Background: Hollow earmolds have become a popular type of earmold used in thin-tube, microsize
hearing aid fittings. It is desirable for clinicians to be familiar with their characteristics
and limitations.
Purpose: This investigation compared the effects of vent diameter between a traditional solid
earmold and a hollow earmold that is used in modern thin-tube hearing aid fittings.
Research Design: A single-blind, 2 × 4 factorial design was used.
Study Sample: Eight adults with a high-frequency hearing loss participated.
Intervention: Custom earmolds for use with thin-tube hearing aids were made for each participant.
Two types of earmolds were made: a solid earmold with a traditional vent length and
a hollow earmold where the thickness of the shell was the length of the vent. Vent
diameters were 0, 1, 2, and 3 mm.
Data Collection and Analysis: The vent effect was evaluated on real-ear aided response, real-ear occluded response
during vocalization, subjective occlusion rating, insertion loss, and maximum available
gain before feedback. Real-ear measurements were made with the Fonix 6500 probe-microphone
real-ear system. Vocalizations from the participants were analyzed with a custom MATLAB
program, and statistical analysis was conducted with SPSS software.
Results: A systematic vent effect was seen with each earmold type as the nominal vent diameter
changed. For the same vent diameter, the vent effect seen with the hollow earmold
was greater than that of the solid earmold.
Conclusions: Because of the difference in vent length (and thus acoustic mass) between a solid
and a hollow earmold, the effect of vent diameter in a hollow earmold is more pronounced
than that seen in a solid earmold of the same nominal vent diameter. Thus, a smaller
vent diameter will be needed in a hollow earmold than in a solid earmold to achieve
similar vent effects.
Key Words
Acoustic mass - earmold - occlusion effect - thin-tube fitting - vent
