Patent Publication Number: US-6223090-B1

Title: Manikin positioning for acoustic measuring

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention concerns the field of auditory localization and more specifically the field of measuring head related transfer functions (HRTFs). 
     Over the past decade many researchers have investigated the role of HRTF in spatial hearing. The HRTF represents the relationship between the audio signal generated at a point source in free space and the sound-pressure generated by that source at the eardrums of a human listener, and is typically measured with microphones in the left and right ear canals of a human listener or anthropomorphic manikin. The HRTF includes the effects of sound energy diffraction by the head and torso, as well as spectral shaping by the outer ear and ear canal resonances. The HRTF is typically a function of both frequency and relative orientation between the head and the source of the soundfield. When a sound source is electronically filtered by a HRTF and presented to a listener through headphones, the listener perceives the sound at the location of the source relative to the head when the HRTF is measured. Such a system is known as a virtual audio display. 
     Traditionally, HRTF measurements have been made in the far field with a sound source disposed at a distance greater than 1 meter. At this distance, an error of a few centimeters in the relative location of the source and head is of no great consequence, since it amounts to no more than a few degrees error in direction. If fact, most loudspeakers used in prior research arrangements measure 7 centimeters or larger in diameter, so the actual effective location of the source is not precisely defined within a few centimeters. When the source is near the head, however, small changes in the relative position of the sound source and the head can have a dramatic impact on the HRTF. The fundamental differences between the “near field” and “far field” as used herein are described in FIG. 1 which is a schematic diagram showing the measurement of the HRTF at close and far distances. In FIG. 1, a sound source is shown at  10  and the dashed lines at  13 ,  14 ,  15 ,  16  and  17  represent the radiating sound wave from the sound source  10 . The thickness of each dashed line indicates the intensity of the radiating sound wave, which is inversely proportional to the distance from the source. In order to measure the HRTF in the far field, a manikin head  12  equipped with microphones in the left and right ears is located 1.2 m from the sound source. Note that in the far field the angle of the source relative to the head is relatively insensitive to small changes in the position of the manikin. A displacement of the manikin by 1 cm will change the direction of the source relative to the head not more than 0.5 degrees. In order to measure the HRTF in the near field, manikin head  11  is located 0.25 m from the source. At this distance, the angle of the source relative to the manikin head is much more sensitive to small changes in the position of the manikin head. A 1 cm displacement of the manikin head can change the angle of the source relative to the head by 2.3 degrees. At closer distances, a small displacement in the location of the head can also generate substantial changes in the intensity of the sound reaching the ear. 
     Because of the precise placement accuracy required for measuring the HRTF at very close distances, the methods that have been used for measuring HRTFs in the far field are not sufficient for making near-field HRTF measurements. It is believed that the best placement solution for near-field HRTF measurements is to place the sound source at a desired distance and elevation relative to the manikin by hand, and then use a motorized stand to rotate the manikin in azimuth. However, it has been discovered that this method could cause large errors in the near field when the center of rotation of the stand is not located directly below the center of the interaural axis of the manikin. When the two centers of rotation are not perfectly aligned, the center of the head moves in a circular pattern as the manikin rotates and the HRTF measurements are corrupted. FIGS. 2 a  and  2   b  show a manikin incorrectly pitched. As used in describing the invention, the orientation of the manikin will be described in relation to a coordinate system with its origin at the point where the manikin  200  is attached to a motorized stand  203 . The x-axis of this coordinate system is parallel to the ground and in the direction of the front of the manikin, and the y-axis is parallel to the ground and perpendicular to the x-axis. The z-axis is perpendicular to the ground and increases with increasing elevation. The azimuth of the manikin will be defined as rotation of the motorized stand about the z-axis, increasing with clockwise rotation. The pitch will be defined as rotation around the y-axis, increasing as the manikin is tilted forward. The roll will be defined as rotation around the x-axis, increasing as the manikin is tilted to the left. 
