Abstract:
Loudspeaker drivers used in audio systems are subject to performance variance caused by production deviation of components and assembly processes and consequently audio system performance is influenced by the mechanical attributes of individual loudspeaker drivers of which the audio systems are comprised. This invention provides a solution to minimize the variance of duplicate audio system performance by rectifying the signal processing to minimize loudspeaker variance in duplicated or mass production audio systems.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/224,858 filed 11, July, 2009, which is expressly incorporated herein in its entirety by reference thereto. 
     
    
     TECHNICAL FIELD 
       [0002]    The present application is in the technical field of audio systems. In particularly, the invention is in the field of measurement, analysis and rectification of loudspeakers used in production audio systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    Audio systems are assembled from a combination of loudspeakers and electronics and are traditionally designed through a development phase that may include computer modeling or simulation followed by development and assembly of a physical reference system that has undergone various iterations to optimize maximum acoustic performance within constraints such as physical size, materials and cost. To quantify the capability of an audio system to accurately reproduce sounds that will be played through it performance characteristics such as frequency response may be measured with acoustic measurement equipment. Along with loudspeakers and amplifiers an audio system often includes analog or digital signal processing that can modify the natural reproduction capabilities of loudspeakers and may be applied to individual channels in a stereo or multi-channel system such as left and right in a stereo system, or the signal processing may be applied to individual channels directly to full range or frequency divided loudspeakers such as a front left mid-range, front left high frequency tweeter and so on. When the reference audio system is finalized the designs for all components such as loudspeakers are finalized for production and the equalization data developed either manually or through an automated tuning process such as described in US patent application 20070025559 “Audio Tuning System” and may be stored in a master record so that it may be recalled for embedding into production audio systems. 
         [0004]    For the acoustic performance of duplicate audio systems such as in mass produced audio systems to closely conform to the acoustic performance of the reference audio system on which it is based the individual loudspeakers should ideally conform within a metric threshold in comparison with the individual loudspeakers used in the reference system to which each loudspeaker is associated. By associated we mean for example the specific type or model of loudspeaker and its installation location in the audio system. For example, a 16 cm loudspeaker where the model information includes all mechanical details of the subcomponents such as cone, spider and voice-coil and so on and their assembly processes is mounted in the front left door of a vehicle audio system. However in actuality performance of mass produced loudspeakers can deviate substantially as described in Audio Engineering Society (AES) Preprint #7530 “Loudspeaker Production Variance”. Consequently variance of individual duplicate production loudspeakers causes performance of duplicate production audio systems to vary from each other and in particular perform differently than the reference audio system. It would be preferable to minimize the physical variance of duplicate production loudspeakers but the required material and assembly process control will add complexity and cost to production loudspeakers. Alternatively sorting loudspeakers into performance categories based on minimal metrics threshold tolerances for grouping similar performance loudspeakers for installation into specific consumer loudspeaker systems as described in AES preprint #1485 “Production Testing of Loudspeakers Using Digital Techniques” adds complexity and cost that is not unpractical for high volume duplicate mass production audio systems and in particular for mass produced audio systems installed in vehicles. 
         [0005]    Loudspeakers are traditionally described as operating within various electro-mechanical and acoustic metric thresholds. Some examples of metric thresholds include a frequency response that usually defines a performance bandwidth and deviation within the bandwidth, sensitivity relative to a distance and power input, voice-coil DC resistance, impedance, harmonic or total harmonic distortion, intermodulation distortion and parameters that describe the interaction of mechanical and electrical components. In addition to loudspeaker production variance loudspeakers are also prone to influences of environmental ambient conditions such as ambient temperature and ambient relative humidity as described in AES preprint 5507 “Ambient Temperature Influences on OEM Automotive Loudspeakers” and U.S. Pat. No. 7,092,536 to Hutt et al. In order to minimize measurement error it is important that ambient conditions be within a reasonable tolerance threshold for loudspeakers to be reliably and repeatably measured. 
