At what point does negative pressure affect the acoustic reflexes? In other words, how much negative pressure is acceptable to obtain accurate acoustic reflexes?
This is a very good and straightforward question but it's more complicated than it seems. First we have to understand what we mean by "negative pressure". Of course we can't directly measure the pressure in the middle ear so we infer the middle-ear pressure from the tympanogram. But the pressure at which the tympanometric peak occurs (the "tympanometric peak pressure") is different from the actual middle-ear pressure. TPP is the pressure at which the pressure difference across the eardrum is zero. When we introduce negative pressure in the ear canal we draw the eardrum back, increasing the middle ear volume, making the pressure more negative than it was with ambient pressure in the ear canal. TPP always overestimates the actual negative middle-ear pressure and can differ from the middle-ear pressure by 100%, depending on the volume of the middle-ear space and the contiguous air spaces in the mastoid. So it's hard to predict the relationship between TPP and the acoustic reflex because of the variable relationship between TPP and the actual middle-ear pressure. Another variable that complicates the relationship is the magnitude of the reflex. A small reflex may be completely eliminated by negative middle-ear pressure where a larger one may be only slightly diminished in amplitude. The easy answer is that any amount of middle-ear pressure (as indicated by TPP) can reduce the reflex amplitude. The safe procedure is to always compensate by adjusting the ear-canal air pressure to TPP when measuring the reflex. Many instruments have an option to do this automatically.
Robert H. Margolis earned bachelor's and master's degrees from Kent State University (1968, 1969) and a Ph.D. degree from the University of Iowa (1974). After a post-doctoral research fellowship at the University of Wisconsin, he joined the faculty of the UCLA Medical School in 1975. In 1980, he was appointed associate professor of communication sciences and disorders and director of the Gebbie Hearing Clinic at Syracuse University. In 1988 he became professor and director of audiology at the University of Minnesota Medical School. In 2000 he established *AUDIOLOGY INCORPORATED to develop improved hearing tests*. Dr. Margolis has over 120 publications in scientific and clinical journals and textbooks. His research has focused on development of methods for evaluating disorders of hearing. He has been awarded research grants from the Deafness Research Foundation, NATO Division of Scientific Affairs, and the National Institutes of Health. He has served as president of the Minnesota Academy of Audiology, the International Hearing Foundation, and the Minneapolis-University Rotary Club. He has been awarded the Honors of the Association by the Minnesota Academy of Audiology, the Humanitarian Award by the American Academy of Audiology, the Editor's Award by the Journal of the American Academy of Audiology, the Rotarian of the Year Award by the Minneapolis-University Rotary Club, the Larry Mauldin Award for Excellence in Education in Audiology, an Honorary Membership by the Vitacura Rotary Club (Santiago, Chile), and the James Jerger Career Research Award by the American Academy of Audiology.