Abstract:
Apparatus and method are disclosed for the recording of distortion product emission (DPE) levels in human beings. At least one microphone and a sound-delivery system is inserted in the external ear canal in a manner similar to that required to position a small hearing aid. Two primary tones T f1 , T f2  are applied simultaneously to the ear. The cochlea of the inner ear produces a DPE tone which is sensed by the microphone. DPE levels are sensed as a function of input frequencies f 1  and f 2 . Such DPE frequency is equal to 2f 1  -f 2 . Such DPE frequencies are collected in 100 Hz steps by adjusting f 1  and f 2  and maintaining a substantially constant ratio between f 1  and f 2 . Two output forms are created: an DPE audiogram and a DPE input/output function. Noise picked up by the microphone is reduced by averaging the DPE signal many times, yet causing each of the DPE signals that is averaged to be of the same phase as every other DPE signal. Random phase noise is reduced by the averaging process. Body noise may be reduced by using two microphones and applying the output of one such microphones to the plus input of a differential amplifier. The output of the other less sensitive microphone, after amplification and phase adjustment is applied to the negative input of such differential amplifier. The output of such amplifier results with the body noise signal substantially reduced, but with the DPE signal substantially unchanged. This abstract of the disclosure-is not intended to define the scope of the invention.

Description:
U.S. GOVERNMENT SUPPORT 
     The invention described and claimed herein resulted from work supported by U.S. government grants from the National Institutes of Health. The Government has certain right in the invention. 
    
    
     This application is a continuation of application Ser. No. 08/064,356 filed May 18, 1993 now abandoned, which is a continuation of Ser. No. 07/471,106 filed Jan. 25, 1990 now abandoned. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention relates in general to hearing testing of a human being. In particular, the invention relates to recording of distortion product emissions (DPEs) of human ears. Still more particularly, the invention relates to apparatus and methods for recording DPE audiograms and input/output functions and to the minimization of random noise in the presence of DPE. 
     2. Description of the Prior Art 
     Otoacoustic Emissions (OAEs), first described in 1978, represent acoustic energy presumed to be generated by stimulus-induced, motile activity of the outer hair cells of the Organ of Corti in the Cochlea of the inner ear of a human being and other mammals. It is believed that mechanical feedback of such outer hair cells into basilar membrane motion and their related cochlear-efferent endings are part of a biomechanical gain system that is responsible for the sharp tuning and high sensitivity associated with normal hearing. 
     Otoacoustic emissions (OAEs) may be classified generally as spontaneous emissions and &#34;evoked&#34; or stimulated emissions. Stimulated emissions can be further separated into three subclasses consisting of transiently evoked emissions (TEEs), stimulus-frequency emissions (SFEs), and distortion-product emissions (DPEs). Each type of stimulus [i.e., clicks (TEEs) or low-level, continuous pure tones (SFEs) or continuous, simultaneously applied, two-tone stimuli (DPEs)] generates evoked emissions. TEEs and SFEs have an appreciable latent or delayed time period with respect to stimulus onset. DPEs, have a nonlatent or instantaneous onset. Based on the response-latency distinction, it is believed that separate subcellular components of the outer hair cell support the generation of delayed versus instantaneous evoked OAEs. For example, the stimulus-induced movements of the stereocilia bundle likely generate the nonlatent DPEs, while the motile activity of the lateral regions of the hair-cell membrane likely produces the latent TEEs and SFEs. 
     D. T. Kemp proposed that a transient OAE could be a diagnostic tool in the examination of impaired hearing. Kemp, Stimulated Acoustic Emissions from within the Human Auditory System, J. Acoust. Soc. Am., Vol 64, No. 5, pp 1386-1391, November 1978. After Kemp&#39;s discoveries became known to the art of hearing research, a number of researchers investigated the status of stimulated OAEs in people with normal hearing and with hearing impairments. 
