Patent Publication Number: US-2020297252-A1

Title: Probe insertion methods and apparatus

Description:
FIELD 
     This application relates to the field of probe insertion methods and apparatus for real ear measurement. 
     INTRODUCTION 
     “Real ear measurement” is the process of measuring sound pressure levels in a patient&#39;s ear canal. This is often done for the purpose of hearing aid verification. The process traditionally involves inserting a probe microphone into the ear canal to record amplified signals. 
     SUMMARY 
     In one aspect, a probe insertion apparatus for real ear measurement is provided. The apparatus may include a probe tube, a reference microphone, a reference speaker, an operator feedback device, a memory storing computer readable instructions, and a processor. The probe tube may have a sound receiving end positionable in an ear canal, and a sound output end sonically coupled to a measurement microphone. The reference microphone may be positionable outside the ear canal. The reference speaker may be positionable to emit reference sound towards the ear canal. The memory may store computer readable instructions. The processor may be configured to execute the computer readable instructions, wherein the computer readable instructions when executed may configure the processor to:
         direct the reference speaker to continuously emit the reference sound,   continuously receive reference signals from the reference microphone, and tympanic reflection signals from the measurement microphone,   continuously make determinations of a tympanic distance between the sound receiving end of the probe tube and a tympanic membrane based on the reference signals and the tympanic reflection signals and absent reference to a calibration measurement , as the operator moves the sound receiving end towards the tympanic membrane, and   automatically direct the operator feedback device to provide indicia of the tympanic distance between the sound receiving end of the probe tube and a tympanic membrane, as the operator moves the sound receiving end towards the tympanic membrane.       

     In another aspect, a probe insertion apparatus for real ear measurement is provided. The apparatus may include a measurement microphone input, a reference microphone input, a reference speaker output, an operator feedback device output, a memory storing computer readable instructions, and a processor configured to execute the computer readable instructions. The computer readable instructions when executed may configure the processor to:
         send directions to the reference speaker output to continuously emit a reference sound,   continuously receive reference signals from the reference microphone input, and tympanic reflection signals from the measurement microphone input,   continuously make determinations of a tympanic probe distance based on the reference signals and the tympanic reflection signals and absent reference to a calibration measurement, and   automatically send directions to the operator feedback device output to provide indicia of the tympanic probe distance between the sound receiving end of the probe tube and a tympanic membrane.       

     In another aspect, a method of performing a real ear measurement is provided. The method may include:
         a) emitting reference sound towards the ear canal,   b) moving a sound receiving end of a probe tube in an ear canal towards a tympanic membrane,   c) receiving tympanic reflection signals from a measurement microphone acoustically coupled to the probe tube,   d) receiving reference signals from a reference microphone located outside the ear canal,   e) continuously determining a tympanic distance between the sound receiving end of the probe tube and the tympanic membrane based on the reflection signals and the reference signals and absent reference to a calibration measurement, and   f) continuously providing indicia of the determined tympanic distance between the sound receiving end of the probe tube and the tympanic membrane,   wherein a) to f) are performed concurrently.       

    
    
     
       DRAWINGS 
         FIG. 1  is a schematic illustration of a probe insertion apparatus in use with a patient&#39;s ear, in accordance with an embodiment; 
         FIG. 2  is a schematic illustration of a probe insertion apparatus in use with a patient&#39;s ear, in accordance with an embodiment; 
         FIG. 3  is a schematic illustration of a probe insertion apparatus in use with a patient&#39;s ear, in accordance with an embodiment; 
         FIG. 4  is a schematic illustration of a probe insertion apparatus in use with a patient&#39;s ear, in accordance with an embodiment; 
         FIG. 5  is a schematic illustration of a probe insertion apparatus in use with a patient&#39;s ear, in accordance with an embodiment; 
         FIG. 6  is a schematic illustration of a controller, in accordance with an embodiment; 
         FIG. 7  is a spectrograph of tympanic reflection sound, measured while inserting a probe into a patient&#39;s ear canal; 
         FIG. 8  is the spectrograph of  FIG. 7  processed through a rank-ordering filter; 
         FIG. 9  is a scatter plot of 5-fold cross validation test predictions compared against manually determined tympanic distance; and 
         FIG. 10  is a line graph comparing tympanic distance predicted by a trained model against tympanic distance manually determined, as an operator moved the probe tube within a patient&#39;s ear canal. 
     
    
    
