Audio: chorus effect (0:06)
chorus effect adds a chorus effect to seven cellos playing different notes. The time delay varies from 20ms to 50ms and back again twice and illustrates a musical application of the Doppler effect. The chorus effect figure, the spectrogram, shows that the peaks and troughs of a chorus effect are more ragged than those in comb filter because the time delay is variable not fixed.
A chorus effect uses a variable time delay.
A chorus effect is achieved by delaying a sound by a small and gradually changing period. The time delay is typically between 20ms and 50ms. A time delay of less than 20ms results in flanging, greater than 50ms produces a noticeable echo.
A fixed time delay produces distinct peaks and troughs in the frequency spectrum, as shown in the comb filter example. Varying the delay time, as in chorus effect, also produces distinct peaks and troughs but they are more ragged in appearance because more frequencies are produced.
The variation in the delay time of a chorus effect is achieved using a Low Frequency Oscillator (LFO). In chorus effect the LFO operates at one third of a Hertz producing an up and down variation in pitch which repeats once every three seconds. This creates a slow shimmering effect. A higher value creates a noticeable vibrato.
The Doppler effect is a perceived change in the pitch of a sound when a sound source is moving relative to the listener. In a sound source approaching a stationary listener the sound waves arrive with a higher frequency than they are being produced by the source and the listener perceives a rise in pitch. Conversely, as the sound source moves away from the listener, the pitch appears lower.
The Doppler effect is used in sound effects to convey an impression of movement. The effect is achieved by varying the time delay. Continuously reducing the delay time causes a rise in pitch, and continuously lengthening the time delay causes a fall in pitch.
The Doppler effect is built into some musical instruments. A Leslie speaker, which rotates an acoustic horn around a loudspeaker, produces a Doppler effect. It is the basis for the justly famous sound of a Hammond organ.
chorus effect has a single chorus effect in which the unprocessed sound is mixed 50:50 with a single delayed copy. A single chorus effect turns a soloist into a duet. Multichorus is achieved by mixing multiple copies with the original sound, each copy having a different and variable time delay. A multichorus effect turns a soloist into a choir.
A chorus effect is used to make one sound like many. Adding a small amount of chorus to a solo voice or instrument, and varying it in the 20-30ms region, produces a full and luscious sound. When noise is processed with a chorus effect, as in chorus effect, the result is a subtle shimmering sound. Increasing the modulation rate changes the chorus effect from a subtle shimmer to a violent vibrato.