Some electronic ear muffs use their battery power only for the purpose of supplying a radio for entertainment, usually AM/FM. Some go a step further and also allow for the plug in of other audio devices through an auxillary port and/or a 3.5mm jack. Usually the volume level of the input will not exceed 82dB. In many cases, the function of noise attenuation on this type of muff is Passive, meaning the noise is reduced only from the structure of the muff itself over the ear.
An Active muff, however, is smarter. It's always working, and it's usually doing two jobs simultaneously. First, because you are wearing the muffs, general sounds and conversation can be difficult to hear, and necessary communication can therefore be challenging. An active ear muff will amplify ambient sounds at the conversation level for easier hearing. Often the level of help it provides is adjustable. At the same time, however, it is also listening for louder sounds to occur. When the muffs detect a sudden noise considered potentially harmful (usually 82dB or higher), it does its best to instantly block that sound from passing through to the ears. This blocking occurs in milliseconds, fast enough to block harmful soundwaves from reaching our eardrums.