         [0006]    U.S. Pat. No. 5,581,621 to Yoshihide et al describes use of a programmable parametric equalizer to automatically adjust an audio system but provides no means for controlling the process to reliably or repeatably correlate results of one operation to another operation or to reliably repeat and correlate to a performance response target based on previously measured reference data. U.S. Pat. No. 5,361,305 by Easley et al describes a vehicle audio test system that utilizes radio transmission to test audio system operation but provides no means to make adjustments to the audio system to rectify loudspeaker production variance nor does it provide a means to reliably and repeatably reproduce test results. 
         [0007]    Therefore there exists a need for a method to reliably and repeatably identify and rectify the influences of loudspeaker production variance on audio systems. 
       OBJECTIVES AND SUMMARY OF THE INVENTION 
       [0008]    It is the objective of the invention to provide a method to rectify the acoustical performance characteristics of individual loudspeakers used in duplicate or mass production audio systems so that the they conform within a specified metrics threshold tolerance of the acoustical performance characteristics of associated loudspeakers in the reference audio system on which the duplicate or mass produced audio system is based. By associated loudspeakers we mean for example the specific type or model of loudspeaker and its installation location in the audio system. 
         [0009]    Audio systems typically include one or more loudspeakers where the type or model of each loudspeaker in the audio system and their mounting locations is known and in the case of production vehicles or architectural layouts such as movie theaters the boundary conditions are also know. By boundary conditions we mean any physical object in the sound field proximate to the loudspeakers in the audio system from which sound may be reflected or partially absorbed such as but not limited to video screens, recording mixing consoles, walls, floors, ceilings, doors, windows, furniture and in vehicles, windscreens, instrument panels, doors, floor and seats etc. When one or more microphones are placed in predetermined locations relative to loudspeakers and boundary conditions of a reference audio system the loudspeakers may be measured individually or collectively in any combination with a computer based audio measurement system that may be embedded in the audio system or in associated hardware such as a vehicle diagnostic computer or in an external device such as a stand alone computer to acquire performance data metrics and metrics threshold tolerances can be specified for each loudspeaker in the audio system. Many audio systems employ a plurality of loudspeakers but individual loudspeakers may be made active for the measurement process by muting all loudspeakers in the system except for the specific loudspeaker to be measured. The performance data metrics acquired during the measurement process will be stored with their specified data metrics threshold tolerances for comparison to the associated loudspeakers in duplicate audio systems where the performance data metrics of the loudspeakers in the duplicate audio system are measured in the same method with same boundary conditions. If loudspeakers performance data metrics in a duplicate audio system do not conform within the specified performance data metrics threshold tolerances additional or modified signal processing may be applied to the signal processing chain and be stored in the signal processing memory so that the loudspeakers performance data metrics in the duplicate audio system are rectified to conform within the specified performance data metrics threshold tolerances. The signal processing required for rectification may be applied directly to the signal processing memory by making manual or automated changes either directly or via a vehicle communication bus or may be applied by an equalization tool external to the duplicate audio system. When performance data metrics for all loudspeakers in the audio system have been rectified as necessary to conform within the specified metrics threshold tolerances the rectified signal processing data is stored in an updated signal processing record in the signal processing memory of the duplicate audio system or an equalization tool external to the duplicate audio system. 
     
