What is "Quiet"? -- Effect of slight increase of ambient sound
I noticed something strange while teaching people to recognize if they are in a Situation of Uncertainty.
In some places, a slight increase in background ("ambient") noise such as from distant lawnmowers and airplanes seemed to have little or no effect on how well we could hear the vehicles approaching.
But at other places, the slightest noise drastically cut back on our ability to hear the vehicles, so that we were unable to hear them until they got much closer.
A good example of this is the crossing where Sue Etters was hit.
As a result, I began to teach people that when they are in situations where they can hear traffic with enough warning when it is quiet, they should notice whether they can still hear it as well when there is a little background noise.
In other words, evaluate how much ambient sound they can tolerate (how "quiet" it has to be) and still hear well enough to be in a Situation of Confidence.
The research that Rob Wall Emerson and I did seemed to validate my observation:
- The chart below shows that for every approach except the hill, increasing the ambient noise as much as 5 db(A) didn't have much effect -- people could still hear the cars just as well.
That is, as the ambient noise went from 36 to 40 or even 42 db(A), detection time fluctuated up and down and back up again before it started to go down permanently.
- It was a different story at the hill approach, however.
When the ambient sound level was 38 db(A), the vehicles approaching from the hill were heard an average of 13 seconds away, but when the ambient sound was increased only 2 db(A), rather than fluctuate up and down, the detection time was reduced to 9-10 seconds, and then it continued to plummet as the ambient sound became louder than 42 db(A).
The graph below shows how the level of ambient sound at each of the 6 approaches affected the ability to hear the vehicles at that site (for a text description of the graph, click here).
The horizontal axis shows the increase in ambient sound, and the vertical axis shows the average time from detection of vehicles to their arrival (in seconds).