Noise

The same object can sound quiet in the hand and loud on a table. A hard surface couples the shell to a large flat panel, which becomes a loudspeaker. Placed on a folded towel or held in a hand, the same object loses much of its apparent volume without any change of setting. This effect is large enough that it swamps most of the differences between products.

A low rumble at moderate volume travels through a wall better than a high buzz at the same volume, because low frequencies pass through building materials more easily. In a flat with thin walls, a high frequency object at low intensity is often the more discreet choice, which runs against the usual assumption that rumble is quieter.

Decibel figures appear occasionally and are rarely comparable, since neither the distance nor the surface is stated. A figure measured at one metre on a soft surface with the motor unloaded is a different measurement from the same object in use. Treat any single number as an indication of intent rather than a specification.

The practical questions are simpler. Does the sound change pitch under load, which suggests a motor struggling. Does anything rattle at particular settings, which suggests a loose component that will get worse. Does the object stay quiet at the setting actually used, rather than at the lowest one.

• Use it on a soft surface rather than a hard one.
• Prefer the setting one step below maximum, where most designs get noticeably louder.
• Keep the shell in good condition, since a cracked or loosened seam adds rattle.
• Add background sound rather than trying to eliminate the source entirely.

None of this is only about privacy. A loud object is distracting, and distraction works against the point. Noise belongs on the same list as weight, grip and controls: the ordinary ergonomics that decide whether something gets used or stays in a drawer, which the first purchase page works through in order.

A new object that rattles at one particular setting has a resonance problem in the shell, and it will not improve. An object that develops a rattle after months has something loose inside, usually a motor mount or a battery that has shifted, and that one gets worse. Neither is repairable in any practical sense.