    
     
       BRIEF DESCRIPTION OF TILE DRAWINGS 
         [0010]    The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
           [0011]      FIG. 1  is a cross-section illustration of an example loudspeaker depicting the individual loudspeaker components. 
           [0012]      FIG. 2  illustrates the frequency response variance of two samples of the same type and model of loudspeaker assembled from components that produce a maximum opposite influence on performance. 
           [0013]      FIG. 3  illustrates the minimum, median and maximum frequency response metrics threshold tolerance deviation of a typical loudspeaker production specification. 
           [0014]      FIG. 4  illustrates a typical vehicle audio system architecture. 
           [0015]      FIG. 5  illustrates performance response characteristics of the same audio system with variations caused by repositioning the acquisition microphone location for each acquisition. 
           [0016]      FIG. 6A  illustrates various example locations for vehicle communications microphones. 
           [0017]      FIG. 6B  illustrates an example microphone mounting fixture. 
           [0018]      FIG. 7  illustrates unequalized frequency response measurement in comparison to the equalized response and the equalization curve. 
           [0019]      FIG. 8  illustrates the typical unequalized response of two similar loudspeakers from a production build. 
           [0020]      FIG. 9  illustrates the comparative results of applying the same equalization curve to two same type loudspeakers. 
           [0021]      FIG. 10  illustrates an original equalization curve, a rectification curve and a curve representing the sum of the equalization plus rectification curves. 
           [0022]      FIG. 11  illustrates the loudspeaker response comparison of applying the rectification curve with the equalization curve. 
           [0023]      FIGS. 12A ,  12 B and  12 C Illustrate the sound propagation path between an example loudspeaker and measurement microphone. 
           [0024]      FIG. 13  illustrates example wave forms used in the rectification acquisition process. 
           [0025]      FIG. 14  illustrates a comparison of impulse response graphs with different boundary conditions. 
           [0026]      FIG. 15  illustrates the flow chart of the process to create the master reference record required to run the rectification process. 
           [0027]      FIG. 16  illustrates the flow chart for the rectification process. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    The invention will be understood by one skilled in the art of transducer design and manufacturing and in particular how production variance in manufactured loudspeakers influences the sound of duplicate audio systems.  FIG. 1  illustrates a cross-sectional drawing of a typical loudspeaker  100 . The moving assembly that contributes to the moving mass is made up of the cone body  101 , voice-coil assembly  102 , suspension  103 , spider  104 , and adhesive  106 . In addition to the mass and stiffness of the loudspeaker moving assembly performance is influenced by the flux density in the magnetic gap  107  determined by the magnetic energy stored in the magnet  108  and the geometry and material of the top plate  109  and T-yolk  110 . Voice-coil resistance is determined by the cross-sectional area of the wire and total length of wire in the voice-coil as determined by the number of turns and voice-coil diameter. When an electrical signal is applied to the voice-coil the voice-coil and magnetic flux combine for an electro-motive force BL where B=magnetic flux and L=length of wire in the magnetic field. Litz wire  111  carries current from the terminal  112  that receives power from an amplifier, not shown. Gasket  113  affects performance by influencing frequencies relative to dimensional geometry or how the loudspeaker is affixed to a baffle and is known as a manufacturing cause of failure in incidents where the gasket is misapplied or missing in production. 
         [0029]      FIG. 2  illustrates the frequency response of two samples of the same type and model of loudspeaker that are assembled with components from the minimum and maximum allowable component metrics threshold tolerance as depicted in table  1 . The frequency response  201  is derived from a loudspeaker assembled with components at the range of metrics threshold tolerance allowing for maximum acoustic output for a given voltage input; minimum moving mass, minimum voice-coil resistance and maximum BL. The frequency response  202  is derived from a loudspeaker assembled with components at the range of metrics threshold tolerance allowing for minimum acoustic output for a given voltage input, maximum moving mass, maximum voice-coil resistance and minimum BL.  FIG. 2  illustrates clearly that there can be considerable difference between two loudspeakers that are supposedly the same type of loudspeaker and where either one could be used in any audio system for which it is specified. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Loudspeaker Components Production Tolerance 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Cone Body Mass 
                 +/−10% 
               
               
                   
                 Suspension stiffness 
                 +/−15% 
               
               
                   
                 Spider stiffness 
                 +/−15% 
               
               
                   
                 Voice-Coil Mass 
                 +/−10% 
               
               
                   
                 Voice-Coil R           
                 +/−10% 
               
               
                   
                 Magnetic Gap flux density B 
                  +/−5% 
               
               
                   
                 Adhesive mass-moving assy 
                 +/−15% 
               
               
                   
                   
               
               
                   
                             indicates data missing or illegible when filed 
               
             
          
         
       
