Resonance explains why one speed shakes and the others do not

This is less a set of instructions about resonance and vibration control than an argument, and it is worth saying so at the start.

Why a specific speed is worse than a higher one

Every structure has natural frequencies at which it prefers to vibrate, set by its stiffness and its mass distribution. When something excites the structure at one of those frequencies, the response builds far beyond what the same force would produce elsewhere. Machines that vary in speed therefore pass through resonance on the way up and down, which is why one point in the range is noticeably rough.

The amplitude at resonance depends on damping, which is the mechanism by which vibration energy is converted into heat rather than motion. Understanding this explains a common observation: a machine that shakes badly at one setting can run smoothly at a higher one.

Imbalance is usually the excitation

Any rotating mass that is not perfectly centred creates a rotating force, and that force grows with the square of rotational speed. Doubling speed therefore roughly quadruples the out-of-balance force, which is why high-speed machines are balanced far more carefully. Loads that shift, such as laundry gathering on one side of a drum, create imbalance that varies from cycle to cycle.

Wear changes balance over time, since material removed unevenly from a rotating part moves its centre of mass. Damaged fan blades, chipped cutting tools and bent shafts all present as vibration long before they present as anything else.

Isolation and damping do different jobs

Isolation mounts decouple a machine from its surroundings by placing a soft element between them, which stops vibration being transmitted outward. An isolator must be soft enough relative to the machine's mass, which is why the correct mount depends on weight and cannot be chosen generically. A mount that is too stiff transmits vibration, and one that is too soft allows the machine to move excessively and to hit things.

Damping converts vibration energy into heat within a material and reduces the peak at resonance rather than moving it elsewhere. Rubber feet that have hardened with age have lost both properties, which is why an old machine can become noisier without anything breaking.

What vibration does to the rest of the machine

Fretting occurs where two surfaces move against each other by tiny amounts, producing wear debris and oxidation in a joint that appears tight. Electrical connectors work loose under vibration, and intermittent contacts produce faults that appear and disappear without a clear cause. Soldered joints and component leads on circuit boards experience cyclic stress, and cracks propagate at the point of highest constraint.

After the warranty ends, fasteners rotate loose under transverse vibration through a well-documented slip mechanism rather than through anyone under-tightening them.

Fluid seals and bearings suffer because vibration disrupts the lubricant film that normally keeps surfaces apart.

Installation and the floor underneath

A machine transmits force into whatever it stands on, and a suspended floor can resonate at frequencies a solid floor never would. Levelling matters because a machine standing on three of its four feet rocks, converting a small excitation into a much larger movement. Rigid connections such as pipes and ducts carry vibration into a building's structure and can make a distant room the noisy one.

Compared like for like, small gaps between a machine and adjacent cabinetry allow contact only during vibration, which produces intermittent buzzing that is hard to locate. Because installation causes so much of this, checking level, feet and clearances should precede any conclusion that a product is faulty.

What to notice and what to do

A change in the character of vibration is more informative than its absolute level, since the change indicates something moved or wore. Locating the speed at which shaking peaks helps identify the mechanism, because resonance is speed-specific and imbalance grows steadily. Touching the structure to feel where amplitude is greatest narrows the search quickly and requires no instruments at all.

Replacing hardened isolation feet is inexpensive and restores a property the machine was designed to have. Persistent vibration in anything with a heavy rotating part is worth investigating promptly, since the loads it creates fall on bearings and mountings.

The takeaway

Find the speed where the shaking peaks, then look for imbalance, a loose foot or a hardened mount.

The question is rarely which is best. It is which is enough.