then the channel's Precision Step Attenuator sets the presentation level in 1 dB steps across a roughly 120 dB range — the defining precision component of the instrument, since the whole audiogram depends on level accuracy of about ±3 dB. An Transducer Output Amplifier matched to each transducer drives the selected output, with routing Relay contacts switching between phones, inserts, bone, and free-field outputs.

The operator presents each tone with the Stimulus Presenter Bar, a spring-return switch that gates the stimulus on only while held, then adjusts level with the channel Encoder dials following the Hughson-Westlake procedure: down 10 dB after each response, up 5 dB after each miss, until the level that yields responses on at least half of ascending presentations is stored as threshold via the Console Keypad. Each press of the Patient Response Button is debounced and timestamped by the Microcontroller against the stimulus window, which helps the audiologist spot false positives.

Air and bone conduction

Air-conduction testing uses the Audiometric Headphone Set. The Supra-Aural Earphone Driver is not an ordinary headphone: its output in dB SPL for a given dB HL setting is defined by reference equivalent threshold sound pressure levels (RETSPL) in ISO 389-1, measured in a standard coupler. The Audiometric Ear Cushion and the 4.5 N force of the Headphone Headband are part of that calibration condition, which is why worn cushions or a stretched band invalidate calibration.

Bone-conduction testing bypasses the outer and middle ear. The Electromagnetic Bone Transducer is an electromagnetic spring-armature vibrator pressed against the mastoid by the Bone Vibrator Headband at 5.4 N; it excites the skull directly so the cochlea is stimulated regardless of the conductive path. Comparing air and bone thresholds separates conductive loss (air worse than bone) from sensorineural loss (both depressed equally). Because the skull transmits vibration to both cochleae with little attenuation, bone testing almost always requires masking noise in the non-test ear from the second channel.

Speech audiometry and talkback

The Speech Audiometry Channel presents word lists either live through the Operator Gooseneck Microphone — levelled against a VU meter so speech peaks hit the calibrated reference — or from standardized recordings through the Recorded Speech Line Input. Speech recognition thresholds and word-recognition scores complement the pure-tone audiogram. The Talkback Microphone in the booth returns the patient's voice to the operator's monitor Speaker, since the booth is otherwise acoustically isolated.

Calibration

An audiometer is a measuring instrument and is calibrated annually. The Calibration Kit couples each earphone to a Reference Measurement Microphone through the IEC 60318 Acoustic Coupler, a cavity with standardized volume and acoustic impedance defined in IEC 60318. Output SPL is verified at every frequency and a range of levels, attenuator linearity is checked in 5 dB steps, and the bone vibrator is verified on an artificial mastoid per ISO 389-3. Frequency accuracy must hold within 1% and harmonic distortion below 2.5% at maximum output. Daily checks are simpler: a biological check on a person of known thresholds, and a listening check of the Wire Bundle leads and Connector joints, which are the most common failure points.