PATENT ABSTRACT
An audio processor device and method is disclosed which measures and provides information relating to the audio level being applied to the ear of a user. The processor device uses a preset or calibrated sensitivity of the applied earphones in combination with an analysis of the audio stream to provide sound-pressure-level or time-weighted exposure information to the user or limit the output when preset levels have been achieved. Also disclosed is the use of microphones, internal or external, to combine an additional audio stream, typically the ambient environment, into the main audio channel.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/879,415, entitled DIGITAL AUDIO SYSTEM WITH SAFETY FEATURES, filed on Jan. 9, 2007. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention is directed to a system and method for providing a user with information related to sound exposure from earphones or headphones, and in particular, to a digital audio processor that displays information regarding sound exposure within a human ear canal in response to an audio input. 
       BACKGROUND OF THE INVENTION 
       [0003]    According to the National Institutes of Health, approximately 28 million Americans have a hearing impairment. Hearing loss covers an age span of approximately 17 in 1,000 children under the age of 18 and approximately 314 in 1,000 adults over the age of 65. This indicates that the incidence of hearing loss increases with age. In addition, ten million Americans have suffered irreversible noise-induced hearing loss, and 30 million more are exposed to dangerous sound levels each day. 
         [0004]    One source of these dangerous sound levels is earphones and headphones. Generally, the user of earphones or headphones has no knowledge of or reference for the actual sound level presented to their ears. Without knowledge of the actual audio levels being applied to the ear canal, the user is at risk of accruing hearing damage with long-term use. With the increased use of insert-earphones (also know as in-ear earphones, isolating earphones, or canal-phones) and portable audio players, it is becoming more commonplace and understood that unsafe audio levels are being played and hearing damage is likely to occur. The Occupational Safety and Health Administration (OSHA) and other governmental and industry organizations have published sound-pressure-level versus exposure-time guidelines which indicate safe limits of exposure to noise. 
         [0005]    In some instances, an earphone user may understand the risk involved in listening to sound at louder levels, but currently does not have a tool available for determining the level of sound to which he or she is actually being exposed. This problem is further exacerbated by the listening experience that is provided by noise-isolating or insert-earphones. In particular, a user of these earphones no longer has the reference of the ambient environment from which to determine the relative level of reproduced audio. In addition, the audio drivers of insert-earphones provide little of the bone-conduction vibration that bigger headphones or speakers create, which may lead the user into believing that the listening level is lower than actual. 
         [0006]    One available method for protecting hearing is a volume limiter that is available on some portable audio devices. However, the protection offered by a Volume limiter is arbitrary because it limits the volume level based solely on the output of the instrument without taking into consideration the sensitivity of the earphones and how efficiently they couple sound to a user&#39;s eardrum. Thus, this method may cause the user to over or underestimate the levels of audio to which they are subject, and may provide a false sense of security. In addition, while it is understood that exposure to high sound-pressure-levels can be harmful to one&#39;s hearing, the duration of the exposure is key to understanding the relative level of danger. However, the volume limiter solution fails to take into account this duration to the exposure of the sound, which can further offer the user a false sense of protection. 
       SUMMARY OF THE INVENTION 
       [0007]    A system and method is provided for measuring and displaying audio level information to an earphone or headphone user. In an embodiment, an audio processor device measures and displays the time-weighted average levels to which the earphone or headphone user has been exposed. 
         [0008]    In another embodiment in accordance with the present invention, an audio processor device implements a measurement of and provides an indication of the audio presentation level of an earphone into the ear canal. The indication can take the form of a graph that shows the audio level as it changes with time input or the level can be indicated numerically, preferably in known units, for example, decibels (dB), or the indication can take the form of a series of colored lights or markers that progressively indicate the relative risk level. This indication is based on a calibration to the sensitivity of the earphones being used with the audio processor device. The calibration is entered into the audio processor device via the user interface and stored in memory. In an alternative embodiment, the calibration is performed using a potentiometer or gain switch. The indication can be displayed in decibels of sound-pressure-level (dB SPL) optionally with a weighing function applied, such as A, B, or C weighting. 
