Sound pressure was mapped in the bony ear canal of gerbils during closed-field sound stimulation at frequencies from . A -diam probe-tube microphone or a -diam fiber-optic miniature microphone was positioned along approximately longitudinal trajectories within the -diam ear canal. Substantial spatial variations in sound pressure, sharp minima in magnitude, and half-cycle phase changes occurred at frequencies . The sound frequencies of these transitions increased with decreasing distance from the tympanic membrane (TM). Sound pressuremeasured orthogonally across the surface of the TM showed only small variations at frequencies below . Hence, the ear canal sound field can be described fairly well as a one-dimensional standing wavepattern. Ear-canal power reflectance estimated from longitudinal spatial variations was roughly constant at 0.2–0.5 at frequencies between 30 and . In contrast, reflectance increased at higher frequencies to at least 0.8 above . Sound pressure was also mapped in a microphone-terminated uniform tube—an “artificial ear.” Comparison with ear canal sound fields suggests that an artificial ear or “artificial cavity calibration” technique may underestimate the in situsound pressure by between 40 and .
The authors thank Michael Stinson for a useful discussion; Douglas Keefe and two anonymous reviewers for many helpful comments; William Peake, Melissa Wood, Kelly Brinsko, and the staff of the Eaton-Peabody Laboratory. Supported by NIDCD.
A. Spatial variations in sound within the ear canal
B. Sound fieldmeasurements in gerbil ear canals
A. Sound pressure distribution and standing waves in a model ear canal
B. Sound pressure spectra at various locations in the model ear canal
A. Animal preparation
1. At Massachusetts Eye and Ear Infirmary (MEEI)
2. At Princeton and Columbia
B. Experimental methods
C. Stimulus generation and response measurement
1. At MEEI
2. At Princeton and Columbia
D. Probe tube and miniature fiber-optic microphone design and calibration
1. Probe-tube microphone (at MEEI)
2. Miniature fiber-optic microphone (at Princeton and Columbia)
3. Calibration coupler and reference microphone
A. Sound field longitudinally within the bony ear canal
B. Sound field medio-axially and transversely across the ear canal
C. Stapes velocity with probe tube in various places in ear canal
D. Sound field in an artificial ear
A. Basic description of EC sound field from measurementsin situ
B. Effects of measurement conditions on ear canal sound field
C. Evidence for longitudinal standing waves within the ear canal
D. Computation of middle-ear power reflectance and absorption from standing wavepatterns
E. Comparisons to ear canal sound pressure field and reflectance in other species
F. Comparison with ear canal sound pressure estimated by other methods
VI. SUMMARY AND CONCLUSIONS
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