Some individuals must wear hearing protectors in order to reduce their noise exposure even after all other avenues of exposure control have been exhausted. However, is it reasonable to expect these individuals to wear earmuffs for long, continuous periods? Measurements of 39 commonly available earmuffs show that in all cases, the pressures experienced on the side of the head are sufficient to restrict blood flow and hence over time produce discomfort. For better results and compliance with earmuff use, breakout times may be necessary to alleviate feelings of discomfort.
Keywords: Circum-aural hearing protectors, clamping pressure, comfort of hearing protectors, noise exposure
|How to cite this article:|
Williams W. Clamping pressure and circum-aural earmuffs. Noise Health 2007;9:45-50
| Introduction|| |
Wearing hearing protectors for the personal reduction of noise exposure is regarded as being the last step in the hierarchy of noise exposure reduction after all other methods have been considered and exposure cannot be reduced by any other means. ,, This is the accepted practice following risk management or the hierarchy for the control of exposure to workplace hazards. This proceeds from the elimination of the hazard down to the use of personal protective equipment.
The wearing of any sort of personal protective equipment can represent an imposition on the wearer for many and varied reasons. For example, while wearing a construction helmet may be hot, heavy, and restrictive of head movement most individuals will accept the disadvantages for the sake of minimizing the effects from being struck by a falling object. Often the significance of preventing an immediate physical injury is perceived as being more important than any comfort or inconvenience issues for the user. This is more so when in situation where the risk of more immediate physical injury takes precedence over the risk of a future possible injury such as hearing loss. This frequently occurs in the construction industry where there is compliance with wearing safety helmets but reluctance to wear hearing protectors due to perceived increased difficulties with communications. ,,
Along with difficulties in communication, discomfort is often given as one of the main factors for either reduced wear time or for not wearing hearing protectors at all. ,,, The possible relationship between the comfort of wearing a hearing protector, specifically earmuffs and the pressure exerted on the sides of the head from the cushions was reviewed by Berger and Mitchell.  From results presented by several authors, Berger and Mitchell concluded that the current methods of calculating ear cup pressure result in uncertainties sufficiently large so as to not provide a useful method for the evaluation of comfort. Comfort was again reviewed by Broughton  who noted that it was very difficult to discuss comfort in a totally objective manner.
Comfort is a complex concept to define in strict and objective scientific terms. One definition interprets comfort as "a state of ease, with freedom from pain and anxiety and satisfaction of bodily wants."  However, if individuals perceive wearing earmuffs to be uncomfortable then there must be some basic underlying cause that is yet to be clearly understood. Expressions of discomfort are frequently associated with the feeling of pressure around the ears while wearing a circum-aural device. Clamping pressure is not a parameter that is usually reported for hearing protectors; however, perhaps a source of discomfort can be attributed between the clamping pressure and blood circulation in the tissue surrounding the ears. This work set out to examine the issue of clamping pressure and possible capillary blood flow interruption.
| Measurements|| |
A total of 39 earmuffs were included in this study. These earmuffs are commonly available in the market for both industrial and home use. All are produced by commercial manufacturers. The data were gathered from hearing protector testing carried out at the National Acoustic Laboratories in accordance with the requirements of AS/NZS 1270  for both mechanical and attenuation testing. Testing is certified by the National Association of Testing Authorities (NATA), Australia.
The attenuation value used for each hearing protector is the mean of the individual attenuation calculated across all test subjects, mi SLC. , The attenuation testing was carried out for each device on a minimum of 16 test subjects, using filtered one-third octave band pink noise at seven octave band centre frequencies (125, 250, 500, 1000, 2000, 4000 and 8000 Hz).
The clamping pressure was calculated from the clamping force and a measure of the effective clamping area of the earmuff cushions. The clamping force was measured with the separation of the ear cup cushions set at 14.5 ± 2 cm. , To calculate the resulting pressure (Pascals or Newtons/m 2 ), the effective surface area of the compressed cushion against the side of the head is required. In order to ensure a consistent method of measurement, a 1-kg mass was used to supply a force (9.8 N) on the rear of one protector cup while the cushion was pressed against a flat, horizontal surface. The resulting surface area was determined by placing a clean white sheet under the pre-inked cushion surface and measuring the area of the resulting cushion impression.
There is a potential for error in the determination of the appropriate surface area to use for the calculation of pressure. This arises from the possible difference between the surface footprint of the external cushion cover and the internal pressure footprint of the cushion filling. If there is a difference, this may introduce an error into the measurement of the actual surface area that transmits the pressure.  For example, in the case of an earmuff cushion constructed from a foam 'donut' contained by an outer 'plastic' cover. The error occurs as the result of the difference in distortion of the two pieces under an externally applied force. The internal foam may simply compress resulting in a very similar sized surface area to the original uncompressed state while the outer cover flattens in shape, resulting in an apparent increase in the external surface area with not all of this surface area bearing a significant load. If this is the case, then the final outcome tends to be an overestimation in the surface area resulting in an under estimation of the pressure calculation of around 12%.  This difficulty does not exist, for example, in the case of fluid-filled cushions as the fluid exerts a consistent pressure over the outer cushion covering.
