4.2 Hearing Aid Styles, Architecture & Transducers

Hearing Aid Form Factors

Hearing aids come in a variety of styles, broadly categorized into custom (in-the-ear) and standard (behind-the-ear) models. The choice depends on audiometric needs, physical limitations, and cosmetic preferences.

Behind-The-Ear (BTE)

BTE devices house all components in a casing that sits behind the ear. Sound is directed into the ear via a tube and a custom earmold. BTEs are highly durable, offer the most amplification, and are the standard for pediatric fittings (as earmolds can be easily replaced as the child grows).

Receiver-In-Canal (RIC / RITE)

RIC or Receiver-In-The-Ear models are currently the most popular style. The microphone and processor sit behind the ear, but the receiver (speaker) is placed directly in the ear canal, connected by a thin wire. This allows for a smaller case behind the ear, smoother frequency response, and an "open fit" that prevents the occlusion effect.

Custom Styles (ITE, ITC, CIC, IIC)

Custom devices are molded to fit the patient's exact ear anatomy:

• In-The-Ear (ITE): Fills the entire concha bowl. Easy to handle, holds larger batteries, and can fit directional microphones and telecoils.
• In-The-Canal (ITC): Fills the lower half of the concha bowl. Slightly less visible but still large enough for directional mics.
• Completely-In-Canal (CIC): Fits deeply in the canal, mostly hidden. Requires good dexterity and often relies on a removal string.
• Invisible-In-Canal (IIC): Placed past the second bend of the ear canal. Virtually invisible but limited in power and lacking directional microphones due to size constraints.

Receiver Power Classifications

Because a single RIC shell can be paired with interchangeable receivers, manufacturers grade RIC receivers by maximum power output (MPO) rather than by shell size alone. The three common tiers are:

Selecting the correct receiver power is a candidacy decision in its own right: fitting a P receiver on a patient with a severe loss leaves the device unable to reach prescriptive targets, while an oversized UP receiver on a mild loss wastes battery life and increases feedback risk.

Dome and Coupling Selection

The dome (or custom earmold) is the soft coupling piece at the end of the receiver wire that seats in the ear canal, and its selection directly affects both comfort and achievable gain:

• Open dome: Vented, allowing natural low-frequency sound and air to pass around it. Reserved for mild-to-moderate losses with relatively normal low-frequency hearing, since venting limits how much low-frequency gain the device can deliver without feedback.
• Closed dome: A sealed but flexible fit that reduces venting, boosting low-frequency gain and reducing feedback compared to an open dome. Appropriate for moderate to moderately severe losses.
• Power dome: A fully occluding, rigid seal used with high-power receivers for moderately severe to profound losses, maximizing usable gain and minimizing feedback at the cost of increased occlusion effect.

Internal Architecture & Transducers

The internal components of a hearing aid work together to capture, convert, process, and deliver sound.

Microphones

The microphone is the input transducer that converts acoustic energy (sound waves) into electrical energy. Modern hearing aids use MEMS (Micro-Electro-Mechanical Systems) microphones for durability and consistency. Most hearing aids utilize dual microphones to create directional microphone systems, which improve the signal-to-noise ratio by reducing amplification of sounds coming from behind the listener.

Signal Processing (ADC and DSP)

The electrical signal from the microphone is continuous (analog) and must be converted into digital data (0s and 1s) by the Analog-to-Digital Converter (ADC). Once digitized, the signal enters the Digital Signal Processor (DSP). The DSP is the "brain" of the hearing aid, executing complex algorithms for amplification, compression, noise reduction, and feedback cancellation. After processing, the signal is converted back to analog form by a Digital-to-Analog Converter (DAC) before reaching the receiver.

The Receiver

In hearing aid terminology, the receiver is actually the output transducer (the speaker). It converts the processed electrical signal back into acoustic energy and delivers it to the ear canal.

Battery Power: Disposable vs. Rechargeable

Hearing aids are powered either by disposable zinc-air batteries or by sealed rechargeable lithium-ion cells. Disposable zinc-air batteries (common sizes 10, 312, 13, and 675) are activated by removing a tab that allows air to enter the cell; they typically last 3-14 days depending on size and streaming use, and their low upfront cost and simple replacement make them attractive to patients who want an easy spare-battery backup. Rechargeable lithium-ion batteries have become the dominant choice in new fittings: a full overnight charge typically provides 18-24 hours of use, and the sealed cell is rated for several years of nightly charging before it needs professional replacement. Rechargeable devices particularly benefit patients with dexterity or visual limitations who struggle to handle small disposable cells.

Special Fitting Applications: CROS and BiCROS

For patients with single-sided deafness (SSD) - no usable hearing in one ear alongside normal or near-normal hearing in the other - a conventional hearing aid on the poorer ear provides little benefit because the cochlea on that side cannot process the amplified signal. Two specialized systems address this:

• CROS (Contralateral Routing of Signals): A microphone worn on the non-hearing ear picks up sound from that side, typically transmitting it wirelessly, and routes it to a receiver worn in or on the normal-hearing ear. This eliminates the head shadow effect for sounds arriving from the deaf side, since the better ear requires no additional amplification of its own.
• BiCROS (Bilateral CROS): Functions the same way as CROS but is used when the "better" ear also has some hearing loss requiring amplification; the routed signal from the poorer ear is combined with amplification of the better ear in a single device.

Connectivity and Telecoils

Telecoils (T-Coils)

A telecoil is a small copper wire coiled around a rod. It functions as an alternative input transducer that picks up electromagnetic signals instead of acoustic signals. This allows the hearing aid to connect directly to telephones or induction loop systems found in theaters, churches, and public venues, providing a clear signal free from background noise.

Wireless Connectivity

Modern hearing aids frequently feature wireless capabilities using Bluetooth or 2.4 GHz technology. This allows for direct streaming of audio from smartphones, televisions, and remote microphones. It also enables binaural synchronization, where the two hearing aids communicate with each other to adjust settings automatically based on the environment.