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METHOD FOR AUTOMATIC DETERMINATION OF AN INDIVIDUAL FUNCTION OF A DPOAE LEVEL

20170332977 · 2017-11-23

    Inventors

    Cpc classification

    International classification

    Abstract

    The automated determination of an individual function of a DPOAE level map with p.sub.dp,I=f(L.sub.1, L.sub.2) of human or animal hearing. The method may include reading into a main memory a model function with model parameters of a DPOAE level map, based upon a number of N DPOAE measurements of a stimulation frequency pair with respectively different level pairs in a population (p) of a population of normally hearing subjects, automatically presenting n different level pairs of a stimulation frequency pair via tone output means to an individual and detecting the corresponding DPOAE's of the individual via tone recording means, wherein at least the first level pair is predefined, iteratively adapting the model function to the measured n DPOAE's until an individual function is obtained with individual parameters of a DPOAE level map of the individual, outputting the individual function and/or its individual parameters.

    Claims

    1. A method for the automated determination of an individual function of a DPOAE level map with p.sub.dp,I=f(L.sub.1, L.sub.2) of human or animal hearing, characterized in that it comprises the following steps: reading into a main memory of a computer unit a model function p.sub.dp,M=f(L.sub.1, L.sub.2) with model parameters of a DPOAE level map, based upon a number of N DPOAE measurements of a stimulation frequency pair f.sub.1, f.sub.2 with respectively different level pairs {L.sub.1.sup.(1 . . . N), L.sub.2.sup.(1 . . . N)} in a population (p) of normally hearing subjects, wherein N≧40 and p≧2, automatically presenting n different level pairs {L.sub.1.sup.(1 . . . n), L.sub.2.sup.(1 . . . n)} of a stimulation frequency pair f.sub.1, f.sub.2 via tone output means to an individual and detecting the corresponding DPOAE's of the individual via tone recording means, wherein at least the first level pair {L.sub.1.sup.(1), L.sub.2.sup.(1)} is predefined, and wherein n<<N, iteratively adapting the model function p.sub.dp,M=f(L.sub.1, L.sub.2) to the measured n DPOAE's until an individual function p.sub.dp,I=f(L.sub.1, L.sub.2) is obtained with individual parameters of a DPOAE level map of the individual by the computer unit, and outputting the individual function p.sub.dp,I=f(L.sub.1, L.sub.2) and/or its individual parameters to an output means of the computer unit.

    2. The method according to claim 1, characterized in that the first level pair {L.sub.1.sup.(1), L.sub.2.sup.(1)} has a level L.sub.1.sup.(1) of 67±10 dB and a level L.sub.2.sup.(1) of 57±10 dB.

    3. The method according to claim 1, characterized in that the model function has a more or less linearly rising ridge to which more or less linearly linked level pairs {L.sub.1.sup.(G), L.sub.2.sup.(G)} are assigned, wherein at least one-half of the measured level pairs {L.sub.1, L.sub.2} lie at least 5 dB to either side of the group of the level pairs {L.sub.1.sup.(G), L.sub.2.sup.(G)} assigned to the ridge (73).

    4. The method according to claim 1, characterized in that the different level pairs {L.sub.1, L.sub.2} are presented in a sequence that is identical for each individual.

    5. The method according to claim 1, characterized in that the predefined, different level pairs {L.sub.1, L.sub.2} are presented in a sequence that has a number of k subsequences whose level pairs {L.sub.1, L.sub.2} are essentially transverse to the linearly linked level pairs {L.sub.1.sup.(G), L.sub.2.sup.(G)} assigned to the ridge.

    6. The method according to claim 1, characterized in that n is ≧5 and ≦12, and, preferably, n is ≧5 and ≦8.

    7. The method according to claim 1, characterized in that k≧2 and ≦8.

    8. The method according to claim 1, characterized in that the level pair {L.sub.1.sup.(2 . . . nk), L.sub.2.sup.(2 . . . nk)} of a subsequence following a first predefined level pair {L.sub.1.sup.(1), L.sub.2.sup.(1))} is determined with n.sub.k measurements by a function {L.sub.1.sup.(i), L.sub.2.sup.(i)}={L.sub.1.sup.(i−1)+μ.Math.ΔL.sub.1, L.sub.2.sup.(i−1)+μ.Math.ΔL.sub.2} from the respectively preceding level pair {L.sub.1.sup.(i−1), L.sub.2.sup.(i−1)}, wherein μ=±1—preferably, +1—and ΔL.sub.1, ΔL.sub.2 is a level difference of two sequential level pairs and has values of ΔL.sub.1=4 to 14 dB—preferably, from 6 to 10 dB—and ΔL.sub.2=0 to −2.78 dB—preferably, ΔL.sub.2=−1.52 to −2.78 dB.