     Early studies established that emissions are present in essentially all normally hearing individuals and that such emissions are reduced or eliminated in regions of sensorineural hearing loss. Moreover, it became apparent that, of the three types of stimulated emissions, SFEs could not be simply applied in practical settings, because they require the utilization of complex methods of analysis in order to separate them from the eliciting stimulus. 
     Of the remaining evoked-emission types, that is TEEs and DPEs, TEEs have received, by far, the most attention as potential clinical measures of cochlear function. The development of transiently evoked otoacoustic emissions (TEEs) has, in fact, reached an advanced level in that a computer-based commercial device is currently available to the audiologist: D. T. Kemp, et al., Acoustic Emission Cochleography--Practical Aspects, Scand Audial Suppl. 25, pp. 71-95, 1986; Peter Bray and David Kemp, An Advanced Cochlear echo Technique Suitable for Infant Screening, British Journal of Audiology, 1987, No. 21, pp. 191-204. 
     The form of a TEE from a given ear is subject to the invariable influence of fixed-frequency emissions that are unique to that ear. In the presence of idiosyncratic frequencies, including the spontaneous and stimulus-frequency otoacoustic emissions, as well as the TEEs themselves, cochlear function, at specific frequencies (e.g., audiometric-test frequencies), cannot be uniquely assessed. Consequently, TEE testing appears most useful as a screening device for estimating the absence or presence of reasonably normal hearing. 
     In contrast to the significant attention that TEEs have received as potential clinical indicators of outer hair-cell cochlea function, DPEs have not been extensively investigated as the basis of an objective test of hearing impairment. 
     3. Identification of Objects of the Invention 
     In view of the inherent problems of using TEEs and SFEs for hearing testing, it is a general object of the invention to provide method and apparatus for using DPEs as the basis of an objective hearing test, both for normal and hearing-impaired ears. 
     It is another object of this invention to provide a method and apparatus by which an ear may be tested, using DPEs, for hearing capability at any frequency between approximately one and eight kHz. In other words, it is an object of this invention to provide a method and apparatus for creating a DPE audiogram for a human being. 
     It is another object of this invention to provide a method and apparatus by which an objective measure of the hearing capability at a particular frequency varies as a function of stimulus level, so as to permit a complete evaluation of cochlear function at both threshold and suprathreshold levels of stimulation. In other words, it is an object of the invention to provide a method and apparatus for creating a DPE input/output function for a human being. 
     It is another object of this invention to provide a method and apparatus by which DPEs may be used to evaluate remaining outer hair-cell function in hearing impared human beings having a hearing loss up to 45-55 dB HL. 
     It is another object of this invention to provide noise reduction method and apparatus for reducing the noise signal which contaminates the measurement of the DPE, thereby providing a system which may be used in a noisy environment such as a doctor&#39;s clinic or other hearing screening facility. 
     SUMMARY OF THE INVENTION 
     To produce DPEs, two related pure tones are simultaneously presented to the ear. Such tones usually, (but not necessarily required) are of equal amplitude. The non-linear response to such tones, believed to occur in the cochlea of the inner ear, generate a lower amplitude tone in the ear. Such tone, believed to be produced by biomechanical elements of the cochlea, is at a frequency lower than the frequency f 1  or f 2  of the two input tones. Such DPE tone is at a frequency of 2f 1  -f 2 , but its amplitude is considerably lower than the amplitude of the two input tones at frequencies f 1  and f 2 . 
     This invention includes providing first and second tones of respective frequencies f 1  and f 2  to the ear canal of the outer ear. Such tones are provided to an eartip via earphones driven by a signal generator. Such eartip includes at least one microphone for sensing not only the two input tones at frequencies f 1  and f 2 , but also the DPE tone at frequency 2f 1  -f 2 . The microphone electrical signal output is amplified and then applied to a spectrum analyzer which produces, a signal output representative of the level of each frequency in the spectrum of frequencies of such microphone electrical signal output. 