         [0009]    In yet another embodiment in accordance with the present invention, an audio processor device provides the measurement and indication of the time-weighed noise (audio) exposure. In this embodiment, an indicator is employed that provides the user with knowledge of the amount of exposure to sound that is being or has been presented to the user&#39;s ears. This indication can give an overall exposure indication or can provide warnings to the user when a particular threshold of exposure has been exceeded. 
         [0010]    In a further embodiment in accordance with the present invention, an audio processor device uses the measured sound-pressure level or the time-weighted exposure to limit the output of the audio stream. This function can be performed by a compression circuit to limit the dynamic-range or sound-pressure-levels, a limiter circuit that prevents the sound-pressure-level from exceeding a preset limit, or as an adaptive function that reduces the output when preset limits of exposure have been met. 
         [0011]    In a further embodiment in accordance with the present invention, an integral microphone is provided with the audio processor that allows the introduction of ambient sound into the audio output. The output of the microphone can be either mixed with a separate audio program, or listened to independently. This audio signal allows the user to audibly interact with the environment while using sound-isolating earphones. 
         [0012]    In still yet another embodiment in accordance with the present invention, the integral microphone is used to calibrate the sensitivity and/or frequency response of a user&#39;s earphones. An audio signal is supplied to the earphones and the response of the earphone measured through a known acoustical coupling volume or known acoustical coupling impedance by the integral microphone. The response as measured is then stored and can be used to provide the calibration for the sensitivity of the earphones. An audio signal such as a chirp or frequency sweep can be used as the source to the earphones in order to measure the transfer function of the earphones. The stored measurement can be used to implement a custom frequency equalization based on the actual frequency response of the earphone and a user-defined frequency response objective. One objective of the earphone calibration can be to normalize the earphone response to that of the frequency response at the tympanic membrane of an average human when exposed to a uniform diffuse sound field. In order to suitably represent the complex frequency dependent impedances of a nominal human ear canal, additional acoustic treatments, for example tuned acoustic dampers, can be used in the acoustical coupling volume. Compensations can also be implemented in the calibration software to assist in properly evaluating the response in view of the nominal human ear canal response. 
         [0013]    In yet another embodiment in accordance with the present invention, the audio processor device uses stored calibration profiles for commonly known earphones. The sensitivity or frequency response of one or more earphones can be stored in the memory of the audio processor as supplied by the manufacturer or can be downloaded into the audio processor device through a computer connection, such as by means of a Universal Serial Bus (USB) connection or wireless USB connection. The earphone user can select the appropriate earphone profile from a selection through the user interface on the digital audio processor device or from a separate computer. The stored earphone can be used to set earphone frequency or amplitude characteristics that are appropriate for the user&#39;s earphones and the user&#39;s preferences. 
         [0014]    In a further embodiment in accordance with the present invention, calibrated electronic equalization functions are provided with the audio processor device that allow the user to adjust the spectral aspects of sound reproduction using conventional bass and treble controls as well as a graphic equalizer, in a package no larger than 3.25 cubic inches. 
         [0015]    Other embodiments, systems, methods, features, and advantages of the present invention will be, or will become, apparent to one having ordinary skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages included within this description be within the scope of the present invention, and can be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention may be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the drawings, like reference numbers designate corresponding parts throughout. 
