Although the open source Audacity audio software is primarily designed for audio editing rather than sound design, you can also use the program to create relatively simple synthesised sounds. Software instruments in the VST and AU formats are not compatible with Audacity; however, several synthesizer plug-ins in the Nyquist and LADSPA formats are available for the program. You can also use Audacity's built-in sound generator as a subtractive synthesizer.
The Computer Music Tools plug-in library for Audacity includes perhaps the most sophisticated synthesizers available for the program. CMT's Analog synthesizer plug-in allows you to set its two oscillators to produce sine, saw, triangle, square and full waves; you can then modify the sound wave with the synthesizer's low-frequency oscillators and attack-decay-sustain-release envelopes. The CMT library also includes a virtual instrument that simulates an organ, as well as a drum synthesizer and a phase-modulated synth plug-in.
Several synthesizer plug-ins written in the Nyquist programming language are available for Audacity. KLSTRBAS is a simple bass synthesizer that runs in Audacity, while the Risset Bell and Risset Drum plug-ins generate synthesised tones that sound like a bell and a drum, respectively. The RNDTONE Audacity Nyquist plug-in synthesises random sound waves.
The CAPS plug-in suite contains several simple synthesizers that are compatible with Audacity. The single-VCO synth produces a simple triangle, saw or square sound wave, while the dual-VCO synth generates two sound waves simultaneously, then allows you to change their pitches and synchronise them together. The CAPS suite also includes the Lorenz and Roessler fractal synthesizers, which generate sound from mathematical algorithms.
Although Audacity does not include a synthesizer per se, you can use the program's built-in sound wave generator as a simple subtractive synthesizer. For example, to make a synthesised bass sound, click "Tone" in the "Generate" menu and set the "Waveform" menu to "Square." Enter a low frequency --- around 100 Hertz --- and click "OK." Duplicate the generated wave, use the "Change Pitch" effect to detune the second wave slightly, then rejoin the waves into a stereo track. Apply a low-pass filter to remove the high frequencies from the synthesised bass sound.