Audiology Fundamentals15 min read

Complete Hearing Aid Fitting Guide

Five device types, core DSP features, and a six-step fitting process

SEER SONUS ResearchFebruary 2025

Figure · Size vs. Visibility

Devices trade off size and power against discretion: BTE is largest and most capable, shrinking toward the canal until nearly invisible. The market leader is receiver-in-canal RIC (62%), followed by behind-the-ear BTE (20%).

Hearing Aid Type Comparison

Type	Suitable loss	Pros	Limitations	Share
BTE (behind-the-ear)	Mild–profound	High power, most features, durable	More visible, prone to wind noise	20%
RIC (receiver-in-canal)	Mild–severe	Discreet, great sound quality, supports open fitting	Receiver susceptible to moisture	62%
ITE (in-the-ear)	Mild–severe	Full features, custom-molded	More visible, shell needs remaking	8%
CIC (completely-in-canal)	Mild–moderate	Nearly invisible	No Bluetooth, small battery	6%
IIC (invisible-in-canal)	Mild–moderate	Completely invisible	Fewest features, needs sufficient canal space	4%

Core Digital Signal Processing (DSP) Functions

The core value of modern digital hearing aids rests on four DSP modules. These algorithms analyze and process sound in milliseconds:

WDRC (wide dynamic range compression)

Compresses an 80–100 dB environmental dynamic range into the 20–40 dB usable by residual hearing. Modern hearing aids use independent multi-band compression (4–20 bands), applying different ratios per frequency to precisely match each individual's hearing-loss profile.

Directional microphone system

Using the time-delay difference across a dual- or multi-microphone array (2–4 mics), it forms a directional beam that enhances speech from the front and suppresses noise from the sides and rear. Adaptive directionality tracks a moving primary speech source.

Digital noise reduction (DNR)

Analyzes the modulation rate in each band: speech carries strong 4–16 Hz modulation, while steady-state noise (air conditioning, traffic) has near-zero modulation. Based on this difference, it selectively attenuates noise bands by 8–12 dB while preserving speech bands.

Adaptive feedback cancellation (AFC)

Continuously monitors the hearing-aid output and uses an adaptive filter to detect feedback-loop signatures, emitting an inverted signal to cancel feedback before it builds. Modern systems respond within 10 ms, providing an extra 12–25 dB of stable gain headroom.

The Fitting Process

Professional hearing-aid fitting is an iterative clinical care process, far more than a product sale. These six steps make up the complete fitting cycle:

Complete hearing assessment

Pure-tone audiometry (air + bone), speech recognition testing (SRT + WRS), tympanometry, and otoscopy. Roughly 45–60 minutes.

Needs and expectations analysis

COSI/APHAB questionnaires assess specific communication challenges. We learn about lifestyle, occupational needs, manual dexterity, cosmetic preferences, and budget.

Model selection and ear impression

Models are chosen by loss type, severity, and canal shape. RIC/BTE can be auditioned on the spot; custom devices (ITE/CIC) require a silicone ear impression sent to the lab.

Initial fitting and REM verification

Load the NAL-NL2/DSL v5 prescription and verify gain targets with real ear measurement. Set initial volume to 70–80% of prescription (an acclimatization strategy).

Acclimatization follow-up (2–4 weeks)

Daily wear time is increased gradually (4 hrs in week 1 → all day by week 4). Regular follow-ups fine-tune high-frequency gain, compression ratio, and directional sensitivity.

Ongoing care and reassessment

Cleaning and maintenance every 3–6 months. Annual hearing reassessment (loss may progress). Hearing aids last 4–6 years, after which replacement is evaluated.

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