“To selectively expel gas when feces are also present in the rectum, the person voluntarily contracts the abdominal muscles and external anal sphincter at the same time. When abdominal contraction raises the pressure against the contracted anal sphincter sufficiently, the pressure gradient forces air out at a high velocity through a slitlike anal opening that is too narrow for solid feces to escape through. This passage of air at high velocity causes the edges of the anal opening to vibrate, giving rise to the characteristic low-pitched sound accompanying the passage of gas.” (Sherwood, 504)
Making farts sound excessively scientific: what a fun writing assignment! I feel like writing resumes every day with high school students about babysitting is almost equivalent.