     DESCRIPTION OF VARIOUS EMBODIMENTS 
     Numerous embodiments are described in this application, and are presented for illustrative purposes only. The described embodiments are not intended to be limiting in any sense. The invention is widely applicable to numerous embodiments, as is readily apparent from the disclosure herein. Those skilled in the art will recognize that the present invention may be practiced with modification and alteration without departing from the teachings disclosed herein. Although particular features of the present invention may be described with reference to one or more particular embodiments or figures, it should be understood that such features are not limited to usage in the one or more particular embodiments or figures with reference to which they are described. 
     The terms “an embodiment,” “embodiment,” “embodiments,” “the embodiment,” “the embodiments,” “one or more embodiments,” “some embodiments,” and “one embodiment” mean “one or more (but not all) embodiments of the present invention(s),” unless expressly specified otherwise. 
     The terms “including,” “comprising” and variations thereof mean “including but not limited to,” unless expressly specified otherwise. A listing of items does not imply that any or all of the items are mutually exclusive, unless expressly specified otherwise. The terms “a,” “an” and “the” mean “one or more,” unless expressly specified otherwise. 
     As used herein and in the claims, two or more parts are said to be “coupled”, “connected”, “attached”, “joined”, “affixed”, or “fastened” where the parts are joined or operate together either directly or indirectly (i.e., through one or more intermediate parts), so long as a link occurs. As used herein and in the claims, two or more parts are said to be “directly coupled”, “directly connected”, “directly attached”, “directly joined”, “directly affixed”, or “directly fastened” where the parts are connected in physical contact with each other. As used herein, two or more parts are said to be “rigidly coupled”, “rigidly connected”, “rigidly attached”, “rigidly joined”, “rigidly affixed”, or “rigidly fastened” where the parts are coupled so as to move as one while maintaining a constant orientation relative to each other. None of the terms “coupled”, “connected”, “attached”, “joined”, “affixed”, and “fastened” distinguish the manner in which two or more parts are joined together. 
     Further, although method steps may be described (in the disclosure and/or in the claims) in a sequential order, such methods may be configured to work in alternate orders. In other words, any sequence or order of steps that may be described does not necessarily indicate a requirement that the steps be performed in that order. The steps of methods described herein may be performed in any order that is practical. Further, some steps may be performed simultaneously. 
     As used herein and in the claims, a first element is said to be ‘communicatively coupled to’ or ‘communicatively connected to’ or ‘connected in communication with’ a second element where the first element is configured to send or receive electronic signals (e.g. data) to or from the second element, and the second element is configured to receive or send the electronic signals from or to the first element. The communication may be wired (e.g. the first and second elements are connected by one or more data cables), or wireless (e.g. at least one of the first and second elements has a wireless transmitter, and at least the other of the first and second elements has a wireless receiver). The electronic signals may be analog or digital. The communication may be one-way or two-way. In some cases, the communication may conform to one or more standard protocols (e.g. SPI, I 2 C, Bluetooth™, or IEEE™ 802.11). 
     As used herein and in the claims, a group of elements are said to ‘collectively’ perform an act where that act is performed by any one of the elements in the group, or performed cooperatively by two or more (or all) elements in the group. 
     As used herein and in the claims, an act is said to occur “continuously” where it occurs continually (i.e. without pause or interruption) or repeatedly (e.g. many times, at a constant or variable frequency). For example, a signal may be received “continuously” over a period of time, where the signal is received as an continuous, uninterrupted analog signal, or an intermittent digital signal (e.g. pulsing between low and high signals to represent digital bits). 
     Some elements herein may be identified by a part number, which is composed of a base number followed by an alphabetical or subscript-numerical suffix (e.g.  112   a , or  112   1 ). Multiple elements herein may be identified by part numbers that share a base number in common and that differ by their suffixes (e.g.  112   1 ,  112   2 , and  112   3 ). All elements with a common base number may be referred to collectively or generically using the base number without a suffix (e.g.  112 ). 
     Within the ear canal, incident sound and reflected sound (e.g. reflected off the tympanic membrane at the proximal end of the ear canal) produce standing waves that cause the sound pressure level to vary along the ear canal. To accurately measure ear canal acoustics with real ear measurement, the probe tube should be placed so that the sound receiving end of the probe tube is within a particular range of distances (e.g. 3-8 mm, and more preferably 4-7 mm) from the tympanic membrane (also referred to as ‘tympanic spacing’, ‘tympanic distance’, ‘tympanic probe spacing, or ‘tympanic probe distance’). 
     Contact between the probe tube and the tympanic membrane can create intense discomfort for patients. Therefore, moving the probe tube into contact with the tympanic membrane and retracting the desired distance is not a good solution for ensuring proper tympanic spacing. Instead, clinicians typically rely on visual inspection and other complex methods for positioning the probe tube with the desired tympanic spacing. Such complexity is one reason why some clinicians are reluctant to adopt real ear measurement. This may be particularly true for clinicians without medical training, such as lay staff at a hearing aid kiosk. 
     Embodiments herein relate to a probe insertion apparatus for real ear measurement that does not require or rely upon calibration measurements of tympanic distance. The apparatus may provide real time feedback to an operator (e.g. clinician) of the tympanic distance as the operator inserts the probe tube into the patient&#39;s ear canal towards the tympanic membrane. For example, the apparatus may include a display of the tympanic distance that updates automatically as the probe tube is moved towards the tympanic membrane. This allows the operator to easily monitor the tympanic distance as the probe tube is being inserted, and to stop inserting the probe tube when the apparatus indicates that the target tympanic distance (e.g. 4-7 mm) has been reached. This avoids any need for the operator to perform visual inspection or other complex techniques to achieve the target tympanic distance. Therefore, the apparatus disclosed herein may reduce the skill required to perform real ear measurement, which may promote greater adoption particularly by operators without medical training (e.g. lay staff at a hearing aid kiosk). 
       FIG. 1  shows a probe insertion apparatus  100 . As shown, apparatus  100  may include a probe tube  104 , a measurement microphone  106 , a reference microphone  108 , a reference speaker  112 , an operator feedback device  116 , and a controller  120 . 
