Tremolo and vibrato are musical terms for amplitude modulation and frequency modulation, respectively. Amplitude modulation, AM, is the changing of amplitude or the volume. Frequency modulation (FM) is the changing of frequency or the pitch. Rotating speaker cabinets can simultaneously add vibrato and tremolo to organs. The vibrato is produced by the Doppler Effect. As the speaker rotates toward the listener the pitch is higher, as it rotates away from the listener the pitch is , because the speaker is at times facing the listener and at times facing away, it also creates a tremolo.