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.
To get a relevant sound reading at a station, measurements are taken at places where patrons are likely to linger — near seating or queuing areas, for example. To determine noise exposure potential, sound levels at those locations are averaged over the typical amount of time between trains and the maximum exposure time.
Metro may check sound levels periodically — particularly if a potential problem is perceived. If corrective measures are implemented (sound walls built, for example) then the sound levels will be checked again and any modifications necessary made.
In train stations background noise levels also are checked so that the intelligibility of the public address announcements can be assessed. Speech comprehension measurements are made to determine compliance with National Fire Protection Agency requirements for voice alarm systems, as well as to help with adjusting announcement volumes.
It’s not just the stations that are monitored. During environmental analysis prior to a construction project, noise measurements are made along a proposed right-of-way to determine pre-project background levels. Before construction can begin, Metro also models expected noise levels along planned rail and busway alignments. Construction noise is monitored during construction. This is followed up with monitoring during the pre-revenue testing period, just before a line is opened.
Metro isn’t the only place sound is monitored. Airports, factories and industrial work areas are measured to ensure that the hearing of workers and patrons is protected. One place where sound is not monitored, however, is on our own personal listening devices. Remember that a high amplitude sound could come from a screeching brake, the roar of traffic on a freeway or from recorded rock music. So think twice before you rock out with your personal music recording device or, at least, keep the duration short.
Categories: How do they do that?