    9. The method according to claim 1, characterized in that, when the first level pair {L.sub.1.sup.(1), L.sub.2.sup.(1)} and the second level pair {L.sub.1.sup.(2), L.sub.2.sup.(2)} produce two DPOAE's with p.sub.dp,I.sup.(12) that each have a signal-to-noise ratio of >=4 dB—preferably, >=10 dB—the level of a subsequent third level pair {L.sub.1.sup.(3), L.sub.2.sup.(3)} is adjusted to differ by at least ΔL.sub.1≧4 dB from the level of the preceding level pair {L.sub.1.sup.(2), L.sub.2.sup.(2)} when p.sub.dp,I.sup.(2)−p.sub.dp,I.sup.(1)>0; on the other hand, the level of a subsequent level pair {L.sub.1.sup.(3), L.sub.2.sup.(3))} is adjusted to differ by at least ΔL.sub.1≦−4 dB from the level of the first level pair {L.sub.1.sup.(1), L.sub.2.sup.(1)}, when p.sub.dp,I.sup.(2)−p.sub.dp,I.sup.(1)≦0.

    10. The method according to claim 1, characterized in that, when the first level pair {L.sub.1.sup.(1), L.sub.2.sup.(1))} does not produce any DPOAE's with p.sub.dp,I.sup.(1) that have a signal-to-noise ratio of ≧4 dB—preferably ≧10 dB—the same search direction is continued until either the maximum or minimum stimulation level L.sub.1.sup.(i) is reached, or a group of three valid DPOAE's with p.sub.dp,I.sup.(i . . . i+2) was produced that have a signal-to-noise ratio of ≧4 dB—preferably, ≧10 dB.

    11. The method according to claim 1, characterized in that, when a group of three valid DPOAE's that have a signal-to-noise ratio of ≧4 dB, and preferably ≧10 dB, is not produced in the first subsequence, another subsequence is started with a higher level pair {L.sub.1.sup.(i+1), L.sub.2.sup.(i+1)}, wherein the start level pair for the new subsequence is set to L.sub.2.sup.(i+3)=L.sub.2.sup.(1)+20±10 dB, L.sub.1.sup.(i+3)=L.sub.1.sup.(1)+20±10 dB, or at most to the maximum achievable level.

    12. The method according to claim 1, characterized in that, after acquiring the DPOAE's of at least 3 level pairs {L.sub.1.sup.(1 . . . 3), L.sub.2.sup.(1 . . . 3)}—preferably, of one subsequence—the position of the ridge {L.sub.1.sup.(G), L.sub.2.sup.(G)} along the line formed by the three level pairs is determined from these three level pairs {L.sub.1.sup.(1 . . . 3), L.sub.2.sup.(1 . . . 3)}, and a fourth level pair {L.sub.1.sup.(4), L.sub.2.sup.(4)} is presented that is placed at a predetermined distance down the ridge, wherein the group average of the ridge direction φ is used, and wherein a slope of the linear ridge of the level map is calculated using the DPOAE's determined from the four presented level pairs {L.sub.1.sup.(1 . . . 4), L.sub.2.sup.(1 . . . 4)}.

