Distortion product otoacoustic emissions measured as vibration on the eardrum of human subjects

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   Fig. 1.

   Dependence of umbo displacement amplitude on SPL for single-tone stimulation (3.5 kHz) measured for an open sound field. The linear regression line of unity slope (1 dB/dB) indicates that the measured umbo response is linear. The dotted lines delineate the maximum noise level in the 100-Hz sidebands adjacent to the stimulus frequency. A reflector was not placed on the umbo. (Subject identifier: JT.)

   
   
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   Fig. 2.

   Amplitude spectra of umbo displacement (A and C) and sound pressure (B and D) for two-tone stimulation with f ₂ = 5.5 kHz and f ₁ = 4.6 kHz, measured simultaneously for an open sound field. (A and B) L ₂ = L ₁ = 65 dB SPL. The DP at 2f ₁ − f ₂ presents displacement amplitude of 5.6 pm on the umbo and SPL of 10.8 dB (arrow). (C and D) L ₂ = 25 dB SPL and L ₁ = 49 dB SPL. The umbo displacement amplitude is 1.25 pm at 2f ₁ − f ₂; the sound pressure is not detectable there. A reflector was placed on the umbo. (Subject identifier: AS.)

   
   
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   Fig. 3.

   Displacement amplitude responses of the umbo expressed for a SPL of 60 dB. Responses were measured before (●) and 30 min after (▵) measuring the DPOAE responses. Notice that the reproducibility is better than 1 dB up to 7 kHz. DPOAE data from this subject are shown in Fig. 2.

   
   
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   Fig. 4.

   Reproducibility of umbo vibration responses from one day to the next, with and without placement of a reflector. The measurements on the first day were made without placement of a reflector on the umbo, whereas a reflector was placed for the measurements 24 h later. (A) Amplitude spectra for two-tone stimulation with f ₂ = 5.0 kHz, f ₁ = 4.2 kHz, L ₂ = 55 dB SPL, and L ₁ = 61 dB SPL. The gray shaded line represents measurements without reflector placement on the first day. The black line represents measurements with reflector placement 24 h later. Notice that (i) all three signals superimpose and (ii) there is a (small) improvement of SNR with the reflector (up to 10 dB below 3.6 kHz). (B) Displacement amplitude responses of the umbo expressed for a SPL of 60 dB. The measurements shown are without reflector placement on the first day (●) and with reflector placement 24 h later (▵). Notice that the reproducibility is better than 1 dB up to 7 kHz. (Subject identifier: SK.)

   
   
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   Fig. 5.

   Dependence of DPOAEs on primary stimulus level. (A) DPOAE sound pressure (P _DP) as a function of SPL (L ₂) at the stimulus frequency, f ₂, of the second primary tone. Pressures were measured in a closed sound field. (B) DPOAE umbo velocity (V _DP) as a function of umbo velocity (V ₂) at f ₂. Velocities were measured in an open sound field. The stimulus parameters were as follows: f ₂ = 5.5 kHz, f ₁ = 4.6 kHz, L ₂ = 25–65 dB SPL, and L ₁ = 0.4L₂ + 39 dB. The lowest measured pressure amplitude at 2f ₁ − f ₂ = 3.7 kHz is 6 μPa, or −10.5 dB SPL. The lowest measured vibration amplitude at 2f ₁ − f ₂ = 3.7 kHz corresponds to a displacement amplitude of 1.18 pm. (A) The intersection of the regression line (r ² = 0.99, slope = 0.92 ± 0.04 μPa/dB SPL) with the abscissa (arrow) yields the so-called (19) EDPT = 14 dB SPL. (B) Correspondingly, the intersection of the regression line (r ² = 0.99, slope = 2.42 ± 0.10 nm/s/dB re. 1 μm/s) with the abscissa (arrow) yields the v-EDPT = 0.0272 μm/s. A reflector was placed on the umbo. (Subject identifier: AS.)

   
   
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   Fig. 6.

   Békésy threshold as a function of the EDPT. The measurements shown are for EDPT derived from conventional pressure in a closed sound field (▵) and for EDPT derived from umbo vibration measurements in an open sound field. n = 30 I/O functions from 14 subjects; f ₂ = 4–9.5 kHz. Regression lines for both the sound (dashed line) and the vibration (solid line) data have fixed slopes at 1.18 dB/dB, equal to the value derived by Boege and Janssen (19) for pressure DPOAEs. Their data set contained 4,236 points and exhibited a standard deviation of the Békésy thresholds from the regression line of 10.9 dB SPL. Here, the standard deviation of the Békésy thresholds from the regression line is 16.7 dB SPL for the sound pressure and 8.6 dB SPL for the umbo vibration measurements.