December 2010: Separation Anxiety: DPOAE Components Refuse to be Apart

 In two relatively recent papers, (Dhar & Shaffer, 2004; Shaffer & Dhar, 2006) have explored the possibility of improving the prediction of behavioral hearing thresholds from distortion product otoacoustic emission (DPOAE) level. DPOAEs are sounds generated in the cochlea (Kemp, 1979) that can be recorded in the ear canal using appropriate recording and analysis equipment. When a normally functioning cochlea is stimulated simultaneously with two pure tones, say at frequencies f1 and f2 (f1 < f2), the cochlea generates energy at various other frequencies related to those of the stimulus tones. The DPOAE produced at the frequency 2f1-f2 is commonly used for clinical purposes as it is easily recorded from human ears under certain stimulus conditions. DPOAEs at frequencies lower than those of the stimulus tones, such as that at the frequency 2f1-f2, can be called ÒapicalÓ DPOAEs as the tonotopic organization of the basilar membrane dictates that their characteristic frequency place lies apical to that of the stimulus tones.

                      In typical clinical applications, DPOAEs are recorded with relatively coarse frequency spacing. For example, a clinical device may default to measuring DPOAEs at 3 frequencies per octave. The resultant DPOAE level versus frequency function is referred to as the DP gram. When DPOAEs are recorded with significantly greater frequency resolution, the resultant DPOAE level versus frequency function exhibits an alternating pattern of peaks and valleys. This pattern is referred to as fine structure in the literature and the peaks and valleys are referred to as maxima and minima, respectively (Kemp, 1979). An example of one such recording is displayed in Figure 1. Note the pseudo periodic variation in DPOAE level over the entire recording frequency range.

Figure 1

Figure 1. Example of DPOAE fine structure measured from a normal-hearing young human adult. DPOAE level, phase, and noise floor are represented using the orange, green, and gray traces, respectively.

Figure 2. Animation of two-source model of DPOAEs. See text for details.


Figure 3. Animation of generation of fine structure due to interference between two DPOAE components. See text for details.




Figure 4. Animation of effects of suppressor tone on DPOAE components and fine structure. See text for details.

Figure 5

Figure 5. Output of inverse FFT operation on DPOAE level and phase recorded from a normal-hearing human ear. The two DPOAE components are separated by color

Figure 6

Figure 6. Differences in the results of iFFT analysis on data obtained between 2200 and 2600 Hz from 16 ears.