     According to one aspect of the invention, a programmed digital computer controls the generation of the input tones and the recording of levels of the various frequencies received from the microphone electrical signal output of the spectrum analyzer. Over a frequency range such as 1 kHz to 8 kHz, input tones are generated and responses recorded by the frequency analyzer. A DPE audiogram is graphically presented by one of a plurality of ways, e.g. on graph paper, on a CRT screen, or in tabular form. Such DPE audiogram may graphically be presented on x-y axes with the response frequency of the DPE tone, 2f 1  -f 2 , plotted as the geometric mean of such two frequencies along the x-axis of the graph. The level of each such frequency, presented in decibels, is plotted along the y-axis of the graph. The response of normal ears may be superimposed on such graph to give the hearing clinician an objective view of the DPE amplitude versus frequency response of the patient being tested as compared to a person of normal hearing. The apparatus and method for DPE audiogram testing is advantageous over traditional hearing tests in that the procedure is completely objective, that is, it does not require a patient to respond as to whether or not a test tone has been &#34;heard&#34;  by such patient. 
     According to another aspect of the invention a programmed digital computer controls the generation of the input tone at particular input frequencies f 1 , f 2 . The input tones, having equal amplitudes, are varied in amplitude over a predetermined range, e.g., 25 to 85 db SPL. For each input amplitude, the DPE response amplitude at frequency 2f 1  -f 2  is recorded after measurement by the frequency analyzer described above. An input/output response for the particular DPE frequency response, usually represented as the geometric mean of f 1  and f 2 , is graphically plotted, either on graph paper or on a CRT screen, or is displayed in tabular form. The input amplitude in db SPL in plotted along the x-axis; the DPE amplitude is plotted along the y-axis. The &#34;noise floor&#34; as measured by the frequency analyzer may also be presented on the graphical display, along with a &#34;band&#34; of output responses which have been determined to be &#34;normal&#34; as a function of input amplitude. The advantage of this DPE procedure over other kinds of stimulated emission testing, is that DPEs have a reasonably wide dynamic range, in terms of growth of response amplitude as a function of stimulus level, thereby permitting evaluation of cochlear function at both &#34;threshold&#34; and suprathreshold levels of stimulation. 
     According to another feature of the invention, methods and apparatuses are provided for reducing the level of background noise sensed in DPE detection. A first method and apparatus, called phase-locked DPE extraction, initiates averaging of the DPE signal from the ear canal only when such DPE signal is at a particular phase. This method assures that all of the DPE signal is added to the averaged waveform, but background noise is significantly reduced. 
     According to another feature of the invention, a method and apparatus for reducing the level of background sensed in DPE detection includes providing two microphones in the eartip by which tones in the ear canal are sensed. The output of the first microphone includes signals representative of the DPE tone as well as patient-induced noise, such as coughing. The output of the second microphone is adjusted such that signals of the DPE tone are not present, yet signals representative of patient induced noise are produced. When applied to the plus and minus inputs of a differential amplifier, the output of such amplifier includes the DPE signal, with a &#34;body&#34; or noise signal significantly reduced due to the subtraction of the two signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, advantages and features of the invention will become more apparent by reference to the drawings which are appended hereto and wherein like numerals indicate like parts and wherein an illustrative embodiment of the invention is shown, of which: 
     FIG. 1 is a prior art illustration of a cross-section through a human ear showing outer, middle and inner ear parts with the cochlea of the inner ear responding to tones transmitted to the middle ear, generating nerve signals via the cochlear nerve; 
     FIG. 2 is a prior art illustration of a cross-section through one portion of the cochlea, showing the placement of the Organ of Corti along the spiral of the cochlea; 
     FIG. 3A is a schematic diagram of the cochlea depicted as unrolled, with a tone entering the oval window and stapes and traveling via the scala vestibuli and back via the scala tympani, while vibrating the scala media with its Basilar membrane and the Organ of Corti; 
     FIG. 3B is a schematic diagram illustrating tones T f1  and T f2  being applied to the ear drum membrane, where they enter the organ of Corti as a traveling wave, which produces a Distortion Product Emission tone of frequency 2f 1  -f 2  which is transmitted by the middle ear ossicles to the ear drum membrane, which vibrates like a speaker diaphram to transduce the DPE into acoustic energy in the ear canal; 
     FIG. 3C is an illustration showing the relative frequencies and amplitudes of the stimulating tones and the DPE tone which is generated by the cochlea; 
     FIG. 4 is a more detailed illustration of a cross-section of the Organ of Corti of the cochlea, in which may be seen the outer hair cells and the inner hair cells with nerve fibers connected thereto; 
     FIG. 5 is a system diagram of the apparatus necessary to produce DPE audiograms and input/output displays for assessing the hearing of a patient; 
     FIGS. 6A, B, C and D illustrate outputs of each DPE signal from the spectrum analyzer (6A) which are stored in computer memory (6B) and then are plotted as a DPE audiogram (6C) or as a DPE input/output display (6D). 
