Earmuffs

This article is about earmuffs that covers a person's ear. “Earmuffs” may also refer to the “earmuff convention” in Common Lisp.

Two elderly persons wearing thermal earmuffs.

A pair of Husqvarna acoustic earmuffs.

Earmuffs are objects designed to cover a person's ears for protection or for warmth. They consist of a thermoplastic or metal head-band, that fits over the top or back of the head, and a pad at each end, to cover the external ears.

Varieties

Ear defenders and visor on a safety helmet

Modern earmuffs come in two basic kinds:

• Thermal earmuffs, worn in cold environments to keep a person's ears warm with pads of cloth or fur.
• Acoustic earmuffs, also known as ear defenders: cups lined with sound-deadening material, like thermal earmuffs and headphones in appearance, which are worn as hearing protection. These may be carried on a head-band or clipped onto the sides of a hard hat, for use on construction sites. Some manufacturers combine headphones with ear defenders, allowing the wearer to listen to music, communication or other audio source and also enjoy protection or isolation from ambient noise. For extra sound attenuation, earplugs can also be used in conjunction with earmuffs.^[1]

Ear defenders protect the wearer from extreme noises. The head-band and outer covering is usually made from a hard thermoplastic or metal. The protection usually comes from acoustic foam – this absorbs sound waves by increasing air resistance, thus reducing the amplitude of the waves. The energy is transformed into heat.

Some ear defenders employ active sound protection, in which a microphone mounted in the headset picks up ambient sounds and transmits them through a dynamic range compression circuit to earphones inside. By virtue of the dynamic compression, the headset can be adjusted to allow the wearer to hear sounds at ordinary volumes normally, while attenuating louder sounds. Similar active earplugs also exist, primarily aimed at musicians.

History

Chester Greenwood invented the earmuff in 1873, at the age of 15.^[2][3] He reportedly came up with the idea while ice skating, and had his grandmother sew tufts of fur between loops of wire.^[4] He was awarded patent #188,292 on March 13, 1877. He manufactured these ear protectors, providing jobs for people in the Farmington, Maine area, for nearly 60 years.^[2][5] Every year, on the first Saturday of December, the town of Farmington celebrates "Chester Greenwood Day" with a variety of activities. A parade in Chester's honor is a part of the festivities. Everyone participating in the parade must wear earmuffs.^[6] Thomas A. Willson of Willson Safety Products in Reading, Pennsylvania invented the ear muffs for sound protection.

Hearing protection

If people are exposed to excessively loud environments (85 dB or more), hearing protection devices are necessary to prevent hearing loss.^[7][8] They should be worn whenever power tools, loud yard equipment, or unsuppressed firearms are used. Tabulated below are the maximum daily exposure times for various noise levels according to NIOSH standards^[9]. Because the auditory system has varying sensitivity to sound as a function of frequency, unprotected exposures to sound in the mid to high frequencies poses a greater risk to hearing than low frequency noises. This frequency dependence is reflected in the use of the A-weighting curve to describe the decibel level of an exposure (dB A).^[10] The A-weighting curve weights the mid frequency content, 500 to 4000 Hz, more than the frequencies outside that range. At lower, non-damaging sound levels, hearing protection will reduce fatigue from frequent exposure to sound.

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.^[11]

Some passive earmuffs have an electronic component and microphones that allow the user to control their access to communication while attenuating background noise.^[12] 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^[13] 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^[11]) low frequency noise.^[14] 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.^[15] 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.^[14]

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^[16], 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. ^[17] 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.^[18]

See also

• Headphones
• Earplugs
• Muffs
• Noise-induced hearing loss
• Noise pollution
• Personal protective equipment
• Dielectric
• National Institute for Occupational Safety and Health

References

External links

Wikimedia Commons has media related to Earmuffs.

• National Institute for Occupational Safety and Health - Hearing Protector Device Compendium
• "Mallock-Armstrong Ear Defender" a 1925 Flight article