    13. The method according to claim 1, characterized in that, when a group of three valid DPOAE's with p.sub.DP I.sup.i−2 . . . i is produced in the first or second subsequence that each have a signal-to-noise ratio of >=4 dB—preferably, >=10 dB—the level pair below the ridge {L.sub.1.sup.(G), L.sub.2.sup.(G)}={L.sub.1.sup.(i−2)+ε.Math.ΔL.sub.1, L.sub.2.sup.(i−2)ε.Math.ΔL.sub.2} is calculated by adapting a mathematical function to the associated DPOAE p.sub.DP I.sup.i−2 . . . i, wherein E must be calculated so that p.sub.DP I(L.sub.1.sup.(G), L.sub.2.sup.(G)) forms a maximum, and on that basis a fourth level pair L.sub.1.sup.(i+1), L.sub.2.sup.(i+1) is presented, with a function {L.sub.1.sup.(i+1), L.sub.2.sup.(i+1)}={L.sub.1.sup.(i)+ΔL.sub.1, L.sub.2.sup.(i)+ΔL.sub.2}, wherein ΔL.sub.2=−15±10 dB is adjusted, and the level pair is preferably adjusted to the projection of the anticipated ridge on the L.sub.1, L.sub.2 level, i.e., adjusted with ΔL.sub.1/ΔL.sub.2≈0.51±0.15, and wherein a slope m of the approximately linear ridge of the level map is determined using the DPOAE calculated from the four presented level pairs L.sub.1.sup.(i−2 . . . i+1), L.sub.2.sup.(i−2 . . . i+1).

    14. The method according to claim 1, characterized in that the level pairs {L.sub.1.sup.(1 . . . n), L.sub.2.sup.(1 . . . n)} are presented as pulsed, wherein each individual pulse is presented with a duration T.sub.D of 2 to 40 ms.

    15. The method according to claim 14, characterized in that the level pairs {L.sub.1.sup.(1 . . . n), L.sub.2.sup.(1 . . . n)} are presented within a measuring block consisting of a plurality of level pairs {L.sub.1.sup.(1 . . . n,m), L.sub.2.sup.(1 . . . n,m)} which are presented sequentially over time in pulses, wherein level pairs {L.sub.1.sup.(1 . . . n,i), L.sub.2.sup.(1 . . . n,i)}; {L.sub.1.sup.(1 . . . n,1+1), L.sub.2.sup.(1 . . . n,i+1)} that follow each other directly in time have different stimulation frequencies {f.sub.2,i,f.sub.1,i}; {f.sub.2,i+1,f.sub.1,i+1}.

    16. The method according to claim 15, characterized in that, in an additional method step, the determined individual function of a DPOAE level map and its parameters are saved by the computer unit in a non-volatile memory.

    17. A system for performing the method according to claim 1, with a computer unit, a main memory, a non-volatile memory for storing a model function p.sub.dp M=f(L.sub.1, L.sub.2) and model parameters of the model function, at least one tone output means controlled by the computer unit for presenting tones to an individual, with at least one tone recording means connected to the computer unit for detecting DPOAE's from the ear of the individual.

    18. The system according to claim 17, characterized in that at least one tone output means is a speaker with a highly linear characteristic.

    19. The system according to claim 17, characterized in that an output means is provided for outputting the individual function of a DPOAE level map to a user.

    Description

    [0038] The invention will be explained in greater detail in the following figures with the help of exemplary embodiments. They show:

    [0039] FIG. 1 A system for the automated determination of an individual function of a DPOAE level map;

    [0040] FIG. 2 A model function whose three-dimensional graph corresponds to a model level map;

    [0041] FIG. 3 Steps according to the invention of a method for the automated determination of an individual function of a DPOAE level map;

    [0042] FIG. 4 Details of the method step according to marking IV in FIG. 3.

    [0043] A system for the automated determination of an individual function of a DPOAE level map of human or animal hearing in one possible embodiment according to the invention is depicted in FIG. 1. A probe unit 20 that can be positioned in an ear—especially, an OAE probe—and a computer unit 10 belong to the system 1. The probe unit has a probe tip 24 which can be inserted into the acoustic meatus of an ear. Arranged in the probe unit 20 is a tone recording means 23 such as a microphone that is configured to record tones coming from the acoustic meatus. Moreover, a first and second tone output means 21 and 22 are provided in the probe unit 20 that function as f1 sound generators (tone output means 21) and as f2 sound generators (tone output means 22). The tone output means 21, 22 can be designed as speakers. In addition, only one tone output means and only one speaker may be provided, which is configured to simultaneously emit two tones f.sub.1, f.sub.2 and, in particular, possesses a highly linear characteristic. The probe unit 20 is, for example, connected by a cable connection 2 to the control unit which contains the computer unit 10. Shielded leads 3, 4, 5 are preferably provided in the cable connection 2, by means of which the tone output means 21, 22 and tone pickup means 23 are connected to an AD/DA converter unit 12 of the control unit. For its part, the AD/DA converter unit 12 is connected to a computer unit 10 by means of at least one lead 6 for a bi-directional data exchange. Alternatively to the cable connection 2, the probe unit 20 can also communicate wirelessly with the control unit or computer unit 10. The wireless connection could, for example, be a Bluetooth wireless link, or another suitable wireless connection that preferably has a short range.

