Hush

Why motor noise is a design problem, not just a defect

There's a particular sound — a thin, electric hum at around 4–6 kHz — that almost every cheap personal product makes. We've been conditioned to accept it as the cost of having a motor in something. It is not.

Hush was a six-month independent research project to characterise where that whine actually comes from in small DC motors (the kind that go into intimate products), and what design decisions can be made upstream of the motor to kill it.

Method

I built a fixture that lets me swap the same motor between four different housing geometries while keeping every other variable constant — power supply, mount torque, ambient temperature, microphone distance and angle. Each housing was then run through a 20–20,000 Hz sweep at three drive levels and recorded.

What surprised me: the motor itself contributed less than I'd assumed. The housing's resonant modes were doing most of the work, amplifying the motor's natural noise floor by 6–11 dB depending on geometry.

What I learned

1. The single biggest lever is the mount interface — how the motor is decoupled from the shell.
2. Soft elastomer mounts work, but only if the geometry is symmetric. Asymmetric mounts introduce their own modes.
3. There's a sweet spot in shell wall thickness around 2.4–2.6 mm where you stop fighting the resonance and start using it as a damper.

This research is going directly into the next prototype I'm working on. Quiet is not a luxury. It's part of how the product speaks to its user.