Continuous noise monitoring and reduction system and method

Details Continuous noise monitoring and reduction system and method Continuous noise monitoring and reduction system and method

2001-02-16

2002-09-24

Mullen, Thomas (Department: 2632)

Communications: electrical

Condition responsive indicating system

Specific condition

C340S540000

Reexamination Certificate

active

06456199

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to a noise monitoring system, and in particular to a noise monitoring system and method for continuously and accurately monitoring an individual's noise exposure during periods when an individual's hearing protective device is occluding the ear, and when such device is being worn in an off-the-ear position.
BACKGROUND OF THE INVENTION
The U.S. Department of Labor Occupational Noise Exposure Standard (29 C.F.R. § 1910.95) specifies that noise dosimetry may be used to measure noise exposure on individuals in the workplace. The standard specifies that individuals exposed to greater than 85 dBA Time-Weighted Average (“TWA”) must be included in a comprehensive hearing conservation program (“HCP”). The allowable exposure to noise is measured in terms of cumulative noise dose, i.e., individuals are considered to be within compliance if they are exposed to less than 90 dBA TWA (a 100% dose) over an 8 hour work day. Total noise dose during the work day is given by D=100 (C
1
/T
1
+C
2
/T
2
+ . . . C
n
/T
n
), where D is the percentage noise dose, C is the total length of the specific exposure, in hours, and T is the reference duration corresponding to the measured sound level (see 29 C.F.R. § 1910.95, Table G-16A, 1999). A TWA of the A-weighted sound level may be calculated from the dose measurement by means of the formula: TWA=16.61 log
10
(D/100)+90. This provides a mechanism for accumulating exposures of varying levels and durations where an “exchange rate” of 5 dB per doubling of time is used to evaluate exposure levels. For example, an exposure of 90 dBA for 4 hours is considered equivalent to either 1) an exposure of 85 dBA for 8 hours, or 2) an exposure of 95 dBA for 2 hours. Noise dosimeters are employed to measure cumulative noise dose by applying the “exchange rate” to the level and duration of exposure.
Noise dosimetry is commonly used in industry, and the measurements are usually intended to indicate the cumulative exposure to noise over the course of a full work shift. In addition to determining which employees should be included in the HCP, these measurements are commonly used to determine hearing protector requirements, and to determine noise control requirements. The information gathered by noise dosimeters is typically used by industrial personnel only, i.e., this information is not intended for interpretation by the worker. In many instances, the readouts of dosimeters are sealed shut so that the wearer has no visible indication of current exposure or dose.
Currently existing hearing protective devices (“HPD”) such as ear muffs, ear plugs, and semi-aural devices, provide widely variable attenuation in the workplace and the laboratory ratings of a HPD's performance may grossly overestimate the protection afforded some individuals. There are several methods of measuring the effectiveness of hearing protectors on the end-users, but these methods are point measurements, i.e., measurements made to determine the attenuation provided by the HPDs after one fitting of the device. Point measurement apparatus and methods are described in Epley (U.S. Pat. Nos. 4,060,701; 4,020,298); Padilla (U.S. Pat. No. 3,968,334); and Seidemann (U.S. Pat. No. 5,757,930). A major disadvantage of evaluating hearing protector effectiveness by point measurement is that no insight is provided into the actual protection afforded the HPD wearer at any time, or period of time, other than during the measurement session.
Continuous monitoring of personal noise exposure with conventional hardware is both cumbersome and prohibitively expensive. Conventional noise dosimeters are not intended for use on every employee during every work shift, and are far too expensive to be used on a daily basis on every employee.
Conventional noise dose measurements are intended to be performed with the dosimeter microphone mounted on the shoulder of the employee. This microphone placement technique accurately measures noise dose, but does not take into account the noise reduction provided by the HPD. While it is possible to mount the conventional dosimeter microphone inside a muff-type HPD to measure noise dose while wearing muffs, the hardware configuration is awkward, prohibitively expensive, and not suitable for everyday use.
Evans (U.S. Pat. No. 4,307,385) describes a noise monitoring device, but it is not designed for accurate measurements when not being worn by the user in an over the ear position. This is a major disadvantage of the Evans system since the overall accuracy of measurements will be compromised during periods when the HPD is not donned. The Evans system measures noise exposure when the HPD is worn, but the system does not measure the overall noise exposure to the hearing protector wearer over the course of an average workday, which includes periods when the HPD is not donned.
Damage-risk criteria for predicting the incidence of noise-induced hearing loss (“NIHL”) among populations of workers as a function of workplace noise levels and exposures are the basis of all current occupational noise regulations in the United States. The development of these fundamental damage-risk criteria is based primarily on an assessment of workplace noise levels as measured in a diffuse field at the worker's center-of-head (“COH”) location, but with the worker absent. Thus, it is of the utmost importance that any determination of a worker's protected, or unprotected, noise exposure be correlated to an equivalent COH dose. For example, the popular top-of-the-shoulder microphone location utilized for conducting a personal noise dosimeter measurement of a worker's unprotected noise exposure is simply a convenient and practical surrogate for the true COH location. Measurement of a worker's noise dose using this substitute location provides a reasonably accurate approximation of the worker's true COH noise exposure for most industrial acoustical conditions. However, other locations in the vicinity of the worker's head, or in the ear, are just as suitable as surrogates, especially with the availability of miniature microphones.
Workers in the United States continue to experience an unacceptably high incidence of NIHL despite the existence of federal legislation designed to prevent such injuries. Much of the current state of hearing conservation can be attributed directly to the reliance, over the last 30 years, on limited or single-shift noise exposure data and personal hearing protection as the first, and only, line of defense against hazardous noise. Moreover, past efforts to protect workers from occupational noise have focused primarily on achieving compliance with the noise regulations, rather than prevention. While a single shift measurement of noise exposure is sufficient for compliance purposes, it fails to account for the highly variable daily noise exposures found in general industry. Numerous studies have also documented that the deficiencies associated with personal hearing protectors and their use make it virtually impossible to accurately predict, based on laboratory-derived performance data, their effectiveness in reducing workplace noise exposures. As a result of this ambiguity in both short and long-term noise exposures, many workers have remained overexposed. No strategy to prevent NIHL will ever be effective until this ambiguity in worker noise exposure is eliminated. Thus, a new solution is needed to resolve this ambiguity and facilitate the upstream prevention of NIHL.
The current invention is a system and method for reducing noise exposure and for the continuous monitoring of personal noise exposure. It involves the integration of personal noise dosimetry with a standard hearing protector in such a manner that, the worker's actual noise exposure is accurately measured under all acceptable wearing conditions. Analytical and empirical techniques demonstrate that the surrogate primary and secondary microphone measurement locations, as defined and employed in the subje

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