     FIG. 7A is a traditional audiogram for a hearing impared person, while FIG. 7B is a DPE audiogram according to the invention of such person; FIGS. 7C-7F are DPE input/output functions for such person; 
     FIGS. 8A and 8B illustrate two embodiments of the phase-locked averaging feature of the invention by which the noise floor of the DPE signal is reduced; and 
     FIG. 9 schematically illustrates a two-microphone embodiment of the apparatus by which body noises such as coughs and the like may be minimized during the measurement of the DPE signal. 
    
    
     DESCRIPTION OF THE INVENTION 
     This invention relates to apparatus and methods for measuring Distortion Product Emission (DPE) tones which are generated in the ear in what is believed to be a non-linear bio-mechanical feedback mechanism of the cochlea of an ear. By inducing such DPE tones, analyzing the level of such tones with respect to input tones, minimizing the noise associated with such tones, and presenting recorded tones in audiogram and input/output displays, hearing of a human being can be objectively assessed. 
     In order to introduce the basis for the apparatus and method of the invention, FIGS. 1, 2, 3A, 3B and 4 illustrate the physiology of the hearing process of a human being. 
     FIG. 1 shows a partial cross-section through a human ear with the outer, middle and inner sections of the ear illustrated. The ear canal is a passage in the outer ear which terminates with an ear drum membrane. Sound in the form of air pressure vibrations of multiple frequencies and amplitudes cause the ear drum membrane to vibrate. 
     Disposed in the middle ear cavity, are ossicles, small boney structures which oscillate in response to the oscillations of the drum membrane. As shown in FIG. 3A the stapes ossicle terminates at the oval window of the cochlea of the inner portion of the ear. 
     The cochlea (see FIGS. 1, 2, 3A, 4) is a spiral or snail like structure of the inner ear. It is a bio-mechanical organ for transforming oscillations of the stapes and the fluid of the cochlea into nerve impulses for recognition by the brain. FIG. 2 shows a cross-section through a portion of the cochlea. FIG. 3A shows a schematic diagram of an &#34;unrolled&#34; cochlea illustrating a sound wave entering the oval window and causing the scala media to oscillate as the sound pressure traverses the scala vestibuli and the scala tympani. As illustrated in FIG. 2, the Organ of Corti is disposed on the basilar membrane of the scala media. Accordingly, vibrations or oscillations of the scala media in response to air pressure vibrations via the outer ear canal and the vibration of the ear drum membrane, cause the Organ of Corti to vibrate. As illustrated in FIG. 4, the Organ of Corti has inner and outer hair cells disposed along the entire spiral length of the cochlea. Each inner hair cell includes nerve fibers which lead to the brain. 