     In use, an operator slowly inserts probe tube  104  into the ear canal  304  of a patient  300  towards their tympanic membrane  308 . At the same time, reference sound  124  is emitted by reference speaker  112  towards the user&#39;s ear  312 . Reference microphone  108  is positioned outside of ear canal  304  adjacent to ear canal  304  where reference microphone  108  senses reference sound  124 , and in response sends reference signals to controller  120 . Reference sound  124  also enters ear canal  304 . Inside ear canal  304 , reference sound  124  reflects off of tympanic membrane  308 . The reflected sound enters probe tube  104 . Probe tube  104  is sonically coupled to measurement microphone  106 , which senses the reflected sound, and in response sends tympanic reflection signals to controller  120 . 
     As the operator is moving probe tube  104  towards tympanic membrane  308 , controller  120  continuously (e.g. repeatedly or continually) determines a tympanic distance  128  between a sound receiving end  132  of probe tube  104  and tympanic membrane  308 , and directs operator feedback device  116  to provide indicia of the determined tympanic distances  128 . For example, operator feedback device  116  may include a display  136 , and controller  120  may continuously direct display  136  to update indicia  140  of the tympanic distance shown on display  136 . The operator may reference indicia  140  as it is updated to determine when probe tube  104  has a targeted tympanic distance  128 , and therefore when to stop moving probe tube  104  towards tympanic membrane  308 . 
     Controller  120  may determine tympanic distance  128  absent reference to calibration measurements of tympanic distance. For example, the operator may not be tasked with taking manual calibration measurements of tympanic distance (e.g. visual or contact-based tympanic measurements), to be used by controller  120  as a basis for subsequent measurements made during probe tube insertion. 
     Probe tube  104  may be any sound conduit suitable for channeling sound from a sound receiving end  132  located within the ear canal  304  proximate a patient&#39;s tympanic membrane  308 , to a reference microphone  108  located outside the ear canal  304 . For example, probe tube  104  may be a flexible hollow tube, such as a silicone or plastic tube. In some embodiments, probe tube  104  has an outer diameter of less than 1 mm (e.g. 0.3 to 0.7 mm), and an inner diameter of less than 0.7 mm (e.g. 0.15 to 0.5 mm). Probe tube  104  may have any length sufficient to extend from proximate tympanic membrane  308  to a measurement microphone  106  located outside of ear canal  304 . For example, probe tube  104  may be at least 5 cm long (e.g. 5 to 30 cm). 
     Microphones  106 ,  108  may be any devices suitable for sensing reflection and reference sound respectively, and sending tympanic reflection and reference signals to controller  120 . For example, microphones  106 ,  108  may include condenser and/or electret microphones. As shown, microphones  106 ,  108  may be located outside of ear canal  304 . This mitigates microphones  106 ,  108  obstructing sound propagation along ear canal  304 . 
     In some embodiments, microphones  106 ,  108  may be provided in a common microphone housing  144  as shown. For example, microphone housing  144  containing microphones  106 ,  108  may be attached (e.g. suspended) from the user&#39;s ear  312 . In the illustrated example, microphone housing  144  is suspended from ear  312  by an adjustable cord  148 . In other embodiments, each microphone  106 ,  108  may be separately housed and supported (e.g. attached) proximate ear canal  304 . 
     Measurement microphone  106  may be configured to sense reflected sound channeled by probe tube  104 . For example, probe tube  104  may include a sound output end  150  sonically coupled to measurement microphone  106 . In the illustrated example, a connector  151  mechanically and sonically joins sound output end  150  to measurement microphone  106  to provide a reliable and efficient transmission of reflection sound from sound output end  150  to measurement microphone  106 . 
     Reference microphone  108  may be positioned and oriented to sense reference sound emitted by reference speaker  112 . For example, reference microphone  108  may be oriented outwardly (i.e. away from the patient) as shown. In the illustrated example, reference microphone  108  is oriented outwardly away from tympanic membrane  308  to receive reference sound with substantially no interference from reflected sound (i.e. sound reflected from tympanic membrane). 
     Reference speaker  112  can be any device suitable to emit reference sound towards a patient&#39;s ear  312 . The reference sound may include broadband sound spanning at least 3000 Hz to 16000 Hz. In general, sound waves below 3000 Hz may be too long to be useful for determining tympanic distance. Although not strictly necessary (and may not be the case in some embodiments), the reference sound is preferably a broadband stimulus that is statistically stationary (i.e. has statistical properties, such as mean, variance, etc. which are relatively constant over time). 
     Operator feedback device  116  can be any device suitable to provide sensible (i.e. perceivable by an operator&#39;s senses) indicia  140  of tympanic distance  128 . Indicia  140  is continuously (e.g. repeatedly or continually) updated at the direction of controller  120  to reflect the tympanic distance  128 , which is changing continually from moment to moment as the operator moves probe tube  104  towards tympanic membrane  308 . Operator feedback device  116  may provide one or more (or all) of auditory, visual, and haptic indicia of tympanic distance  128 .  FIGS. 1-5  illustrate several embodiments of apparatus including operator feedback devices  116  that provide various indicia. These examples may be combined in any combinations or sub-combinations in various embodiments that provide two or more of these indicia. 
       FIG. 1  shows an example in which operator feedback device  116  includes a display  136  that provides visual indicia  140 . Visual indicia  140  may visually represent tympanic distance  128  in any manner, such as for example graphically as shown, symbolically (e.g. with symbols representing proximity to the targeted tympanic distance), numerically (e.g. a distance readout), by color-coding (e.g. continual color spectrum or discrete color bands), or blinking (e.g. blink frequency changes with tympanic distance or proximity to a target tympanic distance). Visual indicia  140  may be digital as shown. Alternatively, operator feedback device  116  may provide analog indicia (e.g. an analog gage), or include both digital and analog indicia. Visual indicia  140  may provide an absolute measurement (e.g. indication of distance from tympanic membrane  308 ) or a relative measurement (e.g. indication of distance to a targeted tympanic distance). 
     In the illustrated embodiment, visual indicia  140  include a graphic representation  152  of an ear canal, and an indicator  156  positioned with respect to the graphical representation  152  (e.g. overlaid on the graphical representation as shown) to indicate tympanic distance  128 . As shown, visual indicia  140  may further include numeric measurements  160  that allow tympanic distance  128  to be determined by reference to the position of indicator  156 . While the operator moves probe tube  104  towards tympanic membrane  308 , controller  120  may direct display  136  to update to move the position of indicium  140  along graphical representation  152  to reflect the tympanic distance  128  from moment to moment. 
       FIG. 2  shows another example in which operator feedback device  116  includes a row  164  of lights  168 . In this example, visual indicia  140  include the illumination of a particular light  168 , and numeric measurements  160  identifying a relative or absolute tympanic distance associated with an illuminated light  168 . While the operator moves probe tube  104  towards tympanic membrane  308 , controller  120  may direct row  164  of lights  168  to update by illuminating a different one of lights  168  to reflect the tympanic distance  128  from moment to moment. 
