# Image Theory Applied to virtual Microphones

Virtual sensing is the art of measuring a quantity at a certain spatial position without having a physical sensor placed at that exact same position. In practical applications, this technique is sometimes very useful, when it is not feasible to put the sensor at the position where the physical quantity should be measured.  In order to get statistical relevant data, the measurement has to be carried out over a period of time. However, it is not a feasible approach to use a large number of microphones hanging from the ceiling at the students head level, over a longer period of time for obvious reasons. One thoughtful solution, in such a situation is to put microphones at the walls and close to the ceiling and using the virtual technique to calculate the noise level at the students head positions based on the measure data. An advantage in such large setup is that it often requires less physical sensors than the number of virtual measurement positions and at the same time features a better validation and sanity check of the data. The most popular application areas for virtual microphones are in active noise control (ANC) or active structural acoustic control (ASAC), where the aim is to move the zone of quietness away from the physical error microphones to the desired location of maximum attenuation. However, there is an important difference between virtual sensing for active
control and virtual sensing for a pure measurement situation concerning the delays introduced by virtual sensing technique.

Contents

INTRODUCTION
CHAPTER 1: FUNDAMENTALS OF ACOUSTICS
1.1 SOUND BEHAVIOUR
1.1.1 Ideal wave propagation in open field
1.1.2 Enclosure spaces
1.2 BUILDING MATERIALS
CHAPTER 2: PROBLEM DEFINITION AND METHODOLOGY
CHAPTER 3: THEORETICAL WORK
3.1 RAY TRACING
3.2 IMAGE THEORY
3.3 IMPULSE RESPONSE
CHAPTER 4: MATLAB IMPLEMENTATION
4.1 RAY TRACING
4.1.1 Ray tracing 2D
4.1.2 Ray tracing 3D
4.2 IMAGE THEORY 3D
4.3 IMPULSE RESPONSE
4.4 MULTI IMAGE THEORY
CHAPTER 5: RESULTS
CHAPTER 6: DISCUSSION / ANALYSIS
REFERENCES
APPENDIX I
APPENDIX II: IMGTHRY3D.M
APPENDIX III: MULTI_IMGTHRY3D.M

Author: Carlos Hernandez Matas and Diana Gomez Olmedilla

Source: Blekinge Institute of Technology

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