     
         [0030]      FIG. 3  illustrates a frequency response graph with example response curves from a production run of a specific type and model of loudspeakers. Frequency response deviation in the same type and model of production loudspeakers is caused by the variance in the materials and manufacturing process of the individual components in addition to geometric alignment of components and process variations during the loudspeaker assembly process.  FIG. 3  illustrates typical acceptable metrics threshold tolerance of frequency response curves for maximum metrics threshold tolerance deviation  301 , median average  302 , and minimum acceptable metrics threshold tolerance deviation  303 . 
         [0031]    Referring to  FIG. 4  components in a typical vehicle audio system architecture are illustrated in an automobile audio system schematic. It should be noted that the audio system architecture in  FIG. 4  is for illustration purposes only and should not be construed as a limitation to vehicle audio system architecture that may utilize the present invention. In the example audio system architecture each of the loudspeakers Center Channel loudspeaker  401 , Right Front Mid loudspeaker  402 ′, Left Front Mid loudspeaker  402 ″, Right High Frequency loudspeaker  403 ′, Left High Frequency loudspeaker  403 ″, Right Mid-Bass Door loudspeaker  404 ′, Left Mid-Bass Door loudspeaker  404 ″, Right Rear Door loudspeaker  405 ′, Left Rear Door loudspeaker  405 ″ and Sub-Woofer loudspeaker  406  would be connected to amplifier  407  or head-unit  408  or combination thereof. Signal processors and memory elements would be installed in the amplifier  407  or head-unit  408 . A telephony microphone  409  may be placed directly in a head-unit  409  or in a location remote from the head-unit as described in  FIG. 6 . Preferably a thermal sensor  410  and humidity sensor  411  should be located within the vehicle to monitor acquire ambient temperature and humidity data during the acoustic acquisition process since ambient atmospheric conditions can influence the propagation of sound. Ideally the ambient condition data is acquired by either by the vehicle sensors or external sensors and are stored in the measurement setup information. One or more test acquisition microphones  412  may be installed in the vehicle temporarily during testing and microphone&#39;s location coordinates should be measured from fixed boundaries  413 ,  414  and the microphone&#39;s test location coordinates stored in the measurement setup information. Alternate acquisition microphone example locations are illustrated  415  and  416 .  FIG. 4  is provided as an example illustration only as production vehicles may place loudspeakers in similar or different locations, may use more or less loudspeakers and may use different size or frequency range loudspeakers and may or may not include an external amplifier with DSP in addition to a head-unit. However once an audio system design is committed for mass production, that design and implementation remains in effect until a design change is executed along with a document control process that accounts for all changes in effect creating a new audio system where a new verification and rectification process would be required. 
         [0032]      FIG. 5  illustrates the variation of frequency response performance that is caused by acquiring measurement data of a specific loudspeaker in a specific vehicle audio system with the microphone in different locations where  FIG. 5  response curve  501  correlates to  FIG. 4  microphone location  412 ,  FIG. 5  response curve  502  correlates to  FIG. 4  microphone location  411  and  FIG. 5  response curve  503  correlates to  FIG. 4  microphone location  412 . The performance variations as illustrated in  FIG. 5  are not caused by audio system or loudspeaker performance variation but are caused only due to the measurement process where different microphone locations were used for each of the response curves  501 ,  502  and  503 . 
         [0033]    Referring to  FIG. 6A  various locations are illustrated for vehicle communications microphones that could be used for voice-commands, telephony, ambient noise acquisition or other. Microphone  601  at top left of windshield  600  near the headliner (not shown) junction,  602  microphone attached to rear-view mirror  610  or included with  603 ,  604 ,  605  in a microphone array attached to the rear-view mirror  610 , microphone  606  at base of windshield near the driver, microphone  607  in center of steering wheel  611  microphone  608  in head-unit  609  are all typical locations that may be utilized for a vehicle microphone. The microphone locations illustrated in  FIG. 6  would not usually be installed in the same vehicle and are included to illustrate example locations only. 
         [0034]    Referring to  FIG. 6B  an example microphone mounting fixture  612  is illustrated that would in this case would attach to the rear-view mirror  613  in such a manner that it would support a test microphone  614  in specific location relative to boundary conditions. Mounting fixtures may be created in different embodiments custom developed for specific vehicles and may be attached to areas other than the rear view mirror. 