     It will be appreciated that operator feedback device  116  may have a limited resolution. For example, the row  164  of lights  168  shown in  FIG. 2  are illustrated with an accuracy of 2.5 mm, such that controller  120  may round determinations of tympanic distance to the nearest 2.5 mm increment when directing operator feedback device  116  to update. Furthermore, controller  120  may not direct operator feedback device  116  to update after every determination of tympanic distance  128 . For example, where a change in tympanic distance  128  would not change the indicia of operator feedback device  116  (e.g. because of the resolution/accuracy of operator feedback device  116 ) then directions from controller  120  to operator feedback device  116  may be withheld. 
       FIG. 3  shows an example in which operator feedback device  116  provides haptic indicia  140 . For example, operator feedback device  116  may include a vibrator  172 . Vibrator  172  may be any device suitable to provide vibrations indicative of tympanic distance  128 . For example, vibrator  172  may include an offset motor (also known as an eccentric rotating mass vibration motor) as shown, a linear resonant actuator, and/or a piezo electric vibrator. Vibrator  172  may indicate tympanic distance  128 , in absolute or relative terms, in any manner, such as for example by vibration intensity, frequency of vibration pulses, or vibration pattern (e.g. Morse code, or similar). While the operator moves probe tube  104  towards tympanic membrane  308 , controller  120  may direct vibrator  172  to update the vibration produced (e.g. intensity, frequency, and/or pattern) to reflect the tympanic distance  128  from moment to moment. 
       FIG. 4  shows an example in which operator feedback device  116  provides auditory indicia  140 . Operator feedback device  116  may provide auditory indicia  140  in any manner that does not conflict with reference sound  124 . In the illustrated example, operator feedback device  116  includes a feedback speaker  176  (e.g. loud speaker, or wearable audio device, such as headphones or earphones). Controller  120  may direct feedback speaker  176  to emit auditory indicia  140 . Auditory indicia  140  may auditorily (i.e. with sound) represent tympanic distance  128  in any manner. Auditory indicia  140  may indicate tympanic distance  128  in absolute terms (e.g. distance from tympanic membrane  308 ) or relative terms (e.g. proximity to target tympanic distance). Auditory indicia  140  may indicate tympanic distance  128  by value (e.g. identifying a specific distance measurement, such as “6 mm”), or qualitatively (e.g. identifying proximity to tympanic membrane or a target tympanic distance in a way that does not specifically communicate a distance measurement). Auditory indicia  140  may indicate tympanic distance  128  with spoken words, such as “seven millimeters”, or “getting closer” for example. Alternatively or in addition, auditory indicia  140  may indicate tympanic distance with non-verbal sounds, such as sound frequency (e.g. pitch increases or decreases based on tympanic distance), sound pattern (e.g. pattern of tones or beeps changes based on tympanic distance), and/or volume (e.g. volume increases or decreases based on tympanic distance). Auditory indicia  140  may include a special alert (verbal or non-verbal) when the target tympanic distance is reached. While the operator moves probe tube  104  towards tympanic membrane  308 , controller  120  may direct feedback speaker  176  to update the sound produced (e.g. verbal and/or non-verbal sounds) to reflect the tympanic distance  128  from moment to moment. An advantage of auditory indicia  140  is that it allows the operator to keep their visual attention on the patient while moving probe tube  104 . 
     Operator feedback device  116  may be configured to produce auditory indicia  140  that does not conflict with reference sound  124 . This allows auditory indicia  140  to be sounded while reference sound  124  continues to play and while the operator continues to move probe tube  104  towards tympanic membrane  308 . In some embodiments, operator feedback device  116  includes a wearable audio device, such as headphones or earphones, that emit indica  140  directly into the operators ears. Alternatively or in addition, feedback speaker  176  may be a loudspeaker oriented to emit auditory indicia  140  away from the patient&#39;s ear  312 . For example, feedback speaker direction  180  may be oriented at an angle of at least 45 degrees (i.e. 45 to 315 degrees) from reference speaker direction  184 . This may substantially mitigate feedback sound  140  from contaminating the reference and tympanic reflection signals that controller  120  relies upon for determining tympanic distance  128 . 
     Alternatively or in addition to orienting feedback speaker  176  to emit auditory indicia  140  away from the patient&#39;s ear  312 , auditory indicia  140  may be composed of sound frequencies that are all outside (e.g. above or below, and preferably below) the sound frequencies of reference sound  124 . This allows microphones  106 ,  108  and/or controller  120  to filter out auditory indicia  140  so that it does not contaminate the reference and tympanic reflection signals that controller  120  relies upon for determining tympanic distance  128 . For example, auditory indicia  140  may be limited to sound frequencies less than 3000 Hz (e.g. 20 Hz to 3000 Hz). Having regard to these limitations, auditory indicia  140  should include sound frequencies with the range of normal human hearing (e.g. 20 Hz to 20000 Hz) so that the operator can perceive the auditory indicia  140 . 
       FIG. 5  shows an example in which controller  120  directs reference speaker  112  to product auditory indicia  140 . For example, auditory indicia  140  may be composed of sound frequencies that are all outside of the sound frequencies of reference sound  124 . In this way, reference speaker  112  may also be an operator feedback device  116 . Therefore, an advantage of this design is that it does not require a separate feedback speaker to produce auditory indicia  140 , which may reduce the cost, size, and weight of apparatus  100 . 
     Reference is now made to  FIGS. 1 and 6 . As shown, controller  120  may include a processor  188 , memory  192 , and a plurality of I/O  196  (also referred to as “interfaces”  196 ). Interfaces  196  may include a measurement microphone input  196   1  for receiving tympanic reflection signals from measurement microphone  106 , a reference microphone input  196   2  for receiving reference signals from reference microphone  108 , and a reference speaker output  196   3  for sending directions to reference speaker  112  to emit reference sound. Controller  120  may also include an operator feedback device output for sending directions to an operator feedback device  116  to provide indicia of tympanic distance  128 . Operator feedback device output may be a discrete interface  196   4  (e.g. where operator feedback device  116  is discrete from reference speaker  112 ), or may be provided by interface  196   3  (e.g. where operator feedback device  116  includes reference speaker  112 , and reference speaker  112  is used to produce auditory indicia of tympanic distance  128 ). Accordingly, measurement microphone input  196   1  may be communicatively coupled to measurement microphone  106 , reference microphone input  196   2  may be communicatively coupled to reference microphone  108 , reference speaker output  196   3  may be communicatively coupled to reference speaker  112 , and operator feedback device output  196   3  or  196   4  may be communicatively coupled to operator feedback device  116  (e.g. which may or may not be/include reference speaker  112 ). 
     In some embodiments, one or more (or all) of interfaces  196  may provide removable connections to the devices (e.g. microphones, speakers, or feedback device) with which they are communicatively coupled. An advantage to a removable connection is that it can allow controller  120  to be supplied (e.g. sold) separately of the device(s), whereby the operator can provide their own device(s) (e.g. microphone, speaker, or feedback device) thereby reducing the size, weight, and cost of the apparatus. This can also allow controller  120  to replaced (e.g. as newer models are released) without having to replace the peripheral device(s) connected by removable connections. Interface(s)  196  may include any removable connection suitable for communicative coupling with the associated peripheral device, such as for example a male or female electrical connector (e.g. RCA, 3.5 mm, HDMI, DVI, DP, USB, banana connector, spade connector, or euroblock). Alternatively, or in addition, one or more (or all) of interfaces  196  may have hardwired, non-removable connections (e.g. soldered wire or PCB trace) to their associated peripheral device. As compared with removable connections, this may improve signal quality. 
     In  FIG. 1 , controller  120 , reference speaker  112 , and operator feedback device  116  are illustrated as being provided in a single housing  204 . An advantage of this design is that it may be more compact, and reduce the need for the operator to decide how to arrange these components. In alternative embodiments, one or more (or all) of controller  120 , reference speaker  112 , and operator feedback device  116  may have separate housings. An advantage of this design is that it provides greater flexibility in the position and arrangement of these components. For example, reference speaker  112  may be positioned and oriented to direct sound towards the patient&#39;s ear  312 , and operator feedback device  116  may be positioned and oriented where the operator can best perceive the provided indicia  140  of tympanic distance  128 . 
     Processor  188  may be any processing device suitable for determining tympanic distance  128  based on reference and tympanic reflection signals from microphones  108 ,  106  respectively. For example, processor  188  may include one or more ARM, RISC, Intel™, or AMD™ microprocessors, or integrated circuits (e.g. fixed or FPGA (field programmable gate array)). 
     Memory  192  may include volatile memory (e.g. RAM) and/or non-volatile memory (e.g. flash memory). Memory  192  may store computer executable instructions (also referred to as computer readable instructions) that when executed by processor  188 , configure processor  188  to perform the functions and methods described herein. Memory  192  may include local storage (connected by wire or wirelessly to processor  188 ), and/or remote storage (connected to processor  188  across a network, such as the Internet). Accordingly, as used herein and in the claims, content is “stored” in memory, where that content is stored in local storage, remote storage, or distributed across both local and remote storage, unless explicitly specified (e.g. “remotely stored” or “locally stored”). In addition, although aspects of controller  120  are described as being stored in memory, one skilled in the art will appreciate that these aspects can also be stored on or read from other types of volatile or non-volatile computer program products or computer-readable media, such as secondary storage devices, including hard disks, floppy disks, CDs, or DVDs; a carrier wave from the Internet or other network; or other forms of RAM or ROM. 
     Returning to  FIGS. 1 and 6 , controller  120  may automatically (i.e. without human interaction) continuously determine tympanic distance  128  based on reference and tympanic reflection signals from microphones  108 ,  106  respectively. As shown, memory  192  may store a model  208  trained by machine learning. Controller  120  may supply inputs (based upon the reference and tympanic reflection signals) to model  208 , and model  208  may output an indication of tympanic distance  128 . Processor  188  may output directions to operator feedback device  116  to produce indicia of tympanic distance  128  based on the output of model  208 . 
     Model  208  is preferably a recurrent neural network model for reasons described as follows. Traditional neural networks have many layers, each layer having several neurons that transform an input to desired a output. Using a mathematical technique known as “backpropagation”, error propagation is calculated, and the error is reduced by an error regression model. Accordingly, traditional neural network models employ a “feed forward” design, in which the same input always generates the same output. 
     In contrast, in a recurrent neural network each neuron is different and depends on (i) a given input, and (ii) the neuron&#39;s internal state (“memory”) at the time of that input. This allows a recurrent neural network to exhibit temporal dynamic behavior. 
     The logic of each neural has two inputs instead of one—the first input is the newly provided input, and the second is the neuron&#39;s previous state. In this way, historical state information is retained within the network. 
     A recurrent neural network is more efficient and stable at making continuous determinations of tympanic distance  128  simultaneously as the operator moves probe tube  104  towards tympanic membrane  308  because each distance determination is highly correlated to the previously determined distance. Probe tube  104  cannot teleport. Probe tube  104  will always be in close proximity to where probe tube  104  was at the previous time interval. This allows the recurrent neural network model to better simulate the physical characteristics of probe tube insertion. This also makes a recurrent neural network model more stable and compact in this application. 
     In alternative embodiments, it is technically possible, if much less efficient, for model  208  to employ a feed forward network design. As compared to a recurrent neural network, the feed forward network would need to be far larger to account for all possible conditions. Further, a feed forward network may be less stable and reliable because the distance determination made at every time step is independent, which could result in erroneous determinations (e.g. probe tube  104  appearing to impossibly teleport). 
     An exemplary method of training recurrent neural network model  208  is described as follows. 
     Recordings were made through the probe tube microphone of a Verifit™ 2 REM system. A clinician inserted the probe tube into the ear canal while a loudspeaker presented a shaped reference sound. Both ear canals of  55  subjects presenting with normal outer ear function were evaluated. From each recording, a spectrogram was generated. An example spectrogram  700  is shown in  FIG. 7 . Spectrogram  700  shows noise  704  caused by handling the probe tube, minima  708  where cancellations occur due to ¼ wavelength reflection, and minima  712  where cancellations occur due to ¾ wavelength reflection. 
     The spectrogram is process through a rank-ordering filter to highlight regions representing the local minima in the spectrogram.  FIG. 8  shows an example spectrogram  800  generated by applying the rank-ordering filter to the spectrogram  700  of  FIG. 7 . Spectrogram  800  includes minima  804  (manually labelled with a dashed-line). The frequency at which these minima occur correspond to the tympanic distance at the corresponding point in time. In this example, a plot digitizer was used to convert the line  804  to tabular data for recurrent neural network training. 
     Although various mathematical techniques may be used to determine the tympanic distance where the minima occur, the following is one example: 
     
