HIS Hearing Instrument Selection & Fitting 2 — Questions and Answers

Question 1: What is the primary purpose of a real-ear measurement (REM) during hearing aid fitting?

To measure the patient's pain threshold
To verify the hearing aid output matches the prescriptive target in the patient's ear canal (Correct answer)
To determine if the patient needs surgery
To calibrate the audiometer

Correct answer: To verify the hearing aid output matches the prescriptive target in the patient's ear canal

Real-ear measurement (REM) verifies that the hearing aid's acoustic output, measured in the patient's actual ear canal, matches the prescriptive gain target (e.g., NAL-NL2 or DSL v5), accounting for individual ear canal acoustics.

Real-ear measurement is the gold standard for hearing aid verification. A probe microphone tube is placed near the eardrum while the hearing aid is worn. The measured output is compared to prescriptive targets such as NAL-NL2 or DSL v5. Without REM, fittings rely only on coupler measurements, which do not account for individual ear canal volume, length, or resonance—meaning up to 20 dB of fitting error is possible.

Question 2: Which hearing aid style is most appropriate for a patient with severe-to-profound hearing loss who also has significant dexterity limitations?

Completely-in-canal (CIC)
Invisible-in-canal (IIC)
Behind-the-ear (BTE) with earmold (Correct answer)
Receiver-in-canal (RIC)

Correct answer: Behind-the-ear (BTE) with earmold

BTE hearing aids with earmolds are easier to handle due to their larger size, and can accommodate the high-power amplification needed for severe-to-profound hearing loss.

BTE hearing aids sit behind the ear and connect to a custom earmold via tubing. For severe-to-profound loss, BTE devices can accommodate large receivers and batteries to deliver high gain without feedback. Their size makes them more manageable for patients with reduced finger dexterity—CIC and IIC styles are tiny and difficult to handle, and RIC styles may not provide sufficient power for very severe losses without custom power receivers.

Question 3: The NAL-NL2 prescriptive formula is designed to maximize which outcome?

Loudness comfort only
Speech intelligibility while maintaining acceptable loudness (Correct answer)
Battery life
Cosmetic appeal of the device

Correct answer: Speech intelligibility while maintaining acceptable loudness

NAL-NL2 (National Acoustic Laboratories Non-Linear 2) prescribes gain to maximize speech intelligibility while keeping overall loudness at an acceptable level, accounting for loudness normalization.

The NAL-NL2 prescription was developed in Australia and is one of the two most widely used prescriptive targets (the other being DSL v5). It calculates frequency-specific gain targets to maximize the audibility of speech across all frequencies while ensuring amplified sound is not uncomfortable. It uses patient factors such as audiogram, age, and experience level to fine-tune targets.

Question 4: What is occlusion effect in hearing aid fitting, and when is it most problematic?

Feedback caused by an open fitting
The boomy, hollow sound of the patient's own voice when the ear canal is blocked (Correct answer)
Excessive high-frequency amplification
A fitting target calculation error

Correct answer: The boomy, hollow sound of the patient's own voice when the ear canal is blocked

Occlusion effect is the perception of one's own voice sounding hollow or boomy when the ear canal is occluded by an earmold or hearing aid shell, trapping low-frequency bone-conducted sound energy.

When the ear canal is occluded, bone-conducted sound from the patient's own voice (especially low frequencies below 1000 Hz) cannot escape and builds up in the canal. It is most problematic for patients with good low-frequency hearing who use traditional, fully occluding earmolds. Solutions include venting the earmold, using an open-fit dome, or reducing low-frequency gain.

Question 5: When selecting a hearing aid for a patient with single-sided deafness (SSD), which device type routes sound from the deaf side to the hearing ear?

CROS or BiCROS hearing aid (Correct answer)
Binaural BTE fitting
IIC in the deaf ear
Bone-anchored hearing device implanted in both ears

Correct answer: CROS or BiCROS hearing aid

CROS (Contralateral Routing of Signals) transmits sound picked up by a microphone on the deaf side wirelessly to a hearing aid on the hearing ear, restoring awareness of sounds from the deaf side.

In CROS amplification, a microphone unit worn on the non-functioning ear picks up sound and wirelessly transmits it to a receiver worn on the better ear. If the better ear also has hearing loss, a BiCROS system is used, which amplifies sound for the better ear in addition to routing signals from the deaf side. These systems cannot restore true binaural hearing but improve awareness of sounds from the non-functioning side.

Question 6: What is the purpose of a vent in a custom earmold?

To increase low-frequency amplification
To prevent moisture from entering the hearing aid
To reduce occlusion effect and allow low-frequency sound to escape (Correct answer)
To increase feedback suppression

Correct answer: To reduce occlusion effect and allow low-frequency sound to escape

Vents are channels built into earmolds that allow low-frequency sound to pass in and out of the ear canal, reducing the occlusion effect and providing a more natural sound quality.

A vent is a small bore (channel) running through or alongside the earmold. It allows low-frequency bone-conducted sound from the patient's own voice to escape the canal, reducing occlusion. Vents also let low-frequency environmental sound bypass the hearing aid for patients with normal or near-normal low-frequency hearing. However, large vents can increase acoustic feedback risk. The vent diameter must be balanced against gain requirements.

HIS Hearing Instrument Selection & Fitting 2

What is the primary purpose of a real-ear measurement (REM) during hearing aid fitting?