Earmuffs

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

Earmuffs are objects designed to cover a person's ears for hearing 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 cushion or cup at each end, to cover the external ears. This article primarily focuses on the earmuffs for hearing protection. The difference between passive and active earmuffs is discussed, as well as a brief history of earmuffs. Finally, principles of hearing protection are introduced and the effectiveness of earmuffs for protecting against noise-induced hearing loss is discussed.

Varieties

Ear defenders and visor on a safety helmet
Two elderly persons wearing thermal earmuffs.

Modern earmuffs come in two basic kinds:

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.

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.[2]

Some passive earmuffs have an electronic component and microphones that allow the user to control their access to communication while attenuating background noise.[3] 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[4] 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[2]) low frequency noise.[5] 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.[6] 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.[5]

History

Chester Greenwood invented the thermal earmuff in 1873, at the age of 15.[7][8] He reportedly came up with the idea while ice skating, and had his grandmother sew tufts of fur between loops of wire.[9] 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.[7][10] 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.[11]

Earmuffs for the purpose of hearing protection are believed to have originated during World War II.[12] Pilots of military aircraft wore leather flaps over their ears, supposedly to protect against noise-induced hearing loss due to engine noise.[12] Prototype versions of earmuffs, composed of a headband and cuffs to fit over the outer ear, were soon after developed. These early versions were not practical due to the discomfort caused by the headbands being tightly fixed against the head.[12] In 1954, an earmuff with a more comfortable cushion design was developed.[12]

Hearing Protection

Basic Info

When persons are exposed to excessively loud environments (85 dB or more), hearing protection devices are recommended to prevent noise-induced hearing loss.[13][14] Hearing protection should be worn whenever power tools, loud yard equipment, or firearms are used. In the workplace, OSHA requires the use of hearing protection devices whenever a person is exposed to an average noise intensity of 90 dBA or greater over an 8 hour shift. The louder the environment, the less time that a person may spend there without the risk of incurring hearing loss. NIOSH has also developed standards for hearing protection.[15] Compared to OSHA, the NIOSH standards are more conservative in their estimates for safe noise exposure times. Tabulated below are the NIOSH standards for the maximum daily exposure times at various noise levels.[16]

Level of noise (dB A) Maximum daily exposure time
85 8 hours
91 2 hours
97 30 minutes
103 7 minutes

Because the auditory system has varying sensitivity to sound as a function of frequency, unprotected noise exposures to mid- to high- frequency sounds pose greater risk to hearing than low frequency sounds. This frequency dependence is reflected in the use of the A-weighting curve to describe the decibel level of an exposure (dB A).[17] 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.

Attenuation Characteristics

A typical earmuff attenuates (decreases) the level of noise by approximately 23 dB when tested under carefully controlled laboratory conditions.[18] The EPA requires that earmuff manufacturers test each device's performance and indicate their specific noise-reduction capabilities on the product labeling.[19] This single number is called the Noise Reduction Rating, or NRR. Unfortunately, when worn in real life, the actual attenuation of the device is likely much lower than the labeled NRR. Experiments have indicated that the actual attenuation achieved by ordinary users of earmuffs is only 33% to 74% of the labeled NRR.[20] Improper fit, device deterioration, and a poor seal with the head all contribute to reduced device performance. Despite these drawbacks, research has shown that the real-world performance of earmuffs is in closer agreement to manufacturers' labels than it is for earplugs.[20] This suggests that earmuffs are more intuitive for users to wear correctly and in some cases may be a more appropriate choice of hearing protection.

When deciding between earmuffs and earplugs, it is also important to consider the noise reduction levels achieved at different sound frequencies. In general, earmuffs provide less attenuation for low-frequency (<500 Hz) sounds than earplugs.[21] Thus, in situations where noise is dominated by low-frequency energy, earplugs are likely to be more effective. Earmuffs also fail to provide any noise reduction at infrasonic frequencies (< 20 Hz),[22] which is energy that cannot be heard because it falls below the range of human hearing sensitivity. In contrast, earplugs can provide some attenuation to infrasonic sounds.[22]

Dual Protection with Earplugs

Most earmuffs can be expected to provide adequate attenuation for noise levels up to 103 dBC.[18] At levels beyond this intensity, it becomes necessary for users to wear earplugs with earmuffs on top in order to achieve adequate protection from hearing damage. The simultaneous use of two forms of hearing protections is known as dual hearing protection. The Mine Safety and Health Administration (MSHA) regulations stipulate that workers must use dual hearing protection when average 8-hour exposures are 105 dBA or greater.[19] Dual protection is also recommended when shooting firearms because of the extremely high-level impulses (140 dB and greater) produced.[23]

The amount of noise reduction from dual hearing protection is NOT a sum of the noise reductions ratings from the two devices.[24] For example, if wearing an earplug with a NRR of 25 dB and an earmuff with an NRR of 20 dB, the combined protection would not be 45 dB. Instead, 5 dB should be added to the higher of the two NRRs.[24] In the preceding example, the combined earmuff and earplug NRR would be estimated at 30 dB (25 dB plus 5).

