# Can't understand the question of a sound problem.

Tags:
1. Jan 29, 2017

1. The problem statement, all variables and given/known data
Two kids are positioned as in the following figure and A emits a long sound. d=100m, x=40m, vsound=340m/s

How long (t) does the sound emitted by A has to be such that B perceives a signal that lasts 2t?

2. Relevant equations

3. The attempt at a solution

Seems like an easy problem. I don't understand what do we mean by "long sound", and what does it mean that a sound lasts t seconds. Does this mean that A can hear the sound for t seconds and B has to hear the sound for 2t seconds?

2. Jan 29, 2017

### Staff: Mentor

Hi kadesh, Welcome to Physics Forums!

By a "long sound" it means a constant sound that is emitted for some time t. Imagine that A has, say, a musical instrument such as a trumpet and he blows a note for a period lasting t seconds, then stops.

3. Jan 29, 2017

### BvU

Bad translation or bad rendering of the problem statement ?: there is no t in there, so 2t is meaningless.

4. Jan 29, 2017

t is the duration in seconds of the sound emitted by A. We need to find that. I just named it t such that we can easily talk about it. Sorry about the misunderstanding.

5. Jan 29, 2017

### Nidum

Is this a primary signal plus echo signal problem ?

6. Jan 29, 2017

Thank you for your greetings! Well, then if B hears the sound for 2t seconds then it hears the original sound for t seconds and the echo for t seconds?

7. Jan 29, 2017

Yes, indeed. My cousin asked me to help her with it. It's been some time since I solved this kind of problems and I forgot how to interpret them.

8. Jan 29, 2017

### BvU

That would be a good approach. Make a sketch: time horizontally and vertically the yes/no (or 1, 0) of B hearing the sound.

9. Jan 29, 2017

### Staff: Mentor

That would depend upon the geometry. B should start hearing sound first directly from A, but later from both A and the reflection from the wall. When A stops making sound, B will still be hearing sound from the wall for some time. The question is, how long should A keep making sound (then stop) in order for B to hear sound for twice as long as A made sound?

You would do well to make some sketches showing the sound's progress over time.

10. Jan 29, 2017

Thanks again for your answer but you kind of lost me there... The graph shouldn't be a horizontal line (y=1) from 0 to 2t?

11. Jan 29, 2017

### Staff: Mentor

Make a few sketches of the setup and superimpose a wave, or block, or arrow, or what ever your choice is to represent the progress of the sound signal. Here's my version for when the sound first reaches B from A: