How do they do that? Measure sound levels in stations

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.

Sound monitoring microphone.

Sound monitoring microphone.

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.

Sound level meter.

Sound level meter.

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.

6 replies

  1. This is the same infamous tunnel that was added at the last moment and was RUSHED because it was never originally planned. It was built on the fast track because the Times published an article (something those of us who follow transit already knew) revealing that the Blue line would NOT connect to the subway station at 7th Street, a mere 6 blocks away, because the LACTC and SCRTD could not “get along.” OK, they hated each others guts and now the transit public was about to suffer because of it. That is the incident that led to the two agencies being merged.

    This is also why 7th Street/Metro Center is the MOST drab, and poorly designed station of the entire system. Note the stupid stairway at the north platform leading over the trains then back down to the south platform and other STUPID elements that were crammed in, especially the Blue Line tracks that were just shoved on top of the subway lines with platforms located on opposite sides. The station was so rushed in RE-design that the Times reported that they forgot to add drains. They were later added at an additional cost.

    Yeah, it is loud. I wonder how much the platens on the pantographs contributes to the noise in the tunnel?

  2. Kim,
    Dimpling the walls (either drilling dimples or putting small bumps on the walls) can help. But for this location, metal panels that are wavy (ideally with various wave lengths, shapes, or heights) and perforated could be fabricated off site and installed during the nightly down time. They would likely do a better job than simple dimples.

  3. Adding dimples to the walls would work. That’s how sound studios disperse the sound to pure silence. If the walls are flat, the noise bounces back. If the walls were dimpled, it disperses the sound.

  4. The noise on the Blue/Expo line when it is underground is bad. The concrete walls are reflecting all of the high frequency sounds. It is especially bad the last part as the trains are about to surface. There is a lot of screeching sounds involved. Last time I rode that section I actually covered my ears, it was that bad.

    • Hi Just a Person;

      I agree–it is very loud in that particular section just before it comes above ground. I’ve never heard the subway that loud. I’ll mention it to the rail ops guys and see if they know what’s going on.

      Steve Hymon
      Editor, The Source

    • Metro operations folks know about the issue, agree and are working on a solution. They tell us the sound is a combination of wheel-rail interaction, air movement due to the train and noise from train equipment itself. Unfortunately, this particular tunnel seems to be particularly bad about reflecting and amplifying noise. A few months ago they tried grinding the rail but it doesn’t seem to have helped. They’re trying a few more approaches, including contacting other agencies that may have the same issue. We forwarded Manuel Esperon’s “dimpled walls” idea to them. Maybe it will work. Stay tuned.