Effectiveness of Earmuffs and Noise-cancelling ...

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Abstract

Objective/Background

The purpose of this pilot study was to examine the effectiveness of standard earmuffs and noise-cancelling (NC) headphones in controlling behavioural problems related to hyper-reactivity to auditory stimuli in children with autism spectrum disorder (ASD).

Methods

Twenty-one children with ASD aged 4&#;16 years (16 boys and 5 girls), after a 2-week nonwearing baseline period, were asked to use standard earmuffs and NC headphones for 2 weeks, in a random order. Parents or teachers rated participants&#; behaviours that were related to their reaction to auditory stimuli.

Results

Four participants refused to wear either the earmuffs or the NC headphones. It was found that the T-score on the Goal Attainment Scaling was significantly higher during the earmuff period than that in the baseline period (Z = 2.726, p = .006). The behaviours of 5 children with ASD improved during the NC headphone period as compared with those in the baseline period; there were no differences in the T-scores on the Goal Attainment Scaling between the NC headphone period and the baseline period (Z = 1.689, p = .091) and between the earmuff and NC headphone periods (Z = &#;0.451, p = .678).

Conclusion

This study demonstrated the effectiveness of standard earmuffs and NC headphones in helping children with ASD to cope with problem behaviours related to hyperreactivity to auditory stimuli, therefore, children with ASD could use earmuffs to help to deal with unpleasant sensory auditory stimuli.

Keywords:

auditory hyperreactivity, autism spectrum disorder, earmuff, noise-cancelling headphone

Introduction

Hyper-reactivity to auditory stimuli is a common problem in children with autistic spectrum disorder (ASD). Bromley, Hare, Davison, and Emerson () reported that 70% of children with ASD exhibit hyper-sensitivity to auditory stimuli. In children with higher functioning ASD, the most common sensory hypersensitivity was auditory hypersensitivity (Futoo et al., ). Various sounds provoked unpleasant sensory experiences in children with ASD. Loud and unexpected sounds such as fire alarms, toilet flushes in public restrooms, dogs barking, other children's crying voices, fireworks, loud coughing or clapping, and microphones with acoustic feedback were the most common examples (Dickie, Baranek, Schultz, Watson, & McComish, ). In their autobiographies, individuals with ASD often described their experiences of hyper-reactivity to auditory stimuli (Grandin & Scariano, ; Hall, ).

Children with ASD who have auditory hyper-reactivity (ASD-AH) are bothered by auditory stimuli that they find intolerable, therefore, it is very important to find a way to manage auditory stimuli in daily life. Earmuffs, earplugs, and noise-cancelling headphones (NC headphones) are examples of equipment designed to protect individuals from harmful auditory stimuli. Some authors suggested earmuffs, earplugs, or headphones to parents or practitioners working with children with ASD (Attwood ; Delaney ; Myles, Tapscott, Miller, Rinner, & Robbins, ). Attwood (, p. 278) described, &#;A barrier to reduce the level of auditory stimulation can be used, as silicone earplugs, kept in the person's pocket&#; These are particularly useful in situations known to be noisy, such as school cafeterias.&#; Myles et al. () suggested that caregivers should provide headphones or earplugs for the child to wear during testing or seatwork after verbal directives are given. Although some specialists have introduced the use of earmuffs, earplugs, and headphones to parents of children with ASD or with sensory processing disorders, there have been no studies so far demonstrating their effectiveness for managing behavioural and emotional problems related to AH in children with ASD. Earplugs and earmuffs can block environmental auditory stimuli, including human speech (Morris, ). Headphones can block some auditory stimuli by providing another auditory stimulus such as music. NC headphones can reduce unwanted ambient sound by using active noise control engineering but cannot cancel human voices and sudden sounds. Different devices protect in different ways and attenuate sound differently. To identify which support equipment is helpful to children with ASD-AH, we need to know which type of device is more effective in improving AH. In this study, we focused on comparing earmuffs and NC headphones to examine which device would be better for blocking all sounds, including human voices, and examining if such equipment might reduce ambient sound using a NC system. We would also like to know which type of hyperreactivity in children with ASD could be controlled by these devices. Although earplugs are also sound-blocking devices, they might cause unwanted tactile stimuli to the ear canal. Therefore, in this study we investigated only two types of ear devices &#;- earmuffs and NC headphones, as earplugs are sound blocking devices that might cause unwanted tactile stimuli to the ear canal.