     FIG. 2 a  shows the manikin  200  tilted slightly so the center of rotation  201  is behind the center of the head  202 . FIG. 2 b  is a top view of the FIG. 2 a  manikin and shows that as the manikin  200  having a center of rotation  201  slightly behind the head is rotated in azimuth, the position of the center of the head  202  does not remain fixed but rather traverses a circle  204 . Note that the manikin connects to the motorized stand  203  at a baseplate located at the waist of the manikin. The center  202  of the interaural axis is approximately 1 m above this connection, so the center of rotation of the head is displaced by approximately 2 cm for each degree of pitch or roll in the manikin  200  relative to the motorized stand  203 , and causes the head to translate through a circle 4 cm in diameter, shown at  204 , as the manikin rotates through 360° in azimuth. FIGS. 2 a  and  2   b  illustrate, therefore, that even a small amount of pitch or roll in the manikin  200  is unacceptable when making measurements less than 25 cm from the center of the head. The present invention provides a method and apparatus for ensuring accurate computer controlled positioning of a manikin for near-field HRTF measuring. 
     SUMMARY OF THE INVENTION 
     The invention provides a computer controlled, three-dimensional closed-loop system for automatically positioning, relative to a stationary sound source, the head of a manikin situated on a motorized stand. The three-axis positioning is responsive to acoustic signals measured from microphones located at each ear of the manikin and is desirable for accurate near field HRTF measuring. 
     It is an object of the invention, therefore, to provide computer control for centering the coordinate axes of rotation of a manikin head over a motorized stand. 
     It is another object of the invention to rapidly and automatically position a manikin in azimuth angle relative to a sound source. 
     It is another object of the invention to rapidly and automatically position a manikin in roll angle relative to a sound source. 
     It is another object of the invention to rapidly and automatically position a manikin in pitch angle relative to a sound source. 
     It is another object of the invention to provide a manikin positioning method for high accuracy HRTF measuring in the near field. 
     These and other objects of the invention are described in the description, claims and accompanying drawings and are achieved by a computer controlled closed-loop three-dimensional iterative method for positioning an acoustic manikin for head-related transfer function measurements, said method comprising the steps of: 
     providing a selectively positioned audio signal from a sound source; 
     receiving said audio signal at left and right ears of said manikin; 
     transforming time domain representations of said audio signal received at left and right ears of said manikin in a manikin selected axis first position thereof to frequency domain phase values; 
     computing from said frequency domain phase values to a first phase difference between left ear and right ear signal representations of said manikin; 
     determining a time delay from said first phase difference from said computing step; 
     rotating said manikin about said selected axis relative to said sound source in directionally determined response to unequal time delay determinations from said determining step; and 
     repeating said transforming, said computing, said determining and said rotating steps until preselected equal left and right ear time delays and optimal position alignment about said selected axis is obtained relative to said sound source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing a comparison between near field and far field localization cues. 
     FIG. 2 a  shows a manikin pitched slightly forward. 
     FIG. 2 b  shows the FIG. 2 a  manikin head translating in a circle during rotation. 
     FIG. 3 shows a block diagram arrangement of the invention. 
     FIG. 4 a  shows a forward facing manikin having an error in roll. 
     FIG. 4 b  shows a backward facing manikin having an error in roll. 
     FIG. 5 a  shows a forward facing manikin having an error in pitch. 
     FIG. 5 b  shows a backward facing manikin having an error in pitch. 
     FIG. 6 shows a computer algorithm used to determine time delay. 
     FIG. 7 shows a computer algorithm for determining least square error. 
     FIG. 8 a  a shows a graph representing a linear relationship between frequency and phase delay. 
     FIG. 8 b  shows a graph representing a typical phase difference spectrum and phase spectrum of estimated time delay. 
     FIG. 8 c  shows a graph representing differences between the actual phase spectrum and the error. 
    
    
     DETAILED DESCRIPTION 
     The invention accurately positions in azimuth, roll and pitch the head of a manikin situated on a motorized rotatable stand relative to a stationary sound source during HRTF measuring so that the center of rotation of the manikin passes through the midpoint of the interaural axis in the manikin. The positioning is iterative, computer controlled and based on the acoustic signals measured from microphones located at the ears of the manikin. The system is helpful for accurate measurement of HRTFs in the near field. 
     FIG. 3 shows an arrangement of the invention. In the FIG., a manikin  301  is shown disposed on top of a motorized stand  300 . The manikin is outfitted with microphones  303  and  302  at the left and right ears, respectively. An amplifier  307  is attached to a sound source  306 . The low frequency sound transducer  306  is placed directly in front of the manikin  301  at the desired near field distance. The sound source  306  provides a low frequency signal, ideally in the range of 0 to 6400 Hertz. A sine wave signal could be used. 
