Few things are more common in a noisy workplace than a hearing protection device (HPD). Since noise is the prevalent occupational hazard, HPDs are found almost everywhere and are known to almost everyone. HPDs come in different shapes, sizes, and options: plugs, muffs, semi-inserts, pre-molded, sponge-like, active, noise-canceling, etc. The only thing they have in common is that they are all intended to reduce sound pressure in the ear of the wearer, and by doing so, prevent damage of the exposed worker’s hearing.
Since their objective is so simple, it is surprising that there is such an array of models and types on the market—many made by the same manufacturer. The variety greatly exceeds what is found among other personal protective equipment (PPE), such as hard hats, respirators, and safety shoes.
This article examines the basic characteristics of a hearing protector: sound attenuation and comfort. Both of them are easy to discuss and difficult to measure.
There is no scientific definition for “comfort.” Highly subjective, comfort is an ergonomic concept. No measurement exists to assess it, nor is there a standard with set values or units to assess comfort in a given situation.
In general, no PPE can be said to be truly comfortable. The lack of comfort can be considered minimal for some PPE, such as safety glasses or safety shoes. Probably the worst offenders are respirators, while HPDs—depending of their type (plugs or muffs)—are in the middle of the discomfort scale.
However, the importance of comfort cannot be emphasized enough: if a protector is not comfortable, it either will not be used, or it will be worn in a more “comfortable” way (wrong in most cases). The net result is that people often believe (or feel) they are protected, while in reality they are not.
Sound Attenuation: The NRR vs the Real World
In general, sound attenuation of a HPD can be defined as the difference between the sound levels at the eardrum measured with and without the protector. Under most current approaches, measurements are performed using human subjects that wear the HPD in a laboratory under tightly controlled test conditions and are asked to indicate the lowest sound (threshold) they can perceive with and without the hearing protector in place. Determining sound attenuation of a protector appears to be a simple concept. However, in practice its application has been a major issue that is still unresolved.
Users would love to see a simple way of describing the sound attenuation of a HPD—a “magic number” that will tell them how much the device reduces the sound a worker is exposed to. Given this information, the choice of the protector would be direct: if the noise exposure level is, say 95 dBA, and the existing regulations stipulate a maximum of 85 dBA, then any protector with an attenuation of 10 dBA would suffice. Simple subtraction, and the problem is solved.
This is what made the Noise Reduction Rating (NRR), prescribed by the US Environmental Protection Agency (EPA) in 1979, so popular. By definition, the simple subtraction of the NRR value found on the hearing protector label from the noise exposure of the worker (measured in dBC) would result in the sound level inside the exposed ear. In other words, if the noise level before using the protector was 95 dBC, an HPD with an NRR of 10 will reduce the noise level to a safe 85 dBA (if the noise level is measured in dBA, then the NRR has to be reduced by 7 dB).
As a result of the simple NRR mathematics, hearing conservationists and safety officers got into the habit of recommending HPDs with the highest NRR values with the assumption that “more protection is better.” Consequently, manufacturers started to look into ways of increasing the NRR values of their products in order to increase their sales. A protector with a NRR 1 dB higher than another was considered by many users as a better product and was, consequently, preferred for purchasing purposes—even though the performance difference between protectors is inconsequential with differences of less than about 3 dB.
Almost since the beginning, the scientific world warned users about the over-optimistic values of NRRs. While there is nothing wrong with the values themselves, a problem resides in the measurement protocol where measurements are performed on human subjects in a controlled environment. Protectors under test are worn in such a way as to provide the highest possible attenuation. Therefore, the result of the measurement is the highest attenuation, which is a good indication of the maximum potential performance of the protector, but is not a good indicator of actual performance in use in real life.
The core of the problem is to determine how the result of a test performed in a laboratory-controlled environment compares to the sound attenuation obtained by real users in the workplace. During the laboratory test, HPDs are worn in a fashion that is strictly controlled by the operator/technician conducting the test. The process takes 15 minutes for completion, and if something goes wrong, the test is repeated again until the result appears to be satisfactory. During this period, subjects are not allowed to move or shift position, and they concentrate hard on listening to the test signals.
|For more information on occupational hearing conservation, also see the September 1999 HR which was guest-edited by Maurice H. Miller, PhD.|
In real life, workers fit themselves with the HPD (correctly or incorrectly), and they wear the devices for several hours, typically without touching or readjusting them. During the work period, a worker talks, chews, or otherwise moves his/her jaw, modifying the seal between the protector and skull or ear canal. It is easy to see that the laboratory and the workplace situations are not the same. This is why numerous studies have concluded that the NRR values as quoted by the manufacturers are overly optimistic and not attained by most wearers in their workplaces.
The NRR is not the only way of calculating the protection resulting from wearing a HPD. There are several other methods, such as the Octave Band method, the High-Medium-Low (HML) method, and the Single Number Rating (SNR), and the latter two are included in the 1994 standard (ISO 4869-2)1 published by the International Organization for Standardization. These measures all use results from similar attenuation measurements to calculate the noise level at the protected ear (ie, noise level under the protector) using different mathematical procedures. Since they all make use of the same type of laboratory-measured results, they are as prone to questioning as the NRR.
