Engineers at the Johns Hopkins Applied Physics Lab have developed a low-cost earplug to help protect military personnel from damage to their hearing caused by explosions. The plugs allow wearers to hear at the usual volume until blast waves hit them, causing a small ball to move into place and protect the eardrum. "The idea was, we needed something that in the event of the unforeseen -- an explosion, an IED that goes off near them -- they needed to have something in place to protect them," said engineer Dan Simon, who came up with the concept.

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