         [0035]    Audio systems typically have electrical equalization applied as necessary to improve the overall acoustic performance.  FIG. 7  is an illustration of the unequalized frequency response measurement  701  of Left Front Mid loudspeaker  402 ″ as illustrated in  FIG. 4  measured in situation in an example vehicle reference audio system. Applying equalization  703  to the electrical signal directed to loudspeaker  402 ″ modifies the loudspeaker output to the frequency response measurement  702 . Equalization curve  703  is stored in the reference audio system master reference record. 
         [0036]    Referring to  FIG. 8  the unequalized frequency response measurement  701  ( FIG. 7 ) of Left Front Mid loudspeaker  402 ″ ( FIG. 4 ) measured in situation in the example vehicle reference audio system is redrawn  801  for comparison to the unaltered frequency response measurement of a typical production loudspeaker measured in the production audio system as represented by curve  802 . The frequency response difference between  801  and  802  is typical of production loudspeakers and is within allowable loudspeaker production metric thresholds tolerance as previously described. 
         [0037]      FIG. 9  illustrates the frequency response with the equalization from the master reference record applied to the production loudspeaker shown in the vehicle audio system of  FIG. 8 . Loudspeaker frequency response  901  illustrates the frequency response with equalization from the master reference record applied to  801  from the reference audio system and  902  illustrates the frequency response with the equalization from the master reference record applied to  802  from the production loudspeaker. The equalization curve from the master reference record is illustrated by  903 . It can be seen that there is considerable deviation between the reference loudspeaker response curve  901  and duplicate production loudspeaker response curve  902  as illustrated by the zones illustrated in circles  904  and  905 . 
         [0038]    Referring to  FIG. 10  the equalization from the master reference record curve  1001  as applied to  801  to result in  901  and applied to  802  to result in  902 . Adding the difference between the frequency responses  901  and  902  to curve  1001  results in curve  1002  which is the rectification adjustment required to bring  902  into closer frequency response compliance with  901 . Rectification curve  1003  is an example of how the rectification would be customized for any specific production loudspeaker so that the combined equalization and acoustic result would closely match the appropriate result from the reference audio system. The rectification process may be done manually or with an automated process to calculate the sum difference between reference target and production response measurements. 
         [0039]    Referring to  FIG. 11  it is shown that there is minimal difference between frequency response of the reference audio system loudspeaker with equalization from the master reference record  1101  and frequency response  1102  of the duplicate audio system with production loudspeaker to which the rectification curve  1003  is applied. 
         [0040]    Referring to  FIGS. 12A ,  12 B and  FIG. 12C  three plan views are illustrated of a vehicle  1201  as an example of how boundary conditions influence sound propagation. The illustration in  FIG. 12  uses a vehicle as an acoustic environment but could also be exemplified by a traditional room such as in a home or a movie theater. Loudspeaker  1203 , windshield  1202  and test acquisition microphone  1204  are illustrated in an example setup in that are each is located in a fixed position relative to the vehicle body  1205 . In this example the lateral distance from base of windshield  1206  to microphone  1204  is indicated by dimension “d 1 ”  1207  and is fixed at a constant distance for each of the example  FIGS. 12A ,  12 B and  12 C. Referring to  FIGS. 12A and 12B  the lateral distance from base of windshield  1206  to the back support of the driver seat  1209  is indicated by dimension “d 2 ”  1208 . Referring to  FIG. 12A  sound propagation paths between loudspeaker source  1203  and microphone  1204  are illustrated as direct path  1211 , a first order reflection  1212  reflecting off driver side window  1213 . Referring to  FIG. 12B  the sound propagation path between loudspeaker  1203  and microphone  1204  are illustrated by a second order reflection path  1214  where the sound first reflects off the back support of the driver seat  1209  then reflects off driver side window  1213  before arriving at the microphone  1204 . Referring to  FIG. 12C  the lateral distance from base of windshield  1206  to the back support of the driver seat  1209  is indicated by dimension “d 3 ”  1210  in figures and in this representation is a dimension greater than  1208 . Based on the dimensional relationships as illustrated the sound propagation time is greater for sound propagation path  1212  than for sound propagation path  1211  and that sound propagation path  1214  is greater than sound propagation path  1212  and that sound propagation path  1215  is greater than sound propagation path  1214 . It should be apparent that sound propagation path  1211  and first order reflection from the driver window  1213  as indicated by  1212  ( FIG. 12A ) will not change regardless of driver seat  1209  position. It should also be apparent that as the driver seat  1209  is moved rearward a greater distance from the base of the windshield  1206  that the second order reflection  1214  ( FIG. 12B) and 1215  ( FIG. 12C ) from the back support of the driver seat  1209  will increase the sound propagation time for sound to travel from the loudspeaker  1203  to the microphone  1204 . 
         [0041]    Referring to  FIG. 13  two waveforms are illustrated where abscissa is in the time domain and the ordinate is amplitude in dBv,  1301  being a signal output from the test source and  1302  being the output from a loudspeaker device under test (DUT) acquired through the test acquisition microphone. Ideally, waveform  1301  is acquired by electrical connection at the amplifier output or loudspeaker input terminals  112 . By calculating the time (t)  1303  interval between the start of test waveform  1304  and the start of loudspeaker DUT waveform  1305  acquired through the test acquisition microphone the sound propagation time between the loudspeaker DUT and test acquisition microphone may be known. The propagation time for sound from each loudspeaker in the audio system to reach the acquisition microphone will be unique in any case that the acquisition microphone is located non-equidistant from any two loudspeakers in the audio system. Knowledge of the sound propagation time for each loudspeaker in the audio system provides a method to confirm that the appropriate loudspeaker is connected to the output channel from the amplifier and conversely that the microphone is in the expected location. An inversion of the acquired waveform from the loudspeaker DUT would indicate that the loudspeaker is connected out of polarity. 
         [0042]    Referring to  FIG. 14  two energy vs. time domain impulse response measurements dashed line  1401  and solid line  1402  are shown as representation of measurements acquired according to the boundary conditions setup as illustrated in  FIGS. 12A ,  12 B where driver seat  1209  is in one position and  12 C where driver seat  1209  is in a second more rearward position. First arrival peak  1403  illustrates the arrival at the microphone  1204  ( FIG. 12 ) of sound most directly from loudspeaker source  1203  represented as sound propagation path  1211  and second peak  1404  is relative to sound propagation path  1212  and as can be seen both peaks are nearly identical for both measurements  1401  and  1402 . Time 0 ms  1410  is analogous to start of waveform  1304  ( FIG. 13 ). The third peak  1405  is related to reflection  1214  and is seen in measurement  1402  but is in fact a slight null  1406  in measurement  1401  because the third peak  1407  arrives slightly later in time due to the greater sound propagation path  1215  of the third order reflection caused by the greater distance  1210  between loudspeaker  1203  and driver seat  1209 . Peaks  1408  and nulls  1409  are typical acoustical influences of other boundary reflections influenced by specific boundary conditions. It is clear that analysis and comparison of time response measurements will indicate physical differences in test setups such as but not limited to which loudspeakers are playing, microphone location relative to boundaries, boundary materials and boundary conditions for example such as seats or open or closed windows or doors. 
         [0043]    Referring to the flow chart in  FIG. 15  the process to create a Master Rectification Record is illustrated. After the reference audio system has been fully developed and tuned per standard equalization and tuning procedures  1502  a Master Rectification Setup Record  1503  is defined for each audio system that includes but is not limited to ambient conditions that include temperature and relative humidity tolerance specification, microphone type and location relative to loudspeakers and boundaries, boundary conditions with furniture or equipment location or in the case of vehicles seat settings including forward/back position, seat-back upright angle position, seat upholstery, seat type, window settings preferably in closed position, doors closed, steering column in default position, armrests position documented preferably in upright position, storage locations documented preferably with covers closed (example; glove box), and in the case of vehicles should include VIN (vehicle information number) series and reference vehicle VIN if applicable. Also included should be the audio system build record with loudspeakers type and location, signal processing record with software version, crossovers, equalization, delay, limiters, amplifier information with model and version and specifications for allowable tolerance of ambient environmental conditions including temperature and relative humidity and allowable ambient noise conditions. The Master Rectification Setup Record  1503  should also include information about how the data acquisition was executed including the test signal type, individual loudspeaker test setups, bandwidth, acquisition signal voltage, method and hardware to store acquisition data and the analysis and rectification algorithms used to analyze conformance and to validate that the data acquisition is within the metric thresholds tolerance. After loading the Master Rectification Setup Record  1503  the routine to create the Master Rectification Record is begun  1504 . Ambient environmental conditions including temperature and relative humidity should be recorded in a log  1505  and verified to be within specification  1506  as defined in the Master Rectification Setup Record  1503 . If ambient environmental conditions are