The ideal number and placement of drivers in small listening rooms has been controversial. Most research has examined only the pressure (loudness) as a function of frequency and source'listener positions. We believe that two additional properties of the sound field'externalization and envelopment'are important to listener preference. Externalization is solely a low-frequency problem, and the interaction between envelopment and loudspeaker position is particularly important at low frequencies. In a previous paper, we proposed a mathematical method of quantifying these two perceptual properties given a measured or calculated binaural impulse response. The average interaural time difference (MTD) is the measure for externalization, and the diffuse field transfer function (DFT) is the measure for envelopment. This paper will present a simple image model for rectangular rooms, and use it to study the interaction between multiple drivers and listening rooms. The image model is used to predict the values of pressure, AITD, and DFT for different combinations of room properties and driver locations. It is found that the low-frequency pressure uniformity and the AITD can be increased in the prime listening area by using multiple low-frequency drivers, especially at the sides of the listeners. When playing material in which the bass energy is primarily monaural, the drivers on the left side of the room should lead or lag the drivers on the right side by a constant phase angle of 90°. In rooms where the lateral reflectivity is below 0.65, DFT is also increased by multiple woofers. Once again a side placement is optimal.
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