           [0017]      FIG. 1  is a simplified functional block diagram of an audio processor device in accordance with the present invention; 
           [0018]      FIG. 2  is a simplified functional block diagram of the audio processor device of  FIG. 1  connected to an audio source and earphones; 
           [0019]      FIG. 3  is a plan view of an embodiment of an audio processor device incorporating the functionality of  FIG. 1 ; 
           [0020]      FIG. 4  is an elevation view of the bottom of the audio processor device of  FIG. 3 ; 
           [0021]      FIG. 5  is a partial cross sectional view of an earphone calibration system in accordance with the present invention wherein an earphone is mounted to the audio processor device of  FIG. 3 ; 
           [0022]      FIGS. 6-12  depict various menu screens that can be presented on the display of the digital audio processor device of  FIG. 3 ; 
           [0023]      FIG. 13  is a chart depicting the recommended maximum exposure time to various sound levels; and, 
           [0024]      FIGS. 14 and 15  are block diagrams depicting a method wherein the sensitivity of earphones can be subjectively determined. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The following descriptions of detailed embodiments are for exemplifying the principles and advantages of the inventions claimed herein. They are not to be taken in any way as limitations on the scope of the inventions. 
         [0026]    Turning to  FIG. 1 , a general functional block diagram is provided of an embodiment of a digital audio processor device  101  in accordance with the present invention. It should be appreciated that the functional blocks of  FIG. 1  can be realized by any number of hardware and/or software components configured to perform the specified functions described herein. For example, the present invention can employ various integrated circuit components, e.g., memory elements, processing elements, logic elements, look-up tables, and the like, which can carry out a variety of functions under the control of one or more microprocessors or other control devices. Moreover, it should be appreciated that the interconnecting lines in  FIG. 1  can be realized by any number of electrically conductive or other communication signal paths such as, but not limited to, electrically conductive wire(s), electrically conductive trace(s), wireless communication (including but not limited to BLUETOOTH), serial bus, parallel bus, or any combination thereof. 
         [0027]    In  FIG. 1 , an audio input jack  12  is provided having a left input  100  and a right input  102  for receiving analog electrical input audio signals into the device  101 . The inputs are electrically connected in a conventional manner to conventional analog-to-digital converters  115  having a digital electrical output  126  representative of the analog electrical inputs  112 , 113 . In an embodiment, dual 24-bit stereo multi-bit sigma-delta codecs are used having part number TLV320AIC23B. 
         [0028]    Receiving the digital electrical signals  126  from the analog-to-digital converters  115  is a 48-bit digital audio processor  140  (e.g., part number TAS3103) having a processed output  142 . Digital-to-analog converters  154  receive the processed signals  142  and produce analog electrical outputs  152 , 156  responsive to the input  142 . The analog outputs  152 , 156  are amplified by output amplifiers  170  wherein the amplified output is provided as electrical output audio signals to an output jack  14  having a left output  104  and a right output  106 . 
         [0029]    A conventional microcontroller  128  (e.g., part number MSP430F133) is electrically connected to the left input  100  of the input jack  12 , the analog-to-digital converters  115 , the output amplifiers  170 , and the digital audio processor  140 . Accordingly, the microcontroller  128  provides for monitoring the electrical input audio signals received at jack  12  and controlling the electrical output audio signals provided at jack  14  wherein the output audio signals are responsive, at least in part, to the input audio signals. Stated another way, the input audio signals have an effect on the audio output signals produced by the device  101 . 
         [0030]    The microcontroller  128  is also electrically coupled to a display  162  that, as explained in more detail further herein, displays the status of the device and audio level generated by headphones electrically connected to the output of the device. The microcontroller  128  determines the audio level based, in part, on the electrical signals received at the input jack  12  of the device and commands entered by a user via the user interface controls  114 . 
         [0031]    In addition to the user interface controls  114 , a programming connector  130  and a USB connector  132  are electrically coupled to the microcontroller  128 . The programming connector  130  provides for ease in coupling the microcontroller to a conventional device (not shown) for programming the microcontroller before final assembly of the device. Further, the USB connector  132  can be coupled to a Universal Serial Bus (not shown) wherein power from the bus can be used to charge the battery  139  contained within the device  101  and, in an embodiment, the device can communicate over the Universal Serial Bus. Alternatively, power to the connector  132  can be supplied from a conventional Universal Serial Bus charging unit (not shown). In an embodiment, but not necessarily, the battery  139  can be a conventional lithium-polymer rechargeable battery. 
