Synth items provide basic sound synthesis capabilities. You can adjust a number of parameters.
Waveform can be set to sine, sawtooth, square or white noise
Attack is the time it takes for the sound the reach maximum value, effectively a fade in.
Decay is the time it takes for the sound to die out, effectively a fade out. Note that the decay occurs within the length of the sound.
Volume between 0 and 100%
Pan turns the right (negative values) or left (positive values) channel down. Setting pan to -20 or 20 completely mutes the right or left channel, respectively.
Length indicates the length of the sound (in milliseconds).
Duration indicates the duration of the synth item, before the next item is presented. This doesn't need to match the length of the sound. For example, the duration of the synth may be set to 0ms, to advance directly to the next item (e.g., a sketchpad), while the sound continues playing. In addition to a numeric value, you can set duration to 'keypress', to wait for a keyboard press, 'mouseclick', to wait for a mouse click, or 'sound', to wait until the synth has finished playing.