A number of years ago, Acentech, along with the late Prof. Richard H. Lyon of M.I.T., partnered with the startup technology company EAROS to explore and evaluate initial concept designs for an “acoustic filter” for use within a fashionable hearing protection device, as a better alternative to an earplug in certain environments. The core audience for this device includes clubgoers, music lovers, fitness class participants, nightlife professionals, and entertainers. Specifically, those who wish to protect their listening experiences and future hearing using a convenient, cost-effective wearable device, but who did not want to suffer the loss of fidelity associated with the use of conventional earplugs. (Most earplugs provide uneven attenuation across frequencies, e.g., sounds at higher frequencies are reduced much more than at lower frequencies – aside from “muffling” and “muddying” the overall sound of music, such attenuation can also interfere with speech communication.)

Acentech’s Product Sound Group started this project by developing a mathematical model of the ear canal with a hearing protection device inserted. Factors affecting sounds heard with the device in place are governed in large part by a combination of the following:

1. Sound transmission through the “acoustically weakest” part of the device, such as a thin screen, diaphragm or perforated septum making up the mechanical/acoustic filter within the device.

2. Air volume between the filter and the eardrum (controlled by exactly where the filter is located).

3. The acoustical impedance of the filter and its supporting sealing structure, and the impedance of the ear canal and eardrum.

In order to estimate how a device with different configurations would affect sound reaching the eardrum, a model was created in which these and other pertinent elements, and the way they interact together could be included in the prediction of acoustic behavior. An “equivalent circuit” type of model was used so that, for a given set of physical parameter values, the sound attenuation provided by the device could be readily calculated at various frequencies over the audio range (expressed as the ratio of estimated sound pressure inside the ear canal to sound pressure at the outside of the device, as a function of frequency). This was the basic output of the model, which enabled us to assess and compare the expected performance of different designs.

Using the model, we undertook a methodical study of how various design parameters affected the expected overall performance of the device. Using the acoustical model, we were able to choose combinations of parameter values that optimized performance in some sense, while at the same time constraining the values to be within feasible limits. In general, these optimizations sought to minimize the tendency for attenuation to increase with increasing frequency, while at the same time providing for sufficient attenuation with minimal variation within certain critical frequency ranges.

Today, the EAROS ONE product is the evolution of this initial study and provides a way for people to fashionably protect their hearing in loud entertainment and hospitality venues while still preserving the fidelity of the sound they want to experience in these environments.

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