to determine when it is "clear to cross"
Teaching students to determine when it is "quiet enough," and understand the effect of masking sounds
Students need to be able to . . .
Noticing masking sounds
- NOTICE masking sounds (sounds that are not part of the ambient residual sound) and
- EVALUATE THE EFFECT of those masking sounds on their ability to hear approaching vehicles.
One strategy that may help is to ask the students to indicate when they think it is "quiet" (or as quiet as it can be in that situation!).
If they are reporting that it is "quiet" when there are actually some masking sounds (such as receding vehicles, distant lawnmowers or airplanes, etc.) help them learn to notice those sounds.
Learning the effect of various masking sounds in different situations on the ability to hear approaching vehicles.
They may not report some sounds because they think the sounds are not loud enough to cause a problem, and they may be right!
But you want to be sure they at least NOTICE the presence of those sounds.
Learning whether those sounds make a difference is the next step.
As we've explained in Section 1, a little masking sound usually isn't a problem, but sometimes even the slightest sounds can drastically reduce the ability to hear approaching vehicles.
Students can learn about the effects of masking sounds by observation, either formal or informal as described below:
- When students can hear a car coming from a distance, have them notice that they can't hear cars coming behind it (or sometimes cars coming from the other direction) until after the first car has passed.
Make sure they understand that this is because the sound of the first car masked the sound of the second car.
- When students are measuring the detection-to-arrival times of approaching vehicles, have them notice how much shorter those times can sometimes be when there is another sound such as an airplane or receding car.
You can help students learn to judge when masking sounds are loud enough to significantly impact their ability to hear approaching vehicles by creating steady masking sounds and letting them observe what happens:
Step 1: Find a situation where the student can hear approaching vehicles with enough warning to know that it is clear to cross when quiet.
For example, perhaps a student judges that in a certain situation she can hear the vehicles with enough warning to know that it's clear to cross whenever it is quiet, and the TMAD measurements indicate that her assessment was correct.
You then can play a recorded steady noise softly and ask the student to judge whether she can still hear the traffic well enough above the noise, while you measure detection-to-arrival time to give her feedback.
Sometimes I ask students to set the volume of the recorded noise themselves, making it just low enough that they think they can still hear the vehicles over that masking sound, and then we use the TMAD to see if the masking sound was loud enough to impact the warning time.
Step 2: Create a steady masking sound (I use a recording of my vacuum cleaner!) and ask the student if he can still hear the vehicles well enough.
Make sure the source of the masking sound (such as the tape recorder) stays in the same place and remains steady throughout this process.
Step 3: Use the TMAD measurements to assess whether the student was correct about being able to hear the approaching vehicles above the masking sound with enough warning.