De-rating the NRR
Several attempts have been made to close the gap between the numerical values of the NRR and the real-life attenuation. One of the most popular de-rating systems is the one recommended by OSHA, where the laboratory-reported NRR is reduced by 50%. For example, in such an approach, an NRR of 28 dB becomes 14 dB. While this is a valiant attempt to align NRR values with real-world performance, there is no scientific evidence for this operation; depending of how the HPD is worn, the NRR can actually be close to 28 dB or much lower than 14 dB.
Additionally, fitting an earplug is much more sensitive to insertion technique as compared to fitting an earmuff. By the same token, during the workday, the cap-mounted earmuff tends to change its position because the safety hat moves quite easily with the head’s movements. As a consequence its attenuation is also compromised.
The above considerations show that applying a single de-rating scheme to all HPD types is simplistic and unrealistic. Some attempts have been made to recommend different de-rating schemes for muffs, plugs, and cap-mounted muffs, where the lower de-rating corresponds to the muffs and the higher to the cap-mounted muffs. Again, it is a purely empirical approach without scientific basis.
In summary, there is no simple numerical algorithm to overcome the problem with the NRR.
A Better Way to Measure Attenuation?
Since the problem of all rating systems derives from the use of optimized measurement results, the obvious solution should be a change in the way the attenuation is measured. This is where the ANSI Standard S12.6-1996 Method B2 comes in. According to this standard, the measurement is performed following a procedure that more closely mirrors real-life situations. As a consequence, the measurement results are also much closer to what is obtained in any workplace with a proper hearing conservation program in place.
While the standard is available for manufacturers to use, there is no regulation that requires its use. The current US regulation pertaining to hearing protector testing was last updated in the late 1970’s, and does not reflect this technological development. Attenuation tests are expensive and time consuming. Unless these tests are required by law or by a regulation, they are typically not conducted and the results are not made available. The reality is that the NRR resulting from the unrealistic measurements is the only attenuation that is printed on the HPD’s packaging.
The Solution: Good Hearing Conservation Programs
Apparently, hearing conservationists and safety officers alike are caught in a situation that has no solution: there are hundreds of HPDs on the market, but there is no realistic information on by how much they reduce the noise level in the workplace.
|TABLE 1. Approximate NRR from different types of HPDs.|
However, there is a way around this problem, and it is a part of every hearing conservation program: it consists of properly choosing HPDs and properly training workers. As mentioned above, comfort is a key issue when dealing with any PPE; if the device is not comfortable, it will be used improperly or not be used at all (the late Aram Glorig, PhD, a leading hearing conservationist, rightly observed that the best protector is the one that is used). Therefore, in the choice of the HPD, the comfort of the worker should be the first factor to be evaluated. A few considerations, such as ambient temperature, occasional or permanent use, need for using a hard hat, etc, will help decide between the three basic types of HPD: plug, muff, or cap-mounted muff.
The next step is to decide on the attenuation characteristics of the HPD. This is where the existing noise exposure levels in the workplace come into play, and one can use de-rated NRR values or approximate attenuation values such as those in Table 1 as a guide. These are presented as approximate values. The real attenuation will be the result of a proper insertion of the plug or the donning of the muff.
The third step is the most important of all: How to encourage workers to wear their HPDs and to wear them properly. There are many pages written on the issue of creating a “hearing conservation conscience.” They all can be summarized as having a proper hearing conservation program in place. This program should deal with the hazard of noise in a holistic way, including not only noise exposure assessment and control, but also awareness of the noise hazard and how to prevent it. (For more information, see Berger et al.3)
In a proper program, only HPDs with the most appropriate attenuation will be selected, avoiding overprotection. In this way, negative effects—such as sense of isolation and increasing hazard of not being able to hear warning sounds—will be avoided.
Alberto Behar, PEng, BASc, is a lead researcher at the Institute for Biomaterial and Biomedical Engineering (IBBME) at the University of Toronto. Correspondence can be addressed to HR or Alberto Behar, IBBME, University of Toronto, Rosebrugh Building, 4 Taddle Creek Road, Toronto, Ontario M5S 3G9; e-mail: .
- International Standards Organization (ISO). ISO 4869-2:1994 Acoustics—Hearing protectors—Part 2: Estimation of effective A-weighted sound pressure levels when hearing protectors are worn. Geneva, Switzerland: ISO; 1994.
- American National Standards Institute (ANSI). ANSI S12.6-1996 Method B: American national standard: determination of occupational noise exposure and estimation of noise-induced hearing impairment. New York: ANSI; 1996.
- Berger EH. Hearing Protection Devices. In: Berger EH, Royster LH, Royster JD, Driscoll DP, Layne M, eds. The Noise Manual, 5th Edition. Fairfax, Va: American Industrial Hygenic Assoc; 2002: 379-454.