outside of the allowable specification tolerance ambient conditions should be adjusted  1507 . Ambient noise conditions are measured by running the acquisition signal from the acquisition microphone with zero input for a nominal time period for example one second and if the ambient noise is determined to be too high the ambient noise should be reduced by changing ambient noise conditions. The acquisition routine is started  1508 . The microphone should be verified to be working within metric thresholds tolerance  1509  and if the microphone is not working within metrics threshold tolerance it should be rectified or replaced  1510 . During the acquisition routine performance data for the loudspeakers is acquired  1511  and the individual loudspeaker performance data is stored  1512  and will include the sound propagation time  1303  for the sound to arrive at the microphone from each loudspeakers. Loudspeaker acquisition data  1511  is cross referenced against the Master Rectification Setup Record  1503  to confirm data from all loudspeakers has been acquired  1513  otherwise the routine loops back to acquire performance data from the next loudspeaker  1511  until performance data has been acquired for all loudspeakers in the audio system and stored in memory  1512 . The Master Rectification Record is then created  1514  and will include all information from the Master Rectification Setup Record  1503  in addition to all data acquisitions organized and stored  1512  before the process is terminated  1515 . 
         [0044]    The flow chart in  FIG. 16  illustrates the verification and rectification process for duplicate or production audio systems. From the start  1601  of the process the audio system should be identified  1602  preferably by VIN for vehicle audio systems then the appropriate Master Rectification Record  1514  ( FIG. 15 ) is loaded  1603 . The vehicle or acoustic space containing the audio system should be prepared for the rectification process  1604  according to the setup instructions contained in the Master Rectification Setup Record  1503  section of Master Rectification Record  1514 . If the audio system does not include a microphone  1605  a microphone should be installed  1606  in compliance with setup requirements of the Master Rectification Record. Ambient conditions such as noise, temperature and humidity should be recorded and logged  1607 . If ambient conditions are not within the allowable tolerance  1608  as specified in the Master Rectification Record  1514  ambient conditions should be adjusted  1609 . Start the rectification routine  1610  and confirm that the microphone is working within metric thresholds tolerance  1611  and if required, rectify or replace the microphone  1612 . Acquire and analyze performance data from the first loudspeaker  1613  according to the Master Rectification Record  1514  routine. If boundary conditions are not within allowable tolerance as indicated by analysis of comparison between the production loudspeaker DUT impulse response and the associated reference audio system loudspeaker DUT impulse response  1614  boundary conditions should be adjusted  1615  to match requirements as specified in the Master Rectification Record  1514 . If the loudspeaker performance is not within the metrics thresholds tolerance  1616  as defined in the Master Rectification Record  1514  the loudspeaker should be rectified accordingly  1617 . When the loudspeaker performs within the metrics threshold tolerance the rectification data should be stored  1618 . After verifying that all loudspeakers in the audio system have been validated  1619  the full audio system may be verified  1620  and the Vehicle Rectification Record data stored  1621  preferably with a backup in a remote memory  1622  and loaded into the audio system signal processing memory  1623  as an updated signal processing record before the process terminated  1624 . 
         [0045]    The invention described in this disclosure is applicable to an audio system in any acoustic space where the methods may be applied to improving conformance between a reference audio system and a duplicated audio system such as a production audio system. The definition of vehicle as used throughout this disclosure is not limited to any one type of vehicle and is not limited to automobile, truck, train, airplane, boat or similar. The invention may apply to any acoustic space with a form where consistent boundary conditions and audio system architecture can be repeated in additional duplicate formation such as movie theaters that utilize the same architectural design and audio system. 
         [0046]    In a first preferred embodiment of the invention a first audio system includes embedded or external or a combination of electronic hardware with signal processing and memory including processing capability required to operate an acoustic performance data acquisition, analysis and a rectification process where after final tuning of a reference audio system a signal processing record that will include but not be limited to multi-channel signal directions, crossover settings, signal delay, equalization, and limiters will be stored in an accessible file format and performance data of the reference audio system will be acquired utilizing a measurement test signal such as but not limited to a sine sweep, log-sweep, pseudorandom noise, or pink noise or cross-correlated music played through the loudspeakers and received