     FIG. 3B illustrates the Distortion Product Emission phenomenon. When two audio tones are applied to the outer ear canal, such tones are simultaneously applied to the cochlea as traveling waves. Such tones, one of lower frequency f 1 , the other of higher frequency f 2 , are preferably applied with equal amplitude, or f 1  10-15 dB larger than f 2 , and of a frequency ratio f 2  /f 1  of about 1.21. Under such conditions a healthy human cochlea 10 generates a lower frequency, lower amplitude tone which is sensed by the ear drum 12. FIG. 3B schematically illustrates that such tone in a human being is at a frequency of 2f 1  -f 2 . 
     It has been discovered, as a result of tests in both normal and hearing impared ears with a number of human beings, that DPEs can form the basis of an objective hearing test. DPE testing has several advantages over the use of Transiently Evoked Emission (TEE) testing. In particular, because of the continuous, short-latency nature of the DPE, essentially any frequency, between approximately 1 and 8 kHz, can be intentionally tested. Such frequency specificity indicates that a DPE &#34;audiogram&#34;, objectively produced, can be a substitute for, or an adjunct to a conventional audiogram, where the patient subjectively responds to tones at different frequencies and amplitudes or levels. 
     Moreover, it has been discovered that compared to other stimulated-emission types of responses of the human ear, the reasonably wide dynamic range of DPEs, in terms of increase of response amplitude as a function of stimulus level, permits evaluation of cochlear function at both threshold and suprathreshold levels of stimulation. This latter feature allows the use of DPEs to evaluate remaining outer hair-cell cochlear function in the ears of patients demonstrating a hearing loss up to 45-55 dB HL. In contrast, TEEs cannot typically be measured in individuals with hearing losses greater then 20-30 dBHL. 
     To produce DPEs, two related pure tones (e.g., f 2  /f 1  =1.21 or 1.22) are simultaneously presented to the ear. The nonlinear audio response, that is distorted primarily in frequency, is believed to be generated by active, biomechanical elements of the cochlea. In particular, these nonlinear elements react to the two-tone signal so that DPEs of additional, different frequencies are created. In human ears, the predominant DPE is generated at the frequency value defined by the simple algebraic expression 2f 1  -f 2 . The testing for DPEs differs from the examination of TEEs in that the emissions are extracted by spectral averaging of the ear-canal signal. The spectral analysis includes emissions not only at the distortion-product frequency, but also at the frequencies of the two stimulating or primary tones, at f 1  and f 2 . 
     FIG. 5 illustrates, by means of a system diagram, the application of equi-level tones to the ear canal 16 of a human ear 14. A foam eartip 20 is placed in the ear canal 16. Two air ways 22, 24 are connected respectively between earphones 26 and 28 and eartip 20. The earphones or speakers 26, 28 are preferably model ER-2 speakers of Etymotic Research Corporation. Such earphones have reasonably flat responses from about 200 Hz to about 10 kHz. 
     The ear tip 20 also includes at least one microphone 30 which is a low-noise, miniature-microphone. Such microphone 30 is preferably model ER-10 13 of the above-mentioned Etymotic Research Corporation. It is specially designed to record low amplitude audio emissions from the human ear canal. 
     As illustrated in FIG. 5, a digital computer 50 preferably a Digital Equipment Computer 11/23 (but any suitable microprocessor such as an IBM PC, or equivalent may be used), is programed to step through a plurality of predetermined frequency pairs at predetermined levels or amplitudes. As mentioned above, because of the physiology of the human ear, a frequency ratio of f 2  /f 1  is preferably about 1.21 or 1.22. The preferred levels of such frequency tones will be discussed below in conjunction with the generation of input/output plots. 
     Under control of the stored program 48 in computer 50, a control signal is sent to signal generator 40 via an IEEE instrumentation bus 44 and lead 42. Signal generator 40 is preferably a dual channel HP3326A synthesizer which produces, on command from computer 50, two equal level sinusoidal electrical signals on leads 32 and 34. Attenuators 36 and 38 advantageously may be placed in leads 32 and 34 between earphones 26, 28 and signal generator 50, to provide a means to precisely control the level of the primary tones, T f1  and T f2  applied to earphones 26 and 28 and via eartip 20 to ear canal 16. (Such attenuators preferably are Wavetek 5P programmable attentuators.) 
