Sources of vibration in rotating machines:

Source Disturbing Frequency fe Hz
Primary out of balance 1 x rpm x 0.0167
Secondary out of balance 2 x rpm x 0.0167
Shaft misalignment 2 x rpm x 0.0167
Bent Shaft 1 & 2 x rpm x 0.0167
Gears(N=number of teeth) N x rpm x 0.0167
Drive Belts (N=belt rpm) N,2N,3N,4N x 0.0167
Aerodynamic or hydraulic forces (N=blades on rotor) N x rpm x 0.0167
Electrical (N=synchronous frequency) N x rpm x 0.0167

Typical support natural frequencies (fn):

Structures Natural Frequency fn Hz Isolator Frequency fni Hz Isolator type
Suspended concrete floor 10-15 3-5 Helical, Air Springs
Ground Floor 12-34 6-8 Helical, Air Springs, Elastomeric
Soft Clay 12 6-8 Helical, Air Springs, Elastomeric
Medium Clay 15 6-8 Elastomeric Isolators
Stiff Clay 19 8-10 Elastomeric Isolators
Loose Fill 19 8-10 Elastomeric Isolators
Very dense mixed grain sand 24 10-12 Elastomeric Isolators
Limestone 30 10-12 Elastomeric Isolators
Hard Sandstone 34 10-12 Elastomeric Isolators

Significant problems occur when the disturbing frequency fe is near to or coincident with the natural frequency of the supporting structure (floor, foundation or subsoil).