Popular Woodworking 2004-04 № 140, страница 66
foam in the earcup. If your muffs aren't quite comfortable, you shouldn't alter the headband or foam to make them fit better - this will reduce or eliminate their effectiveness. Instead, you should purchase a more comfortable set of muffs. Muffs can be divided even further into two designs - passive muffs and active muffs. Passive muffs are the most common and can be found at your local hardware or sporting goods store. They should sit flush to the head without hair or glasses compromising the seal. The headbands usually are intended to be worn over the top of the head; changing the orientation can reduce the muff's effectiveness, so be sure to check the manufacturer's labels before changing the way you wear them. Active muffs are battery-pow- ered with a volume control and are either level-dependent or have Active Noise Reduction (ANR) technology. Level-dependent active muffs reduce the noise level when the environment or equipment noise level gets above a certain decibel level set by the manufacturer (typically 85dBA). ANR muffs work by monitoring sound at the ear. A microphone measures or predicts the noise spectrum and creates SAFE EXPOSURE TIMES FOR COMMON NOISES NIOSH* max. time dB unprotected exposure Sounds and noises
*National Institute of Occupational Safety "Noise Exposure From Leisure Activities: A & Health "Protection recommended, but not required. More available in Review" in Journal of Acoustical Society of America (1991) - asa.aip.org an out-of-phase signal to reduce the sound spectrum. This technology is practical only for continuous noise spectra (i.e., airplane noise, manufacturing, trucking, etc.) where the noise is on for a sufficient time, allowing the circuitry to detect it. It isn't practical for random or intermittent noise because the circuitry may not have enough time to actively control for short duration noise exposures (such as when using a miter saw). There is an added risk with these kinds of muffs - you must be sure they don't cancel warning or alarm noise, which would be a huge safety issue. Check with the muffs' manufacturer for more information on this. All active muffs allow you to hear voices and other sounds in your work environment and are best used where there are multiple pieces of equipment running continuously. They also are helpful for people with existing hearing loss because the muffs let them hear their environment while still protecting their ears from further damage. Caution should be used in setting the volume control on these devices. While it sounds like a good idea, stereo headphones should not be worn under any muffs. The wires of the headphones (much like eyeglass frames) will compromise the seal on the muffs and reduce or eliminate effectiveness. More likely, people usually set the volume high enough to be heard over the background noise, effectively increasing the noise level experienced so it's louder than the machines and moving the noise right up to the ear. There are, of course, muffs designed with stereo devices built into the headset. If you're considering this type of device, you need to be cautious and set the volume to the point where you 64 Popular Woodworking April 2004 |