     The microphone 30 of eartip 20 is connected to pre-amplifier 54 via lead 52. Pre-amplifier 54 is preferably an Etymotic Research model ER-10-72. The DPE tone T 2f1-f2  as well as tones T f1  and T f2  from the ear canal 16 are amplified by preamplifier 54 and then applied to measuring amplifier 58, a Bruel and Kjaer model 2610. Next, such tones are applied to frequency spectrum analyzer 62, preferably a Hewlett Packard model 3561A, where the amplitude level of each tone signal is determined in decibels SPL. Such levels, as well as the frequencies of the input tones, f 1  and f 2 , and the DPE tone 2f 1  -f 2  are applied to computer 50 memory 46 via lead 64 and IEEE instrumentation bus 65. Note: (IEEE signals are digital.) 
     FIGS. 6A and 6B schematically illustrate the storage of the tone levels of tones T 2f1-f1 , T f1  and T f2 . The level of the noise floor is also stored. 
     It has been determined that the primary cochlear frequency position that contributes to the generation of the DPE at 2f 1  -f 2  is the frequency region near the geometric mean of frequencies f 1  and f 2 . The geometric mean of two frequencies f 1  and f 2  is (f 1  ×f 2 ) 1/2 . Accordingly, the levels of DPE frequencies 2f 1  -f 2  are stored in memory and plotted as described below as a function of the geometric mean frequency, f geo  =(f 1  ×f 2 ) 1/2 . 
     According to this invention, DPE activity of a human being is specified in terms of two response measures. In the first form, illustrated at FIG. 6C, the frequency extent of cochlear function is expressed in terms of DPE amplitude as a function of stimulation frequency. Such a graphical display is called a DPE &#34;audiogram&#34;. To obtain an objective DPE audiogram, DPEs are recorded, using the apparatus of FIG. 5, in 100 Hz steps, at three primary tone levels of 65, 75, and 85 dB SPL. The frequency ratio of the f 1  and f 2  tones are adjusted to be about 1.22 or 1.21, i.e., f 2  /f 1  =1.21 or 1.22. 
     The second measure of DPE activity is shown at FIG. 6D which indicates the response/growth or input/output (I/O) aspects of DPE activity. To determine the dynamic range of the distortion-generation process, the I/O functions are determined over a 60 dB range of stimulus levels, (i.e., from 25 to 85 dB SPL). Such functions are preferably acquired at 11 discrete test frequencies, distributed in regular, 1/4 octave intervals, from 1-8 kHz. The display of FIG. 6D is for one particular test frequency and shows the level response of the DPE tone as a function of increasing input tone amplitude. From the various I/O curves at the various test frequencies, information concerning the function of an ear under test can be determined. Specifically, an ear&#39;s outer hair cells at either threshold or suprathreshold sound levels can be determined. 
     One of ordinary skill in the digital programing art can rapidly prepare a stored program 48 and data arrangement for memory 46 to automatically collect the data for an audiogram like that of FIG. 6C and Input/Output response like that of FIG. 6D. The source code program used in a laboratory setting during the creation of this invention is attached hereto as appendix A. Such program is written in Fortran language. As indicated above, such program controls a DEC mini-computer, but the ultimate commercial version of the system of FIG. 5 will likely include a dedicated microcomputer with a stored program similar to that of Appendix A for collecting data for DPE audiograms and I/O functions. 
     Returning now to a more detailed explanation of the DPE audiogram of FIG. 6C, the amplitude vs. frequency bands 80, 82 represent the ±one standard deviation of a database of human DPE response as a function of DPE frequency (geometric mean of f 1  and f 2 ). Such bands were determined from &#34;audiograms&#34; of 44 ears displaying normal aural-acoustic immittance and hearing sensitivity. In FIG. 6D, bands 84 and 86 represent I/O ±one standard deviation bands at a particular frequency of such database. The noise floor of FIG. 6D represents the noise floor illustrated in FIG. 6A as recorded for the various input levels of tones T f1  and T f2 . The bands 88, 90 of FIG. 6C represent ±one standard deviation of the noise floor. 