     In operation, the sound source  306  generates an audio signal, which is recorded by the microphones  302  and  303  located at the ears of the manikin  301 . The signals recorded by the microphones  302  and  303  are communicated to a spectrum analyzer  308 . The spectrum analyzer  308  is coupled to a control computer  309 . The control computer  309  is also coupled to a motor controller  310  for motorized positioning control of the stand  300  through control of the motors  311  controlling the azimuth of the stand and  311  controlling the pitch of the stand. The procedure of positioning the manikin  301  relative to the sound source  306  may be accomplished in a three step process, the first step positioning the manikin in azimuth, the second step positioning the manikin in roll and the third step positioning the manikin in pitch. The procedure and device is iterative both within each step and between the three steps. Each step verifies that the manikin  301  is adequately positioned in one dimension and the positioning process is repeated within each dimension until a preselected degree of error is obtained. Each adjustment may displace the manikin  301  in the other two dimensions, so the entire procedure must be repeated until data from all three steps indicate that the manikin  301  is accurately positioned. 
     The microphone  303 ,  302  signals are communicated to the spectrum analyzer  308  by two channels shown at  304  and  305 . The spectrum analyzer  308  has a channel for processing the recorded signal at each ear. Channel  1  of the analyzer at  305  is connected to the output of the left ear microphone  302  and channel  2  of the analyzer at  304  is connected to the right ear microphone  303 . The spectrum analyzer  308  receives the time domain data from microphones  302  and  303  and calculates the phase difference between the microphone signals  304  from the right ear and  305  from the left ear. This is accomplished by setting the analyzer in a frequency response measurement mode that calculates the ratio of the Fourier transform of the channel two signal  305  to the Fourier transform of the channel one signal  304 . The resulting data are represented as a series of complex coefficients representing the amplitude and phase of the ratio of the Fourier transforms of signals  304  and  305  as a function of frequency. The amplitudes of these coefficients are disregarded, and only the phase angles of each coefficient, representing the phase differences between the signals at the left and right ears  302  and  303  of manikin  301  at each frequency are considered. Alternately, the Fourier transforms of the left and right ears could be calculated separately, and the phase angle of the transform at the left ear could be subtracted from the phase angle of the transform at the right ear to determine this phase difference. 
     The difference between the phase data from channel  1  and channel  2  is then used to verify that the manikin is correctly positioned in azimuth, elevation and roll. That is, if the manikin head is not correctly positioned relative to the sound source, there is a delay in time at which the sound reaches one ear relative to the other ear and this delay may be represented by a phase difference determined by the spectrum analyzer  308 . The system of the invention uses such phase difference or phase delay measurements from the spectrum analyzer  308  to calculate the time delay in the dimensions of azimuth, roll and pitch for the sound signal received at one ear relative to the other ear. An approximately linear relationship exists between the difference in phase data of channels  1  and  2 , and the frequency spectrum, and this relationship permits a representative time delay to be calculated from the phase delay data. The relationship can be described graphically as shown in the graph of FIG. 8 a.  In FIG. 8 a,  the y-axis at  801  represents phase difference in degrees between channels  1  and  2 , shown at  304  and  305  of FIG. 3, and the x-axis at  802  in FIG. 8 a  represents frequency. The phase delay as a function of frequency for an ideal time delay of 169 microseconds is shown in line  803 . Note that the slope of this ideal line is directly related to the time delay and is equal to (360 degrees * delay)/Hz. Plotting the phase delay for a typical measurement from the manikin of FIG. 3 against a frequency range results in the line of  804 . The actual phase difference data of the form shown in line  804  is used to calculate the best estimate of the ideal time delay shown in  803  using a software algorithm written in C++ code and provided as an Appendix hereto and implemented by the control computer  309  in FIG.  3 . 
     The FIGS. 6 and 7 drawings show flow charts summarizing the software algorithms used to determine interaural time delay values and linear least squares error, respectively. The algorithm represented by the flow chart of FIG. 6 calls and uses the FIG. 7 linear least square error algorithm to generate the interaural time delay. The software algorithms represented in FIGS. 6 and 7 are implemented in the control computer, shown at  309  in FIG.  3 . The control computer  309  implements these algorithms after receiving the phase difference data calculated by the spectrum analyzer shown at  308  in FIG.  3 . 
