In a .22 LR, the powder is consumed in 14 ~ 18 inches. after that, the pressure is decreasing. Therefore, if the barrel is but 15" long, the pressure being vented is at or near its maximum, thus resulting in a louder "Bang".
The longer the barrel, the lower the pressure is when the bullet exits the barrel.
Further; subsonic ammunition (less than 1100 fps) will not have the supersonic "Crack" and will be perceived as being less noisy.
Subsonic ammunition is generally more accurate at ranges over 75 ~ 100 yds because it does not destabilise as it transitions the sound barrior.
Roger