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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.
Contents
Varieties
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.
Some passive 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 dB 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) low frequency noise. 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.
History
Chester Greenwood invented the thermal earmuff in 1873, at the age of 15. He reportedly came up with the idea while ice skating, and had his grandmother sew tufts of fur between loops of wire. 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. 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.
Earmuffs for the purpose of hearing protection 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. 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. In 1954, an earmuff with a more comfortable cushion design was developed.
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. 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 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. 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. 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 manufacturer's labels than it is for earplugs. 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. 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.
Dual Protection with Earplugs
Most earmuffs can be expected to provide adequate attenuation for noise levels up to 103 dBC. At levels beyond this intensity, it becomes necessary for to wear earplugs with earmuffs on top in order to acheive adaquate 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. 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. In the preceding example, the combined earmuff and earplug NRR would be estimated at 30 dB (25 dB plus 5).
Fit
A proper fit of the earmuffs on the head is essential to providing adequate hearing protection. 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. 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. Even eye-glasses with thinner frames can reduce the effectiveness of hearing protection by 3-7 dB.
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.
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 loose their tension or ability to properly adjust to the head, which could lead to a decrease in device effectiveness.Physical changes could create an opening to the ear, allowing sound through and reducing attenuation.