     The control computer  309  in FIG. 3 implements the algorithms represented by FIGS. 6 and 7 first for the azimuth axis in order to find the interaural time delay and reposition the manikin as needed. It continues to run the algorithms until the preselected time delay and associated azimuth position relative to the sound source is attained. The control computer  309  in FIG. 3 then implements the algorithms represented by FIGS. 6 and 7 for the roll axis in order to find the interaural time delay and repositions the manikin as needed. It continues to run the algorithm until the preselected time delay and associated position relative to the sound source is attained. 
     The control computer  309  in FIG. 3 again implements the algorithms represented by the flow charts of FIGS. 6 and 7 with the manikin considered for accurate pitch positioning to find the interaural time delay and reposition the manikin as needed. It continues to run the algorithm until a preselected time delay and associated position relative to the sound source is attained. Each subsequent adjustment of the manikin in azimuth, roll and pitch may displace the manikin with respect to the axis in the previously considered. The entire procedure of attaining a preselected time delay and associated positioning relative to the sound source in each dimension is preferably repeated until the preselected time delay is attained with respect to each axis and an indication that the manikin is optimized in each axis is obtained. 
     The FIG. 6 algorithm determines the interaural time delay as stated in box  600 . The first step, shown in block  601 , initializes a large arbitrary minimum error of 10 to the 15 th  power and a time delay of −1000 microseconds. The minimum error is a large value to ensure that actual errors produced by the algorithm will be less than the initialized value. The time delay is a preselected estimated value based on the assumption that the actual time delay is within the range of −1000 microseconds to +1000 microseconds based on the geometry of the head and physical constraints. That is, the initialized time delay is based on the theory that time propagates at the speed of 1 ms per foot and the maximum interaural time delay is approximated by the distance between the two ears measured across the surface of a manikin head, approximately half the diameter of a manikin head. A time delay of 1000 microseconds, therefore, represents a time delay for an extremely large diameter head of a manikin or human subject. 
     After the error and estimated interaural time delay (itd) are initialized in block  601 , the linear least square (lls) error algorithm is called in block  602 . The linear least square error algorithm is disclosed in flow chart form in FIG.  7  and is described in detail infra. Generally, this linear least square error algorithm calculates the error between the initialized time delay value set in block  601  of FIG.  6  and the time delay value of the current position of the manikin determined by the measured interaural phase difference. After a least square error value is obtained using the algorithm represented by the flow chart of FIG. 7, block  603  of FIG. 6 shows that the linear least square error value is compared with the initialized minimum error from block  601 . If the least square error value is less than the minimum error value, then the algorithm proceeds as shown in block  607  of FIG.  6  and the minimum interaural time delay and minimum error are reset to be the current estimated time delay value and the least square error value. 
     The algorithm then proceeds to block  604  where 10 microseconds are added to the interaural time delay estimate so that the next estimate of the interaural time delay is the former estimate plus 10 and this new value will then be considered. Adding 10 to the former estimate eliminates the need to consider the former estimate plus every microsecond value between the former estimate plus 10. It allows one to find the closest error in a 10 microsecond interval. Block  605  of FIG. 6 continues the loop until the interaural time delay estimate is equal to 1000 microseconds, and then terminates the loop. In block  606 , the time delay estimate is set to the delay generating the minimum least-squared error minus 10 microseconds. The linear least square error algorithm of FIG. 7 is then called as shown in block  608  of FIG.  6 . The linear least square error algorithm again calculates the error between the estimated time delay and the actual value. As shown in block  609 , the algorithm next evaluates whether the least square error calculated in block  608  is less than or equal to the minimum error set in block  607 . If the error is less than or equal to the minimum error, then as shown in block  613  the minimum error is reset to the least square error calculated in block  608  and the output variable of the function (find_itd) is set to the current delay estimate. 
     The algorithm proceeds to block  610 , which increases the estimate of time delay by 1 microsecond. Block  611  repeats the loop until the estimate of time delay has been incremented through a series of 21 values, in one microsecond steps, within 10 microseconds of the best estimate of interaural delay found in the first part of the algorithm and set in block  607 . Once all the time delay estimates within 10 microseconds of the previous best guess have been tested, the function returns the best estimate of the interaural delay (find_itd). Note that the first loop of this algorithm finds the estimate of interaural time delay producing the smallest error relative to the measured interaural phase with 10 microsecond steps. The second loop of the algorithm narrows the search to the vicinity of the best time delay estimate found in the first part of the algorithm and finds the best estimate of the interaural time delay within 1 microsecond. 
