Some controversy exists as to the length of time required to confirm or exclude bowel sounds, with suggested durations up to seven minutes. Bowel obstruction may present with grumbling bowel sounds or high-pitched noises. Healthy persons can have no bowel sounds for several minutes and intestinal contractions can be silent. Hyperactive bowel sounds may be caused by partial or complete bowel obstruction as the intestines initially try to clear the obstruction. Absence of sounds may be caused by peritonitis, paralytic ileus, late-stage bowel obstruction, intestinal ischemia or other causes. Some authors suggest that listening at a single location is enough as sounds can be transmitted throughout the abdomen.