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Binaural Hearing- Human Ability of Sound Source Localization MEE09:07 Binaural Hearing- Human Ability of Sound Source Localization Parvaneh Parhizkari Master of Science in Electrical Engineering Blekinge Institute of Technology December 2008 Blekinge Institute of Technology School of Engineering Department of Signal Processing Supervisors: Dr. Nedelko Grbic Erik Loxbo Examiner: Dr. Nedelko Grbic Blekinge Tekniska Högskola SE–371 79 Karlskrona Tel.vx 0455-38 50 00 Fax 0455-38 50 57 Abstract The purpose of this project is to desig a systematical method in order to measure human directionality ability in horizontal plane with a single sound source. A completely virtual auditory model has been created in Matlab. The project consists of modeling binaural cues, designing digital filters, designing a test workbench, measuring listener's directionality and analyzing the data. The head related transfer function (HRTF) is computed by calculating the two most important binaural cues, interaural level difference (ILD) and interaural time difference (ITD). The platform is made in Matlab and all results have been shown by plots produced from Matlab code. The directionality test has been done with real human subjects and the results have been analyzed and presented. I II Table of Contents Page Abbreviation 1 Introduction 3 Background 5 1. Binaural Perception 7 1.1 Binaural cues 7 1.1.1 Interaural Time Differences 7 1.1.2 Interaural Level Differences 9 1.2 Head Related Transfer Function 10 1.3 Minimum Audible angle 13 1.4 Cone of Confusion 14 2. The Spherical head model 15 2.1 Modeling ITD 15 2.2 Modeling ILD 20 2.2.1 ILD Approximation in Spherical Head Model 21 2.3 The HRTF in SHM 23 3. The Virtual Auditory Model 25 3.1 Calculating ITD 26 3.1.1 Time Delay Filtering 27 3.1.2 The FD-MF all pass filter 30 3.2 Calculating ILD 31 3.3 The Generated HRTF 32 III 4. The Directionality Test Work Bench and Test Equipments 4.1 The GUI Interface 35 4.2 Test Requirements 37 4.2.1 The ASIO Sound Card 38 4.2.2 The Matlab Audio Processing Framework 38 4.2.3 The Calibration 39 4.2.4 The Test Environment 40 5. The Directionality Test and The Error Calculation 43 5.1 The Measurement Method 43 5.2 The Test Procedure 43 5.2.1 The Test Signals 44 5.2.2 The Subjects 45 5.3 The Experiment 46 5.3.1 Average Directionality Error 46 5.3.2 The Audiogram 49 5.4 Data Analysis 48 5.5 Improvement 50 -Conclusion 51 - Future Work 51 -Appendix A 53 -References 55 IV Abbreviations ASIO: Audio Stream Input/ Output FD: Fractional Delay GUI: Graphic User Interface HRIR: Head Related Impulse Response HRTF: Head Related Transfer Function IID: Interaural Intensity Differences ILD: Interaural Level Differences IPD: Interaural Phase Differences ITD: Interaural Time Differences MAA: Minimum Audible Angle MF: Maximally Flat SHM: Spherical Head Model 1 2 Introduction Binaural hearing is human and other animal's ability to judge direction of a sound source. As long as man has lived on Earth he/she has been able to localize the sound source(s) by using two ears. Wide research has been done on binaural hearing in many advanced laboratories during last century. Many of them have worked with dummy heads and some of them have worked with humans. This thesis has focused on some of the recent researches and uses one of the existing models to determine a method for measuring human’s directionality. The thesis scope is the horizontal plane and the binaural cues (ITD and ILD) have been simulated in azimuth. The "spherical head model" is one of the oldest and the easiest but the most powerful model that has been considered for creating the virtual auditory model. This thesis does not discuss about physiology of hearing and hearing organ. The investigated area is just between a sound source and entrance of pinna. 3 The assumptions are using a single sound source, working on horizontal plane in the front semicircle. We also suppose that 0 is at right ear, 180 is at left ear and 90 is in front of the head. The details of the work are discussed in following sections. In the background section there are some turnovers on recent researches. Binaural perception, binaural cues, head related transfer functions (HRTF) have been discussed in chapter 1. In chapter 2 the Spherical head model is been explained. The virtual auditory model and digital filter design and some calculations, have been put in section 3. The test workbench and the test equipment are presented in chapter 4 and chapter 5 consists of the binaural measurement and analysis of the results. 4 Background Lord Rayleigh (John William Strutt) found the localization process during 1877-1878. He noted that if a sound source is in ipsilateral ear, then the head makes a shadow cast in contralateral ear. Therefore, the signal in the contralateral ear is been more attenuated than the ipsilateral one. He also noted that different parameters affect on localization at low and high frequencies. His theory was named "Duplex theory" and it is valid to now, of course with some extensions. Many models of binaural processing were created over the last century. "Spherical head model" (Lord Rayleigh, 1907 and Woodworth/Schlosberg, 1954), “direct Cross-correlation of the stimuli model” (Sayers and Cherry, 1957), “The binaural cross- correlation model ” (Jeffress, 1956), “direct comparison of the amount of the left-sided and right-sided internal response to stimuli model” (Bergeijk, 1962), ”interaural comparison auditory–nerve activity” model( Colburn, 1973, 1977) and many other models were created [12]. Many other researchers studied other aspects of the binaural hearing such as multi channel sound sources, moving sound sources, noise reduction and so on. Spherical Head Model (SHM) that will be presented in this project is the first binaural model and it was born in the first of the last century. Rayleigh's SHM (1907) was so simple. Woodworth 5 and Schlosberg (1954) calculated binaural cues in polar coordinate system [5]. Joel David Miller (2001) modeled the spherical head in Cartesian coordinate system [10]. 6 1. Binaural Perception 1.1 Binaural cues There are two important binaural physical cues in the horizontal plane. These two cues are: 1. Interaural time differences (delays), ITD and 2. Interaural level (intensity) differences, ILD or IID. 1.1.1 Interaural Time Differences The difference in arrival times from a sound source in ipsilateral and contralateral ear is called ITD. ITD happens because sound waves arrive to one ear earlier than another one. ITD is the dominant cue at frequencies lower than 1500 Hz. The wavelengths of frequencies lower than about 1.5 KHz are comparable with the human head size. The minimum ITD is zero and the maximum perceptible ITD is about 600-800 µs. Figure 1.1 shows a simple single source spherical head model with head radius a and azimuth θ. In Rayleigh's spherical head model with a sound source at infinity, ITD has a simple explanation. He obtained the following formula for ITD: 7 Horizontal M P plane e l a d n i a e n θ θ θ a a Ipsilateral lateralContara Ear Ear Figure 1.1- Rayleigh's spherical head model in horizontal plane a ITD ( sin) 2/ 2/ c (1) Here c is speed of sound (approximately 343 m/s) and θ is the angle between the line which has connected the sound source to the head center and the median plane in radian. With this formula the ITD is zero when the sound source is in front of the head and is .257a/c , when the sound source is located at one of two ears at the sides. ITD is more sensitive in near-field (less than 1 meter source distance) than far-field. It is seen in the formula that ITD is frequency independent, but in some other binaural models it is dependent on frequency. The position of a sound source at distance dis from the center of the head in a SHM has been shown in Figure 1.2. 8 dis θ Vertical plane Ipsilateral a Contaralateral Ear Ear Figure 1.2- A sound source at distance dis from the center of the head in spherical head model in horizontal plane 1.1.2 Interaural Level Differences The difference in sound pressure levels or intensities in ipsilateral and contralateral ear is called ILD or IID respectively. ILD is a dominant cue at frequencies higher than about 1500 Hz but generally affects the contralateral signals of all frequencies. ILD happens because the head makes a shadow cast in contralateral ear. The ILD dependency to frequency is illustrated in Figure 1.3. ILD is nonlinear with frequency and is strongly dependent on frequency over audible spectrum because sound waves are scattered when the head diameter is larger than the wavelengths and diffraction increases rapidly with increasing frequency. 9 250 Hz Head Shadow 6 KHz Figure 1.3- The head-shadow effect at high frequencies and ILD dependency to frequency and position The smallest detectable ILD is about 0.5 dB, regardless of frequency. The far-field ILD doesn't exceed 5-6 dB whereas the near-field ILD, for example, at 500 Hz exceeds 15 dB [2]. 1.2 Head Related Transfer Function The transformation of a sound signal from a sound source to a listener's ears is called Head Related Transfer Function (HRTF) or Anatomical Transfer Function (ATF). HRTF is a function that characterizes and captures the binaural cues for sound localization. HRTF is an individual function for every person and every sound source location.
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