How do they do that? is a series for The Source that explores the technology that helps keep Metro running and passengers and other commuters moving. Some of it applies directly to the trains, buses and freeways and some of it runs in the background — invisible to nearly everyone but essential to mobility in our region.
Buses, trains, cars and construction all make noise. That’s why Metro monitors sound levels at Metro properties and projects.
How do they do that?
Sound level measurements are taken throughout the Metro system … but not everywhere. The measuring can be part of routine maintenance. It can be in preparation for a construction project. Or it can be the result of a question or complaint from a patron or someone who lives or works near a Metro project or facility. Whatever the reason, the analysis is done pretty much the same way.
Typically an acoustical engineer measures noise levels with a microphone connected to a sound level meter or other sound recording device that collects the sounds for later analysis.
Noise levels measured at a moderately busy downtown bus stop generally are about 70 decibels. The highest noise levels collected at Metro stations are found at trains running down of the middle of a freeway. Those could be 85 to 90 decibels — by far the noisiest places in the system because of the surrounding vehicle traffic but still safe for human ears in part because the sound exposure doesn’t last long.
By comparison, the humming of a refrigerator is 45 decibels; normal conversation is approximately 60 decibels. Noise-induced hearing loss can result from short bursts of sound from firecrackers or small firearms emitting sounds of 120 to 150 decibels. But sounds of less than 75 decibels, even after long exposure, are unlikely to cause hearing loss, according to the National Institutes of Health. Since bus and train noise is brief and noise level takes into consideration duration as well as intensity, stops and stations are well below what would be considered harmful to the human ear. And that, of course, is what’s important.