Sidebands
Sidebands appear as ‘family of spikes’ either side of the gear meshing frequency in a spectral analysis – FFT – they are the result of amplitude or more often frequency modulation and are an indicator of potential problems. If a gear in a pair of meshing gears is running slightly eccentrically it will vary the tooth loading during the period of one revolution as it becomes tight and then loose this will cause minor variations in the rotational speed.

Sine Wave
A sine wave is a wave form which is generated by the path of a point moving around on a circle as it rolls along a line.