acoustic earmuffs.
History Acoustic earmuffs, also known as
ear defenders, are believed to have originated during
World War II. Pilots of military aircraft wore leather flaps over their ears, supposedly to protect against noise-induced hearing loss due to engine noise. MSHA requirements are similar to OSHA. OSHA and MSHA require the use of hearing protection for workers who have not had a baseline hearing test or have had a shift in hearing thresholds, called a standard threshold shift. But this does not mean that OSHA considers HPDs to be effective. Hearing protection earmuffs have 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. 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. Earmuffs can be used in the workplace or recreationally for loud activities, e.g., concerts, shooting firearms, heavy machinery, mowing, etc. When persons are exposed to excessively loud environments (85 dB or more), hearing protection devices are recommended to prevent
noise-induced hearing loss. Hearing protection should be worn whenever power tools, loud yard equipment, or firearms are used. Any noise greater than 140 dB can cause permanent hearing loss. Firearms range from a noise level of 140 dB to 175 dB depending on the firearm type. It is recommended to use dual hearing protection (earmuffs and earplugs together) when using firearms. Exposure to loud noises damages the hair cells in the inner ear that are essential for sending neural impulses to the brain in order to perceive sounds. Loss of these hair cells leads to hearing loss that may cause speech and sounds to be muffled or distorted. Tinnitus is often associated with hearing loss; there is no cure for
tinnitus. 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. 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. 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). 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. The EPA requires that earmuff manufacturers test each device's performance and indicate their specific noise-reduction capabilities on the product labeling. This single number is called the Noise Reduction Rating, or NRR. The attenuation is higher when measured in laboratory testing than worn in the field, or in real world settings. However, earmuffs had the least variability compared to
earplugs. Discrepancies between the field and lab results could be due to improper training of how to wear the device. 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. 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), 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.
Active earmuffs have an electronic component and
microphones that allow the user to control their access to communication while attenuating background noise. 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
dBNRR), 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 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. 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. Dual protection is also recommended when shooting firearms because of the extremely high-level impulses (140 dB and greater) produced. The amount of noise reduction from dual hearing protection is not a sum of the noise reductions ratings from the two devices. 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. Individuals will require earmuffs of differing sizes. 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. Even eyeglasses with thinner frames can reduce the effectiveness of hearing protection by 3-7 dB. (right image). 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.
Styles There are different earmuff style options for HPD users. Styles include: cap-mounts for hard hats, neckbands for use with welding helmets and faceshields, folding earmuffs are meant to be portable and easy to store, and multi-position earmuffs worn in varies positions are useful for versatility to wear both earmuffs and other safety ware, such as glasses or masks.
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.
Detriments Wearing earmuffs makes it difficult to communicate because it blocks speech noise which may make speech sound muffled or unintelligible. It also makes it difficult to localize sound.
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. This occurs due to the
hearing protection device (HPD) attenuating the signals/noises below the level that the worker is able to hear. OSHA regulations require individuals to wear HPD regardless of their amount of hearing loss, even if they have a severe to profound hearing loss. Workers that have sustained a standard threshold shift are required by OSHA to wear HPD at an 85 dB TWA. 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. 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. 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. 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. ==See also==