Low-frequency characteristics of human and guinea pig cochleae
Reconstruction of the modulation period pattern (MoPP) with 90-Hz modulation. (A) The spectrum of the recorded signal shows the stimulus tones with black circular markers (modulator tone, fm; primary tones, f1 and f2), and modulated DPOAE, with gray circular markers. The DPOAE consists of the 2f1-f2 carrier and modulation sidelines. Due to the appropriate rectangular buffer length of exactly 1/5 second, they show no spectral splatter and first and second order modulation sidelines stick clearly out from the noise floor. (Further singular spectral lines representing other distortion products can be identified.) (B) The time course of the 2f1-f2 DPOAE is reconstructed from the 2f1-f2 carrier and its first and second order pair of modulation sidelines. The black modulation period pattern (MoPP) represents the sound pressure level of the 2f1-f2 DPOAE in decibel as a function of time. The horizontal gray line indicates the DPOAE level without modulation. For phase reference, the sinusoidal sound pressure of the modulator tone in the ear canal is indicated as a dashed line (arbitrary linear scale). The difference between the DPOAE minimum and the unmodulated DPOAE level defines maximum DPOAE suppression (gray double arrow), called “modulation depth.”
Same as Fig. 1 but with 350-Hz modulation. (A) At higher modulation frequencies the modulation sidelines of the distortion product 2f1-f2 overlap with those of the primaries. Careful choice of stimulus frequencies avoids coincidence of the modulation sidelines and other spectral lines representing stimuli or other distortion products. The interleaving of the spectral components enables the reconstruction of the time course of the 2f1-f2 DPOAE for modulation frequencies close to or even greater than f2-f1 (B).
Distortion product isomodulation curves (DPIMC) from the right ear of subject TM. DPIMC for various primary parameters are shown. Interpretation of the legend is “subject ear (unmodulated DPOAE level–modulation depth), , .” For example, “TM right , , ” means that subject TM’s right ear was exposed to two primary tones of at SPL and at SPL and produced a 2f1-f2 DPOAE level of SPL. In this example, LF tones at levels indicated on the ordinate in (A) for the frequencies shown on the abscissa resulted in a DPOAE modulation depth of (in this figure, all curves stem from isomodulation). For comparison, slopes of 6 and are indicated by straight lines with arbitrary offsets. (B) Phases of maximum DPOAE suppression with reference to the pressure maximum of the modulator tone in the ear canal are shown as a function of modulation frequency. Phase values are compensated for DPOAE reverse travel times. See text for details.
Distortion product isomodulation curves (DPIMC) for four ears of three subjects with almost constant primary parameters. See legend and caption of Fig. 3. For comparison, the 80-phon equal-loudness contour (ISO226:2003) is shown in gray.
Distortion product isomodulation curves (DPIMC) from one guinea pig ear for DPOAE modulation depths of , , and . The primary parameters are constant. See legend and caption of Fig. 3.
Distortion product isomodulation curves (DPIMC) for four ears of two guinea pigs obtained with various primary parameters and DPOAE modulation depths. See legend and caption of Fig. 3.
Comparison with other published results measured with a variety of techniques that reveal cochlear impedance features. Curves are selected for containing a similar irregularity as seen in DPIMC. Data by Franke et al. (1985) and Nedzelnitski (1980), obtained as iso-output function, have been inverted. Following Dallos’ (1970) species classification, single-lined graphs represent data from viscosity-dominated cochleae; double lined graphs represent data from inertia-dominated cochleae (at lowest frequencies).
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