The knuckle test misjudges cabinets because contact excitation differs from internal pressure.

Evidence

On one of my visits to Japan, my attention was drawn to a popular brand that had an inch of concrete poured into the top of every loudspeaker. This was so that it would pass the “knuckle test” exercised by shoppers as they moved around the store floor tapping the loudspeakers. That was clever, but the knuckle test itself is not. It excites the panel at the point of contact, not as it is by sound inside the cabinet, which is pressure distributed over the surface. A well-designed loudspeaker might not pass the knuckle test. More folklore. — Toole, Floyd E, Sound reproduction- the acoustics and…, 107

Categorization

• SEED: A strong anecdote for an essay on how consumer ‘tests’ (tapping, heft) measure the wrong physical quantity, and how manufacturers cynically engineer to pass them.
• ADDENDUM: Reinforces the ‘Grafting & Growth’ theme of objects being engineered to perceived tests rather than actual function—here literally concrete added to pass shoppers’ knuckle tap.