# Finding plane distance from ground, with only a sound recorder and a stopwatch

1. Oct 4, 2012

### cheahchungyin

Hey! Was discussing with my friends back then about the interesting usage of physics in daily life when we suddenly had some interesting ideas that would be exciting to experiment on.

One of the ideas we had was to calculate the distance and velocity of a flying airplane without directly looking at it. We know that there is a plane flying across a particular area, so we can get prepared early before its coming.

Given that we can only record the sound wave produced by the airplane (with minimum background noises and sound diffraction) with a SIMPLE sound recorder, we would be able to get information on the sound waves. With the aid of physics calculations on the waves, we think that we should be able to deduce the velocity, direction of travel, and distance from ground of the airplane. However, after some intense brainstorming, we noticed that we do not have sufficient physics knowledge to deal with this.

To make our lives easier by not getting through all those extra physics studying (as we are not taking it as major), I decided to post the question here to get some help from the pros . Hey guys, is it practically possible to find how fast the plane is flying? Or do we lack information here?

2. Oct 5, 2012

### mathman

General principle: The doppler shift (which is what you would measure) will depend on the velocity component in your direction only.

Further you need to know what the sound frequency would be if the airplane wasn't moving, although you could deduce it if the plane first approaches you and then passes and continues away. The average of the before and after frequency shifts gives you the stationary airplane sound frequency.