Hearing thresholds are mapped in 5 dB steps to balance precision against test duration

Evidence

Tests are started at a level that can be readily heard (usually around +30 dBHL), which is presented for 1 to 3 s using the output key switch and then involves watching for the subject to light the lamp or LED. If this does not happen, the level is increased in 5-dB steps (5 dB being a minimum practical value to enable tests to be carried out in a reasonable time)—presenting the sound and awaiting a response each time. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2353