Much has been said how Interstate 70 is a physical barrier between downtown and the Arch grounds—a moat that is crossable in only a few select locations. Less has been said about the psychological barrier the highway presents, particularly when it comes to noise pollution. After a quick visit to the Arch, I can conclude that not construction of a one-block lid over I-70 nor the removal of large segments of Memorial Dr will noticeably reduce noise pollution at the JNEM.
Using my iPhone (assuming it is accurate), I measured noise levels during the morning rush at various points along I-70 within and near the Arch grounds.
As a point of comparison, I first measured the noise levels of a nearby downtown street, Broadway between Chestnut and Pine. When traffic was passing by, my iPhone recorded an average of 73 decibels; without traffic, a relatively peaceful 61 decibels.
On the Arch grounds, the quietest spots I measured were behind the Old Cathedral and at the base of the east side of the berm separating the Arch from the highway. Noise levels averaged 58 decibels in these locations. Slightly noisier was the center of Luther Ely Smith Park at 63 decibels.
Between Walnut and Pine in line with the middle of the Old Cathedral, I consistently measured noise levels averaging about 67 decibels. This measurement was remarkably consistent over time even during the frequent lulls in traffic passing by on Memorial Dr.
To verify that Memorial Dr was an insignificant contributor to noise levels in the vicinity of the highway, I went a little farther north to where Interstate 70 is level and at-grade with the surrounding terrain directly. At this location, my iPhone revealed the obvious roar of the highway reading 75 decibels with occasional 2-3 decibel spikes from passing trucks. Twenty feet up from the balcony of the “ground floor” of the Mansion House, the highway droned even louder at 77 decibels.
For reference, MoDOT considers the installation of soundwalls whenever noise levels exceed 65 decibels.
Thank you for this comparitive study of noise metrics. This was a useful tool, especially the comparison of MoDOT’s soundwall criteria, to understand noise levels. It would be interesting to know the ambient decibel level on the other side of the Arch (river side).
If that is around the 58 decibels you measured at the “quietest spots”, then we need some other way to understand the difference between 58 acceptable decibels and 73 highway noise decibels. IE, what does an incremental 15 decibels translate to ?
I believe that the definition of a decibel is the smallest difference in sounds that a human ear can detect. 10db increase represents a 10 fold increase in sound energy, which is perceived to be two times as loud. Thus a noise 15 decibels louder than another has 10 x 5 = 50 times the sound energy.
So from the quietest parts of the Arch grounds to the highway, the sound increase would be perceptible about 3 times as loud.
The lid, being a solid barrier, would indeed reduce some sound energy coming from the highway, and its effectiveness would depend on the width of the lid and thickness. Other strategies typically used by landscape designers would be white noise waterfalls.
Comments are closed.