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workers, building, architect

NORA Manufacturing Sector Strategic Goals

921Z6KN - Hearing Protection for Hearing Impaired Workers

Start Date: 10/1/2006
End Date: 9/30/2009

Principal Investigator (PI)
Name: David Byrne
Phone: 513-533-8414
Organization: NIOSH
Sub-Unit: DART
Funded By: NIOSH

Primary Goal Addressed

Secondary Goal Addressed


Attributed to Manufacturing


Project Description

Short Summary

The overall objective of this project is to identify and/or predict when the audibility of important sounds can be improved while wearing hearing protectors. Currently, no standard test method exists for the evaluation of a hearing protector's effect on speech intelligibility. Therefore, data regarding the effects of a particular protector on speech intelligibility are not readily available from the manufacturer. Without the benefit of knowing to what extent each hearing protector affects speech intelligibility for a given acoustic environment (and employee's hearing status), the best device may not always be selected. This project will generate the necessary but currently unavailable data through controlled laboratory studies, and will integrate pertinent information obtained from the scientific literature and other hearing loss prevention professionals.


This project will evaluate various styles/types of hearing protection devices for effects (either advantageous or detrimental) on speech intelligibility in industrial noise environments. In terms of a hypothesis to be tested, it is expected that under certain conditions (e.g., fluctuating or relatively low ambient noise levels), electronically enhanced or uniformly attenuating hearing protectors will out-perform conventional passive devices (i.e., produce higher speech intelligibility scores), and in other situations (e.g., very high ambient noise levels) the differences will be significantly less pronounced. Hearing protectors that perform well on the speech intelligibility tests while providing sufficient noise attenuation will be rated the highest.

The primary purpose is to conduct a systematic investigation into speech intelligibility concerns that arise when a hearing-impaired worker is required to wear hearing protection. Laboratory evaluation of the acoustical characteristics of various hearing protective devices, with an emphasis on optimizing the balance between under-/over-protection, effectiveness, and communication ability, will be conducted using the existing acoustical test facilities at the NIOSH Taft Laboratories. Results of these evaluations will allow recommendations to be made for accommodating noise-exposed workers so that they can perform their jobs safely while still being protected against noise-induced hearing loss. This project will be conducted with an emphasis on manufacturing sector workers and their work environments; however, the results of this investigation will be applicable to workers from other occupational sectors.

This project will be carried out in three phases. The first two phases are expected to take approximately two-and-a-half years, and the third phase will last approximately six months. A draft outline of the project timeline is as follows:

PHASE I: Test protocol development and validation.

a) Conduct an updated literature review on assessment techniques for hearing protector evaluation and speech intelligibility testing.

b) Establish working relationships with other laboratories/agencies (e.g., US Army Aeromedical Research Laboratory, US Air Force Research Laboratory, Canadian Defense Research & Development Center – Toronto) conducting similar research.

c) Investigate the strengths/weaknesses of existing speech intelligibility tests (e.g., MRT, DRT, PB monosyllabic words, HINT, QuickSIN, Callsign Acquisition Test, FAAF, Connected Speech Test).

d) Conduct laboratory testing using human subjects and an electro-acoustic test head to identify features/characteristics of the best or most appropriate speech intelligibility test.

e) Perform pilot testing of the selected test and evaluate the results.

f) Refine laboratory test methods/protocols as necessary; establish an SOP for administering all future speech intelligibility testing.

g) Prepare a journal article outlining the test protocol development and validation process.

PHASE II: Development of a hearing protector/speech intelligibility database.

a) Perform a market survey to compile a current list of the hearing protection devices most commonly used in noise-hazardous industrial environments.

b) Establish a panel of listeners to serve as subjects and perform the speech intelligibility testing.

c) Establish a database to track the performance characteristics of different types/styles of hearing protectors.

PHASE III: Data analysis, report writing, and development of recommendations.

a) Compute appropriate statistics and analyze data.

b) Develop recommendations for optimizing hearing protection usage and speech intelligibility.

c) Prepare journal and trade publication articles for widespread dissemination of the recommendations developed from this research.

d) Provide input for the development of a new ANSI standard.

e) Present research findings at professional conferences (e.g., the annual National Hearing Conservation Association Conference; American Industrial Hygiene Conference & Exposition).


On-site safety or hearing conservation program managers typically do not have access to the necessary equipment for the measurement of speech intelligibility. This project will focus on providing a method by which this important information can be made available and appropriately used by practitioners outside of the research community. An underlying goal is to develop the necessary research foundation that could form the basis of a new national standard for evaluating the ability to communicate while wearing hearing protection. The principal investigator is the Chair of an American National Standards Institute working group (ANSI S12 Committee on Noise, Working Group 13) that is tasked with developing this new standard.

Project Goals:

a) Further refine and validate the previously developed assessment protocol designed to evaluate a worker's hearing ability, communication needs, ability to process signals in noise, and hearing protection requirements.

b) Develop and test a strategy for determining the most appropriate form of hearing protection to maximize perception of speech and warning signals.

c) Provide recommendations for accommodating noise-exposed, hearing-impaired workers so they can continue to perform their jobs safely without incurring additional hearing loss.

This project will be evaluated on the basis of whether accurate recommendations can be developed to suggest what type(s) of hearing protective device(s) will optimize the speech communication ability for individuals working in a noise-hazardous environment.

Mission Relevance

The National Center for Health Statistics has reported that 58% of persons who are unable to hear and understand normal speech are currently employed (NCHS, 1994). Workers in service and "blue-collar" occupations are over-represented among those individuals reporting hearing difficulty. Statistics also show that more than 30 million US workers are exposed to hazardous noise levels and noise-induced hearing loss is the most common occupational disease in the United States (NIOSH, 1996). Therefore, several million hearing-impaired workers in manufacturing, construction, and other occupational sectors are required to wear hearing protection, yet the combined effect of their hearing loss and the type of hearing protector worn may interact to provide a less-than-optimal listening condition. This problem can have serious consequences, considering that a study of older workers with disabilities indicated hearing loss as a risk factor for occupational injury (Zwerling et al, 1998). Current hearing conservation standards, regulations, and practices do not address the problem that a hearing loss could be both a health and a safety issue in the workplace.

Hearing loss prevention was on the master list of priority research topics under the original National Occupational Research Agenda (NIOSH-NORA, 1996). In the 1998 revised criteria document for occupational noise exposure (NIOSH, 1998), attenuation of hearing protectors and management procedures for workers identified with substantial hearing impairment were listed as research needs.

This project will contribute to achieving the NIOSH Hearing Loss Research Program Strategic Goal 3: Reduce hearing loss through interventions targeting personal hearing protection devices. Specifically, it will contribute to meeting Intermediate Goal 3.3: Develop methods for assessing speech intelligibility when hearing protection devices are worn in noise.