         [0032]    A microphone  110  is also provided for monitoring of environmental sounds with the device. The microphone  110  is electrically connected to an analog-to-digital converter  111  wherein analog signals from the microphone are converted into digital signals  119  that are received by the digital audio processor  140  to generate processed output signals  142 . As indicated previously, the digital-to-analog converters  154  receive the processed signals  142  and produce analog electrical outputs  152 , 156  responsive to the processed signals  142 . The analog outputs  152 , 156  are amplified by output amplifiers  170  wherein the amplified output is provided as electrical output audio signals to an output jack  14  having a left output  104  and a right output  106 . 
         [0033]    When the microphone  110  input is selected by a user via the user interface controls  114 , the microcontroller  128  determines the input audio level based, in part, on the electrical signals produced by the microphone  110  in response to external or environmental audio signals received by the microphone. The microcontroller  128  controls the display for depicting the audio level generated by headphones electrically connected to the output of the device  101  wherein the audio level is based on commands entered by the user via the user interface controls  114  and the ambient audio level received by the microphone  110 . 
         [0034]    Also provided in the device  101  of  FIG. 1  are a battery management and voltage regulator  150 , a power module interface  114 , and a power module  168  for controlling power distribution within the device  101 . In addition, within the functional blocks shown in  FIG. 1 , the device provides a three-band equalizer, overload detection, sound level meter, and hearing safety monitor as described in detail further herein. 
         [0035]    Turning to  FIG. 2 , a simplified functional block diagram is provided of the device of  FIG. 1  connected to an audio source  212  and earphones  216 . As in  FIG. 1 , it should be appreciated that the interconnecting lines in  FIG. 2  can be realized by any number of electrically conductive or other communication signal paths such as, but not limited to, electrically conductive wires. 
         [0036]    Moreover, the audio source  212  can be any conventional device having an audio output. For instance, but not necessarily, the audio source  212  can be a portable media player such as the iPod manufactured by Apple Inc. of Cupertino, Calif., which can play MP3, AAC/M4A, Protected AAC, AIFF, WAV, Audible audiobook files. 
         [0037]    Preferably, but not necessarily, the audio output of the audio source  212  is a conventional earphone jack  218  wherein one end of an interconnect cable  214  is plugged into the earphone jack of the audio source and the other end of the cable is plugged into the input jack  12  of the audio processor  101 . 
         [0038]    The earphones  216  can be conventional in design. For instance, but not necessary, the earphones can be manufactured by one or more companies such as Etymotic Research under the model designation ER-4S, ER-4P, and ER-6i having a sensitivity of 108 dB/V, 120 dB/V, and 125 dB/V at 1 kHz, respectively. Further, but not necessary, the earphones can be manufactured by Shure under the model designation E2C, E3C, E4C, E5C, and E500 having a sensitivity of 123 dB/V, 129 dB/V, 124 dB/V, 132 dB/V at 1 kHz, respectively. Moreover, but not necessarily, the earphones can also be manufactured by Ultimate Ears under the model designation Super.fi3 studio, Super.fi5 Pro, and Super.fi5 EB having a sensitivity of 134 dB/V, 136 dB/V, and 136 dB/V at 1 kHz, respectively. 
         [0039]    Turning to  FIG. 3 , a plan view is provided of an embodiment of a digital audio processor device  301  incorporating the functionality of  FIG. 1 . The device  301  includes an outer housing  320  constructed of a generally rigid plastic, metal, or metal alloy. The housing  320  also includes a select switch  315  and two buttons  312 , 314  as part of the user interface controls  114  of  FIG. 1 . In an embodiment, button  312  can be used to turn off and on device  301 , enable the audio input  12 , and also select audio functions of the device as described in detail further herein. Further, button  314  can be used to turn on and off device  301 , enable the internal microphone  110  ( FIG. 1 ), and also select audio functions of the device as described in detail further herein. Either or both the audio input  12  and microphone  110  can be selected at any time. However, if neither input is selected, then the device  301  will turn off. 
