Which of the following is NOT a type of hearing protection device?

Noise-cancelling headphones are primarily designed to reduce ambient sounds and improve audio quality for the listener rather than to serve explicitly as hearing protection devices. Their main function is to create a quieter listening experience by using active noise control technology to cancel out background noise. While they do provide some level of passive sound isolation, they are not classified as hearing protection devices in the same way that earmuffs, earplugs, and custom-fitted devices are, which are specifically designed to reduce hazardous noise levels to protect hearing from potential damage.

Earmuffs and earplugs are well-established hearing protection devices that are actively used in various environments, such as industrial settings or during loud activities, to limit sound exposure. Custom-fitted devices are also created specifically based on an individual's ear shape and are tailored to provide effective protection against excessive sound levels.