All of the devices tested had foam-filled cushions. The pressure calculations undertaken here made no specific treatment for this error besides noting its presence.
| Results|| |
The mean individual attenuation ( mi SLC) is plotted against clamping force for the 39 earmuffs and presented in [Figure - 1]. The mi SLC is a convenient method of providing a single value hearing protector attenuation performance figure (in principle, the following discussion is independent of attenuation parameter used as long as it is applied in a consistent manner) that does not include the standard deviation as, for example, with the parameters NRR and SRN.  The mean value of the clamping force was 10.9 N ( n = 39, SD = 2.2) ranging over the values 7.0-17.3 N.
The distribution of the standard deviation of the mi SLC versus clamping force is presented in [Figure - 2]. It would be reasonable to assume that if the clamping force of an earmuff is increased, then both the attenuation and variation in performance, as measured by the standard deviation, should decrease. However, this is not shown to be the case. In the case of both clamping force and standard deviation, there is no significant statistically predictive or useful correlation with attenuation.
The average clamping pressure for the 39 devices was calculated to be 2.5 kPa with a standard deviation of 0.5 kPa. The surface areas ranged from 31.7 to 54.3 cm 2 (mean = 43.3 cm 2 , SD = 5.3 cm 2 ). Again there is no statistically significant or useful predictive value between clamping pressure and attenuation. However, as will be discussed below, it is useful to show on the graph the variation in capillary blood pressure from a mean input (arterial) pressure of 3.3 kPa to the mean capillary output (venous) pressure of 1.3 kPa.  Blood pressure can be measured either directly or indirectly. By direct means it is measured by the insertion of an instrument directly into the blood flow. Indirectly, the maximum pressure is measured by the external pressure required to cause the blood vessel to close down and stop the flow of blood as in the common use of a pressurized cuff during a regular blood pressure check. 
| Discussion|| |
From [Figure - 1], attenuation versus force, the spread of results clearly contradicts the popular conception that an increased clamping force will necessarily produce greater attenuation.  As the graph illustrates for a similar clamping force, there can be a wide range of attenuation amongst a variety of devices. Similarly, sometimes, it is anecdotally suggested  that if the clamping force or pressure was increased for a device, then the performance would improve by decreasing the variance or standard deviation. [Figure - 2] shows a plot of standard deviation versus increasing clamping pressure. Analysis shows that although there is a tendency for the opposite to occur this is not statistically significant ( P < 0.05).
The pressure results presented in [Figure - 3] do not exhibit any clear correlation between clamping pressure and attenuation. As is the case with clamping force, a wide range of attenuation can be expected from hearing protectors with very similar clamping pressures. These results draw the same conclusion as Berger and Mitchell,  and Broughton  that there is no apparent strong correlation between comfort and pressure.
What is worth discussing is a possible cause that makes hearing protectors uncomfortable for many users. When the comfort of earmuffs is discussed, there is usually a mention or link drawn to clamping force and/or clamping pressure; ,,,, thus, it would seem reasonable to assume that feelings of discomfort could arise from the pressure exerted around the ear from a circum-aural device.
[Figure - 3] also shows the normal average capillary blood pressure for arterial side (the inlet of the capillary) at 3.3 kPa and the venous side (the outlet of the capillary) at 1.3 kPa in the average adult.  These specific pressures imply that if a mean pressure greater than 3.3 kPa is applied to the skin surface above the capillaries, blood flow into the capillaries from the arterioles will be limited, and if a pressure greater than 1.3 kPa is applied, blood flow out of the capillaries into the venules will be reduced for the average individual. Effectively, overall, if a pressure greater than 1.3 kPa is applied, capillary blood flow to the area in question will be restricted due to congestion in the capillaries and the disruption of the perfusion of nutrients to the surrounding tissue. This disruption of blood flow may give rise to feelings of discomfort.
As illustrated in [Figure - 3], any pressure greater than 1.3 kPa will restrict capillary blood flow to some extent and result in a feeling of discomfort for most users. The experience of discomfort will vary from person to person due to individual variations and variations in design and performance between hearing protectors. However, it is almost inevitable that the majority of individuals will experience discomfort after they have been wearing earmuffs for some significant time. This will probably result in the protector being removed for the rest of the time it should have been worn. Alternatively, if the discomfort is sufficiently great, the earmuff may not be used at all in the future.
