Image Source Method

The image source method is a computational technique for calculating early reflections in room acoustics by creating virtual "image" sources mirrored across each reflecting surface. For a first-order reflection, the image source is located at the mirror position of the real source relative to the reflecting wall. Higher-order reflections require cascaded mirroring across multiple surfaces. The method produces geometrically exact reflection paths, arrival times, and energies, making it ideal for calculating early reflections (first 50–80 ms) that are critical for clarity and intelligibility metrics. The computational cost grows exponentially with reflection order, so practical implementations limit calculations to 2nd–4th order reflections. The image source method is typically combined with ray tracing in hybrid algorithms: image sources handle early reflections accurately while ray tracing efficiently computes the late reverberant field. This hybrid approach is standard in professional simulation software including ODEON, CATT-Acoustic, and Ramsete.