It generally disappears by 6 to 7 months, although the startle response to loud noises is permanent. Infants cry at loud noises because those sounds are unfamiliar to them and it's a normal part of their neurological development. As your baby matures and becomes used to different sounds, he may stop crying when he hears them.
Babies also respond according to their temperament.
An easygoing baby may take blaring sounds in stride, while a more sensitive child may cry at the slightest whisper. In general, though, babies handle interruptions and noise better as they get older. As with any source of medical or health care information, you should as a preliminary matter satisfy yourself that the credentials and experience of the source are suitable. The information provided through the Ask the Experts service is a general educational aid designed to help you discuss your unique medical or health concerns with your physician or qualified health care provider. You should not rely on the information as a substitute for personal medical or health care advice, or for diagnosis or treatment purposes.