         [0040]    In an embodiment, buttons  312  and  314  are constructed of a generally clear rigid plastic wherein light emitting diodes are mounted in proximity behind the buttons. Preferably, but not necessarily, a blue light emitting diode  306  is mounted in proximity behind button  312  and a red light emitting diode  308  is mounted in proximity behind button  314 . 
         [0041]    Switch  315  is a conventional switch for allowing the user to make selections as explained in detail further herein. In an embodiment, the switch  315  can be depressed in the direction of arrow  300  and rolled in the direction of arrows  302  and  304 . 
         [0042]    Visible through the housing  320  of the device  301  is the display  162  comprising a conventional liquid crystal display. In an embodiment, the housing  320  includes a generally clear rigid plastic window  330  mounted over the display  162  to protect it. 
         [0043]    Located towards the bottom  342  of the housing  320  and extending through the housing are a plurality of slits  316 . Mounted behind the slits  316  is a convention microphone  110  ( FIG. 1 ). Preferably, but not necessarily, the slits  316  are located in a circular indentation  346  formed in the housing  320 . 
         [0044]    Also mounted about the indentation  346  in the housing  320  is a charge indicator  310  comprising a light emitting diode wherein the diode is controlled by regulator  150  ( FIG. 1 ). In an embodiment, the charge indicator is a red light emitting diode that will illuminate when the charge cycle for the battery  139  begins and will extinguish when the charge cycle is complete. 
         [0045]    Turning to  FIG. 4 , an elevation view is provided of the bottom  342  of the housing  320  of the processor device  301 . Preferably, but not necessarily, the bottom  342  includes the inputs and outputs associated with the device  301 . In particular, the bottom  342  of the device  301  includes the audio signal input  12 , the audio signal output  14 , and the digital interface connection  132 . As indicated previously, the audio signal input  12  can be, for example, a stereo jack for the connection of a stereo audio signal. The audio signal output  14  can be, for example, a stereo jack for the connection of earphones. In an embodiment, the digital interface connection  132  can provide for digital communication with an audio player, computer, or other digital device. This connection can, for example, be a Universal Serial Bus (USB) connection. 
         [0046]    Turning back to  FIG. 1 , in operation a user enters into the digital audio processor device  101  a sensitivity level that represents the acoustic sensitivity of the earphones  216  ( FIG. 2 ) the user will attach to the device. This setting is entered into the user interface controls  114  which is then entered into the flash memory of the microcontroller  128 . A signal source  212  ( FIG. 2 ), or optionally a stereo signal source, such as the earphone output  218  ( FIG. 2 ) of a music player for example, is operatively connected to an audio processing device  101 . As indicated previously, the signal source applied to the audio processing device  101  can be music, speech, or any other audio source that can be applied to an earphone. The analog-to-digital converter  115  receives the analog audio signal and converts it to a digital signal, which is then sent to the digital audio processor  140 . The digital audio processor  140  queries the microcontroller  128  for the previously stored earphone sensitivity setting which is contained within the flash memory stage of the microcontroller  128 . The digital audio processor  140  calculates the level being applied by the earphones by measuring the rms level of the audio stream and adding in a correction factor based on the stored earphone sensitivity level. This measurement is done while the audio stream passes through the digital audio processor  140  uninterrupted, allowing the user to ascertain the audio level while concurrently listening to the audio source(s). The digital audio processor  140  used within the device  101  passes the measured level information to the microcontroller  128  which then drives the display  162  to provide a bar graph indication and/or numerical indication. 
         [0047]    Referring again to  FIG. 1 , as the digital audio processor  140  within the device  101  passes the audio level measurements to the microcontroller  128 , the audio level is sampled and averaged resulting in a time-weighted measurement. The result of this time-weighted measurement can be stored in the flash memory on the microcontroller  128  as it is calculated for later summation and/or provided to the user via the display  162 . When the time-weighted measurement exceeds a particular threshold, a warning or automatic gain reduction can be triggered by the microcontroller  128 . 
