Below is a program designed for a workshop dealing with Fourier Analysis and Transforms (click here for the basic manual)
The program allows a user to either record or import a sound wave file. Then the user can modify the sound (and play it back) using the below list of filters:
High Pass Filter - Allows only the frequencies above a set level through.
Low Pass Filter - Allows only the frequencies below a set level through.
Threshold Filter - Removes frequencies whose Fourier Amplitude is below a set percentage of the largest Fourier Amplitude.
Signal Killer - This performs the opposite function of the threshold filter. It removes frequencies whose Fourier Amplitude is above a set percentage of the largest Fourier Amplitude.
Notch Filter - This removes a set width of frequencies around a center frequency.
Notch Pass - This only allows a set width of frequencies around a center frequency.
Comb Filter - This is similar to the notch filter, but it is designed to remove a set of harmonic frequencies.
Phase Botcher - This switch keeps the Fourier Transform graph untouched; however, the phase of each frequency is assigned a random phase.
The Zip files (will need Winzip or some other program to get to the install files; then simply double click on install) are HERE. (NOTE: This program hasn't been updated since 2003 and there are no plans to update it)
Things to try:
1) Whistle one note into the microphone. Then use the notch filter and notch pass around the largest peak. One can also apply the phase botcher to this signal for comparison to future items.
2) Speak a sentence into the microphone. Then try the phase botcher. Also try seeing how long one can recognize the sentence using the threshold filter and the signal killer (the answer might surprise you).
3) Sing a single note into the microphone. Use the notch filter to remove only the largest frequency. Then play back the sound.
If you have any comments about the program, please e-mail me.