     One advantage of DPE testing is that DPE emissions have the capability of accurately delimiting the boundary between normal and abnormal function. This property is illustrated best in patients exhibiting the effects of noise damage in which discrete notches and sharp reductions in high-frequency hearing commonly occur. FIGS. 7A through 7F are test results of a patient with noise-induced hearing loss due to the excessive use of recreational firearms. The standard audiograms, for each ear, are shown at FIG. 7A At FIG. 7B, the corresponding DPE &#34;audiograms,&#34; in response to 75-dB SPL primaries, are shown. A comparison of the DPE audiogram of FIG. 7B with that of a standard audiogram of FIG. 7A indicates that the frequency pattern of the reduction in DPE amplitudes follows very closely the details of the hearing loss depicted by standard audiometrics. That is, the function for the more damaged left ear (triangular and square symbols) declined to the level of the noise floor at a frequency that was lower than that at which the curve for the better-hearing right ear (circular symbols) descended to these levels. In addition, the finer-frequency steps of the DPE &#34;audiogram&#34; demonstrate that the emissions for the right ear also returned to the average range of amplitudes, at a frequency that was lower in value than that at which the responses recorded from the poorer-hearing left ear reached normal emission levels. 
     FIGS. 7C through 7F represent several I/O functions selected to show the outcome of the discrete-frequency, I/O testing, between 1 and 6 kHz. For example, as expected from the behavioral threshold estimated for 1.5 kHz (i.e., by interpolating between 1 and 2 kHz on the behavioral audiogram), DPE magnitudes for the right ear are within normal limits. In contrast, the left ear exhibited an estimated hearing loss between 10 and 55 dB SPL. At 1.5 kHz, DPEs were essentially nonexistent. Thus, the observed asymmetry in the frequency extent of DPE I/O activity supported the asymmetry noted in the hearing for the two ears, around 1-2 kHz. By 6 kHz, (FIG. 7F) the behavioral measures (FIG. 7A) indicate that the right ear has slightly better hearing than the left. Similarly, the DPE I/O curves for 6 kHz (FIG. 7F) support the behavioral observation in that the detection &#34;thresholds&#34; for the left ear are about 5 dB poorer than the comparable measures for the right ear. In general, DPEs track hearing loss due to noise exposure accurately. This is believed to be so because excessive sound injures the outer hair cells preferentially, especially in the beginning stages. It is believed that DPEs selectively test outer hair-cell functioning. 
     Returning to FIG. 5, both DPE audiograms and I/O functions may be graphically displayed by means of a plotter 68 under control of digital computer 50. The plotter used in the laboratory embodiment of the invention was an HP 7470A plotter, but of course, a wide variety of commercial plotters could be used as would be known to one of ordinary skill in the data processing art. Likewise printer 70, under control of computer 50 serves to provide a print-out of DPE audiograms or I/O functions. Such DPE audiograms or I/O functions may also advantageously be displayed on a CRT or the like (not illustrated). 
     Turning now to FIG. 8A, an illustration of the phase-locked averaging of the DPE signals is shown which is performed in order to reduce the noise level or &#34;floor&#34; shown at FIG. 6A. A signal generator 40 as shown in FIG. 5 is provided to generate signals S f1  and S f2  on leads 32, 34. The master clock of signal generator 40 (Hewlett Packard 3326A) is connected to the master clock of single channel signal generator 40&#39;. Signal generator 40&#39; is set to the DPE frequency 2f 1  -f 2 . Connection of the master clocks of signal generators 40 and 40&#39; insures that there is no phase drift among any of the three signals, i.e., S f1 , S f2 , S 2f1-f2 , because all clocks are synchronized. 
