The museum's new audio system was installed by the lowest bidder, which is always a promising start. Ceiling speakers every six meters. Handheld wireless mic for the guide. No acoustic treatment because the curator said foam panels would clash with the Impressionist collection.

The guide gathered her group of 30 visitors in the main gallery — a room with marble floors, plaster walls, and a glass skylight. She raised the mic and began: "Welcome to the—" A piercing 3.2 kHz ring exploded from every ceiling speaker simultaneously. Monet's Water Lilies seemed to ripple.

The guide pulled the mic away from her mouth. The ringing stopped. She tried again, more carefully. It returned the instant her voice exceeded a whisper. She resorted to projecting her voice naturally, which worked fine except the entire point of having a PA system was that she shouldn't have to.

Ceiling speakers pointing downward at a reflective marble floor create an efficient acoustic mirror that bounces energy right back up to the mic. The 3.2 kHz resonance matched the spacing between ceiling speaker and floor — a half-wavelength standing wave that would ring at any gain above barely audible.

The Moral: Even simple installations need measurement. Use SonaVyx Problem Detection to identify feedback frequencies during installation, not during the guided tour.