A widely used audiometric standard was calibrated roughly 10 dB off

Evidence

In 1952, Dadson and King conducted a similar survey in England and found thresholds to be approximately 10 dB better than those reported in the United States. Studies were then conducted in a number of other countries, all of which conformed to the English measurements, and not those made in the United States. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 104