         [0048]    As indicated previously, a typical use of the device as shown in  FIG. 1  includes applying an input from an audio source to the input channels  100  and  102 , and attaching an earphone to the output channels  104  and  106 . The analog audio signal stream from the external audio source is digitized by the A/D converter  115 . The resulting digital stream is sent to the digital audio processor  140  where it is measured and selected processing functions are applied to the digital signal. For example, the signal can be filtered according to an equalization setting selected by the user, or a microphone input can be digitally added to the audio stream. Signal limiting, signal compression, frequency equalization, and digital delays and crossfeeds are also functions that can be processed by this stage, for example. Audio level information is passed from the digital audio processor  140  to the microcontroller  128 . The signal is then fed from the digital audio processor  140  to the digital-to-analog converter  154 . The digital-to-analog converter  154  transforms the digital audio stream into an analog audio signal to be fed to the output amplifier  170 . The output amplifier  170  provides any amplification required and drives the output  104  and  106 . 
         [0049]    Continuing with  FIG. 1 , in an embodiment, the audio input to the device  101  is preset to provide a gain of +6 dB that may be adjusted. Further, the microphone output can be processed through an A-weighting filter. When the microphone  110  is on and the audio channel  12  is off, the display  162  will show the sound level in the environment as measured by the microphone. The levels can be shown in dBA SPL (A-weighted sound pressure level in dB) 
         [0050]    Turning to  FIG. 3 , as previously indicated the device  301  includes the housing  320 , the display  162 , and the user controls  312 ,  314 , and  315 . The user controls  312  and  314  are switches, in the form of buttons, which allow the user to selectively engage or disengage the audio and microphone inputs. Integrated within the user controls  312  and  314  are indicators  306  and  308  which visibly display the status of the controls. Indicators  306  and  308  can be, for example, light-emitting-diodes (LEDs) which can be seen through a transparent portion of the controls  312  and  314 . Another aspect of the present invention is the microphone  110  ( FIG. 1 ), which is contained within the housing  320 . Control  315  allows the user to navigate the user interface which is selectively shown on the display  162 . The user can actuate the control  315  in the down direction  304  to navigate in one direction through the user interface, in the up direction  302  to navigate in the other direction, or the user can press the control  315  inwards  300  towards the housing  320  in order to make a selection in the user interface. Accordingly, display  162  shows various menu selections and visual indicators, depending on the functions selected by the user and the status of the user controls  312 ,  314 , and  315 . 
         [0051]    In an embodiment, as shown in  FIG. 3 , the main screen  612  presented on the display  162  depicts a battery condition indicator  614 , the audio level indicator  616  in dB SPL and a graphical display  618  of the audio level. The battery condition indicator  614  depicts the approximate amount of life in the battery  139  ( FIG. 1 ) wherein the unfilled area  620  of the indicator enlarges as the battery charge depletes. 
         [0052]    The audio level indicator  616  indicates, with a number, the approximate audio level being reproduced by the earphones  216 . The number indicates the level of the audio from the audio input  12  and the microphone  110 , depending on which source(s) is active. 
         [0053]    The graphical display  618  can be a bar graph that moves from left to right, indicating the listening level in dB SPL in 3 dB steps. In an embodiment, if the bar graph reaches the far right a “+” sign appears indicating that the output is at or near clipping. 
         [0054]    As indicated previously, there are several user-adjustable settings in the device  301 . These can be accessed by pressing the menu select switch  315  directly inwards (i.e., in the direction of arrow  300 ). Once the menu is accessed, the user can scroll through the selections by rolling upward  302  or downward  304  on the select switch  315 . On the menu screens the setting can be changed using the audio button  312  and microphone button  314  on the front of the device. When the user is finished making changes to a setting, rolling upward or downward on the select switch  315  moves to the next setting. The settings are saved in the memory of the microcontroller  128  ( FIG. 1 ). The user can exit the settings menu by waiting a short time (e.g., three seconds) for the menu to automatically time out or by pushing in on the select switch  315 . 
