in fig1 , a pair of hearing aids are inserted into charger 20 for charging and conveying information among the hearing aids and a hand held control ( not shown in fig1 . hearing aid 10 includes at least one microphone , such as microphone 11 , a flex circuit or integrated circuit 12 containing a microprocessor for signal processing and other tasks , speaker 14 , and rechargeable battery 16 for power . beneath battery 16 is inductor 17 , which is electrically coupled to circuit 12 , as are the other electrical components . dedicated electronics can be used instead of programmable electronics but programmable electronics are preferred . the lower portion of hearing aid 10 , containing speaker 14 , fits easily within chamber 21 . the middle portion of hearing aid 10 is located in chamber 22 within inductor 23 . inductor 17 and inductor 23 are approximately concentric but , as one of the advantages of the invention , alignment and position are not critical . the outer or upper portion of hearing aid 10 fits within conical depression 25 , which provides a self - centering action for the type of hearing aid illustrated . conical depression 25 terminates in chamber 22 . the preferred medium for communication with the hearing aids is sound , using the microphone and speaker already in the hearing aid . in accordance with a preferred embodiment of the invention , charger 20 includes speaker 31 and microphone 32 for this purpose . given the two - way communication between the charger and the hearing aid , there is no limit on the content of the communication . for example , the charger could also serve as an interface for programming a microprocessor in the hearing aid . using suitable tones , or sets of tones , to represent logic ones and zeros , the hearing aid can transmit a first code indicating the level of charge and a second code indicating the rate of charge . if , for example , the coupling between inductors 17 and 23 happened to be particularly good , the hearing aid could “ ask ” the charger to reduce the current through inductor 23 to reduce the rate of charge , thereby preventing overheating . as illustrated in fig1 , speaker 31 and microphone 32 are located adjacent chamber 21 . hearing aid 10 does not form a seal with charger 20 and there is sufficient coupling between speaker 31 and microphone 11 . speaker 31 can be located closer to microphone 11 , if desired . if one wanted the charging to be inaudible , one could position speaker 31 closer to microphone 11 and put the speaker and the hearing aid in an enclosed space . power supply 33 provides charging power to hearing aid 10 by way of inductor 23 . a signal at a current of a few tens of milliamperes and a frequency of 100 khz - 500 khz is effective . power supply 33 is controlled by and communicates with microprocessor 36 by way of input - output ( i / o ) interface 37 . interface 37 also drives speaker 31 and receives signals from microphone 32 . while shown as separate elements , it is known in the art that many commercially available microprocessors have analog inputs and include analog to digital ( a / d ) converters on the same semiconductor chip as the computer portion of the microprocessor . thus , “ microprocessor ” is intended to include computing and logic capability and suitable i / o , whether on a single semiconductor chip or on plural chips . as illustrated in fig1 , charger 20 includes receptacles for two hearing aids . hearing aid 40 includes battery 41 , which is charged by power supply 43 under the control of microprocessor 36 . although the operation is the same , the charging of battery 41 is completely independent of the charging of battery 16 . the presence of a hearing aid can be detected by power supply 33 or power supply 43 , for example , by sensing a change in inductance in inductor 23 or inductor 53 . alternatively , presence can be sensed acoustically by recognizing the sound of a hearing aid being inserted into charger 20 or by a sound in microphone 32 or 45 . other acoustic or magnetic presence detectors can be used instead . more simply , one can simply use a switch ( not shown ) for each receptacle to alert microprocessor 36 that a hearing aid has been inserted and to begin a charging cycle for that receptacle . charger 20 further includes receptacle 61 for receiving a hand held controller ( not shown in fig1 ). i / o circuit 37 provides two way communication over bus 62 with the controller . an acoustic coupling , as used for the hearing aids , can be used instead and is preferred . in accordance with one aspect of the invention , the charger , the controller , and the programmer ( fig3 ) all have microphones and speakers for two way communication among the devices . fig2 is a perspective view of controller 70 resting in receptacle 61 in charger 20 . controller 70 provides day to day operation of one or more hearing aids and includes at least left volume control 71 , right volume control 72 , and keypad 74 for selecting a frequency response suited to the named environments . in one embodiment of the invention , keypad 74 includes four keys labeled “ quiet ,”“ dinner ,” “ party ,” and “ music .” these frequency responses are derived from the test results when the hearing aid is calibrated . controller 70 preferably also includes “ reset ” switch 76 , for returning a hearing aid to its calibrated setting after testing , “ bass ” control 77 , and “ treble ” control 78 . the latter controls affect frequency response as the names indicate . controller 70 also includes speaker 79 for communicating with the hearing aids . a microphone ( not shown in fig2 ) is located in receptacle 61 for receiving signals from charger 20 or from the hearing aids . when the hearing aids are worn by the user , controller 70 communicates with the hearing aids by way of speaker 79 . in a preferred embodiment of the invention , integrated circuits 12 and 49 ( fig1 ) each include a narrow band pass filter having a center frequency at some audible frequency , e . g . 7 khz . communication is preferably asynchronous . controller 70 transmits a sonic signal at this frequency and the presence or absence of a signal at a particular time indicates a one or a zero . thus , for example , increasing volume in the left ear is achieved by pressing the upper portion of button 71 . an address prefix is included for selecting the appropriate hearing aid , followed by the command for increasing volume . if only a single hearing aid is used , the prefix is omitted and either button 71 or 72 increases volume . because of the data shared among the units , controller 70 “ knows ” whether there is one hearing aid in use or two hearing aids . other modulation techniques can be used but asynchronous amplitude modulation is preferred for simplicity . check sums and other error detecting techniques known per se in the art can also used . if a user , for example , pushes a button for increasing volume in a hearing aid , and there is an error , the hearing aid is preferably programmed to indicate an error for the user by an alarm such as a beep or a blinking light