       
         
           
             λ 
             = 
             
               c 
               f 
             
           
         
       
     
     where λ is wavelength, c is an estimate of the speed of sound in the ear canal (approximately 350 meters per second), and f is sound frequency. In this example, tympanic distance (d) may be equal to ¼ of the wavelength of the frequency at which the minima occurs: 
     
       
         
           
             d 
             = 
             
               
                 λ 
                 4 
               
               = 
               
                 c 
                 
                   4 
                    
                   f 
                 
               
             
           
         
       
     
     When a sound wave travels a total distance of λ/2, the phase of the wave is exactly inverse. At the position of the probe tube, there is a wave at some frequency f that travels λ/4 to the ear drum. A portion of the energy is reflected back and travels λ/4 to the probe position for a total distance of λ/2. Since the incident wave and reflected wave at the position of the probe tube sound receiving end have inverse phase, they destructively interfere producing the minima seen in spectrogram  800 . 
     A Gated Recurrent Unit recurrent neural network model was trained on sequences of input spectra to produce an estimate of the tympanic distance for every time step (many-to-many configuration). In this example, the selected model architecture included 77 inputs, 60 neurons at layer 1, 40 neurons at layer 2, 40 neurons at layer 3, 20 neurons at layer 4, and 1 output. The data generated from the 55 subjects was used to train the model. To validation the model, 5-fold cross-validation was used—in which 5 models were generated, each based on 80% of the patient spectrum data.  FIG. 9  shows a scatter plot  900  of the 5-fold cross validation test predictions compared against manually labelled tympanic distance. This scatter plot validated the accuracy of the model&#39;s predictions of tympanic distance. 
       FIG. 10  shows a line graph  1000  illustrating an example of the trained model  208  ( FIG. 6 ) successfully tracking the tympanic distance as an operator moved the probe tube within a patient&#39;s ear canal. As shown, the tracking error is consistently very low (e.g. less than 1 mm). 
     Returning to  FIGS. 1 and 6 , apparatus  100  may include one or more user inputs  212 . A user input  212  may be any device that can receive input from a user, such as for example a dial  212   1 , a button  212   2 , a switch  212   3 , or a touch screen. Controller  120  may include additional interface(s)  196  to receive signals from user input(s)  212 . In use, an operator may interact with user input(s)  212  to direct the operation of apparatus  100 . For example, a user input  212  may be manipulated by an operator to direct controller  120  to begin automatically continuously determining tympanic distance  128 . 
     Still referring to  FIGS. 1 and 6 , in some embodiments controller  120  may monitor the tympanic reflection signals for indications that the probe tube has become blocked in the ear canal. The blockage may prevent proper reception of the tympanic reflection sound by measurement microphone  106 , and therefore prevent controller  120  from accurately determining tympanic distance  128 . For example, a volume characteristic of the tympanic reflection signals may drop off in the event that the probe tube becomes jammed on a wall of the ear canal  304  or encounters a bundle of earwax (cerumen). In some embodiments, controller  120  (e.g. processor  188  may execute computer readable instructions to) detect a blockage of probe tube  104  based on a volume characteristic of the tympanic reflection signals. For example, controller  120  may detect a blockage based on one (or, more preferably both of):
         (a) the tympanic reflection signals indicating a sound level at the probe tube sound receiving end  132  that is less than the reference sound level (as indicated by the reference signals) by a threshold difference (e.g. by at least 5 dB, by at least 10 dB, or by at least 50% of the reference sound level), and   (b) the tympanic reflection signals indicating a sound level at the probe tube sound receiving end  132  that is less than a threshold level (e.g. less than 10 dB).       

