Mechanical Shakers utilize an extremely simple principle to generate sine vibration in the range of 5 Hz to 60 Hz, with vibration levels as high as 10g.

Powered by an electrical motor, the Sine vibration forces originate from a deliberately out-of-balance shaft (or shafts) underneath the Shaker top plate.
The frequency of vibration is tied to the rotational speed of the motor.
The amplitude of vibration is tied to the extent of shaft imbalance.
(Both frequency and amplitude can be adjusted by the user).

With a single rotating shaft, the force vector rotates, and the simply-sprung top-plate vibrates in a circular pattern. For Shakers with twin rotating shafts, the top-plate can be configured to vibrate only up-and-down, OR only side-to-side.

Furthermore, the speed of the motor can be automatically varied, to accomplish controlled Sine Sweeps thru a frequency range.

So there are numerous variations on the one simple theme.
Mechanical Shakers have a number of compelling advantages, for low-frequency Sine testing: