Mistake me if I am wrong. But the DB scale is not linear like temperature is, but an order of magnitude. So something 9 DB higher not just 9 equal units higher, but is nine times as loud as the lower sound.
Technically correct... in the old days we used to equate of 3dB increase in "loudness" with 2x increase in power.
But in terms of perception and hearing damage/loss, the type of noise and the length of exposure has more to do with it. For example, well powered, tube amplification, stereo systems played loud with even harmonic distortion tend not to be harsh (even pleasing). Odd harmonic distortion can be painful even at low levels played out of a boombox. But to be on topic, 9 dB is well beyond perceptible even for old guys like me.