Audio: tremolo (0:06)
tremolo plays a tremolo effect simulating the sound of a steam train leaving a station. The tremolo figure shows the waveform.
Tremolo is a rapid change in amplitude. Tremolo trembles, hence the name.
tremolo is created by modulating pink noise with a LFO.
A Low Frequency Oscillator (LFO) generates subaudio. It might seem a little strange to design an oscillator that plays sound you cannot hear. But that is not the purpose of a LFO. A LFO is a sound controller not a sound generator. Its sole purpose is to control another sound. The output of a LFO cannot be heard, only the effect it has on the modulated sound.
The LFO frequency starts at 1Hz and builds up to 6Hz by the end of the sound. This varies the amplitude up and down once a second (1Hz) at the beginning of the sound, rising to six times a second (6Hz) at the end.
The actual shape of the amplitude envelope is determined by the LFO waveform. The shape of tremolo is an inverse saw wave. This upside down saw wave has a rapid attack and a slow release and the waveform looks like a series of right pointing arrows. This gives the sound its punchy effect.
Pink noise is the chosen sound source because it is rich in low frequencies. A similar effect can be achieved with white noise, which will make the sound brighter, or with red noise to make the sound even more industrial.