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Despite the marvelous electronic advances in digital hardware, sound must eventually travel the acoustic analog path from loudspeaker to our ears. Since we also have a brain attached to our ears, a very complex psychoacoustic process is involved in sound perception.

The sound that we hear in a critical listening room is determined by complex interactions among the quality of the electronics, the quality and placement of the loudspeakers, the hearing ability and placement of the listener, the room dimensions (or geometry if non-rectangular) and the acoustical condition of the room’s boundary surfaces and contents. All too often these factors are ignored and emphasis is placed solely on the quality of the loudspeakers. However, the tonal balance and timbre of a given loudspeaker can vary significantly, depending on the placement of the listener and loudspeaker and the room acoustic conditions. In some cases the differences between different loudspeakers located in the same location in a room can be less than the differences introduced by moving the same loudspeaker to different locations in a room.

The causes of acoustic distortion are:

Modal Coupling - The acoustical coupling between the loudspeakers and listener with the room’s modal pressure variations or room modes

Speaker-Boundary Interference - The coherent interaction between the direct sound and the omnidirectional early reflections from the room’s adjacent boundaries


Comb Filtering - The coherent constructive and destructive interference between the direct sound and early reflections

Sound Diffusion - The spatial and temporal reflection pattern due to mid and late arriving reflectionsThe acoustic distortion introduced by the room can be so influential that it dominates the overall sonic impression.

The home theater is essentially a "small" room acoustically. The volume is approximately 3,000-4,000 cubic feet). The decay time is roughly 100 to 400 ms. The room’s acoustical signature is strongly characterized by its lowfrequency modal response and speaker-boundary interference, strong early reflection interference from surfaces, consoles and equipment racks, flutter echoes from untreated parallel reflective surfaces, sparse late reflection density and spatiality leading to poor sound diffusion and envelopment.

This presentation will describe the causes and effects of acoustical distortion, but depart from traditional pedagogy, by introducing a new computer algorithm, made possible by the continuing increase in desktop processing power, to automatically minimize these effects.

While there has been much discussion about the influence of room dimensions on the frequency distribution of room modes, less attention has been given to a parallel effect caused by the coherent interaction between the loudspeakers and the early reflections from surrounding surfaces. Typically these two orthogonal effects have been discussed independently, when in fact that must be minimized simultaneously. This can be accomplished, simply by properly positioning the loudspeakers and listener. By combining the speed and power of computer image modeling in a rectangular room, with multi-dimensional optimization techniques, i.e. intelligent search engines, one can simultaneously minimize the speaker-boundary interference and modal responses. The metric that is minimized is the standard deviation of the combined room responses, which has been perceptually equated to listener preference.

In addition to minimizing the low-frequency distortion caused by room modes and speaker- boundary effects, the image model can also identify the locations for acoustical surface treatment to minimize comb filtering and increase the diffusion.

A systematic approach to home theater design will be presented in which the room dimensions, and locations for loudspeakers, listener, and acoustical absorptive and diffusive surface treatments are optimized. Examples of the two current surround formats, namely dipole surrounds and matching monopoles in an International Telecommunicatio ns Union (ITU) configuration will be presented.