At particular risk are those who work in areas where noise levels are not considered to be high. This is a subjective measure by the individuals who actually work there and may not be governed by any objective measure. The tendency to not wear hearing protectors in what are considered low-noise environments or while performing less noisy tasks has previously been observed and discussed. ,,, For example, farmers will usually wear earmuffs when using a chain saw (high noise) for short periods of time but will tend not to wear earmuffs when using heavy machinery (low noise) for much longer periods even though the overall noise exposure risk may be the same or greater from the use of heavy machinery for the longer period. 
Daniell et al.  commented that it "is not surprising that workers would be more likely to wear protection in higher noise, if only to reduce ear discomfort from the noise," implying that hearing protectors are more likely to be worn if the discomfort from the noise is greater than the discomfort from the hearing protector. Thus, for the individual, the decision is "which produces more discomfort: the hearing protector or the noise?"
None of the earmuffs measured had a clamping pressure below the required pressure of around 1.3 kPa beyond which circulation can be impaired. The minimum calculated pressure was 1.6 kPa. A low clamping pressure will mean that maintaining the ear cup position over the ears will be difficult as the earmuff cups will have a greater tendency to shift with constant head movement.
Individuals who work in high-risk noise environments and hence wear hearing protectors as part of their noise management program will at some stage feel discomfort, the time until discomfort varying between individuals. When hearing protector programs are implemented, break out time should be allocated so that individuals can proceed to quiet areas and remove their protectors for a short time in order to relieve any increasing feelings of discomfort. It is unreasonable to expect individuals to consistently wear personal protective equipment, earmuffs, for extended periods without relief.
'Low' noise areas with A-weighted, 8-h exposure levels in the range of 85-90 dB have recently demonstrated the highest degree of age-corrected hearing threshold shifts or hearing loss.  "Workers with the greatest risk for OHL (occupational hearing loss) may be those employed at companies where a moderate risk or low percentage of workers are overexposed to noise but whose use of protection is low, rather than at companies where noise is most prevalent and protector use is high,"  the implication being that the perceived risk from the noise hazard is low compared to the obligation to wear hearing protectors. Hence, earmuffs are not worn as rigorously as they should be, leading to greater noise exposure and subsequent hearing loss.
In a summary of a subjective analysis of comfort while using hearing protectors, Arezes and Miguel,  in an effort to generalize their results, suggested that there "are no significant differences in the sensation of comfort between the same type (i.e., muff or plug) of HPD (hearing protection device)," and that there is a positive correlation between the measured 'comfort index' of a particular hearing protector and the length of time of use. Further, Arezes and Miguel continue and interpret their results to imply that individuals find the comfort of earplugs similar; the comfort of earmuffs similar; the comfort of earplugs significantly different from the comfort of earmuffs; and that the longer a device is worn, the greater the probability that it will be experienced as being uncomfortable. Although later comments by Edwards  suggest that more work needs to be carried out using a larger range of hearing protectors, the last observation emphasizes the need for break periods during which hearing protectors can be safely removed in order to provide periods of relief from discomfort.
The difficulty of devising a 'comfort index' or 'comfort guideline' has been examined by Bhattacharya et al.  and more recently by Hsu et al .  Bhattacharya et al.  concluded "that the feeling of comfort is a psychological sensation" and that small variations in physical parameters can produce significant alterations in comfort. Hsu et al.  established comfort indices, designed a "comfort tester" and produced guidelines for the design of hearing protectors "which may help increase workers' willingness to wear hearing protection." However, considering the relatively low clamping pressure required for an earmuff that would not restrict blood flow around the ears, a truly comfortable device may be difficult to design. In the future, the design of more form-fitting cushions may assist by minimizing the pressure while retaining seal integrity. Currently, the most obvious solution may be limited wear time or regular breaks during noise-exposed work shifts.
| Conclusion|| |
While many researchers have indicated that it does not currently appear possible to assign a single objective measure of the relative comfort of hearing protectors, the pressure experienced around the ears has been suggested one possible measure. However, while attenuation may not be highly correlated with clamping pressure, it is almost certain that this pressure around the ears can restrict blood flow in the area, thus creating unpleasant or uncomfortable feelings.
Those responsible for supplying hearing protectors as part of an occupational noise management program should be aware that when individuals claim to not like wearing earmuffs because of discomfort there may be valid, scientific reasons. Consideration should be given to the possibility of providing regular 'break-out' time away from noisy locations so that earmuffs can be safely removed and some measure of comfort restored.
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National Acoustic Laboratories, Chatswood, NSW
Source of Support: None, Conflict of Interest: None
[Figure - 1], [Figure - 2], [Figure - 3]