What amount of hearing protection does it afford?

Francis X. Baur, AuD, is the owner of Amplified Hearing LLC, located in Hanover Township, Pa. Thomas R. Zalewski, PhD, is an associate professor of audiology in the Department of Audiology and Speech Pathology at Bloomsburg University, Bloomsburg, Pa.

Headphones with noise-cancellation technology indicate that background noise can be attenuated to allow comfortable and safe listening. Since noise-cancellation technology is a relatively new hearing protection device, minimal research is available on this topic. The purpose of this study was to determine the attenuation value provided by the Bose® QuietComfort® 2 Noise-Cancelling Headphones in the presence of high-level white noise.

Noise-induced hearing loss (NIHL) is the second most common form of acquired hearing impairment,1 following age. It can be prevented by avoiding extended exposure to increased noise levels or by using protective devices to reduce the intensity level of the noise. According to the National Institute on Deafness and Other Communication Disorders (NIDCD), 22 million Americans suffer from NIHL to some degree. NIHL is due to extended exposure to sounds at or above 85 dB that can damage the structures of the inner ear.2 The intensity of the sound and the duration of the exposure will determine the rate the auditory structure is damaged as well as the extent of the damage.

The action level (AL), measured in decibel time weighted average (TWA), is the level of sound intensity exposure that requires implementation of a hearing conservation program, hearing testing, and the use of hearing protection.3 Time weighted average refers to a noise dose normalized to an 8-hour period. The Occupational Safety and Health Administration (OSHA) AL is set at 85 dB TWA because permanent cochlear damage may result from extended exposure at or above this intensity level. The process of permanent metabolic cochlear damage occurs when the hair cells within the cochlea swell from being exposed to excessive levels of sound, causing a possible metabolic disruption in the physical orientation of the hair cells in relation to the tectorial membrane.4

OSHA instituted standards for recommended noise exposure in 1983 to protect the population exposed to occupational noise.5 The standards were based on a 5 dB exchange rate, meaning for every 5 dB increase in sound level, recommended allowable exposure time decreases by half.

OSHA

NIOSH

Duration/day (hours)

Sound level (dBA)

Duration/day (hours)

Sound level (dBA)

8

90

8

85

4

95

4

88

2

100

2

91

1

105

1

94

0.5

110

0.5

97

0.25 or less

115

0.25

100

TABLE 1. Permissible noise exposures for the National Institute for Occupational Safety and Health (NIOSH) and the Occupational Safety and Health Administration (OSHA). dBA = decibel level using the A-weighted scale.

The National Institute for Occupational Safety and Health (NIOSH) re-evaluated these standards and recommended a 3 dB exchange rate (Table 1). For every 3 dB increase in sound level, recommended allowable exposure time limits are decreased by half. These guidelines are stricter than the OSHA 5 dB exchange rate criteria and closely correspond to the phenomenon that a true doubling of sound power occurs for every 3 dB increase in sound level.6 The NIOSH goal was to protect a greater percentage of the population exposed to excessive sound levels. Further, OSHA’s exchange rate is based on the premise that individuals are not exposed to a constant intensity level throughout the entire 8hour period. That is, the individual will have periods when the ear is exposed to levels below 85 dB, which provides the auditory system a rest and perhaps time to recover from the hazardous intensities.

NIOSH recommends a maximum permissible noise level exposure time of up to 8 hours TWA at an 85 dBA industrial noise level without the use of hearing protection. Decibel A scale refers to the decibel level using the A-weighting scale. The A-weight-ed scale approximates loudness perception by the human ear based upon a pure-tone frequency response at 40 dB sound pressure level (SPL) and provides the best estimation of the potential for noise exposure damage.6

OSHA sets its exposure level at 90 dBA for 8 hours of exposure time. However, an individual can be exposed to environments with sound levels that exceed 85 dB if hearing protection is used. Hearing protection, when fit and worn properly, can provide an appropriate amount of attenuation to allow individuals to be exposed to hazardous sound levels without risk of damaging the auditory system.

