How Do Vents Affect Hearing Aid Performance?

with an occluding earmold; data above “0” suggest a gain increase (from resonance) while that below “0” suggest gain reduction with the specific vent dimension (length and diameter). The solid line shows the result of a 6 mmlong vent, while the dotted line shows that of a 22 mm-long vent. For both vent lengths, one sees more low-frequency gain reduction as the vent diameter increases. For example, one sees that the output at 200 Hz is reduced by 7-8 dB with a 1 mm vent diameter, but as much as 28 dB reduction with a 3 mm vent diameter. Thus, an increase in vent diameter leads to a reduction in low frequency output below 1000 Hz. A vent is a tube. As such, it is subject to tubing resonance. Figure 2 also shows that a change in vent diameter leads to a shift in the ventassociated resoW e typically consider sounds at the eardrum to be a function of the output of the hearing aid moderated by the residual volume between the tip of the hearing aid/earmold and the eardrum. To a large extent, this is true for an occluding hearing aid (one without any vents or leakage) and when the wearer listens to sounds from their environments. On the other hand, with a vented hearing aid and when the wearer talks, the overall sound pressure level at the eardrum also includes direct sounds that enter (or leave) through the vents (and any unintentional leakage) and bone-conducted sounds generated from the wearer’s voice. The contribution of each source varies depending on the state of the wearer (speaking versus listening) and the size of the leakage (or venting), in addition to the gain settings on the hearing aid. Figure 1 shows a simplified diagram of the three sources of sound. In the extreme case of someone with a high frequency hearing loss who is speaking while wearing a closed earmold, the low frequency SPL at the eardrum is dominated by the bone-conducted sounds. In an open-fitting situation, sounds entering directly through the vent opening will have a larger contribution to the SPL at the eardrum.