The mechanical reason behind high-pitched whining in budget designs

That sharp electronic whine in some budget air purifiers usually isn't the filter. It often comes from the motor and its control system working at the edge of what the design can comfortably handle.

Cheaper units tend to use smaller motors, lighter bearings, thinner housings, and simpler PWM control. At certain speeds, the switching frequency can excite the motor windings, bearings, fan imbalance, or even the plastic housing itself. Instead of producing a broad “whoosh,” you get a narrow, irritating high-frequency tone.

I've also noticed that manufacturers sometimes chase airflow numbers by running a small fan faster rather than using a better-balanced, more substantial assembly. That can make the purifier look good on paper while making the acoustic behavior much worse.

A good design doesn't just ask, How much air can this motor move? It asks, How quietly and consistently can it move that air for years?

When comparing purifiers, listen at several fan speeds. A clean, stable change in pitch is usually a better sign than a unit that suddenly develops a whistle or whine at one particular speed.