     Signal generator 40&#39; produces a square wave output on lead 43 such that its positive going pulse is synchronized with the positive going sine wave of frequency 2f 1  -f 2 . Such square wave of frequency 2f 1  -f 2  is applied to the trigger input of spectrum analyzer 62. A time average is determined by real-time spectrum analyzer 62 each time it is triggered by the positive going zero crossing signal on lead 43. Accordingly, the time average of the DPE signal is initiated for each sample of the DPE signal at the same phase of such DPE signal. Such procedure assures that all of the DPE signal is added to the averaged waveform, but noise signals of different frequencies than that of the DPE signal will be out of phase from sample to sample. Accordingly, the noise signal, dissimilar in phase, is substantially reduced during the averaging process. 
     Once the time averaged waveform has been collected, spectrum analyzer 62 uses standard Fourier-transform techniques to determine the level of the DPE signal, and of course the noise floor for frequencies other than that of the DPE signal. Using the method and apparatus of FIG. 8A to augment that of FIG. 5, background noise is reduced by approximately 15 dB below that observed without the phase-locking method and apparatus. 
     The phase-locking feature of the invention described above provides a means for repeatedly initiating a time sample at the same phase of the DPE signal. An alternative method for accomplishing such phase-locking is illustrated in FIG. 8B. Rather than using a stand-alone signal generator 40, signals S f1  and S f2  are generated by microcomputer 50. Two wave form buffers are established in the memory of computer 50&#39;. The buffers 101,102 store the digitized time values for S f1  and S f2 . 
     Each buffer is set to an integer multiple of the length of the DPE period. After output of each point in the buffer, exactly one period of an integer multiple of the period of the DPE signal has elapsed. For example, if a buffer contains 1,000 points for an output signal, representing 1 micro-second per point, such buffer is equivalent to a DPE frequency of 1 kHz each time the information in the buffer is completed. 
     Another alternative approach, one not involving computer generation of the primary-tone signals, provides a computer clock running at the period of the DPE frequency. For example, if the clock is set to &#34;tick&#34; once per microsecond, 1,000 ticks would be equivalent to 1 millisecond, i.e., the period of a 1-kHz DPE. At the end of every 1,000 ticks, an interrupt would be generated to instruct the computer to initiate another time sample. If the time sample is longer than one period of the distortion-product frequency, the computer would simply wait until the next 1,000 ticks have elapsed. With the clock running continuously, the phase of the distortion product would be constant from sample to sample. 
     FIG. 9 illustrates still another apparatus and method for reducing noise when measuring DPE signals. A significant amount of body noise is created by the patient during the measurement. Coughing, sneezing and other body noises contaminate the DPE signal. FIG. 9 shows that two microphones 30A, 30B are placed in ear canal 16 to pick up the DPE signal. The output of one microphone 30A is applied via lead 120 to the plus input of differential amplifier 130. The output of a second less sensitive microphone 30B is applied via lead 122 through variable gain amplifier and phase shifter circuits 124, 126 via lead 128 to the minus input of differential amplifier 130. The output of microphone 30B is of less sensitivity so that the frequency component of S 2f1-f2  (the DPE signal) is missing from the signal applied to the minus input of differential amplifier. The gain and phase of this signal via lead 128 are then adjusted to yeild maximum cancellation when applied to differential amplifier 130. That part of the lower frequency signal appearing on lead 128 due to the bodily noise is relatively unchanged. As a result, the output of differential amplifier 130 produces on lead 132 a signal comprising the DPE signal, with the body noise signal greatly reduced. 
     Various modifications and alterations in the described methods and apparatus will be apparent to those skilled in the art of the foregoing description which does not depart from the spirit of the invention. For this reason, these changes are desired to be included in the appended claims. The appended claims recite the only limitation to the present invention. The descriptive manner which is employed for setting forth the embodiments is to be interpreted as illustrative but not limitative. ##SPC1##