     Conditions (a) and (b) above may be determined based on any portion or the entirety of the broadband sound wave frequencies. In some embodiments, conditions (a) and (b) are assessed solely based on sound frequencies below 500 Hz (e.g. 20 Hz to 500 Hz). Without being limited by theory, it is thought that at such low frequencies, there ought to be substantially no difference between levels indicated by the reflection and reference signals since the wavelengths are substantially larger than the distance between the reference microphone  108  and probe tube sound receiving end  132 . If there was a probe tube blockage, then the tympanic reflection signals would indicate a substantially lower sound level (condition (a) above). 
     However, condition (a) may also arise from an increase in measured level of the reference microphone  108 , which may be the result of microphone  108  being bumped by a user&#39;s hand during probe insertion. In addition, when the signals at measurement microphone  106  are at or near the noise floor (i.e. ambient noise level), then a blockage may be indicated (condition (b) satisfied). However, this could also result from other conditions causing little low frequency energy from reaching the subject. In these situations the condition (a) will not be satisfied. Therefore, controller  120  may provide better reliability in determining probe tube blockages by making the determination based on both of conditions (a) and (b) being satisfied. 
     In response to determining a blockage, controller  120  may automatically direct operator feedback device  116  to provide an indicia (e.g. visual, auditory, or haptic) of the blockage. The operator may take corrective action in response to the indicia (e.g. clear the blockage by removing the earwax or readjusting the probe tube  104 ). 
     While the above description provides examples of the embodiments, it will be appreciated that some features and/or functions of the described embodiments are susceptible to modification without departing from the spirit and principles of operation of the described embodiments. Accordingly, what has been described above has been intended to be illustrative of the invention and non-limiting and it will be understood by persons skilled in the art that other variants and modifications may be made without departing from the scope of the invention as defined in the claims appended hereto. The scope of the claims should not be limited by the preferred embodiments and examples, but should be given the broadest interpretation consistent with the description as a whole. 
     Items 
     Item 1: A probe insertion apparatus for real ear measurement, the apparatus comprising:
         a probe tube having a sound receiving end positionable in an ear canal, and a sound output end sonically coupled to a measurement microphone;   a reference microphone positionable outside the ear canal;   a reference speaker positionable to emit reference sound towards the ear canal;   an operator feedback device;   a memory storing computer readable instructions; and   a processor configured to execute the computer readable instructions, wherein the computer readable instructions when executed configure the processor to:
           direct the reference speaker to continuously emit the reference sound,   continuously receive reference signals from the reference microphone, and tympanic reflection signals from the measurement microphone,   continuously make determinations of a tympanic distance between the sound receiving end of the probe tube and a tympanic membrane based on the reference signals and the tympanic reflection signals and absent reference to a calibration measurement, as the operator moves the sound receiving end towards the tympanic membrane, and   automatically direct the operator feedback device to provide indicia of the tympanic distance between the sound receiving end of the probe tube and a tympanic membrane, as the operator moves the sound receiving end towards the tympanic membrane.   
               

     Item 2: The probe insertion apparatus of any preceding item, wherein:
         each determination of the tympanic distance is based solely on the reference and tympanic reflections signals.       

     Item 3: The probe insertion apparatus of any preceding item, wherein:
         each determination of the tympanic distance is based on current reference and tympanic reflection signals as well as reference and tympanic reflection signals used to determine a previous tympanic distance.       

     Item 4: The probe insertion apparatus of any preceding item, wherein:
         the memory stores a neural network model, and   said continuously making determinations of the tympanic distance comprises continuously supplying the neural network model with inputs based on the received reference and tympanic reflection signals, and continuously receiving from the neural network model outputs corresponding to determined tympanic distances.       

     Item 5: The probe insertion apparatus of any preceding item, wherein:
         the neural network model is a recurrent neural network model.       

     Item 6: The probe insertion apparatus of any preceding item, wherein:
         the indicia comprise visual indicia.       

     Item 7: The probe insertion apparatus of any preceding item, wherein:
         the operator feedback device comprises a display that provides the visual indicia.       

     Item 8: The probe insertion apparatus of any preceding item, wherein:
         the visual indicia identify the determined tympanic distance.       

     Item 9: The probe insertion apparatus of any preceding item, wherein:
         the display presents a graphical representation of an ear canal and an indicator positioned with respect to the graphical representation to indicate the tympanic distance.       

