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Passive vs. active
There are two different types of earmuffs used to protect the user from loud sounds based on the acoustical properties and materials used to create them: passively attenuating and actively attenuating earmuffs.
The ability of a passive earmuff to attenuate a signal is based on the materials used. Materials, such as a cupped foam coated in hard plastic, will block sound due to the thick and dampening properties of the foam.[1]
Some passive earmuffs have an electronic component and microphones that allow the user to control their access to communication while attenuating background noise.[2] When in loud, hazardous settings, the wearer may still be required to listen to outside sources, such as machinery work, their supervisor's commands, or talk to their colleagues. While the material and design of the muff allows for a reasonable attenuation (roughly 22 dB[3] NRR), the user has the option to allow some sounds in that are necessary for their job. These earmuffs incorporate a volume control to increase and decrease the attenuation.
Active noise reduction earmuffs incorporate Electronic Noise Cancellation or Active Noise Cancellation to attenuate (roughly 26 dB NRR[1]) low frequency noise.[4] A microphone, circuit, and speaker inside the muff are used to actively cancel out noise. As a signal enters the microphone, the electronics within the earmuff cast a signal back that is 180° out of phase with the signal, thus "cancelling" this signal.[5] This opposing signal reduces the amplitude of the waveform and reduces the signal. These earmuffs are designed to protect against a continuous signal, particularly low frequency sounds, such as diesel locomotives, heavy tractors, or airfields.[4]
Fit Risks
Earmuffs are comprised of a headband connecting two cups designed to fit over the entire outer ear. The purpose of protective earmuffs is to protect the user from dangerously loud sounds, thus preventing noise-induced hearing loss. While earmuffs create a simple, subjectively comfortable approach to protection against dangerously loud settings, they do not always provide an adequate fit. Some wearers may use their earmuffs when hair is covering their ears or while wearing a thick set of glasses. Simply placing the earmuffs over obstructing hair or glasses may reduce the earmuff attenuation by 10-15 dB[6], rendering the earmuff incapable of appropriately protecting the user. Additionally, old and flattened foam may create an opening to the ear, allowing sound through and reducing attenuation. A headband that is incorrectly adjusted, has reduced tension and cannot firmly cup the ear, reducing the attenuation of the muff. An inappropriately sized ear muff placed on a child reduces the attenuation of the ear muff as well, as it cannot firmly seal around the ear. [7] Alternatively, while there may be an appropriate fit to the earmuffs, certain environmental circumstances may alter the attenuation. Earmuffs worn while firing weapons may lift off the ear, as the blast from the firearm is very powerful. When shooting, as the blast affects the efficacy of the earmuff, the user should consider pairing insert earplugs with the earmuffs as additional protection.[8]
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- ^ a b "Ear Muffs: A Field Guide -- Occupational Health & Safety". Occupational Health & Safety. Retrieved 2016-10-28.
- ^ Leight, Howard. "Noise Cancelling Ear Muffs | Howard Leight". Howard Leight. Retrieved 2016-10-28.
- ^ Leight, Howard. "Noise Cancelling Ear Muffs | Howard Leight". Howard Leight. Retrieved 2016-10-28.
- ^ a b Lipper, Joanna (2007-06-05). "Active Noise Reduction". Occupational Health & Safety. Retrieved 2016-12-3.
{{cite web}}: Check date values in:|access-date=(help) - ^ "How do active noise-cancelling headphones work? || Audio-Technica US". www.audio-technica.com. Retrieved 2016-10-28.
- ^ Witt, Brad (2009). "Bad Assumptions About Hearing Protection" (PDF). Howard Leight. Sperian Hearing Conservation, LLC. Retrieved October 27, 2016.
- ^ Patient Health Information. "Noise and Hearing Protection". American Academy of Otolaryngology- Head and Neck Surgery.
- ^ Stewart, Michael. "Recreational Firearm Noise Exposure". American Speech-Language-Hearing Association.