    [0044] The computer unit 10 has a working memory 15 and a non-volatile memory 16 in which a model function p.sub.dp,M=f(L1, L2) for a model level map of human or animal hearing is stored, along with the parameters belonging to this model function. Likewise, the instructions for performing the method according to the invention are saved in the non-volatile memory 16. Moreover, the system 1 has an output means 11 or a display unit such as a display, monitor, etc., by means of which a determined individual function of a DPOAE level map of human or animal hearing and its parameters are output by the system 1 and can be made accessible to a user. The output means 11 can also be realized in the form of an interface by means of which an external output device such as a printer or monitor can be connected to the system.

    [0045] To perform an automated measuring procedure for generating an individual function of a DPOAE level map of human or animal hearing, the probe unit 23 is inserted in the direction of the arrow 40 into the acoustic meatus 31 of an ear 30 (indicated in FIG. 1). The method according to the invention will be explained below with reference to FIGS. 3 and 4.

    [0046] Initially, however, a model function, by way of example, is depicted in FIG. 2 whose three-dimensional graph 70 corresponds to a model level map. The model function depicted as an example is based upon measuring data of p≧2—in the present case, p=6—normally hearing individuals with N≧40—in the present case, N=47—different measured level pairs {L.sub.1.sup.(1 . . . N), L.sub.2.sup.(1 . . . N)} at stimulation frequencies f.sub.2=2 kHz and f.sub.1=1.67 kHz. The stimulation frequencies f2 and f.sub.1 of a level pair {L.sub.1, L.sub.2} are preferably linked by a frequency ratio f.sub.2/f.sub.1=1.2. In the process, a certain distortion product was evaluated, which is preferably at the frequency f.sub.dp=2f.sub.1−f.sub.2. The uppercase indices in parentheses correspond to the 1st to Nth measuring point.

    [0047] The model function defines a more-or-less linearly rising ridge 73, to which more-or-less linearly linked level pairs {L.sub.1.sup.(G), L.sub.2.sup.(G)} are assigned. Lines transverse to the ridge can be defined by the relationship L.sub.2+aL.sub.1=C, wherein C is any constant, and wherein a is the slope parameter of the projection of the ridge onto the {L.sub.1, L.sub.2} plane. In mathematical terms, the position of the ridge is defined by a sequential set of gradient vectors of the scalar field that is formed by the DPOAE's, wherein all other field lines formed by gradient vectors run toward and enter this ridge. In the {L.sub.1, L.sub.2} plane 71 below the model level map, shifted for the sake of clarity by p.sub.dp=100 μPa, the transformed {L′.sub.1, L′.sub.2} coordinate system 72 is drawn that arises by shifting the origin toward {L.sub.1,edpt, L.sub.2,edpt} and rotating on arctan (a), as well as height lines of the level map at 20 μPa intervals. The L′.sub.2 axis corresponds to the projection of the ridge of the level map onto the {L.sub.1, L.sub.2} plane. The L′.sub.1 orthogonally intersects the model hill approximating the level map. This section of the hill transverse to the ridge is approximated by a second-order parabola whose spread is indicated by a parameter c, and the input is expressed in the following equation:


    L′.sub.dp=−C(L′.sub.1).sup.2+L′.sub.dp.sup.(G)


    with


    L′.sub.dp.sup.(G)=20 log.sub.10(m(L.sub.2′))

    [0048] L′.sub.dp and L′.sub.dp.sup.(G) are the level of any DPOAE, or a DPOAE located on the ridge, and m is the slope of the ridge along the L′.sub.2 axis.