Barriers to effectiveness

Fit

A proper fit of the earmuffs on the head is essential to providing adequate hearing protection. Individuals will require earmuffs of differing sizes.[25] This is especially important to remember when considering earmuffs for children. Muffs should make a good seal against the head and should fully cover the outer ear without pushing against the ears. Additionally, the headband should be the correct length to hold the cushions over the ears.[25] Otherwise, sound can leak under the muffs and will reach the users' ears. Some wearers may use their earmuffs when hair is covering their ears or while wearing glasses. Prior to placement on the head, hair should be carefully pulled back and away from the cushions. Placing earmuffs over obstructing hair or safety glasses with thick frames may reduce the earmuff attenuation by 5-10 dB.[26] Even eye-glasses with thinner frames can reduce the effectiveness of hearing protection by 3-7 dB.[25]

One simple method for checking earmuff fit is to lift one or both muffs away from the head while in a noisy environment. If the noise is considerably louder with the adjustment, then the earmuffs are providing at least some degree of noise reduction.[25]

Improper earmuff fit can cause discomfort, which in turn may cause the individual to avoid wearing the hearing protection device, reducing its effectiveness. Characteristics of a comfortable earmuff include: lightweight material, soft and removable circumaural cushions, low heat and humidity buildup, easy maintenance, reduction in low-frequency noise, no resonances of sound within the earcup, wide headband, and large enough earcups to allow for full coverage of the outer ear. If the individual finds the hearing protection device to be uncomfortable, he or she should explore other options for hearing protection, such as a different style of earmuff or earplugs. [27]

Structural Transmission

It is possible for sound to transmit through the earmuff materials, reducing the device's effectiveness. This transmission is primarily seen above 1000 Hz.[27]

Vibration of the Earmuff

In loud enough environments, the ear canal can vibrate, causing the air trapped inside the earcup to vibrate as well. This typically only occurs with low frequency noise, but can reduce the effectiveness of the hearing protection device. [27]

Readjustment

During the amount of time an individual wears earmuffs, the device can be jostled and displaced from the proper position that allows for the highest attenuation. This can be common in the workplace, as many individuals are in motion during the time they are wearing the hearing protection device. Moving the jaw while chewing or talking and perspiration are examples of ways in which readjustment can occur, causing the seal to be broken between the earcup and skin and allowing sound to leak in.[28]

Deterioration

It is also important to consider the age and physical condition of earmuffs. Earmuffs should be inspected regularly for cracks and changes in shape or firmness. Headbands may also lose their tension or ability to properly adjust to the head, which could lead to a decrease in device effectiveness.[28] Physical changes could create an opening to the ear, allowing sound through and reducing attenuation.

Specific Considerations for Hearing Protection for Workers with Hearing Loss

Workers with hearing loss face additional risk factors on the job site such as an inability to hear warning signals or alarms, an increased difficulty to tell where sounds are coming from, and increased difficulty communicating with co-workers.[29] This occurs due to the hearing protection device (HPD) attenuating the signals/noises below the level that the worker is able to hear.[30] OSHA regulations require individuals to wear HPD regardless of their amount of hearing loss, even if they have a severe to profound hearing loss.[31] Workers that have sustained a standard threshold shift are required by OSHA to wear HPD at a 85 dB TWA.[32] There are special considerations to take into account when fitting HPD on workers with a hearing loss. These factors include comfort, degree and configuration of the worker's hearing loss, the necessary communication demands in the workplace (verbal vs. nonverbal), the ease of communication, and the noise exposure levels of the worker.[33]

Workers may want to wear their hearing aids under an earmuff. According to OSHA, hearing aids should not be used in areas with dangerous noise levels. However, OSHA allows for the professional(s) in charge of the hearing loss protection program to decide on a case-by-case basis if a worker can wear their hearing aids under an earmuff in high-level noise environments. However, workers are not permitted to wear their hearing aids (even if they are turned off) instead of using HPD. OSHA specifies that hearing aids are not "hearing protectors" and do not attenuate enough sound to be used instead of HPD.[32]

Devices that provide both communication enhancement and hearing protection can be used to attenuate loud sounds and amplify soft-level sounds. These are available with both wireless and wired options.[32][33] The effects of these will vary based on the degree and configuration of the worker's hearing loss. Dual hearing protection with electronic/communication elements may aid a person with hearing loss in hearing warning signals and help with communication. Workers with a high frequency hearing loss may benefit more from HPD that attenuates sounds equally across the pitch range. This is helpful because traditional HPD will attenuate the higher frequencies (where these individuals have a hearing loss) more than the mid- and low-frequencies. Whereas, HPD that attenuate equally across the pitch range, can provide more comfort and balancing of loudness across the pitches for these individuals. This type of HPD are commonly referred to as "musicians plugs."[33] NIOSH provides a "Hearing Protector Device Compendium" with information on the different types of HPD.[34]

