Hearing Instrument Technology and Electroacoustics
Signal processing
Modern instruments digitize sound, apply gain via compression, and output through a receiver. Key parameters: gain (output minus input), maximum output (SSPL90), frequency response, compression threshold, compression ratio, attack and release times, and expansion for low-level noise. Wide dynamic range compression (WDRC) is standard; slower compression preserves temporal cues, faster compression suits fluctuating environments.
Feedback and coupling
Feedback occurs when amplified sound re-enters the microphone. Manage via venting choices, feedback cancellation algorithms, and appropriate dome/coupling. Occlusion effect is worst for low frequencies with tight seals.
Electroacoustics
ANSI/ASA S3.22 specifies measurement of gain, frequency range, harmonic distortion, equivalent input noise, and battery current. Coupler measurements (2 cc) approximate but do not equal real-ear performance. Directional microphones improve SNR in noise; telecoils access loop systems.
Traps
Questions confuse compression ratio with gain, or attack with release. Another trap: assuming coupler data equal real-ear data. Also watch for confusing SSPL90 (maximum output) with full-on gain.
Rule of thumb
Compression ratio = input change ÷ output change above threshold. A 2:1 ratio means a 10 dB input increase yields a 5 dB output increase.