David C. Byrne, MS CCC-A, and Dennis P. Driscoll, PE
Associates in Acoustics, Inc., Evergreen, Colorado
Implications for Engineering Noise Controls
A majority of noise sources encountered throughout many plants are relatively steady-state, which means that their sound levels do not fluctuate over time. However, periodic steam or compressed air releases may temporarily exceed the ambient level by 10 dB or more. These (and other) types of intermittent noise sources may interfere with effective communication by temporarily masking the alarm signal or notification message.
Designing an emergency warning system for use in steady-state noise levels is much easier than trying to account for sporadic fluctuations in the background sound level, particularly when the intermittent sounds are high intensity and of relatively short duration. Therefore, consideration should be given to eliminating or reducing the noise from these types of sources, as another method to ensure adequate voice communication. Properly selected and installed silencers or mufflers may solve most of these problems; however, it is recommended that a detailed acoustical engineering analysis be conducted to define the appropriate control technique.
Although implementation of engineering noise controls may not be required to maintain compliance with the OSHA Occupational Noise Exposure Standard (29 CFR 1910.95), reduction of noise levels may still be desirable. In addition to treating intermittent noise sources, engineering control treatments should be investigated for certain high intensity steady-state noise sources as well. Lower noise levels will allow for more effective communication, provide a better working environment, and may enable hearing conservation measures to be eliminated in some buildings or areas.
It is important to recognize that all contingencies may not be able to be accounted for in all circumstances. In particular, the hearing and auditory processing abilities of an individual employee may vary from day to day, which may cause an alarm signal to go undetected or unrecognized. Therefore, persons with known or suspected hearing deficiencies or problems may have to be accounted for on an individual basis.
Significant communication problems are often encountered by individuals with various degrees of hearing loss. High-frequency hearing loss is usually the consequence of long-term exposure to excessive noise levels, as well as a typical result of the aging process. As previously mentioned, high-frequency hearing ability is essential to the understanding of spoken language. Therefore, hearing-impaired indi-viduals are at a disadvantage even before the effects of an inadequate communication system or high ambient noise levels are introduced. The problem is exacerbated when an employee with hearing loss must wear hearing protection in the work environment.
Many industrial employees dislike wearing hearing protectors based on their complaints that the protectors interfere with necessary speech communication. However, hearing protectors attenuate both the speech and background noise by equal amounts, and therefore should not adversely affect speech reception ability for normal-hearing listeners. In fact, wearing hearing protection in high noise areas (above 85 dBA) actually improves speech recognition by lowering the overall sound level reaching a listener's ear, which reduces the potential for auditory distortion.
There are situations, however, where a particular hearing protector may attenuate high-frequencies substantially more than the low-frequency sounds will mask or obscure the high-frequency components, and cause the important consonant sounds to be unintelligible. Similarly, when hearing-impaired employees wear hearing protection, the higher-frequency sounds may be attenuated to a point below the level of audibility. Therefore, too much attenuation (i.e., inadequate hearing protector selection) may be the cause of communication problems for normal-hearing as well as hearing-impaired employees.
These considerations provide support for reducing background noise levels through the implementation of engineering controls. Until such controls are in place, hearing protectors should be selected that provide adequate but not excessive attenuation. Octave-band noise data from the workplace should be used to select the appropriate hearing protector for use in a particular noise environment. The degree of protection provided may be determined by using the National Institute for Occupational Safety and Health (NIOSH) Method 1 or "Long Method." This technique uses the measured spectral information, and is the most accurate procedure to estimate the hearing protection provided by a particular device as used in a specific noise environment. Over-protection, or similarly, wearing hearing protection in areas with sound levels below 80 dBA will interfere with speech communication and notification message intelligibility.
As indicated in a previous section, the alarm signal or the speech message should be 15 to 25 dB above an individual's masked threshold. In areas with high background noise levels, this will necessitate sounding of the alarm very high intensities. However, in these cases hearing protection will already be required, so that the overall sound level reaching the employee's ear should not pose a hearing hazard. Hearing protectors should continue to be worn during a broadcast of the warning signal and notification message. When the sound level of the ambient noise is greater than 110 dBA, a secondary alerting device (e.g., visual signal) should also be used.
The National Fire Alarm Code (NFPA 72) discusses the requirement for a fire alarm signal to be distinctive in sound from other signals and indicate that this sound not be used for any other purpose. Effective July 1, 1996, this required the use of the signal pattern described in ANSI S3.41-1990 "Audible Emergency Evacuation Signal." Toward the ANSI requirements, the signal shall consist of a "three-pulse" temporal pattern. Three successive "on" phases, lasting 0.5 second each, must be separated by 0.5 second of "off" time. Then, at the completion of the third "on" phase there must be 1.5 seconds of "off" time before the full cycle is repeated. Therefore, the total cycle shall last 4.0 seconds (0.5 second "on," 0.5 second "off," 0.5 second "on," 0.5 second "off", 0.5 second "on," 1.5 seconds "off"). However, this signal pattern is to be used only to notify personnel of the need to immediately evacuate the building. In many plants total evacuation is not always desirable or necessary during an emergency. The National Fire Alarm Code states that the ANSI S3.41 Audible Evacuation Signal "...shall not be used where, with the approval of the authority having jurisdiction, the planned action during a fire emergency is not evacuation, but relocation of the occupants from the affected area to a safe area within the building, or their protection in place."
The acoustical environment of the interior of a car or truck is highly variable, depending on the type of vehicle, whether the windows are open or closed, the condition of the vehicle, weather conditions, etc. Most of these variables cannot be evaluated to the extent necessary to realistically expect that an alarm signal will be heard in all types of vehicles and in all situations. Therefore, visual signaling devices should be used to alert all vehicle occupants that the alarm has sounded. Instructions should be given to all drivers to immediately stop the vehicle and roll down the window (or exit the vehicle) to listen for the subsequent voice notification message.
Requirements contained in NFPA 72, Chapter 7, should be followed for alarm system inspection, testing, and maintenance. These requirements should be supplemented by recommendations supplied by the equipment manufacturer and installer. When plant operating parameters change or process machinery is added or removed, the background sound levels may change. Therefore, testing and maintenance personnel should be aware that loudspeaker outputs may require adjustment from time to time.
system operation should be protected by an independent secondary
or standby power supply. Additionally, a backup alarm system
is necessary when the primary alarm system is temporarily out
of service. This backup system must be capable of providing
alarm service equivalent to the primary system, although it
does not need to be an exact duplicate in terms of mechanical
and electronic equipment. If a portion of the alarm system (e.g.,
one floor in a building) is undergoing maintenance or has an
equipment failure, the backup system may involve the use of
personal radios, telephones, or employee runners to adequately
notify and inform all personnel of the emergency situation.