    [0049] The {L′.sub.1, L′.sub.2} coordinate system is located in the area created by the known coordinate system {L.sub.1, L.sub.2} of the primary tone level. The above-addressed coordinate transformation can be expressed, for example, as follows:


    L′.sub.1=(L.sub.1−L.sub.1,edpt)cos(φ)−(L.sub.2−L(.sub.2,edpt)sin(φ)


    L′.sub.2=(L.sub.1−L.sub.1,edpt)sin(φ)+(L.sub.2−L(.sub.2,edpt)cos(φ)

    [0050] The projection of the ridge of the L.sub.dp hill corresponds to the {L.sub.1, L.sub.2} plane of the L′.sub.2 axis. Moreover, the point {L.sub.2,edpt, L.sub.1,edpt} corresponds to the base point of the ridge of the L.sub.dp hill, and φ is the angle between the L.sub.2 axis and the projection of the ridge of the L.sub.dp hill onto the {L.sub.1,L.sub.2} plane, established by the aforementioned L′.sub.2 axis. The angle φ is accordingly the angle at which the L′.sub.2 axis is rotated relative to the L.sub.2 axis. In a broader sense, the base point of the ridge can be interpreted as being equivalent, but not identical with, the “estimated distortion product level” (edpt), as is known from [P. Boege and T. Janssen., J. Acoust. Soc. Am., 111(4): 1810-1818, 2002].

    [0051] The model function for the level map, for the validity range of positive L.sub.dp, can be described by five free parameters: a; b; c; L′.sub.2,edpt; m. To calculate this surface from measured values, at least 5 DPOAE's are therefore needed.

    [0052] The method according to the invention is based upon the adaptation of the three-dimensional model function to a roughly sampled three-dimensional DPOAE level map with, preferably, at least 5 measurements. In a first exemplary embodiment of the method according to the invention for the automated determination of an individual function of a DPOAE level map with p.sub.dp=f(L.sub.1, L.sub.2) of human or animal hearing, a stimulation level pair {L.sub.1, L.sub.2} 51, 52, 53, 54, 55, 56 is presented to the hearing of an individual that is predefined from the system n, e.g., n=6, which can be seen in FIG. 1. These six predefined stimulation pairs {L.sub.1, L.sub.2} 51, 52, 53, 54, 55, 56 are, for example, marked in FIG. 2 in the {L.sub.1, L.sub.2} plane. From these 6 predefined stimulation level pairs {L.sub.1, L.sub.2} 51, 52, 53, 54, 55, 56, which are presented in two subsequences 57, 58, DPOAE's are then determined which are used in the method according to the invention to determine the individual function of a DPOAE level map.

    [0053] In a first step 110 of the method according to the invention according to FIG. 3, the above-described model function is first read out of the non-volatile memory 16 into the computer unit 10 of the system 1, or the working memory 15 of the computer unit 10. After the model function is read in, a number of different level pairs {L.sub.1.sup.(1 . . . n), L.sub.2.sup.(1 . . . n)} of a stimulation frequency pair {f.sub.1, f.sub.2} are output by the system 1 in a second step 120 by the tone output means 21, 22 of the probe unit 20, or are presented to an individual, and the corresponding DPOAE's of the individual are detected by the tone recording means 23, wherein at least the first level pair {L.sub.1.sup.(1), L.sub.2.sup.(1)} is predefined, and wherein n<<N The corresponding DPOAE's are fed by the AD/DA converter unit 12 to the computer unit 10 for further processing. The uppercase indices in parentheses correspond to the 1st to nth measuring point.

    [0054] In a first version of the method according to the invention, which can be termed an adaptive variant, the second step 120 contains a series of substeps 121 to 127 which will be explained in greater detail below with reference to FIG. 4.