through a measurement microphone  410  located in a documented location for example as indicated by measuring the relative distance from the microphone to fixed points in a horizontal location measurement  411  and vertical location measurement  412  such that a measurement may be repeated utilizing the same microphone location in the same or second audio system. The performance data may be acquired with all or some of the loudspeakers in the audio system operating simultaneously or more ideally with each of the loudspeakers  401  to  406  operated individually where separate data sets will be acquired and stored independently for each data acquisition. Source of the test signal may be generated during each test cycle or may be stored on any of CD, DVD, hard drive or solid state memory, or on any external memory device such as but not limited to USB drive, SD or compact flash and can be in any format such as but not limited to WAV, AU, MP3, OGG such that it may be converted to a data format that can be mathematically analyzed by a software program such as Matlab™ or Octave or similar. Relative detailed information of the test signal source, storage type, microphone information such as type and location and method will be stored with each data acquisition set. The acquisition data may be stored on a medium attached to the acquisition system directly or on a remote memory such as that attached to a remote server. The analysis of the test acquisition data may include but not be limited to frequency response and time domain analysis and an acceptable metrics threshold tolerances will be specified. Spatial attributes will be apparent in energy vs. time measurements such as an impulse response or derivatives such as energy time curve that may be captured directly in the case of a pseudorandom noise excitation or calculated by executing an Inverse Fast Fourier Transform of a swept frequency response measurement. The verification measurement process of mass production or duplicate audio systems will follow the same measurement process such that the data acquisition process utilized on the first audio system to acquire performance data metrics is also utilized on the second audio system to acquire performance data metrics. The second audio system measurement process may take place on or near a vehicle assembly production line or after completion of assembling of vehicles Signal processing data to rectify loudspeaker performance data metrics of a second audio system may be loaded into the signal processing section of an audio system via but not limited by CD, USB, WiFi, Bluetooth, directly or via a vehicle communication bus. 
         [0047]    A second preferred embodiment of the invention uses an external computer to manage all of the signal acquisition processing required to execute the verification and rectification process. 
         [0048]    Another preferred embodiment of the invention may use the microphone that is included in a vehicle audio system that is originally intended for communications, telephony, ambient noise characterization, active noise cancellation or similar as an acquisition microphone. Preferably the vehicle microphone has been rectified to its own metrics threshold tolerance the rectification data stored in the master rectification record. When the reference audio system has been measured through the vehicle&#39;s microphone, the production audio systems will be measured in the same manner. For accurate repeatability it is recommended that the vehicle&#39;s microphone be measured and if necessary be rectified to documented performance characteristics. 
         [0049]    Another preferred embodiment of the invention utilizes a test signal such as but not limited to a sine sweep, log-sweep, pseudorandom noise, or pink noise sent sequentially through each loudspeaker in a reference audio system and utilizes an acquisition test microphone placed at a specific location relative to the loudspeakers and room boundaries preferably in a location that may be repeated within a 50 mm distance. In this embodiment the acquisition microphone could be mounted by measuring distances to predefined coordinates in the room or would utilize a mounting fixture for example as illustrated in  FIG. 6B  so that it can be placed in a specific location which in the case of a vehicle may be the rear-view mirror or a location such as instrument panel, windshield, steering wheel, proximate the apex where the windshield meets the instrument panel or a location such that in subsequent tests the same location may be accurately repeated. 
         [0050]    Another preferred embodiment includes analysis of the boundary conditions by comparing the time domain data such as impulse response between two measurements to verify if boundary reflections as viewed in the time domain information occur in coincident or unrelated fashion. 
         [0051]    Any of the acquisition microphone setups previously described may be connected to the test apparatus by cable or wireless transmission such as FM, UHF, Bluetooth or WiFi. Wireless transmission adds the advantage of not requiring operators to manage cables. 
         [0052]    Variance in production loudspeakers causes duplicated audio systems to perform with wide variance and that rectifying loudspeakers to perform within a metrics threshold defined by analysis of a reference audio system is of great value. While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention.