Shown below is a
video clip (QuickTime movie, with captions) of the closed lab.
Depending on your network connection, it may take a few seconds to a few
minutes to download this demo.

In this exercise, students generated two pure tones
at frequencies of 2100 Hz and 480 Hz and mixed them into one audio
file with a sampling
rate of 44,100 Hz. They then downsampled the file at a sampling rate
of 3100 Hz. The purpose of this exercise was to have the students
observe that the 2100 Hz frequency component aliases as 1000 Hz because
the sampling rate of 3100 Hz is not greater than twice the frequency
of the 2100 Hz frequency component. The students had to apply the
following formula to compute the frequency of the aliased wave: Given
that f is the highest frequency component of an audio file, fs is
the sampling rate, and ½f_{s} < f < f_{s}, then the frequency
of the aliased wave will be f_{s} – f. 