    [0055] According to FIG. 4, in a first substep 121 of the second step 120, a start level {L.sub.1.sup.(1), L.sub.2.sup.(1)} (preferably a level L.sub.1 of 67±10 dB and a level L.sub.2 of 57±10 dB) for a first level pair is read out of the non-volatile memory 16 into the computer unit 10. Moreover, in this substep 121, the increments ΔL.sub.1, ΔL.sub.2 for the additional level pairs {L.sub.1.sup.(2 . . . n), L.sub.2.sup.(2 . . . n)} and the thresholds for the search direction decisions L.sub.dp,min.sup.(G,1); SNR.sub.min) are read into the computer unit 10. SNR.sub.min designates the desired SNR (signal-to-noise ratio), and L.sub.dp,min.sup.(G,1) designates the DPOAE level on the ridge that must exist at least along a first subsequence of k subsequences so that a following subsequence below the first one with a sufficient SNR can be anticipated. If this value is not achieved, then the next subsequence above the first one, i.e., up the slope, is sampled, in order to avoid excessive measuring time in achieving a sufficient SNR. The increments ΔL.sub.1, ΔL.sub.2 designate the level difference between two sequential level pairs, wherein ΔL.sub.1 has in particular a value of ΔL.sub.1=4 to 14 dB—preferably, from 6 to 10 dB—and wherein ΔL.sub.2=0 to −2.78 dB—preferably, ΔL.sub.2=−1.52 to −2.78 dB.

    [0056] In a second substep 122 of the second step, the measurements of the first subsequence of k subsequences are performed transversely to the assumed ridge of the individual function of a level map. The DPOAE's are then measured at the above-described stimulation frequencies f.sub.2=2 kHz and f.sub.1=1.67 kHz. The stimulation frequencies f.sub.2 and f.sub.1 of a level pair {L.sub.1, L.sub.2} are preferably linked by a frequency ratio f.sub.2/f.sub.1=1.2. The subsequence belongs to a number of k subsequences, wherein k≧2 and ≦5. In each subsequence, a number of n.sub.k level pairs {L.sub.1, L.sub.2} is measured.

    [0057] Corresponding to the established increments ΔL.sub.1, ΔL.sub.2, the start level {L.sub.1.sup.(1), L.sub.2.sup.(1)} is varied corresponding to the formula L.sub.1.sup.(n+1)=L.sub.1.sup.(n)+ΔL.sub.1′ cos(φ) and the additional formula L.sub.2.sup.(n+1)=L.sub.2.sup.(n)−ΔL.sub.1′ sin(φ). If a descending edge, or no ascending edge, is measured in the measured subsequence, the search direction is reversed, and ΔL.sub.1′=−ΔL.sub.1′.

    [0058] A third substep 123 checks whether at least three valid DPOAE's have been measured. If this check is positive, i.e., three valid DPOAE's were measured, the procedure advances to the next substep 124. If three valid DPOAE's were not measured, then the measurement is repeated, in which, based upon the original stimulation level L.sub.1.sup.(old), L.sub.2.sup.(old), a new stimulation level L.sub.1.sup.(1), L.sub.2.sup.(1) is determined by:


    L.sub.2.sup.(1)=L.sub.2.sup.(old)+ΔL.sub.2 cos(φ),


    L.sub.1.sup.(1)=L.sub.1.sup.(old)+ΔL.sub.1 sin(φ).

    [0059] In the following fourth substep 124, the position of the ridge of the individual function is determined using the three valid determined measured values, e.g., by solving a parabolic equation and finding the individual maximum according to the function:


    L′.sub.dp.sup.(G,1)=f(L.sub.1.sup.(1 . . . 3),L.sub.2.sup.(1 . . . 3))

    [0060] Here, L′.sub.dp.sup.(G,1) means the point on the estimated ridge of the individual model function whose associated stimulation pair lies on the line formed by {L.sub.1.sup.(1 . . . 3), L.sub.2.sup.(1 . . . 3)}. The position of the ridge of the individual function at the higher stimulation levels L.sup.(1 . . . 3) is already known from substep 124; however, the slope of the ridge, i.e., the parameter m, is not known.

    [0061] In the following fifth sub step 125 of the second step 120, a second subsequence is measured along the assumed ridge, wherein only one measured value is determined. The measurement is performed using the formula L.sub.1.sup.(4)=L′.sub.dp.sup.(G,1)−ΔL′.sub.2 sin(φ).

    [0062] If L′.sub.dp.sup.(G,1) falls below a preset limit (L′.sub.dp,min.sup.(G,1)), this step is performed on a higher level (ΔL.sub.2=−ΔL.sub.2).

    [0063] The value there measured of the DPOAE (L.sup.(4).sub.dp) is then used in the sixth substep 126 to determine the individual slope of the ridge m.

    [0064] In the sixth substep 126 of the second step 120, the individual slope of the ridge is calculated using the formula m=f(L.sub.dp.sup.(G,1), L.sub.1.sup.(G,1), L.sub.2.sup.(G,1), L.sub.dp.sup.(4), L.sub.1.sup.(4), L.sub.2.sup.(4)). Using the calculated slope m of the ridge, a start level L.sub.1.sup.(5), L.sub.2.sup.(5) is then determined for the third subsequence.

    [0065] In the seventh substep 127 of the second step 120, the measurements of the third subsequence are then performed transversely to the assumed ridge of the function: a variation of ΔL′.sub.1 in L.sub.1.sup.(n+1)=L.sub.1.sup.(n)+ΔL′.sub.1 cos(φ) is performed.

    [0066] Preferably, at least one-half of the level pairs {L.sub.1, L.sub.2} used in the measurements lie at least 5 dB to either side of the group of the level pairs assigned to the ridge (of the model function) {L.sub.1.sup.(G), L.sub.2.sup.(G)}.

    [0067] Using the measured values determined in the second step 120 and its substeps 121 to 127, the above-described model function is adapted to the obtained measured values in a third step 130 in the computer unit 10. In so doing, the three-dimensional model function p.sub.dp,M=f(L.sub.1,L.sub.2) is adapted to the measured DPOAE values. The adaptation is performed using the mathematical methods of equalization calculus, e.g., using the method of least squares, i.e., with the iterative minimization of the difference between n measured values and the values of the model function p.sub.dp,M=f(L.sub.1, L.sub.2) for the measured n DPOAE's (corresponding to the associated L.sub.1, L.sub.2 coordinates) until an individual function p.sub.dp,I=f(L.sub.1, L.sub.2) is obtained by the computer unit 10 with individual parameters of a DPOAE function and level map of the individual. Accordingly, an individual function/level map of the hearing of an individual is thereby quickly obtained with much less measuring effort.

    [0068] In a fourth step 140, the individually adapted function and its function parameters are output on output means 11 of the system 1, such as a display, monitor, printer, etc. The output function parameters contain, in particular, the above-described parameters a; b; c; L′.sub.2,edpt, and the slope of the ridge m. As mentioned, the output means 11 can also be realized in the form of an interface by means of which an external output device such as a printer or monitor can be connected to the system 1.

    [0069] In a possible additional method step, the determined individual function of a DPOAE level map and its parameters are, advantageously, saved by the computer unit 10 in the non-volatile memory 16. Likewise, the measured raw data of the computer unit 10 can be stored in non-volatile memory 16. The stored data can be used by the computer unit 10 to, for example, continuously expand the data set underlying the model function of a level map.

    [0070] The following information can be gleaned from the individually adapted function and the associated function parameters obtained according to the invention: [0071] An approximated distortion product threshold can be calculated that provides information on the threshold of the input signal for the inner hair cells of the measured hearing. L.sub.2,edpt can be considered a corresponding parameter. [0072] The width of the ridge, described in the function by the parameter c, is a measure of the compression, and thus of the frequency resolution of the underlying traveling waves in the measured hearing. [0073] The position and the angle expressed in the function by the parameters a; b contains information on the nature of a hearing loss: In the event of a pure conductive loss, the angle (expressed in the function by the parameter a) does not change; instead, the hill shifts in a first approximation to the same extent toward a higher L.sub.1 and L.sub.2 level. When, for example, the shift of the hill (relative to standard values, or relative to a reference measurement of the individual at an earlier time) coincides with the degradation of the distortion product threshold, i.e., ΔL.sub.2≈ΔL.sub.1≈ΔL.sub.2,edpt, a pure conductive loss can be assumed. [0074] The slope of the ridge expressed by the parameter m provides information about a potential conductive loss. As long as the hearing loss lies below 30 dB, it can be assumed that the slope corresponds to standard values in the event of a pure conductive loss, whereas, when there is a deviation from the standard value, a proportional reduction of the retrograde middle ear transmission at f.sub.dp is indicated.

