1 Sune Mushendwa Aalborg University – Institute of Media Technology and Engineering Science Master Thesis: Semester 9 and 10, 2008-2009 Enhancing Headphone Music Sound Quality i Abstract Stereo music played through headphones comprises a narrow acoustic field which can sound unnatural and even unpleasant at times. Various attempts to expand the acoustic field have contained flaws which lead to deterioration in some aspects of the sound, such as excessive coloration and other unwanted tonal changes. In the present research a new way of achieving better sound quality for headphone music is presented. The proposed method diverts from the familiar techniques and aims to correct some of the current problems by using a balanced combination of assorted sound expansion methods. These include the use of amplitude panning, time panning, and particularly binaural synthesis. Using the stereo format as a reference context, preference selection tests are conducted to measure the validity of the proposed methods. Conclusions from the tests showed that there are several factors contributing to our preference inclinations in regards to spatial sound quality perception. 2 ii Table of Contents i Abstract............................................................................................................................... 1 ii Table of Contents ................................................................................................................ 2 iii Table of Figures ................................................................................................................. 5 iv Appendices ........................................................................................................................ 6 1 Introduction ......................................................................................................................... 7 1.1 Externalization v/s Lateralization .................................................................................. 7 1.2 3D sound technology in music.................................................................................... 10 2 Aspects of audio signals.................................................................................................... 12 2.1 Human Sound Perception .......................................................................................... 12 2.2 Impacts of acoustic environments on sound perception.............................................. 15 2.3 Sound Localization ..................................................................................................... 18 2.3.1 Interaural differences........................................................................................... 18 2.3.2 Head Related Transfer Functions........................................................................ 19 2.3.3 Source and head movements.............................................................................. 20 2.3.4 Distance Perception ............................................................................................ 21 2.3.5 Familiarity as a factor to sound localization ......................................................... 21 2.4 Effects of background noise on sound localization and externalization....................... 22 2.5 Effects of audio file sizes on localization and externalization ...................................... 23 3 Current sound expansion techniques ................................................................................ 25 3.1 Amplitude panning...................................................................................................... 26 3.2 Phase modification – Time panning............................................................................ 28 3.3 Binaural synthesis ...................................................................................................... 29 3.3.1 Reproducing binaural sound................................................................................ 29 3.3.2 Tonal changes in HRTF cues .............................................................................. 30 3.3.3 Externalizing music using binaural synthesis....................................................... 31 4 Evaluating sound quality.................................................................................................... 37 4.1 Sound quality ............................................................................................................. 37 3 4.2 What is music?........................................................................................................... 39 4.3 Sound quality evaluation tests .................................................................................... 40 4.3.1 Results from spatial sound quality evaluations .................................................... 40 4.3.2 Relevant sound quality evaluation test methods.................................................. 42 4.4 Considerations for sound quality evaluation in this project.......................................... 43 5 Research question ............................................................................................................ 46 Can a wider acoustic field enhance the listening experience of music played through headphones?........................................................................................................................ 46 6 Design and implementation of stimuli ................................................................................ 48 6.1 Design........................................................................................................................ 48 6.1.1 Choice of Stimulus .............................................................................................. 48 6.1.2 Song composition................................................................................................ 49 6.2 Implementation........................................................................................................... 50 6.2.1 Creating 3D sound by recording and by digital signal processing ........................ 51 6.2.2 Overview of arrangements in a soundfield........................................................... 53 6.2.3 Mixing and monitoring ......................................................................................... 53 6.2.4 Placement of vocals in the 3D soundfield............................................................ 55 6.2.5 Placement of the instruments in the 3D soundfield.............................................. 57 6.2.6 Mastering ............................................................................................................ 58 7 Testing and evaluation ...................................................................................................... 59 7.1 Pilot test ..................................................................................................................... 59 7.1.1 Implementation.................................................................................................... 59 7.1.2 Analysis, results and conclusions from pilot test.................................................. 62 7.2 Main Test ................................................................................................................... 64 7.2.1 General outline of test ......................................................................................... 64 7.2.2 Preference selection test – short sound clips....................................................... 65 7.2.3 Preference selection test – full song.................................................................... 66 7.2.4 Additional questions ............................................................................................ 67 8 Test results........................................................................................................................ 69 8.1 Preference selection test results – short sound clips .................................................. 69 8.2 Preference selection test results – full song................................................................ 74 4 8.3 Additional questions ................................................................................................... 76 9 General discussion, conclusions and future directions ...................................................... 80 9.1 General Discussion .................................................................................................... 80 9.2 Conclusions................................................................................................................ 82 9.3 Future research.......................................................................................................... 83 10 References .................................................................................................................... 85 5 iii Table of Figures Figure 1 - Externalization v/s Lateralization .............................................................................. 10 Figure 2 - Simplified diagram of an impulse response. ............................................................. 17 Figure 3 - Signal routing used in Wave Arts Acoustic Environment Modeling. .......................... 33 Figure 4 - Neumann KU 100 – Dummy-head Binaural Stereo Microphone. .............................. 34 Figure 5 – Sound image of the song "good friend".................................................................... 50 Figure 6 – Diesel Studios graphical