# Reflection and transmission of waves

1. Apr 7, 2009

### begbeg42

1. The problem statement, all variables and given/known data

A river canyon is 147 m across as shown in Figure P.32. You are located on the river surface and clap your hands once to generate a brief acoustic pulse. You detect a time interval of 0.407 s between the twin echoes recorded on tape, analyzed when back home (?) in a physics lab. Where were you relative to the canyon walls? (Neglect multiple echoes.)

Answer is m (from the closer wall)

2. Relevant equations

Not sure
3. The attempt at a solution
not sure

2. Apr 7, 2009

### tiny-tim

Hi begbeg42!

Show us what you've tried, and where you're stuck, and then we'll know how to help!

3. Apr 7, 2009

### begbeg42

is this problem anything like where you drop a rock into a well and hear the sound ___ sec. later and have to find the depth? I know how to do this....but this problem confuses me b/c it involves horizontal distances...

I know the speed of sound in 20 degree Celsius air is 343 m/s
I'm not sure physically/intuitively what .407 sec. actually means? is it the time it takes you to hear the echo after you clap?

im utterly baffled. and yes I have read about reflections of sound/echoes etc.
sorry...and any guidance is appreciated

4. Apr 7, 2009

### tiny-tim

Horizontal and vertical are the same …

but in this case you have two sounds, the .407 s is the gap between the two echoes

5. Apr 7, 2009

### begbeg42

t1(sound to travel to wall) + t1(sound to return to you)=a=2t(sound to travel to wall)
t2(sound to travel to wall) + t2(sound to return to you)=b=2t(sound to travel to wall)
4t(sound to travel to wall)=0.407 sec?
t(sound to travel to wall)=.407/4?

d(to wall)=343(.407/4)=34.9...which seems plausible but

i mean this seems too simple...how does the actual width of the canyon=147 m come into play?

Last edited: Apr 7, 2009
6. Apr 7, 2009

### tiny-tim

Draw a diagram

what is a + b?

what is a - b?

7. Apr 7, 2009

### begbeg42

t(echo1) - t(echo2)=0.407
t(echo1)=t(sound to travel to wall) + t(reflected sound to return to you)
t(echo2)=???

d(from wall)=343t(sound to travel to wall)

does the sound hit the wall, return to you, and then bounce back or....how exactly is the second echo produced?

8. Apr 7, 2009

### tiny-tim

ah … you're not getting this canyon thing, are you?

there are two walls … one echo comes from each wall.

Try again!

9. Apr 7, 2009