Limitations on insertion gains with vented earmoulds imposed by oscillatory feedback

Area of Science:

• Audiology
• Acoustics
• Hearing Aid Technology

Background:

• Earmould venting is common for hearing aid users to reduce blockage sensations or alter frequency response.
• Current prescription of vented earmoulds often relies on empirical methods (rule-of-thumb or trial-and-error) due to limited systematic study of feedback gain.
• Understanding feedback limits in vented hearing aid systems is crucial for optimizing performance and user satisfaction.

Purpose of the Study:

• To systematically investigate the effects of earmould venting parameters on hearing aid feedback oscillation.
• To quantify the impact of vent size, microphone orientation, and acoustic stimuli on the maximum stable gain.
• To provide data-driven insights for more precise prescription of vented earmoulds.

Main Methods:

• Laboratory study using a KEMAR mannikin simulating hearing aid use.
• Testing hearing aids with gains up to 64 dB.
• Varying parameters: parallel vent sizes (0.8 mm and 2.0 mm), microphone orientation (forward-facing and downward-facing), and acoustic stimuli (speech-shaped noise, wide-band noise, narrow bands of noise).

Main Results:

• A forward-facing microphone orientation resulted in less susceptibility to oscillatory feedback compared to a downward-facing one.
• A 2.0 mm vent imposed greater feedback limitations than a 0.8 mm vent, indicating the smaller vent also has acoustic effects beyond simple pressure equalization.
• The spectral characteristics of ambient acoustics significantly reduced the available gain before feedback onset, decreasing it from over 40 dB to less than 20 dB in some conditions.

Conclusions:

• Hearing aid feedback gain is demonstrably influenced by earmould vent characteristics and microphone placement.
• The acoustic environment plays a critical role in determining feedback limits, suggesting that standard gain prescriptions may be insufficient without considering ambient sound.
• Systematic study reveals that vented earmould prescription requires a more nuanced approach than current rule-of-thumb methods, considering vent size, microphone orientation, and acoustic conditions.