and sending a message to the controller or programmer , or both , that there was an error . simple messages take less than one second to transmit and feedback is therefore relatively immediate . a programmer constructed in accordance with a preferred embodiment of the invention is illustrated in perspective view in fig3 . programmer 80 includes a display panel 81 , preferably a backlit , liquid crystal display that can provide both text and images , such as graphs , and keypad 83 , including arrows arranged as shown . the keypad and display 81 are programmable , that is , the function assigned to a given key can change depending upon the operation taking place and the display can contain text and graphics in any arrangement . programmer 80 also includes selection keys 84 , 85 , and 86 that , when actuated , provide the response indicated ; viz . “ yes ,” “ no ,” or “ back .” programmer 80 also includes speaker 87 and microphone 88 for two way communication with the other devices . the microphone and speaker are illustrated on the front of programmer 80 but are preferable on opposite sides for greater acoustic isolation . in fig1 , hearing aid 10 and hearing aid 40 are programmed with a unique serial number stored in memory in each hearing aid . when microprocessor 36 detects the presence of two hearing aids , the hearing aids are requested to transmit at least the lower order bits of their serial numbers , i . e . a portion of the serial number . the number of bits actually transmitted is a trade - off between the probably of accidental duplication and transmit time . for example , eight bits can uniquely identify two hundred and fifty - six hearing aids . the probability of two hearing aids having the same last eight bits is thus one in two hundred and fifty - six — a reasonably low probability . in accordance with one aspect of the invention , microprocessor 36 then sends the eight bits from one hearing aid to the other hearing aid , and vice - versa . now the hearing aids are paired and the microprocessor can detect a new hearing aid if a hearing aid is lost or destroyed . if a hearing aid does not transmit a second serial number , or transmits numbers not recognized by the programmer , then a new hearing aid must be present . if the programmer 80 is turned on , then the user is told a new hearing aid was detected and is asked if the new hearing aid should be programmed . if the answer is yes , then the programmer sends data to the appropriate hearing aid , reproducing the missing hearing aid . in accordance with another aspect of the invention , a left and right convention is also used . in a preferred embodiment of the invention , the lowest order bit indicates hand , left or right . this enables the controller to provide a further check on the hearing aids before programming . it also simplifies and provides much more rapid control of the hearing aids in use because only a single bit distinguishes the two . thus , the address of a hearing aid in use is either an odd number or an even number . in a preferred embodiment of the invention , serial numbers are not programmable by a user but are stored in read only memory . programming is transparent to the user , who merely says “ yes ” or “ no ” to the prompts on display 81 . in accordance with another aspect of the invention , each hearing aid is calibrated during manufacture to a specific spl ( sound pressure level ) in each band . this provides a user with a calibrated instrument for noise cancellation and hearing test . the initial , default , or “ unprogrammed ” state of a new hearing aid is a uniform amplitude response across the available spectrum . the calibration can take place in apparatus substantially like that shown in fig1 , except that a cover is added to create a chamber for the hearing aids and exclude noise . a hearing test is initiated by selection from a menu . the display then asks “ which ear ” and the user presses , for example , the left arrow on button 83 to select the left ear . the display asks if hearing aids are inserted in each ear and turned on . if the answer is “ yes ,” programmer 80 issues a noise cancellation command for the left ear . optionally , programmer 80 issues a command for background noise in the right hearing aid to reduce the chance of cross - coupling during the test . the user is then told to press selection button 84 when a tone is heard in the left ear . programmer 80 issues a command to produce a tone at a particular frequency and progressively increases the amplitude until button 84 is pressed or until an internal safety limit is reached . a sequence of tones is produced , preferably in random order . in accordance with another aspect of the invention , the tones produced match the center frequencies of the band pass filters in the hearing aid . in this way , there is no interpolation of five or six data points to sixteen or more bands . the data matches the hearing aid as closely as possible and provides a better fit for the user . after the last tone , the hearing aid is programmed in accordance with the results of the test . throughout the test , the user is given an opportunity to start over on a given tone or to start over from the beginning . after the first hearing aid is programmed , the process is repeated for the other hearing aid , if any . the invention thus provides a device that is both hearing aid and master hearing aid . a more accurate test and more accurate compensation for hearing impairment is provided than obtainable in the prior art . the hearing test accurately tests each ear independently and minimizes the effect of cross - talk . thus , the invention provides a user controlled hearing test that is accurate even when the user has one ear with distinctly better hearing than the other ear . the invention also provides a hand held controller for hearing aids that is easy to use and a system that identifies hearings aids and is easy for the user to operate . the system recognizes when a new hearing aid is added and can program a replacement hearing aid according the same parameters as the hearing aid being replaced . the test and corrective device are one and the same , thereby enabling greater accuracy than available in the prior art . most importantly , a haring aid can be programmed wherever the user is , within reason . obviously , a relatively quiet room is needed for best results . having thus described the invention , it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention . for example , even though the controller performs a hearing test , this feature or other features can be locked out to prevent accidental or inappropriate use . the invention is particularly useful for hearing aids , wherein space is at a premium , but the invention can be used for other devices for assisting hearing . any bit in the identification code could be used for indicating handedness . for example , instead of using the least significant bit ( odd / even ), one could use the most significant bit ( positive / negative ) of a signed number . using the least significant bit is preferred .