     The loop identified by blocks  602 ,  603 ,  604 ,  605  and  607  is performed 201 times in any single implementation of the algorithm represented by the flow chart of FIG. 7, because the algorithm performs calculations from −1000 microseconds to +1000 microseconds in increments of 10 which is equivalent to 201 calculations. The loop identified by blocks  608  to  611  is performed 21 times because in any single implementation of the algorithm represented by the flow chart of FIG. 7 performs calculations within the range −10 to +10 around the reset minimum interaural time delay, this comprises 21 calculations. 
     FIG. 7 is a flow chart of the algorithm which calculates linear least square error when implemented by the control computer at  309  in FIG.  3 . The discussion of FIG. 6 supra shows the interaural time delay algorithm location where the FIG. 7 linear least square error algorithm is used. The algorithm represented by the flow chart of FIG. 7 first initializes both the frequency count and the error output determination of the algorithm, both values being initialized to zero in block  701 . The term “count” in the algorithm keeps track of the frequency variable in the algorithm. The relationship set forth in block  702  states what the phase relationship is at the frequency identified by the variable count. The block  72  relationship is identified as the interaural time delay phase and is equivalent to the interaural time delay multiplied by 360 degrees multiplied by 16 multiplied by count. The interaural time delay is multiplied by 360 degrees to obtain a full period of delay and multiplied by 16 * count, which represents the frequency of the current value in the 400 point frequency spectrum. 
     In block  703  of FIG. 7, the variable delta is defined as the phase one obtained with a pure delay minus the interaural time delay phase and this variable is the error at the frequency that count represents. The next step of the algorithm represents a deviation from traditional least square error calculations and is believed novel to the invention. Blocks  704  and  705  of FIG. 7 show how the delta value manipulated in the algorithm is within 180 degrees. Block  704  calculates whether the delta value is greater than or equal to 180 degrees and if so, the algorithm proceeds to block  710  which resets delta as delta minus 360 degrees. If the block  704  result is that delta is not greater than or equal to 180 degrees, the algorithm is directed to block  705  where the computer calculates whether delta is less than or equal to 180 and if so, delta is reset in block  709  to delta plus 360 degrees. 
     This process is illustrated in FIG. 8 b.  The graph of FIG. 8 b,  with y-axis  806  the phase difference in degrees and x-axis  807  representing frequency, shows in line  808  the difference between the phase of the estimate of time delay  803  and a typical measured phase difference spectrum  804 . This line is equivalent to the value of delta after the execution of block  703  in FIG.  7 . Note that this error changes frequently from values near 0 degrees to values near −360 degrees, complicating evaluation of the error. In FIG. 8 c,  line  812  represents the difference  808  between the actual phase difference and the error (delta) after processing by the loop represented by blocks  704 ,  705 ,  709  and  710  of FIG.  7 . Note that the errors represented by  812  are useful for evaluating the accuracy of the time delay estimate  803 , while the unprocessed errors represented by line  808  are not. 
     Block  706  resets the values of count and error, both initially zero in block  701 . Count is reset to count +1, which represents the next frequency value. Error is reset to the equivalent of error plus delta multiplied by delta. If variable count, or frequency value is less than 400, as determined in block  707 , then the algorithm loops back up to block  702  to again calculate a pure delay value. Once the entire 400 point frequency spectrum is considered, the algorithm ends as shown in block  708 . The 400 point frequency spectrum is arbitrarily chosen to match the capabilities of the frequency analyzer used, and is not critical to the algorithm. In order to average out variability, at least 200 points representing frequencies from approximately 100 to approximately 5000 Hz should be used. 
     Considering the first step of properly positioning the manikin in azimuth relative to the stationary sound source, if there is a positive time delay between channel  1  and channel  2 , or the left and right ears, respectively, i.e., a left ear lag, the control computer  309  then communicates a signal to the motor controller  310  to rotate the manikin  301  to the right one degree for every 15 microsecond of delay. Similarly, if there is a negative time delay, or a lag at the right ear, the control computer  309  then communicates a signal to the motor controller  310  to rotate the manikin  301  to the left one degree for every 15 microsecond of delay. This process is iterative and repeated using the algorithms represented in FIGS. 6 and 7 until the interaural delay is reduced below 2 microseconds and the attained location is defined as the zero degree azimuth. Motor controller  310  accomplishes manikin rotation by way of motor  311 . 
