jaw to mimic singing in order to account for ear canal changes during performance. Therefore, if the impression is not properly constructed, then it will need to be redone. These changes can be felt by feeling with a finger just at the entrance to the ear canal while moving the jaws sideways, up and down or anterior and posterior. Most molded earplugs are made from silicone but other materials may be used, including thermoplastics, plastic, nylon and even 3-D printed earplugs. Electronic earplugs The noise reduction of passive earplugs varies with frequency but is largely independent of level (soft noises are reduced as much as loud noises). As a result, while loud noises are reduced in level, protecting hearing, it can be difficult to hear low level noises. Active electronic earplugs exist, where loud noises are reduced more than soft noises, and soft sounds may even be amplified, providing dynamic range compression. This is done by having a standard passive earplug, together with a microphone/speaker pair (microphone on outside, speaker on inside; formally a pair of transducers), so sound can be transmitted without being attenuated by the earplug. When external sounds exceed an established threshold (typically 82 dBA SPL), the amplification of the electronic circuit is reduced. At very high levels, the amplification is turned off automatically and you receive the full attenuation of the earplug just as if it were turned off and seated in the ear canal. This protects hearing, but allows one to hear normally when sounds are in safe ranges – for example, have a normal conversation in a low-noise situation, but be protected from sudden loud noises, for example at a construction site or a while hunting. Nonlinear earplugs Nonlinear earplugs provide similar advantages to electronic earplugs but do not require electricity. They are designed with a thin diaphragm which allows the amount of noise reduction to increase in proportion to the sound level to which the wearer is exposed. This makes them useful for applications where situational awareness is required but noise protection is also necessary, such as the military or police. Sleep Earplugs for sleeping are made to be as comfortable as possible while blocking external sounds that may prevent or disrupt sleep. Specialized earplugs for such noises as a partner's snoring may have sound-dampening enhancements that enable the user to still hear other noises, such as an alarm clock. To determine the