Consider a digital audio system and a steadily rising sine wave
tone. At lower frequency, the tone is sampled with many points
per cycle. As the tone rises in frequency, the cycles get shorter
and fewer points are available to describe it.
At half the sampling rate, only two samples are available per
cycle. We are at the limit stated by Nyquist. Still, those two
samples are sufficient to reconstruct the original signal.
If the tone keeps on rising, the number of points per cycle
becomes inadequate to represent the signal. Those samples will be
misinterpreted by the digital-to-analog converter and will be
associated with a wrong frequency. This phenomenon is referred to as
In order to avoid aliasing, all signal content above half the
sampling rate should be removed prior to the sampling (i.e.
the anti-aliasing filter).
The test file
44.1kHz Sample Rate
After the three beep tones, a -3 dbFS sweeping sine tone from 22 kHz
(half the sampling rate) down to 1 kHz.
As the test starts by playing frequencies that are probably beyond
your audible range, you won't hear anything for a moment, except
the three starting beep tones. When the frequency under test
reaches your audible range, a high pitched sine tone will be
heard, steadily decreasing down to 1 kHz.
If your hear anything different, your sound system likely
suffers from (severe) aliasing. In this test, aliasing will appear
as frequencies increasing instead of decreasing.
48kHz Sample Rate
To check if the aliasing is genertated by your browser's audio
plug-in or your sound card, download the original file (click
the download arrow) and play the test tone straight from your hard disk
through your favorite audio application.
Alternatively, you can try the 48 kHz version of this test (beware
: all other test files available on this site are 44.1 kHz).