Category	Definition	Regulatory Requirements	Approximate Price Range
Prescription (Medical-Grade) Hearing Aid	Requires professional fitting by a licensed audiologist or hearing instrument specialist (HIS)	Registered and approved by national medical device regulatory authorities; must be purchased through authorized professional channels	USD 300–3,000+ per device
Over-the-Counter (OTC) Hearing Aid	Intended for adults aged 18 and older with perceived mild-to-moderate hearing loss; does not require professional fitting	In the United States, the FDA established a separate OTC hearing aid category in 2022; regulations continue to evolve in other markets	USD 100–800 per device

Note: If you have completed the self-assessment in Article 3 and suspect hearing loss, we recommend scheduling a professional audiological evaluation before selecting or purchasing any hearing aid device.

┌──────────────┐ Electrical signal ┌─────────────────────┐ Amplified signal ┌──────────────┐
│ Microphone │ ──────────────────→ │ Digital Signal │ ─────────────────→ │ Receiver │
│ (Input sound)│ │ Processor (DSP) │ │(Output sound)│
└──────────────┘ └─────────────────────┘ └──────────────┘

DSP Function	Description
Multi-channel compression	Divides the audio signal into multiple frequency channels and independently adjusts the gain of each, preventing large sounds from being amplified excessively and causing discomfort
Noise reduction	Identifies and attenuates background noise (e.g., wind, traffic) while preserving speech signals
Feedback management	Eliminates the acoustic feedback whistle caused by sound re-entering the microphone
Directional processing	Automatically identifies the direction of the primary talker and applies adaptive beamforming
Scene/environment recognition	High-end models can classify listening environments (quiet, noisy, music) and automatically switch processing programs

[Figure 4-1: Hearing aid signal processing flow diagram — Microphone → DSP → Receiver acoustic chain]

3.1 Behind-the-Ear (BTE)

Physical design: The main housing is worn behind the auricle and connected to a custom earmold via a clear acoustic tubing. Sound is conducted through the earmold into the ear canal.

Applicable hearing loss range: Mild to profound (approximate gain range: 40–80+ dB SPL; covers WHO 2021 grades from mild through profound)

Indicated for:

• All degrees of hearing loss, particularly moderate-severe to profound
• Children (the ear canal changes with growth; only the earmold needs replacing, not the device)
• Elderly users and individuals with limited manual dexterity (large battery, easy to handle)
• Individuals with narrow ear canals or auricular anomalies

Advantages:

• Highest available output power; can compensate for profound hearing loss
• Larger housing accommodates larger controls and batteries for ease of use
• The main body is separated from the ear canal, reducing the impact of moisture on the device
• Easy to service; highly durable
• Rechargeable versions available

Disadvantages:

• Most visible of all styles
• Perspiration behind the ear may affect the device
• Acoustic tubing may dislodge during vigorous physical activity

3.2 Receiver-in-Canal (RIC / RITE)

Physical design: The main housing (containing the microphone and DSP chip) is worn behind the ear, while an ultra-thin wire carries the miniature receiver (loudspeaker) directly into the ear canal, coupled to a soft dome or custom earmold.

Applicable hearing loss range: Mild to severe (approximate gain range: 20–65 dB SPL; covers WHO 2021 grades from mild through severe)

Indicated for:

• Mild-to-moderate through severe hearing loss, with preference for a discreet appearance
• High-frequency sloping (ski-slope) hearing loss — the open-fitting dome minimizes the low-frequency occlusion effect
• Users seeking high sound quality and advanced noise reduction features

Advantages:

• Small and discreet (main body is a thin strip behind the ear)
• Receiver placement close to the tympanic membrane produces natural, low-distortion sound
• Open-dome fitting minimizes the occlusion effect; comfortable to wear
• The thin wire is nearly invisible
• Most premium flagship models adopt this style, offering the richest feature sets

Disadvantages:

• The receiver sits inside the ear canal and is susceptible to cerumen (earwax) and moisture; regular cleaning is required
• The thin wire is relatively fragile and may break under excessive pulling force
• Output power is slightly lower than BTE; may not be suitable for profound hearing loss

3.3 In-the-Ear (ITE)

Physical design: The entire device is housed within the concha (the bowl-shaped cavity at the entrance of the external auditory canal). The shell is custom-fabricated from an individual ear impression to conform precisely to the contours of the outer ear.

