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Aliasing is a situation in which the signal being sampled contains frequencies which are greater than 1/2 the sampling rate.  If this is the case, the signal will be detected by the analyzer but the frequency will be incorrect.  For instance, if the sampling rate is 6,000 Hz and a 3,500 Hz signal is applied to the input, the analyzer will show the signal at 2,500Hz.
All sound cards provide low pass filtering to prevent aliasing; however, when decimation is used you should either use an external low pass filter or check the box labeled "Apply Low Pass Filter when Decimating" in the Settings dialog box.

See also:  Sampling Rate, Measurement Accuracy , Decimation Ratio , File Decimate