     Item 10: The probe insertion apparatus of any preceding item, wherein:
         the indicia comprise one or more of auditory and haptic indicia.       

     Item 11: The probe insertion apparatus of any preceding item, wherein:
         the operator feedback device comprises a notification speaker that emits auditory indicia, and   the notification speaker and reference speaker emit sound in different directions.       

     Item 12: The probe insertion apparatus of any preceding item, wherein:
         the operator feedback device comprises the reference speaker, that when activated outputs the auditory indicia composed of feedback sound frequencies lower than the reference sound.       

     Item 13: The probe insertion apparatus of any preceding item, wherein the computer readable instructions when executed further configure the processor to:
         detect a blockage of the probe tube in the ear canal based on a volume characteristic of the tympanic reflection signals.       

     Item 14: The probe insertion apparatus of any preceding item, wherein the computer readable instructions when executed further configure the processor to:
         detect a blockage of the probe tube in the ear canal based on a volume characteristic of the tympanic reflection signals being indicative of (a) a reflection volume below a threshold volume and (b) the reflection volume being less than a reference volume by a threshold volume difference.       

     Item 15: The probe insertion apparatus of any preceding item, wherein:
         the volume characteristic, the reflection volume, and the reference volume only relate to sound frequencies of 500 Hz or less.       

     Item 16: The probe insertion apparatus of any preceding item, wherein the computer readable instructions when executed further configure the processor to:
         automatically direct the operator feedback device to provide an indicium of the blockage in response to detecting the blockage.       

     Item 17: A probe insertion apparatus for real ear measurement, the apparatus comprising:
         a measurement microphone input;   a reference microphone input;   a reference speaker output;   an operator feedback device output;   a memory storing computer readable instructions; and   a processor configured to execute the computer readable instructions, wherein the computer readable instructions when executed configure the processor to:
           send directions to the reference speaker output to continuously emit a reference sound,   continuously receive reference signals from the reference microphone input, and tympanic reflection signals from the measurement microphone input,   continuously make determinations of a tympanic probe distance based on the reference signals and the tympanic reflection signals and absent reference to a calibration measurement, and   automatically send directions to the operator feedback device output to provide indicia of the tympanic probe distance between the sound receiving end of the probe tube and a tympanic membrane.   
               

     Item 18: A probe insertion apparatus for real ear measurement, the apparatus comprising:
         a probe tube having a sound receiving end positionable in an ear canal, and a sound output end sonically coupled to a measurement microphone;   a reference microphone positionable outside the ear canal;   a reference speaker positionable to emit reference sound towards the ear canal;   an operator feedback device;   a memory storing computer readable instructions; and   a processor configured to execute the computer readable instructions, wherein the computer readable instructions when executed configure the processor to:
           direct the reference speaker to continuously emit the reference sound, and automatically perform several iterations of:
               receiving reference signals from the reference microphone, and tympanic reflection signals from the measurement microphone,   determining, one or many times, a tympanic distance between the sound receiving end of the probe tube and a tympanic membrane based on the reference signals and the tympanic reflection signals and absent reference to a calibration measurement, and   directing the operator feedback device to provide an indicium of the tympanic distance between the sound receiving end of the probe tube and the tympanic membrane.   
               
               

     Item 19: A method of performing a real ear measurement, the method comprising:
         a) emitting reference sound towards the ear canal,   b) moving a sound receiving end of a probe tube in an ear canal towards a tympanic membrane,   c) receiving tympanic reflection signals from a measurement microphone acoustically coupled to the probe tube,   d) receiving reference signals from a reference microphone located outside the ear canal,   e) continuously determining a tympanic distance between the sound receiving end of the probe tube and the tympanic membrane based on the reflection signals and the reference signals and absent reference to a calibration measurement, and   f) continuously providing indicia of the determined tympanic distance between the sound receiving end of the probe tube and the tympanic membrane,   wherein a) to f) are performed concurrently.       

     Item 20: The method of any preceding item, wherein said providing indicia comprises providing visual indicia of the determined tympanic distance. 
     Item 21: The method of any preceding item, wherein:
         the visual indicia comprise a graphical representation of an ear canal and an indicator positioned with respect to the graphical representation to indicate determined tympanic distance.       

     Item 22: The method of any preceding item, wherein:
         the indicia comprise one or more of auditory and haptic indicia.       

     Item 23: The method of any preceding item wherein:
         said providing indicia comprises emitting auditory indicia of the tympanic distance.       

     Item 24: A probe insertion apparatus for real ear measurement, the apparatus comprising:
         a probe tube having a sound receiving end positionable in an ear canal, and a sound output end sonically coupled to a measurement microphone;   a reference microphone positionable outside the ear canal;   a reference speaker positionable to emit reference sound towards the ear canal; an operator feedback device;   a memory storing computer readable instructions and a neural network model; and   a processor configured to execute the computer readable instructions, wherein the computer readable instructions when executed configure the processor to:   direct the reference speaker to continuously emit the reference sound, and automatically perform several iterations of:
           receiving reference signals from the reference microphone, and tympanic reflection signals from the measurement microphone,   determining, one or many times, a tympanic distance between the sound receiving end of the probe tube and a tympanic membrane by supplying the neural network model with inputs based on the received reference and tympanic reflection signals, and receiving from the neural network model output corresponding to a determined tympanic distance, and   directing the operator feedback device to provide an indicium of the tympanic distance between the sound receiving end of the probe tube and the tympanic membrane.