Measuring Amplitude of sound wave FFT Please, pardon my ignorance. This may be a simple question, but I am still unable to explain what is happening.

I am using a turtlebot to listen to sound emitted from a stationary sound source.

My experiment setup is this:


One stationary sound source emits tone of 500Hz.
A turtlebot robot located about 3.5m away listens and measures the amplitude of the sound source (used FFT and filtered out 500Hz) at a rate of 100Hz while moving linearly toward the sound source at a velocity of 0.1m/s.


The result of the measurements is shown below.
NB: X axis is distance in metres, Y axis is amplitude measured by the turtlebot.


My question is this:
1. Why is there a noticeable dip in the amplitude as the robot moves closer to the sound source?
2. Please can you help me with links to resources that can help me explain this behaviour?
These are interesting results.  Your comment "The X axis represents the distance travelled in metres (from 3.5m away to 0.5m away from the sound source). " seems to contradict "With this explanation, '0' represents 0.5m from sound source, while '3.0' represents 3.5m from sound source."  The latter seems to be the one that fits your data better.

What is more pronounced is the fluctuations you are getting.  Taking the speed of sound at 343 meters per second:

$$ \frac{343~\text{meters}/\text{second}}{500~\text{cycles}/\text{second}} = 0.686~\text{meters/cycle} $$

With standing waves, the nodes are at half the wavelength, which corresponds closely to the size of your fluctuations in the graph.  So as your robot is moving through the room, it looks like you are measuring the standing wave nodes.