The information presented here is based on clinical examples as well as modeled patterns. Text and accompanying. Absorbance sketch is authored by Navid Shahnaz, PhD, Aud, Associate Professor of Audiology in the School of Audiology & Speech Sciences at the University of British Columbia (UBC).
It should be noted, that recordings on ears with negative middle ear pressure will vary between patients – the shown Absorbance pattern is a sketched example only.
Absorbance characteristics to look for:
The Absorbance at lower frequencies tends to be at a very low level – close to a flat reading. This effect might increase with the amount of fluid in the middle ear. With pressures as high as plus or minus 300daPa, the low absorbance might extend up to the 2 kHz area.
Higher frequencies are often not affected by positive pressure in the ear canal. Negative pressure might cause a higher Absorbance at higher frequencies. These high frequency effects are not consistent across all ears.
In general, it can be difficult to distinguish between the condition of Negative Middle Ear Pressure and the condition of partial Fluid in the Middle Ear. If, however, the Absorbance reading is made at tympanometric peak pressure, for purely Negative Middle Ear Pressure condition the Absorbance reading, may not be distorted by the effects of a Negative Middle Ear Pressure.
Consequences of probe fit
All Absorbance measures need to have a good probe fit to be reliable. Evaluating Absorbance as provided by the 3D Tympanometry test ensures that a reasonably air tight probe seal was accomplished, as the air pressure sweep would not have been performed otherwise. In addition to an air tight probe seal, a deeper rather than a shallower probe insertion ensures the most accurate Absorbance measures. Shallow insertions tend to provide more elevated Absorbance readings at lower frequencies. This is somewhat similar to normal Tympanometry measures that are also influenced by probe fit and probe insertion depth.
Effects of Middle-Ear Disorders on Power Reflectance Measured in Cadaveric Ear Canals, Voss, Susan E., Merchant, Gabrielle R.,Horton, Nicholas J., Ear & Hearing. 33(2):195-208, March/April 2012.
Acoustic Immittance Measures, Basic and Advanced Practice, 2013, Lisa Hunter, Phd, FAAA, Navid Shahnaz, PhD, Aud. (C), Plural Publishing. ISBN10: 1-59756-437-0, ISBN13: 978-1-59756-437-3.