Anechoic chambers lose accuracy below roughly 80 Hz and require correction.

Evidence

Anechoic chambers cease to be perfectly echo free at very low frequencies, and errors arise. The NRCC chamber had 1 m long wedges, and a space behind them, so it was usefully anechoic down to about 80 Hz. To compensate for errors at lower frequencies, reference measurements were made using a closed-box woofer on a 10 m tower and then the digital post-processing made corrections so that the same woofer located at the axis of rotation in the chamber yielded the same result. Henceforth, any loudspeaker with a similar bass configuration would have free-field corrected bass measurements. — Toole, Floyd E, Sound reproduction- the acoustics and…, 168

Categorization

• DEFINE: Clarifies the physical mechanism limiting anechoic chambers: wedge length sets the low-frequency cutoff.