Devatier “Low-Frequency Optimization Using Multiple Subwoofers

At low frequencies the listening environment has a significant impact on the sound quality of an audio system. Standing waves within the room cause large frequency-response variations at the listening locations. Furthermore, the frequency response changes significantly from one listening location to another; therefore the system cannot be equalized effectively. However, through the use of multiple subwoofers the seat-to-seat variation in the frequency response can be reduced significantly, allowing subsequent equalization to be more effective. Three methods to reduce seat-to-seat variation are described, including a novel approach based on simple signal processing. The desired result in each case is to allow the system to be equalized over a seating area rather than just one seat. Results are shown for several listening rooms. INTRODUCTION The advent of home theater and multichannel audio has placed significant demands on the low-frequency performance of audio systems. First, theatrical low-frequency effects combined with digital media have increased the maximum output requirements well beyond those of the stereo LP era. Second, home theater and multichannel audio have transformed listening from a solitary event into a social one. As a result the low-frequency performance needs to be optimized over a seating area rather than at a single location. Low-frequency equalization over an area is complicated by the wide seat-to-seat frequency response variations that are caused by the standing waves within the listening environment. A fundamental precept of this paper is that optimization is achieved when the responses at multiple seats are as similar to each other as possible. Making them flat is then simply a matter of global equalization. This consistency of amplitude response is a fundamental goal for the current investigation. A common feature of consumer multichannel audio systems is bass management, which redirects the lowfrequency information from all the audio channels into a single subwoofer channel. This low-frequency signal can be manipulated and distributed to a number of subwoofers located strategically in the listening room. From an intuitive standpoint it seems likely that putting a large number of subwoofers at different locations in the room might excite room modes in a more "balanced" manner than a single source. Typical approaches to this problem have involved exciting the standing waves within the room equally but out of phase, or trying not to excite them at all. There is not much agreement on how many subwoofers are required, or where to place them. This paper outlines three methods of using multiple subwoofers to reduce seat-to-seat amplitude response variations so that the system can then be equalized effectively over the listening area. One method assumes a rectangular room and involves using standardized subwoofer locations. We may refer to this as standardized positional optimization. The second method can be used for any shape room, and uses analytically or adaptively derived highorder filters to try to match a target curve at the seats. The third method, sound field management (SFM), 1 can also be used for any shape room but uses only subwoofer placement and very simple signal processing. The second and third methods are based on in-room measurements. This paper focuses on positional optimization and SFM because of their relative ease of implementation and certain other advantages, which are outlined in this paper. *Manuscrip