On Quiet: The Underrated Engineering Problem

Here is the unglamorous truth about working with small DC motors: the motor itself is rarely the loudest part of your product.

I learned this the long way. I spent the first month of my acoustic-damping research project (codename Hush, more about it here) hunting for a quieter motor. I tested seven of them. I found maybe a 2 dB difference between the best and the worst — within the noise floor of my measurement setup, basically. I was solving the wrong problem.

The right problem is the housing.

What's actually happening

A small DC motor produces broadband mechanical noise — a soft hiss with peaks at the commutation harmonics. It's not pleasant, but on its own it is quiet. What you hear when you hold a finished product is that noise, multiplied through the housing, which has its own resonant modes happily amplifying the frequencies they like best.

In four out of the four prototypes I disassembled this winter, the housing was contributing 6–11 dB of gain at the most offensive frequencies. That's the difference between I can hear it across the room and I can hear it in the next room.

What helps

A short, opinionated list:

• Decouple the motor mechanically. Soft elastomer mounts work. Foam disks under the bearings work. A printed lattice with calibrated cell size works (this one is fun).
• Avoid axial symmetry in the shell. Symmetric shells have a small number of strong modes. Asymmetric shells have many weaker modes that cancel each other out.
• Pick wall thickness deliberately. There's a thickness band — for the silicones I've been using, around 2.4–2.6 mm — where the wall stops amplifying the motor and starts absorbing it. Below the band: drum. Above: dead weight that adds cost without adding silence.
• Damp the cavity, not just the surface. A 4 mm felt liner inside the shell did more for me than any external coating.

Why this matters

A product that whines tells its user: I am cheap, I am industrial, I do not care if you can hear me from the next room. A product that runs near-silent says the opposite. Sound is a UI. We just don't usually call it that.