The ideal number and placement of low frequency drivers in small listening rooms has been controversial. Most research has assumed listener satisfaction is determined by the sound pressure as a function of frequency and source-listener position. We believe two additional properties of the soundfield, externalization and envelopment, contribute to listener preference. We proposed mathematical methods for quantifying these two perceptual properties given a measured or calculated binaural impulse response. The average interaural time difference (AITD) is our measure for externalization, and the diffuse field transfer function (DFT) is our measure for envelopment. An image model for small rectangular rooms is used to predict the values of pressure, AITD, and DFT for different room properties and driver locations. It is found that the low frequency pressure uniformity, the AITD, and the DFT can be increased in the prime listening area by using multiple low frequency drivers - especially at the sides of the listeners. When playing material where 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 degrees. Listening tests confirm the results of the calculations.
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