This Canadian list confirms what all the other similar lists I've seen indicate : 9mm. is significantly louder (more decibels ) than .45acp (the decibel numbers are logarithmic so the difference is more than it seems.) CENTERFIRE PISTOL DATA .25 ACP 155.0 dB .32 ACP 153.5 dB .380 157.7 dB 9mm 159.8 dB .38 Spl 156.3 dB .357 Magnum 164.3 dB .44 Spl 155.9 dB .45 ACP 157.0 dB .45 COLT 154.7 dB The above averages are for all types of ammunition used in these firearms, and should be considered fairly representative. No wonder we hear numerous reports about hearing loss as a result of firearms , including acoustic traumas that destroy hearing completely as a result of one shot. Imagine what the noise levels must be when we incorporate porting into firearms, or have a gun fire near the ear .