         [0055]    As indicated previously, the device  301  includes a three-band parametric equalizer that can be used to provide a customized frequency response. The adjustable bands include a low-frequency (bass), mid-frequency, and high-frequency (treble) setting. These filters can be adjusted as to frequency and level for each band. 
         [0056]    Turning to  FIG. 6 , the low gain setting in menu  640  allows the user to adjust the gain of the low frequency band of the equalizer. In an embodiment, it is adjustable from +9 to −9 dB in 1 dB steps. The low frequency setting in menu  642  allows the user to adjust the corner point of the low frequency filter. In an embodiment, the frequency options are 110 Hz, 220 Hz, and 345 Hz. 
         [0057]    Turning to  FIG. 7 , the mid gain setting in menu  740  allows the user to adjust the gain of the mid frequency band of the equalizer. In an embodiment, it is adjustable from +6 to −6 db in 3 dB steps. The mid frequency setting in menu  742  allows the user to adjust the mid point of the mid-frequency filter. In an embodiment, the frequency options are 2.0 kHz, 2.5 kHz, and 3.0 kHz. 
         [0058]    Turning to  FIG. 8 , the high gain feature in menu  840  allows the user to adjust the gain of the high frequency band of the equalizer. In an embodiment, it is adjustable from +9 to −9 dB in 1 dB steps. The high frequency setting in menu  842  allows the user to adjust the corner point of the high-frequency filter. In an embodiment, the frequency options are 2.8 kHz, 5.5 kHz, and 8.3 kHz. 
         [0059]    Turning to  FIG. 9 , the earphone sensitivity setting in menu  940  indicates the value that corresponds to the type of earphone the user is using with the device  301 . Desirably, the setting is adjusted by the user to match the sensitivity of the user&#39;s earphones. 
         [0060]    Turning to  FIG. 10 , the 3DX soundfield expansion mode provides an enhanced listening environment, simulating the environment of listening to music through a pair of stereo speakers. When in a typical listening environment, in front of a pair of stereo speakers, both ears will hear the sound from both speakers. However, with earphones a person loses this ability and only the signal coming from each earphone is heard in each corresponding ear. As such, earphones cause the normal “crossfeed” to be lost. The 3DX feature attempts to recreate the experience of listening to sounds as if the user is in front of a pair of speakers. This feature and its implementation are well known to those having skill in the art. Through menu  1040 , the user can enable and disable the feature. 
         [0061]    Turning to  FIG. 11 , a menu  1140  is provided wherein the voltage gain of the audio channel can be adjusted by the user from −20 dB to +20 dB. In an embodiment, the default setting is +6 dB. 
         [0062]    Turning to  FIG. 12 , a display setting menu is provided wherein the setting can be toggled by a user between selecting either level  1240  or time  1242 . When “level” is selected, the display  162  will show the sound level in the user&#39;s ear. When “time” is selected, the display  162  will shown an estimate of the length of time the user could listen at a sound level before risking hearing damage due to the intensity of the sound. In an embodiment, the time is indicated in minutes that it is relatively safe to listen at the given sound level for a 24 hour period based on the National Institute for Occupational Safety and Health (NIOSH) workplace limits. 
         [0063]    Turning to  FIG. 13 , a chart adapted from NIOSH 98-126, incorporated herein by reference, is depicted wherein the time indicated by the device  301  that it is safe to listen at a given sound level is derived therefrom. Accordingly, the device  301  provides the user with information to make safe choices about the level and the amount of time at which to listen to a given sound level. Preferably, once the user has set his or her earphone sensitivity in the device  301 , both the levels and time shown on the display are based on the levels being produced in the ears of the user. However, it is recognized that typically only an estimate can be provided of the sound level since earphones are manufactured within various tolerances levels determined by the manufacturer. 
         [0064]    Turning to  FIG. 5 , a partial cross sectional view of an earphone calibration system in accordance with the present invention is provided wherein an earphone is mounted to the device of  FIG. 3 . The calibration system  501  includes a generally cylindrical earphone coupler  504  having a known inner volume  514  and known acoustic properties. The earphone coupler  504  which for example, can be removable or can be an integral part of the housing  320  of the digital audio processor  301 , is attached to the processor  301  via an annular sealing portion  512  which can or can not be compliant. This sealing member  512  can be made, for example, to snap into the recess  346  in the housing  320  of the digital audio processor  301 . With this system  501 , an earphone  216  can be actively calibrated using the integrated microphone  110  in the digital audio processor  301 . The earphone  216  with its compliant eartip  516  to be calibrated is proximately sealed into the open end  502  of the earphone coupler  504 . A wide-band audio chirp, frequency sweep, tone, or other known audio signal can be applied to the earphone  216 , via device output  14  ( FIG. 1 ), and the resultant audio output of the earphone measured by the microphone  110 . The digital audio processor device  301  receives the signal from the microphone  110  and can store the response in memory or use the response to calibrate the output  14  of the device  301 . As will be appreciated by those having ordinary skill in the art, this calibration system can also use an external microphone connected to the digital audio processor  301  via an external connection such as, but not limited to, an input for the external microphone. 
         [0065]    In an alternative embodiment, headphone sensitivity can be determined using both the headphone and the sound level received by microphone  110 . In particular, in this embodiment, the user inserts an earphone into only one ear canal while the other ear canal remains unobstructed. Next, the audio processor  301  generates a sound level in the ear canal of the user, via the inserted earphone, wherein the sound level is based upon the electrical signals  124  ( FIG. 1 ) generated by the microphone  110 . As such, the user adjusts the volume control of the microphone until the perceived sound level in both ears match. Accordingly, as will be appreciated by those having ordinary skill in the art, the sensitivity of the earphone can be determined based on the electrical signals  124  generated by the microphone and the signal strength required to make the earphone sound level match the environmental sound level perceived by the user. 
         [0066]    Turning to  FIGS. 14 and 15 , a block diagrams are provided depicting the method wherein the sensitivity of earphones can be subjectively determined by a user. In block  1412  of  FIG. 14 , the device  101  ( FIG. 1 ) is set in a calibration mode where P m  represents an arbitrary sound pressure level to which the integral microphone and a user&#39;s uncovered ear are exposed. In block  1414 , P e  represents the reproduced sound pressure level at the user&#39;s other ear due to the combined effects of sound pressure level P m , the microphone sensitivity M s  (in volts/sound pressure level), a reference gain setting Gm, an adjustable gain setting G s , the output amplifier gain G a , and the sensitivity of the user&#39;s earphones S e  (in sound pressure level/volts). In block  1416 , if the user adjusts the calibration gain control G s  such that equal sound pressure levels are perceived in each ear, P e  is then equal to P m . Under these conditions, the sensitivity of the earphones are a function of the microphone sensitivity and the identified gain settings. In block  1418 , by noting these values the sound level or exposure time display can be calibrated to read the correct sound pressure level or safe exposure time for a given voltage applied to the user&#39;s earphones. Accordingly, in block  1420 , once calibrated using this procedure, the sound pressure level or safe listening time associated with an audio source  212  ( FIG. 2 ), microphone  110  ( FIG. 1 ), or any combination of the two can be displayed on display  162  ( FIG. 1 ). 
         [0067]    Turning back to  FIG. 2 , in an embodiment, the audio signal source  212  and the audio processing device  101  can be attached to each other using conventional hook and loop fasteners such as, but not limited to, VELCRO. Alternatively, the functionality of the audio signal source  212  and the audio processing device  101  can be combined into a single device  220 . 
         [0068]    It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are possible examples of implementations merely set forth for a clear understanding of the principles for the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without substantially departing from the spirit and principles of the invention. All such modifications are intended to be included herein within the scope of this disclosure and the present invention, and protected by the following claims.