Whether a surface reflects specularly or diffusely depends on wavelength, not polish

Evidence

If the surface is instead rough, on the scale of or somewhat larger than the wavelength, the reflection becomes diffuse, meaning the waves head off in a range of directions, not just in the specular direction. For example, a brushed metal surface reflects light rather diffusely because the wavelength of visible light is smaller than the imperfections and grooves in the metal. The same surface will be mirror-like for microwaves with wavelengths on the order of centimeters. Similarly, an ordinary bathroom mirror is not in fact perfectly smooth, but rather just smooth enough—the surface of the silvered or aluminized glass that forms the mirror is quite rough, but the scale of the roughness is small compared to a wavelength of visible light. — Eric J. Heller, Why You Hear What You Hear- An…, 170

Categorization

• XREF: Connects to the reader’s prior interest in concert-hall acoustics and sound reflection mechanics from the listening/soundscape books.
• CANDY: The fact that a brushed-metal surface is mirror-like to centimeter microwaves is a fun, tweetable tidbit.