See also

References

  1. ↑ Stephenson, Carol Merry. "Choosing the Hearing Protection That's Right For You". Retrieved 2009-07-30.
  2. 1 2 "Ear Muffs: A Field Guide -- Occupational Health & Safety". Occupational Health & Safety. Retrieved 2016-10-28.
  3. ↑ Leight, Howard. "Noise Cancelling Ear Muffs | Howard Leight". Howard Leight. Retrieved 2016-10-28.
  4. ↑ Leight, Howard. "Noise Cancelling Ear Muffs | Howard Leight". Howard Leight. Retrieved 2016-10-28.
  5. 1 2 Lipper, Joanna (2007-06-05). "Active Noise Reduction". Occupational Health & Safety. Retrieved 2016-12-03.
  6. ↑ "How do active noise-cancelling headphones work? || Audio-Technica US". www.audio-technica.com. Retrieved 2016-10-28.
  7. 1 2 "Maine Secretary of State Kid's Page - Famous People". Maine.gov. Retrieved 2013-06-22.
  8. ↑ Bellis, Mary (1936-12-29). "Chester Greenwood - Earmuffs". Inventors.about.com. Retrieved 2013-06-22.
  9. ↑ Long, Tony. "Dec. 4, 1858: It Was Very Cold the Day Chester Greenwood Was Born". Wired.com. Retrieved 2013-06-22.
  10. ↑ Ament, Phil (2005-08-31). "Earmuff History - Invention of Earmuffs". Ideafinder.com. Retrieved 2013-06-22.
  11. ↑ Sharp, David (December 5, 2015). "Ear, Ear: Maine Town Hails Earmuff's Inventor". Chicago Tribune. p. 3.
  12. 1 2 3 4 Acton, W. Ian (1987). "History and development of hearing protection devices". Journal of the Acoustical Society of America. 81: S4.
  13. ↑ "Noise And Hearing Loss Prevention". Cdc.gov.
  14. ↑ "Occupational Noise Exposure". Cdc.gov.
  15. ↑ "NIOSH Criteria for a Recommended Standard" (PDF). 1997.
  16. ↑ Kardous, Chuck (2016-02-08). "Understanding Noise Exposure Limits: Occupational vs. General Environmental Noise". Centers for Disease Control and Prevention. Retrieved 2016-12-04.
  17. ↑ A-weighting
  18. 1 2 Berger, Elliot (1979). Single number measures of hearing protector noise reduction. EARlog 2. Aearo Company: Indianopolis, IN.
  19. 1 2 Rawool, Vishakha (2012). Hearing Conservation. New York, NY: Thieme. ISBN 978-1-60406-256-4.
  20. 1 2 Berger, Elliot (1993). The Naked Truth About NRRs. EARlog 20. Aearo Company: Indianapolis, IN.
  21. ↑ Berger, Elliot (1996b). The performance of hearing protectors in Industrial Noise Environments. EARlog 4. Aearo Company: Indianapolis, IN.
  22. 1 2 Berger, Elliot (1984). Protection for Infrasonic and Ultrasonic Noise Exposure. EARlog 14. Aearo Company: Indianapolis, IN.
  23. ↑ "Recreational Firearm Noise Exposure". www.asha.org. Retrieved 2017-02-24.
  24. 1 2 Berger, Elliot (1984). Attenuation of earplugs worn in combination with earmuffs. EARlog 13. Aearo Company: Indianapolis, IN.
  25. 1 2 3 4 Berger, Elliot (1988). Tips for fitting hearing protectors. EARlog 19. Aearo Company: Indianapolis, IN.
  26. ↑ Witt, Brad (2009). "Bad Assumptions About Hearing Protection" (PDF). Howard Leight. Sperian Hearing Conservation, LLC. Retrieved October 27, 2016.
  27. 1 2 3 Vishakha, Rawool (2011). Hearing Conservation: In Occupational, Recreational, Educational, and Home Settings. New York, NY: Thieme Medical Publishers, Inc. pp. 156–158.
  28. 1 2 Berger, Elliot (1996a). Hearing protector performance: how they work-and- what goes wrong in the real world. EARlog 5. Aearo Company: Indianapolis, IN.
  29. ↑ "Innovative Workplace Safety Accommodations for Hearing-Impaired Workers".
  30. ↑ Berger, Elliott (2000). The Noise Manual, Fifth Edition. Fairfax, VA: AIHA. pp. Chapter 10.
  31. ↑ "OSHA Regulations (Standards - 29 CFR)".
  32. 1 2 3 Rawool, Vishakha Waman (2012). Hearing Conservation in Occupational, Recreational, Educational, and Home Settings. New York, NY: Thieme Medical Publishers, Inc. pp. 146–148.
  33. 1 2 3 Berger, Elliott (2000). The Noise Manual, Fifth Edition. Fairfax, VA: AIHA.
  34. ↑ "Hearing Protector Device Compendium". Centers for Disease Control and Prevention.
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