Research addressing attenuation capabilities of various hearing protection devices should be interpreted with caution since the findings can be easily influenced by methodological errors. The variability may be a result of device improper use or fit, especially plugs inserted by the participant.7 Further, custom-molded plugs may vary due to the mold impression errors, as well as manufacturing issues. Earmuffs can distortion both the pinna and earphone cushion assembly, which can lead to variability between subjects. In general, slow-recovery earplugs have been found to provide the greatest amount of attenuation from 125 Hz to 8000 Hz, followed by earmuffs with active noise reduction and finally earmuffs without active noise reduction.7 The attenuation differences were primarily seen in the frequencies below 1000 Hz, while the values were similar in the frequencies above 1000 Hz across the types of hearing protection.7

The Bose QuietComfort 2 Noise Cancelling Headphones features technology that can be used as a noise reductor. In order for any hearing protection device to be used appropriately, the attenuation values must be known. Unfortunately, the researchers were unable to locate attenuation values regarding this technology. The Bose QuietComfort 2 Noise Cancelling Headphones uses an active noise reduction system for attenuation purposes. Microphones in the ear cups of the headset measure the amount of background noise that is interfering with the desired signal.8 A correction signal is produced in 180° opposite phase of the unwanted signal and, when added together, reduces the intensity of the unwanted signal.8 This noise reduction technology is reportedly coupled with QuietComfort ear cushions, which provide an acoustic seal with the ear. The QuietComfort ear cushions are circumaur-al, fitting around the outside of the pinna, as opposed to supra-aural, which sit on top of the pinna, or insert earphones that are designed to fit directly into the ear canal.9

Why We Need to Know About This Technology

Awareness regarding excessive noise exposure is important for consumers with no prior noise-exposure education. Chung et al10 found that adolescents and young adults would be more inclined to use hearing protection if they received appropriate education. Further, health professionals play an important role in the educational process, requiring these individuals to have adequate knowledge on this topic.

The absence of attenuation values on these types of systems (including the Bose system examined in this article) prevents appropriate discussions concerning the use of this device as hearing protection. Therefore, identifying the attenuation values provided by the Bose Noise-Cancelling Headphones will allow professionals to make appropriate recommendations for its safe usage. It will also allow the consumer to make informed decisions regarding the types of environments in which this noise reduction system should be used.

As this technology becomes more popular, health care professionals should be kept informed on the specifications of these devices to promote education, answer questions, and make appropriate usage recommendations.

Methodology

Participants. A total of 30 volunteers (M = 22.87 years of age; SD = 1.28; 26 females, 4 males) agreed to participate in this project. The participants underwent an evaluation consisting of otoscopy to evaluate the status of the external auditory meatus and the tympanic membrane (TM). Real-ear measurements were taken to determine the natural resonance of the external auditory meatus and the sound pressure level within the canal to determine the noise reduction provided by the device. The inclusion criteria consisted of an unoccluded external auditory meatus and an intact tympanic membrane.

All participants who met the inclusion criteria were seated in a chair facing away from the Audioscan Verifit system to prevent any visual cues that may bias the testing. A probe tube attached to the Verifit system was placed 31 mm from the tip for an adult male and 28 mm for an adult female into the ear canal, representing the length of the average male and female ear canal. The probe-tube length indicator was placed at the intratragal notch of the ear to ensure appropriate placement.11

Measurement of the ear canal natural SPL was completed using the probe tube of the real-ear system. White noise of 85 dBA produced by the Virtual Audiometer 1.1 computer program was presented via the speaker system into the soundfield of the test room at a distance of approximately 1 meter from the participant. The reference microphone from the real-ear system verified the intensity of white noise level for the participants at the level of the ear. The 85 dBA white noise level was used, as it is the maximum allowable noise level without use of hearing protection as stated by NIOSH.3 Measurements of the SPL in the ear canal of both ears were taken individually by the probe tube attachment to determine the real-ear unaided gain (REUG).

The headset of the Bose QuietComfort 2 Noise Cancelling Headphones was then carefully placed over the ears of the participant. The QuietComfort ear cushions provided an acoustic seal around the ear, with the exception of the slit the probe tube made under the cushion. The SPL within both external auditory canals was measured individually in the presence of the white noise using the Audioscan Verifit system. These measurements represented the attenuation of the white noise provided by the headset. The difference between the dBA levels in both ear canals with and without the headset represented the level of noise reduction (NR) provided by the headset.12

FIGURE 1. The average amount of NR by the headset in 85 dBA of white noise.

Results

The results indicate a significant reduction in the 85 dBA white noise level in the ear canal upon placement of the noise-cancellation headset. The amount of NR is shown in Figure 1, while the ear canal sound pressure values with and without the headset and the standard deviations are shown in Table 2. Statistical analysis indicates the amount of reduction to be significant for dBA (F = 1200.852; df 1, 116; P < 0.0001), with no significance when comparing ears dBA (F = 2.379; df 1, 116; P = 0.126).

