Comparison of vent effects between a solid earmold and a hollow earmold.

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 x 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.