     Looking again at FIG. 3, channel  1  at  304  and channel  2  at  305  of the analyzer remain connected to the left and right ears for the second step of the positioning process, positioning the manikin in roll, as illustrated in FIGS. 4 a  and  4   b.  FIGS. 4 a  and  4   b  illustrate positioning of the manikin in roll with the manikin facing forward in FIG. 4 a  and the manikin facing backward in FIG. 4 b.  In FIGS. 4 a  and  4   b,  the sound source is shown at  400 , the center of rotation is shown at  401 , the center of the head is shown at  402  and the shoulders of the manikin are shown at  403 . FIG. 4 a  shows the manikin facing forward with the center of rotation  401  of the manikin head aligned with the sound source for a zero time delay. In the FIG. 4 b  position, the manikin is then rotated 180 degrees as shown in FIG. 4 b  to determine whether any additional positioning is required. The FIG. 4 b  rotation indicates that the sound source  400  is on the right side of the manikin. The manikin is displaced from alignment with sound source  400  by Θ degrees, shown at  404  in FIG. 4 b.  Once the FIG. 4 b  position is achieved, the spectrum analyzer shown at  308  in FIG. 3 then communicates the phase difference information to the control computer  309  in FIG. 3 which calculates the interaural time delay by implementing the algorithms represented in FIGS. 6 and 7. The interaural time delay for the manikin in FIG. 4 b  is negative indicating the manikin needs to be tilted slightly to the manikin&#39;s right. Similarly, if there is a positive delay between the left and right ears, the computer tells the operator to tilt the manikin slightly to the left. The measuring process and positioning is repeated until the manikin is positioned such that the time delay is less than 5 microseconds in magnitude. 
     The third step of the reiterative process considers the manikin&#39;s positioning in pitch, that is its forward/backward positioning relative to the sound source as illustrated in FIG.  5 . To consider the manikin&#39;s position in pitch relative to the sound source, channel  1  at  304  in FIG. 3 is disconnected from the left ear and connected to the electrical input of the sound source  501 . In considering pitch, the interaural time delay is no longer measured, rather, the time delay between the sound signal leaving the source and reception at the ear is measured. For pitch angle determination, the phase difference between the front transfer function and the rear transfer function is used to calculate a delay, which corresponds to the difference between the distance the source to the right ear when the manikin faces forward and the distance from the source to the right ear when the manikin is facing backward. 
     FIG. 5 a  shows a forward facing manikin located at accurate pitch angle relative to the sound source  501 . FIG. 5 b  shows a manikin facing backward having an error in pitch. In FIGS. 5 a  and  5   b,  the sound source is shown at  501 , the center of rotation of the manikin is shown at  503  and the center of the head is shown at  502  with the distance between the manikin and the sound source shown at  505  and  504 , respectively. The delay from the source  501  to the right ear is longer in FIG. 5 b  where the manikin is facing backward than in FIG. 5 a  where the manikin is facing forward. The relatively longer delay produced by the configuration of FIG. 5 b  at an azimuth position of 180° indicates that the manikin has an error in pitch and should be tilted backwards by a part of the motorized stand  300  in FIG. 3 after receiving direction from the control computer  309 . Similarly, if the distance between the sound source and the manikin illustrated at  504  is less than the distance illustrated at  505  indicates that the manikin has an error in pitch and should be tilted forward. This measuring is repeated until the difference between the forward-facing and backward facing delays is less than 10 microseconds. 
     When all three criteria of the three step reiterative process are met—a time delay less than 2 microseconds in azimuth, a time delay less than 5 microseconds in roll and a time delay less than 10 microseconds in pitch—the manikin at  301  in FIG. 3 is considered to be centered. The preselected time delays are different for each axis and are chosen to ensure that the manikin is within 0.25 degrees in azimuth of directly facing the sound source and the center of the head is displaced no more than 0.25 cm from the axis of azimuthal rotation of the motorized stand. 
     If it is desired to move the source to a different elevation for further testing, the manikin can be positioned by employing step 1 of the three-step process without checking positioning for roll and pitch. 
     The invention provides a computer controlled, three-dimensional closed-loop system for automatically positioning the head of a manikin situated on a motorized stand relative to a stationary sound source. The positioning is responsive to acoustic signals measured from microphones located at each ear of the manikin and is desirable for accurate near field HRTF measuring. The invention fills a void in the art for measuring near-field HRTF because positioning techniques used in far-field HRTF measuring cannot be used with acceptable degrees of accuracy in near-field measuring. 
     While the apparatus and method herein described constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus or method and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.