The purpose of this study was to examine the effectiveness of earmuffs and NC headphones in controlling behaviours related to hyper-reactivity to auditory stimuli in children with ASD. To the best of our knowledge, this was the first study that examined the effectiveness of these devices in controlling behavioural and emotional problems in children with ASD-AH.

Methods

Participants

Participants were recruited from 220 families who were members of the Autism Society of Nagasaki, Japan, from April to September . We informed the parents about our plan to hold workshops on sensory problems in children with ASD, then two workshops were run during the study period. In the workshops, after explaining the study methods and inclusion criteria, we invited 65 parents (mothers or fathers) who were present in these workshops to participate in the study with their verbal and written consent. The same procedure was repeated for eight families who were not present at these workshops. Inclusion criteria were: (1) the child was diagnosed with autistic disorder; (2) Asperger's disorder; (3) pervasive developmental disorder not otherwise specified; (4) was aged between 3 years to 17 years; and (5) had hyper-reactivity to auditory stimuli. Parents of 25 children with ASD who met the inclusion criteria (age range 4&#;16 years, mean age 8 years 2 months ± 36 months; 19 boys and 6 girls) expressed their willingness to participate in this study. All participants had already been diagnosed by paediatricians based on Diagnostic and Statistical Manual of Mental Disorders-IV criteria (American Psychiatric Association, ). We asked the parents to report the grade of intelligence disabilities of their children as described in the &#;Rehabilitation Certificate Handbook for Individuals with Intellectual Disabilities&#; by the prefectural government (Nagasaki Prefectural Government, ).

This study received prior approval from the Human Investigation Committee of the Nagasaki University Graduate School of Biomedical Sciences (Number ).

Equipment

Earmuffs (PELTOR Optime 1 Earmuffs 3M H510A-401-GU, ) and NC headphones (SONY Digital Noise Cancelling Headpone MDR-NC500D, ) were used. Earmuffs are headphone style devices designed to reduce sound levels. The attenuation rating (noise reduction rating) and weight of the earmuffs (PELTOR H510) were 27 dB and 180 g, respectively. Earmuffs protect children from loud noises that may bother them or even be harmful to their hearing and attenuate not only bothersome environmental sounds but also human speech (Morris, ). NC headphones reduce unwanted ambient sounds using active noise control engineering. The noise reduction rating and weight of the NC headphones (Sony MDR-NC500D) were 20 dB and 195 g, respectively. NC headphones primarily work well on low frequency bands, such as motor and air duct noises (Sony Noise Cancelling Headphones, ) but do not work well on sounds that continuously change in frequency and amplitude, such as human voices or thunder.

Outcome measure

Goal Attainment Scaling

To evaluate changes in behaviour during the baseline control period, earmuff period, and NC headphone period, Goal Attainment Scaling (GAS) was used. The GAS provides a standardised means to capture the diversity of meaningful functional outcomes (Kiresuk & Sherman, ). In various studies, GAS has been determined to be an effective outcome measure and has been used to evaluate the effectiveness of intervention in children with developmental disabilities (Mailloux et al., ; Miller, Coll, & Schoen, ; Schaaf et al., ). Ruble, McGrew, and Toland (, p. ) demonstrated that the GAS was a valid and reliable method for the measurement of progress on individualized goals for children with ASD, and concluded that the GAS is a &#;promising ideographic approach for measuring intervention effectiveness.&#; Palisano () demonstrated the content validity and responsiveness of the GAS, and concluded that the GAS could be recommended for use in clinical practice and treatment outcome research. In the GAS, a specific goal is selected on a composed scale that ranges from least to most favourable outcomes. The GAS has rating scale ranging from &#;2 to +2; 0 being the anticipated performance by the end of the study intervention (Kiresuk, Smith, & Cardillo, ). Negative numbers represent less-than-expected outcomes, and positive numbers represent greater-than-expected outcomes. After carefully reading each child's prior assessment reports, the first author developed the goals together with the parents and teachers to ensure that the goals were relevant and at appropriate levels for the participants. The first author was blinded to the experimental schedule assignment, because the third author was responsible for randomization and was blinded to the assigned schedules. The second author was responsible for a double-check on each GAS item to ensure that it met all quality criteria based on the GAS literature (Turner-Stokes, , p. 363).

The attainment levels for the chosen personal goals were then combined in a single aggregated T-score by applying the recommended formula which accounts for variable numbers of goals, inter-correlation of goal areas, and variable weighting (Kiresuk & Sherman, ; Turner-Stokes, ):

Total GAS=50+{(10&#;&#;(wixi))/(0.7&#;wi2+0.3(&#;wi)2)1/2}

where wi = weight assigned to the ith goal and xi = the score of the ith goal.

Procedure

Participants were randomly assigned to one of two experimental schedules that consisted of a sequence of a 2-week control period followed by 2 weeks of earmuff use and 2 weeks of NC headphone use, or a sequence of a 2-week control period followed by 2 weeks of NC headphone use and 2 weeks of earmuff use. Randomisation of the experimental schedule was done using the RAND and SORT functions in MS Excel (Microsoft Excel ) by the third author. Each participant had to decide whether to use earmuffs or NC headphones for the whole day or part time, however, the parents and teachers had to record the total time of using earmuffs or NC headphones per day.

Because it was necessary to observe the children's behaviour at all time, the children's GAS rating was checked by the teacher at school and by the parent at home.

Statistical analysis

The GAS T-scores for each period were calculated for each participant. The T-scores of all the participants were compared between the control period, the NC headphone period, and the earmuff period. The Shapiro&#;-Wilk test indicated that the T-scores in the baseline period were not normally distributed (W = 0.826, p = .021), therefore, we used the Wilcoxon rank sum test, which is used for nonparametric comparisons. If there were missing values due to participants refusing to wear earmuffs or NC headphones or not encountering disliked auditory stimuli during the assessment period, the data were excluded from the analysis.

Sample size calculations were performed using the G-Power software version 3.1 (G*Power: Universität Dusseldorf, Düsseldorf, Germany; &#;) with &#;a priori calculation&#; that is an analysis method to calculate sufficient sample sizes to achieve adequate power prior to the research study. For this calculation, an alpha value of 0.05, an effect size of 1, and a power of 80% were set, which identified that 11 participants were sufficient to detect a significant change in the GAS T-score.

Results

Medians and quartiles of the GAS scores in each period are presented in .

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Four participants (3 boys and 1 girl) refused to wear either the earmuffs or NC headphones. Their mothers reported that their children disliked the tactile impression or pressure. No children wore earmuffs or NC headphones continuously during waking hours. Among the participants who used the earmuffs and NC headphones, the use time per day ranged from 65 minutes to 360 minutes (mean = 136.9 minutes, standard deviation = 69.4 minutes) for earmuffs and from 30 minutes to 360 minutes (mean = 94.6 minutes, standard deviation = 50.6 minutes) for NC headphones according to the parents' and teachers' reports. Three participants (Cases A, K, and M) used them at school only. Five participants (Cases C, F, H, I, and J) used them at home only. Other participants used them both at home and at school.

shows the age and sex of each participant, the auditory stimuli that induce behaviour problems, and problem behaviours induced by auditory stimuli in each child, excluding participants who had refused to wear either earmuffs or NC headphones. One to four goals were prepared for each child.

Table 1

ParticipantsSexAge (y)IntelligenceAuditory stimuli that induce behaviour problemsBehaviours after soundsGAS scores1st period2nd period3rd periodControl periodUse of earmuff or NC headphonesCase AFemale13NormalMany voices of other studentsThe child becomes nervous and aggressive-1-10Loud musicThe child becomes nervous and aggressive2 2 1Grinding sound of sharpening a pencilThe child becomes nervous and aggressive2 0 0Case BMale11NormalHigh-pitched voices of girlsThe child covers his ears with his hands-1-1 0 Sounds from a televisionThe child covers his ears with his hands-11 1 Sounds of construction workThe child covers his ears with his hands-11 1 Sound of an electric sharpenerThe child covers his ears with his hands-1-1-1Case CMale8SevereSound of a motorcycle engineThe child covers his ears with his hands01 2 Sounds of sirensThe child squats, groans, and covers his ears with his hands22 2 Names of certain personsThe child hates hearing the name of a certain person. He cries and runs away11 1 Sound of the engine of big trucksThe child covers his ears with his hands12 2 Case DMale7SevereSound of a chimeThe child flurries, bites his fingers, and talks furiously-122Sounds of thunder, rain, and stormThe child flurries and runs around.-1 1 2He becomes pessimistic.Singing voice of another personThe child flutters and disturbs1 1 1Sound of a sirenThe child withdraws to his room-1 1 -Case EMale7NormalSounds of construction machinesThe child covers his ears with his hands1 0 Sounds of dental treatment machinesThe child covers his ears with his hands-1--Sound of dance music used during physical education and musical instrumentsThe child covers his ears with his hands01Barking of dogsThe child covers his ears with his hands0--1Case FMale4MildHigh-pitched voices of childrenThe child complains by yelling-12-2Loud conversation voicesThe child says, &#;Shut up!&#;-1 1 -Case GFemale4NormalSounds of an engine, slamming doors, construction work, high-pitched metallic sounds, drums, cymbals, fireworks, whistles, and a child's screamingThe child covers her ears with her hands, cries, or stands motionlessly-11 0 Case HMale9NormalNoise of a crowdThe child lingers close to his mother or becomes absentminded-1 0 -1Children's voices outside his houseThe child stops the ongoing activity and hides-1 1 -1Father's voiceThe child screams in order to deaden his father's voice. If the father does not stop talking, the child has a & of rage-12-1Sound of the toilet flushThe child avoids using the toilet or hits family members who flush the toilet-1 1 Case IFemale11NormalSound of a sprayThe child expresses disgust or irritation-21-1Sounds of scratching, writing on, and erasing the blackboardThe child's face becomes tense-11-1Classroom noiseThe child's face shows displeasure-10-1Sounds of a drum, starter pistol, or crackerThe child covers her ears with her hands-10-1Sounds of motorcyclesThe child covers her ears with her hands-10-1Case JMale7Most severeSound of a baby cryingThe child runs away shouting &#;Ah!&#; Sometimes, he hits people around him-1--Noisy crowded place (such as a hotel lobby)The child cries and runs around-1-1Scolding with a loud voiceThe child looks like he is angry, anxious, or is going to cry. Sometimes, he throws objects-1 -1 Crowing of a cockThe child runs away shouting &#;Ah!&#;-1&#;- &#;- Case KMale4MildSounds of a starter pistol or fireworks (also in a movie or story)The child screams and has a & of rage&#;-1-1 -1 Case LMale15NormalSound effects before presenting an answer in a television quiz programThe child covers his ears with his hands and moves away from the television&#;-1Refusal 0 Just before a character was scolded by his/her parent in an animation programThe child covers his ears with his hands and changes the channel-1RefusalCommercial music in the department store or supermarketThe child covers his ears with his hands or says unrelated things (to himself)&#;-1Refusal 0 Case MMale8SevereOlder sister's screamingThe child cries and runs away covering his ears. The child bites or hits his older sister&#;-1 -1 DiscontinuationLoud sound in the large hallThe child covers his ears with his hands and freezes or runs away-1 0 Music or sound of a jet towelThe child cannot enter the bathroom or leaves from there-1Case NMale7Most severeSneezeAfter crying, the child becomes aggressive0 -1 DiscontinuationSound of a motorcycle engineThe child cries and rolls around on the floor 2 Sound of snifflingThe child becomes aggressive and hits those around him 2 Noise in an assembly hallThe child cries and angrily wishes to leave the hall2 0 Sound of a baby cryingThe child grabs anyone standing nearby&#;- -2 Music with a quick tempoThe child growls&#;- -1 Case 0Male8NormalOther children's voice in the classroomThe child covers his ears with his hands0-1 Discontinuation Sound of alarm bellThe child covers his ears with his hands or leaves the place&#;&#;Case PMale8NormalSound of thunderThe child shudders, covers his ears with his hands, and clings to the teacher&#;&#;DiscontinuationLoud voice when child was directed or scoldedThe child hits his own face1 1 White noise on the television screenThe child freezes-1 0 Sound occurring suddenlyThe child freezes after being startled-1 &#; Case QMale8NormalOther children's voice in the classroomThe child covers his ears with his hands0 2 DiscontinuationSound of alarm bellThe child covers his ears with his hands or leaves the place-1 2 Open in a separate window

One child refused to wear earmuffs because he disliked the pressure they exerted. Five children refused or discontinued to wear NC headphones because they could hear human voices better and disliked other students' voices. As a result, we compared the T-scores between the baseline control and earmuff use period in 16 children with ASD, and between the baseline control and NC headphone period in 12 children with ASD.

Fifty-six behaviours induced by auditory stimuli were evaluated using the GAS. Since most of the participants exhibited some problem behaviours induced by auditory stimuli, the GAS T-score for each participant was calculated from the GAS scores in each period.

The GAS T-score was significantly higher for the earmuff period than that for the control period (Z = 2.726, p = .006). There were no significant differences in the GAS T-scores between the NC headphone period and the control period (Z = 1.689, p = .091) and between the earmuff period and NC headphone period (Z = &#;0.451, p = .678). Although there were no significant differences between the NC headphone period and control period, the GAS T-scores of five children improved during the NC headphone period.

Parents of participants whose GAS T-scores during the NC headphone period were better than those during the earmuff period reported the following: &#;He was pleased by the diminished sound of the piano,&#; &#;He could tolerate noise from the big trucks,&#; &#;He disliked the pressure of the earmuffs, but he tolerated the NC headphones.&#; Parents of participants whose GAS T-scores were better during the earmuff period than those during the NC headphone period reported the following: &#;If he wore earmuffs, he could enter the bathroom&#; (this participant disliked the sound of water flushing), &#;He could tolerate the noise of the air towel,&#; &#;He may have felt stressed when he wore the NC headphones because he could hear human voices clearly when the other noises diminished.&#;

Discussion

Development of treatment or support for individuals with ASD-AH is an important issue; however, evidence supporting therapies such as auditory integration therapy and sound therapy for children with ASD has not been reported (Sinha, Silove, Hayen, & Williams, ).

Special education directors reported that occupational therapists provide relatively more service in assistive technology consultation (30.3%) and task or environment modification (25.8%) to improve student performance (Spencer, Emery, & Schneck, ). Spencer, Turkett, Vaughan, and Koenig () also stated that occupational therapy intervention focused on changing or adapting the performance environment was perceived to be helpful. Although these opinions were not solely about occupational therapy for ASD, they indicated that occupational therapists should work on modifying learning environments and provide assistive technology to enable better performance in children with ASD, especially for those with ASD-AH. Providing earmuffs and NC headphones, for example, are possible ways to support children with ASD-AH through occupational therapy.

To adapt earmuffs and NC headphones for use in children with ASD-AH, treatment evidence is necessary. To date, a few researchers have reported the benefits of earmuffs and earplugs for children with central auditory processing disorders who have difficulty concentrating and processing auditory input in busy environments (Hasbrouck, ; Willford & Burleigh, ). However, the effectiveness of these devices for children with ASD-AH has not been demonstrated.

Our results showed improvement of GAS T-scores during earmuff use, therefore, earmuffs can have a positive effect for coping with behavioural problems related to hyperreactivity to auditory sensory stimuli in children with ASD. Since earmuffs reduce auditory stimuli from the environment, they might reduce the stress or anxiety caused by auditory stimuli. Kinnealey et al. () demonstrated that sound-absorbing walls and halogen lighting can benefit students with sensory hyper-sensitivity and improve their attention and engagement in the classroom. Although the control methods were different between the sound-absorbing walls in their study and the earmuffs in our study, both strategies might be effective in helping children with ASD-AH because both could diminish intolerable sounds. Since earmuffs might be effective equipment for children with ASD-AH and could be used in various situations, practitioners, including occupational therapists, could recommend earmuffs to individuals with ASD-AH and their parents. Morris () pointed out that because sound isolators such as earmuffs have fairly nonspecific broadband sound-attenuating characteristics, bothersome environmental sounds are attenuated, but so are the speech sounds, which are very important to the individual. Therefore, practitioners should also consider this disadvantage of sound reduction equipment.

Although an improvement in the GAS T-scores was observed in some participants, there was no effect of NC headphones on behavioural problems related with hyperreactivity to auditory sensory stimuli. The parents of participants whose GAS T-scores during the earmuff period were better than those during the NC headphone period suggested that earmuffs improved hyper-reactivity behaviour to air towels and flushing water. One mother complained that her child felt stressed when he wore NC headphones because he could hear human voices clearly when other noises diminished. Since NC headphones cannot eliminate auditory stimuli except for low frequency noises, human voices and some other sounds might not be reduced. Therefore, NC headphones may not be effective for participants who have auditory sensitivity to human voices. Interestingly, our study found that behaviours of five children with ASD (Cases C, D, E, G, and I) improved during the NC headphone period as compared with those in the baseline control period, and that the intolerable sounds related with the behaviours of these five participants were not voices but were &#;noisy sounds in the classroom.&#; Thus, NC headphones might not be effective in coping with behavioural problems caused by human voices. Occupational therapists should consider the specific sounds related to hyper-reactivity when recommending earmuffs or NC headphones to individuals with ASD who exhibit hyperreactivity to auditory stimuli.

This study has some limitations. One of them is the small number of participants. Since four participants refused to wear either earmuffs or NC headphones, and another five refused NC headphones, the behavioural data from the NC headphone period were limited. Further study should be conducted with larger samples. Additionally, we did not examine adverse and long-term effects of earmuff and NC headphone use. Habitual use of sound isolators may actually exacerbate sound sensitivity over time, as suggested in tinnitus patients (Jastreboff & Hazell, ). Therefore, we should further examine the benefits and disadvantages of prolonged use of earmuffs and NC headphones in children with ASD. Furthermore, age, sex, general intelligence, functional level of participants, frequency of intervention, and duration of using the devices were not controlled. These factors should be considered in future randomised, controlled studies when comparing the effectiveness between different devices using intervention and control groups. We did not investigate other sound isolation devices such as earplugs or headphones without a NC system, therefore, further study should be conducted to examine the effects of other sound isolation devices.

Conclusion

This was a pilot study, and although there were some limitations in this study, the usefulness of earmuffs for children with ASD-AH, even for such a short period of wearing time, was demonstrated. Although the effectiveness of NC headphones was not statistically significant, we concluded that earmuffs that block sound might be useful for children with ASD-AH, and that NC headphones, which reduce ambient sounds, might also be useful for children with ASD-AH who are not affected by human voices.

Funding/Support:

This study was supported by a grant-in-aid for Scientific Research (C) no. from the Japan Society for the Promotion of Science.

Conflicts of interest:

All contributing authors declare that they have no conflicts of interest.

For the practice in Lisp programming, see earmuff convention

Ear-protecting headgear worn over ears to protect from cold or loud noise

Earmuffs that are also hearing protection

Earmuffs refer to two different items. Both items 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 usually cover both ears. The cups can either be clothing accessories designed to cover a person's ears for warmth or personal protective equipment designed to cover a person's ears for hearing protection.

Cold weather

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Woman wearing cold-weather thermal earmuffs

History

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Thermal earmuffs were invented by Chester Greenwood of Farmington, Maine in , when he was 15.[1] He reportedly conceived the idea while ice skating, and asked his grandmother to sew tufts of fur between loops of wire.[2] His patent was for improved ear protectors, which he and his local employees manufactured in the Farmington area for nearly 60 years.[1]

Earmuffs vs. hats

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Two people wearing behind-the-neck earmuffs

Thermal earmuffs are worn for protection from the cold. Because the ears extend from the sides of the head to gather sound waves, they have a high skin surface-area-to-volume ratio, and very little muscle tissue, causing them to be one of the first body parts to become uncomfortably cold as temperatures drop. Some people experience this discomfort even if most of the body is comfortably warm, especially during strenuous activity. Wind can often cause the ears to be much colder than the rest of the head. When the ears are uncomfortably cold and the rest of the body is much warmer, using a winter hat or the hood of a jacket to cover the ears may cause the head or body to be uncomfortably hot, possibly inducing perspiration of the head, a dangerous condition in cold weather. Earmuffs can be used to warm the ears only, avoiding overheating other parts of the body or trapping exhaust heat from strenuous movement.

Types of thermal earmuffs

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There are two main types of thermal earmuffs. One type has a structure similar to large headphones, with a band going over the top of the head. Another type has two round earpieces made from a material that can produce heat, connected to a thick headband going around and behind the head. Some headbands are thick and wide enough to warm the ears, and are referred to "earmuffs" when used this way.

Hearing protection

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A pair of Husqvarna acoustic earmuffs. A hard hat with attached face shield and ear defenders.

History

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If you want to learn more, please visit our website soundproof earmuffs.

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Acoustic earmuffs are believed to have originated during World War II.[3] Pilots of military aircraft wore leather flaps over their ears, supposedly to protect against noise-induced hearing loss due to engine noise.[3] 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.[3] In , an earmuff with a more comfortable cushion design was developed.[3]

Overview

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Hearing protection in the workplace in the United States is regulated by organizations such as the Occupational Safety and Health Administration (OSHA), the Mine Safety and Health Administration (MSHA), and the National Institute for Occupational Safety and Health (NIOSH). Hearing protection can be included in hearing conservation programs if noise exceeds a certain criteria. OSHA recommends the use of hearing protection devices (HPD) when an employer is exposed to an average noise intensity of 85 dBA over an 8-hour shift. HPD is required when exposed to average noise intensity of 90 dBA or greater over an 8-hour shift.[4] 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.[4][5] But this does not mean that OSHA considers HPDs to be effective.[6]

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.[7] 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.[8][9] 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.[10] 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.[11] 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.[12] 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.[13]

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).[14] The A-weighting curve weights the mid frequency content, 500 to  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

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The variability in the effectiveness of the earmuffs when used by 8 participants

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(REAT method)

A typical earmuff attenuates (decreases) the level of noise by approximately 23 dB when tested under carefully controlled laboratory conditions.[16] The EPA requires that earmuff manufacturers test each device's performance and indicate their specific noise-reduction capabilities on the product labeling.[17] 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.[18] Experiments have indicated that the actual attenuation achieved by ordinary users of earmuffs is only 33% to 74% of the labeled NRR.[18] 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.[18] 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.[19] 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),[20] 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.[20]

Passive vs. active

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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. The material and structure of the earmuff device is used to decrease the sound level that is transmitted through the ear canal. Materials, such as a cupped foam coated in hard plastic, will block sound due to the thick and dampening properties of the foam.[21]

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[21]) low frequency noise.[22] 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.[23] 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.[22]

Dual protection with earplugs

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Most earmuffs can be expected to provide adequate attenuation for noise levels up to 103 dBC.[16] 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 MSHA regulations stipulate that workers must use dual hearing protection when average 8-hour exposures are 105 dBA or greater.[17] The US Department of Defense recommends use of dual protection when exposed to noise ranging from 108-118 dBA.[24] Dual protection is also recommended when shooting firearms because of the extremely high-level impulses (140 dB and greater) produced.[25]

The amount of noise reduction from dual hearing protection is NOT a sum of the noise reductions ratings from the two devices.[26] 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.[26] In the preceding example, the combined earmuff and earplug NRR would be estimated at 30 dB (25 dB plus 5).

Barriers to effectiveness

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Fit

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A proper fit of the earmuffs on the head is essential to providing adequate hearing protection.[27][28] Individuals will require earmuffs of differing sizes.[29] 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.[29] 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.[30] Even eyeglasses with thinner frames can reduce the effectiveness of hearing protection by 3-7 dB.[29]

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.[29]

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.[31]

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.[21]

Structural transmission

It is possible for sound to transmit through the earmuff materials, reducing the device's effectiveness. This transmission is primarily seen above  Hz.[31]

Vibration of the earmuff

Exposure to high level noise (190 dB SPL) may cause the earmuff to vibrate off the external ear causing a leak which would allow hazardous exposure to dangerous levels of noise.[32] 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.[31] Technology in earmuffs is developing and shows promise in reducing the effects of airflow vibrations in the ear muffs.[21]

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.[33]

Deterioration

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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.[33] Physical changes could create an opening to the ear, allowing sound through and reducing attenuation. According to some manufacturers, ear cushions should be replaced every 6-8 months if used regularly. If earmuffs are used very frequently then the cushion should be replaced every 3-4 months.[34]

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.[35]

Specific considerations for hearing protection for workers with hearing loss

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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.[36] This occurs due to the hearing protection device (HPD) attenuating the signals/noises below the level that the worker is able to hear.[37] OSHA regulations require individuals to wear HPD regardless of their amount of hearing loss, even if they have a severe to profound hearing loss.[38] Workers that have sustained a standard threshold shift are required by OSHA to wear HPD at an 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.[39]

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.[32] Wearing hearing aids alone, without the use of earmuffs, could potentially cause additional noise-induced hearing loss. It is recommended that workers should not use their hearing aids without the use of an earmuff when exposed to sound levels over 80 dBA.[40]

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][39] 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 with hearing loss. This type of HPD are commonly referred to as "musicians plugs."[39] NIOSH provides a "Hearing Protector Device Compendium" with information on the different types of HPD.[41]

See also

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References

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