Discussion

The purpose of this study was to determine the amount of NR provided by the Bose QuietComfort 2 Noise Cancelling headset in the presence of 85 dBA of white noise. The results indicate that the NR produced by the headset was 13.3 dBSPL.

Headset Noise Reduction (NR) in dBA Open Ear

Ear

N

M

SD

Left

30

89.55

1.69

Right

30

89.03

1.92

Combined

60

89.29

1.81

Headset

Ear

N

M

SD

Left

30

76.33

2.57

Right

30

75.67

2.11

Combined

60

76.00

2.3

The amount of NR provided by this technology is sufficient to allow for extended exposure to noise levels up to approximately 98 dBA to remain within a safe listening level, providing the attenuation does not vary in higher levels of noise. Environments such as an average office or home (75 dBA), a machine shop (85-95 dBA), or average street noise (70-85 dBA) would be suitable for using this hearing protection system without limiting exposure.13 Louder environments, such as inside of a subway car (95-105 dBA), use of a power mower (105-115 dBA), or exposure to loud street noise (90-110 dBA),13 would require limited time exposure when using this device as a noise reductor.

The current research found that the headset did provide a statistically significant amount of NR. The headphone technology provides appropriate NR for some environments, and hearing care professionals should be a proponent for the use of this technology. However, the safe use of this device is dependent upon the level of noise and the duration of exposure.

Since this project did not examine the amount of reduction produced by the Bose system at levels that exceed 85 dBA, it is not known if the active noise suppression system will produce a greater amount of attenuation at these levels. Therefore, consumers and health care professionals need to be made aware that the potential does exist to damage the auditory system if the noise levels and duration exceed the allowable limits established.

References

  1. Royster JD. Noise-induced Hearing Loss. 3rd ed. Needham Heights, Mass: Allyn & Bacon; 1996.
  2. National Institute on Deafness and Other Communication Disorders (NIDCD). NIDCD Fact Sheet. Available at: http://www.nidcd.nih.gov/ health/hearing/noise.asp. Accessed May 7, 2008.
  3. Suter AH. Standards and regulations. In: Berger EH, Royster LH, Royster JD, Driscoll DP Layne M, eds. The Noise Manual. Fairfax, Va: American Industrial Hygiene Assn; 2003: 647
  4. NIH Consensus Statement. Noise and Hearing Loss. JAMA. 1990;263:3185-3190.
  5. Occupational Safety and Health Administration (OSHA). Dept of Labor Occupational Noise Exposure Standard. Washington, DC: OSHA; 1983. Publ No. 29 CFR 1910.95.
  6. Earshen JJ. Sound measurement: instrumentation and noise descriptors. In: Berger EH, Royster LH, Royster JD, Driscoll DP, Layne M, eds. The Noise Manual. Fairfax, Va: American Industrial Hygiene Assn; 2003:70.
  7. Berger EH. Hearing protection devices. In: Berger EH, Royster LH, Royster JD, Driscoll DP, Layne M, eds. The Noise Manual. Fairfax, Va: Am Industrial Hygiene Assn; 2003:379-454.
  8. Bose Corporation. Bose Product Information. Available at: http://www.bose.com/controller?event=VIEW_STATIC_PAGE_EVENT&url=/popup/tech_details/pop_qc2_noisecancelling.jsp. Accessed September 15, 2007
  9. Katz J. Handbook of Clinical Audiology. 5th ed. Baltimore: Lippincott Williams & Wilkins; 2002:90-92.
  10. Chung JH, Des Roches CM, Meunier J, Eavey RD. Evaluation of noise-induced hearing loss in young people using a Web-based survey technique. Pediatrics. 2005;115(4):861-867
  11. Moodie KS, Seewald RC, Sinclair ST Procedure for predicting real-ear hearing aid performance in young children. Am J Audiol. 1994;3:23-31.
  12. American National Standards Institute (ANSI). Methods for Measuring the Real-Ear Attenuation of Hearing Protectors, S12.6-1997 New York: Acoustical Society of America; 1997
  13. Feuerstein JF Occupational hearing conservation. In: Katz J, ed. Handbook of Clinical Audiology. Baltimore: Lippincott Williams & Wilkins; 2002:567-583.
  14. US Department of Labor Occupational Safety and Health Administration. Occupational noise exposure-1910.95. Available at: http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=standards&p_id=9735. Accessed May 27, 2008.

Correspondence can be addressed to HR at HREditor or Thomas R. Zalewski, PhD, at tzalewsk’,’bloomu.edu.