Applicable hearing loss range: Mild to severe (approximate gain range: 30–55 dB SPL; covers WHO 2021 grades from mild through severe)

Indicated for:

• Users with reduced manual dexterity (larger body size, larger controls)
• Users requiring extended battery life (accommodates Size 312 or Size 13 batteries)
• Individuals who cannot use BTE (e.g., unusual ear canal anatomy) but require higher power than a RIC
• Users who prioritize ease of operation over maximum discreteness

Advantages:

• Larger than RIC/CIC, with bigger buttons and volume controls for ease of use in elderly patients
• Larger battery capacity provides longer operating time
• Can accommodate dual directional microphones (ITC typically only supports a single microphone)
• Stable retention; less likely to dislodge during movement

Disadvantages:

• Moderate discreteness; the full-shell device is visible in the ear
• Custom fabrication requires an ear impression; manufacturing takes approximately 1–2 weeks
• Requires sufficient concha volume; not suitable for all ear shapes

3.4 In-the-Canal (ITC) / Completely-in-Canal (CIC)

Both ITC and CIC are custom in-canal devices. The distinction lies in physical size and degree of concealment:

CIC-specific limitations: Because the device sits deep within the ear canal, output power is the lowest of all custom styles. Not suitable for severe or profound hearing loss. Requires adequate ear canal dimensions. Some users experience a pronounced occlusion effect (own voice sounds hollow or reverberant). Most CIC devices do not support Bluetooth wireless connectivity.

3.5 Invisible-in-Canal (IIC)

Physical design: The most discreet hearing aid style currently available. The entire device is positioned at or beyond the second bend of the ear canal (second bend / bony canal), making it completely invisible from the outside.

Applicable hearing loss range: Mild to moderate (approximate gain range: 15–35 dB SPL; covers WHO 2021 grades of mild and part of moderate)

Indicated for:

• Mild to moderate hearing loss with very high discreteness requirements
• Socially active individuals who are sensitive to others noticing a hearing aid
• Individuals with favorable ear canal anatomy (sufficiently straight and wide canal)

Advantages:

• Maximum concealment — completely invisible to observers
• Deep placement yields a short sound transmission path, resulting in highly natural sound quality
• Minimal wind noise interference
• No conflict with face masks or eyeglass frames

Disadvantages:

• Lowest available output power — not suitable for moderate-severe or greater hearing loss
• Strict anatomical requirements; not universally compatible with all ear canal shapes
• Must be inserted and removed daily (continuous overnight wearing is generally not recommended)
• Very small vent; noticeable occlusion effect
• Extremely small battery (Size 10A); approximately 40-hour runtime requires frequent replacement
• The vast majority of IIC models do not support Bluetooth wireless connectivity

3.6 Side-by-Side Comparison of All Six Styles

Note: Power ranges above are reference values; actual products vary by manufacturer and model. Degree-of-loss classifications follow the WHO 2021 grading standard: mild 21–35 / moderate 36–50 / moderate-severe 51–65 / severe 66–80 / profound ≥81 dB HL.

Specification	Meaning	Reference Values (Quality Products)	Selection Guidance
Saturation Sound Pressure Level (OSPL90)	The maximum output sound pressure level when the input is 90 dB SPL; reflects maximum output capability	BTE: 120–140 dB SPL; ITE/ITC: 105–130 dB SPL	Must be selected by the audiologist based on degree of hearing loss; excessive output risks additional hearing damage
Full-On Gain (FOG / HF-FOG)	The amplification in dB for a 60 dB SPL input signal with the volume control at maximum; typically averaged at 1000/1600/2500 Hz	Mild-loss devices: 20–35 dB; severe-loss devices: 50–80 dB	Insufficient gain = poor speech clarity; excessive gain may damage residual hearing
Equivalent Input Noise (EIN)	The internal self-noise of the hearing aid expressed as an equivalent input sound pressure level; lower is better	High-quality devices: ≤26 dB SPL	Elevated EIN produces an audible background noise floor in quiet environments
Total Harmonic Distortion (THD)	The percentage of harmonic distortion components in the output signal; lower is better	High-quality devices: <3%; standards require ≤10% at 500/800 Hz, ≤20% at 1600 Hz	Excessive distortion causes a "muddy" or "clipped" sound quality
Frequency Response Range	The frequency bandwidth over which the device provides effective amplification	Standard: 200–5000 Hz; broadband: 100–8000 Hz	A wider frequency response better reproduces high-frequency consonants (e.g., /s/, /ʃ/, /θ/), improving speech intelligibility
Directivity Index (DI)	A measure of how effectively the directional microphone attenuates noise from the sides and rear in noisy environments; higher is better	High-end devices: 6–10 dB	Users who frequently operate in noisy environments should prioritize models with strong directionality

[Figure 4-3: Technical parameter chart — frequency response curve and gain curve comparison]

Dimension	Analog Hearing Aid	Digital