What is "Quiet"? -- Effect of changes in ambient sound
As you already learned from the research that Dr. Rob Wall Emerson and I did, the factor that most impacted the ability of the blind subjects to hear vehicles was the ambient sound.
Let's look at those results more closely, in view of what I explained on the last page about residual noise levels (the sound level of "quiet") in various communities and at various times of the day.
The graph below shows the effect of the ambient sound level on the subjects' ability to hear the vehicles at our research site where the street had no bends or hills or barriers (for a verbal description of the graph, click here).
The vertical axis shows how many seconds passed from the moment that the subjects heard the vehicle until the vehicle arrived (the detection-to-arrival time), and the horizontal axis shows how loud was the environment when they heard the vehicle (for example, was there noise from a lawnmower, airplane, receding traffic, etc.).
The pink line shows the average of the vehicle detection-to-arrival times.
Notice that as the ambient sound got noisier, it became more and more difficult to hear the approaching vehicles.
Now consider the fact that in cities, the residual noise level (the sound level of "quiet") during the day is more than 55 db(A).
What does that tell you about the ability of blind pedestrians to hear the approaching vehicles when it is "quiet" in the city?
They are likely to be in a Situation of Uncertainty during the day because they probably cannot hear approaching vehicles until they are too close.
- When it was very quiet (37 db(A) or less), all the vehicles were heard more than 8 seconds away.
- At 49 db(A), the pink line indicates that average vehicles were not heard until they were less than 8 seconds away.
- At 55 db(A), none of the vehicles could be heard more than 8 seconds away.
And consider that the residual noise level varied during the day at each site.
For example, the last page gave you a sample from a "Typical Residential Community" where the sound level of "quiet" at night was about 30 db(A) and in the afternoon it was about 48 db(A).
According to the graph above, when the ambient sound was less than 38 db(A), the subjects heard all the vehicles at least 10 seconds away,
but when it was 48 db(A) at the same site, half the vehicles could not be heard until they were less than 10 seconds away.
So what would happen if our research site has the same variation in "residual noise level" that the typical residential community had?
That would mean that at night, when the sound level of "quiet" is 30 db(A), pedestrians whose crossing time is 10 seconds could be confident that it is clear to cross when quiet (if there was a vehicle less than 10 seconds away, they would have heard it).
But in the afternoon at that same crossing, they would probably be in a Situtation of Uncertainty because when it was "quiet," the ambient sound level would be about 48 db(A) and at that level in our research, half the vehicles could not be heard until they are less than 10 seconds away.
This effect of variation in the sound level of "quiet" may explain the difference in the ability to hear traffic at different times of the day at the crossing where Gordon Parks was killed (this is explained later in this section).