    [0075] In an alternative variant of the method according to the invention, instead of substeps 121 to 127 during the measurements in the second step 120, a number of n determined or predefined, but different, level pairs {L.sub.1, L.sub.2} (where n is preferably ≧5 and ≦12—in particular, ≧5 and ≦8) is output by the system, and the reaction of the hearing of an individual to these level pairs {L.sub.1, L.sub.2} is detected. This version can be termed a rigid method. The level pairs {L.sub.1, L.sub.2} can, in turn, be measured in a number of k subsequences (57, 58; cf. FIG. 2), wherein k≧2 and ≦12. The n level pairs are then largely static, and there is no adaptation of the second and possibly third subsequence to the results of the measurements in the first subsequence, as is the case in the above-described method. The number n level pairs {L.sub.1, L.sub.2} is determined based upon a consideration of the measuring time (the fewest possible measuring points) relative to the achievable precision (the most possible measuring points). In this rigid method with predetermined stimulation levels, L′.sub.1=0±6 dB can always be measured, e.g., with L′.sub.2=40 dB for the three higher stimulation levels, and L′.sub.2=25 dB for the three lower stimulation levels, and within a group of three stimulation levels. In the {L.sub.1, L.sub.2} coordinate system, this would correspond to the excitation levels L2=68.1; 65.6; 63.1; 42.8; 45.3; 40.3 and L.sub.1=68.0; 73.5; 79.0; 63.3; 57.8; 68.7 (cf. FIG. 2). The aim of the choice of L′.sub.1=0±6 dB is to measure the position of the ridge with three points that are transverse to the assumed position of the ridge: given f.sub.2=2 kHz, the DPOAE with ΔL′.sub.1±6 dB typically decreases to approximately 50% of the maximum value. If, primarily, individuals with normal hearing are to be measured, as is usually the case in screening tests, the fixed arrangement will provide favorable results. Exceptions must, however, also be discerned. This is feasible using the root mean square error in the model adaptation. If the error is too high, i.e., the rms error (rms: root mean square) is, for example, greater than 5 μPa, additional level pairs must be measured until the error is sufficiently low.

    [0076] Additional ΔL′.sub.1 steps are recommended in this case. The same procedure is required when individual measuring points cannot be recorded because the signal-to-noise ratio is too low.

    [0077] In conclusion, it should be noted that, in contrast to the employed and above-described frequency ratio f.sub.2/f.sub.1 of 1.2, another frequency ratio can be chosen. Accordingly, the frequency ratio f.sub.2/f.sub.1 can, for example, be set at a different suitable value between 1.15 and 1.35. Moreover, the frequency ratio f.sub.2/f.sub.1 could be a function of f2.

    LIST OF REFERENCE NUMBERS

    [0078] 1 System [0079] 2 Cable connection [0080] 3 First line [0081] 4 Second line [0082] 5 Third line [0083] 6 Fourth line [0084] 10 Computer unit [0085] 11 Output means [0086] 12 AD/DA converter unit [0087] 13 DA converter [0088] 14 AD converter [0089] 15 Main memory [0090] 16 Non-volatile memory with a saved model function [0091] 20 Probe unit, OAE probe [0092] 21 First tone output means, f1 sound generator [0093] 22 Second tone output means, f2 sound generator [0094] 23 Tone recording means, microphone [0095] 24 Probe tip [0096] 30 Ear [0097] 31 Acoustic meatus [0098] 40 Arrow [0099] 51 Stimulation level pair {L.sub.1, L.sub.2} [0100] 52 Stimulation level pair {L.sub.1, L.sub.2} [0101] 53 Stimulation level pair {L.sub.1, L.sub.2} [0102] 54 Stimulation level pair {L.sub.1, L.sub.2} [0103] 55 Stimulation level pair {L.sub.1, L.sub.2} [0104] 56 Stimulation level pair {L.sub.1, L.sub.2} [0105] 57 First subsequence [0106] 58 Second/additional subsequence [0107] 70 Graph/model level map [0108] 71 {L.sub.1, L.sub.2} plane [0109] 72 Transformed {L′.sub.1, L′.sub.2} coordinate system [0110] 73 Ridge (of the DPOAE model level map) [0111] 110 First method step [0112] 120 Second method step [0113] 121 First substep [0114] 122 Second substep [0115] 123 Third substep [0116] 124 Fourth sub step [0117] 125 Fifth substep [0118] 126 Sixth substep [0119] 127 Seventh substep [0120] 130 Third method step [0121] 140 Fourth method step