Etymotic HD-15 electronic earplugs only block the loud noises

Etymotic's HD-15 High-Definition electronic earplugs let users hear normally when things are quiet, but kick in when sudden loud noises occur
If you work someplace where sudden loud noises frequently but intermittently occur, it can get kind of frustrating - you pretty much have to choose between protecting your hearing with ear plugs, or being able to hear what people are saying when it isn't noisy. Your basic earplugs, unfortunately, don't let you hear when things are quiet, but then activate when loud noises occur. Non-basic earplugs, however, do that very thing. Etymotic Research's HD-15 High-Definition electronic earplugs contain tiny microphones, that instantaneously cause the plugs to block incoming sound waves when they detect noises over a given threshold. When the noise has ceased, the earplugs let the user hear normally again.
Not only do the HD-15s not impede hearing when it isn't noisy, but with the flick of a built-in switch, they can also amplify ambient sound by 15 decibels. In either setting, they will still block sudden, loud impact sounds when they occur. They have a noise reduction rating of 25 decibels.

These earplugs aren't the first hearing protection devices to use such technology. The QUIETPRO+ Intelligent Hearing System also uses an integrated microphone to detect loud noises, at which point its noise-reduction function kicks in. It additionally features a conductive microphone that is located inside the user's ear canal, so they can talk to other people by radio without unfiltered background noise drowning out their voice.
Sensear utilizes a similar system, in which directional microphones enhance speech, but block other sounds. It appears that one advantage that the HD-15s have over QUIETPRO+ and Sensear is that everything is contained with the plugs themselves, with no additional hardware required.
The HD-15 High-Definition electronic earplugs are available for US$499 via the Etymotic website. The company also makes versions optimized for people shooting guns, and for military applications.