Multi-Sub Optimization Multi-Sub Optimization (Multiple Subwoofer Optimization)

Mechanism

Multi-Sub Optimization (MSO) refers to the use of multiple subwoofers, positioned and filtered specifically to reduce room-mode-driven seat-to-seat variation in low-frequency response, rather than simply adding output. A single subwoofer can be positioned and equalized to produce a smooth frequency response at one listening location, but locations even a few feet from that "sweet spot" can differ greatly from each other and from the main seat. With only one subwoofer, this is unavoidable.

The underlying acoustic principle (using multiple loudspeakers in a room to reduce standing waves through destructive interference) was first suggested by Floyd Toole; some have called this "mode canceling." With two subwoofers positioned appropriately, each excites a series of standing waves, but the acoustical response of the odd-order modes at one side of the room is 180° out of phase relative to the other side, so the odd-order modes are driven destructively and the two subwoofers effectively cancel each other for those modes. The even-order modes from each subwoofer, however, remain in phase, so no mode canceling occurs for those standing waves in a simple two-sub arrangement.

Key metrics and optimal placement

A 2006 Journal of the Audio Engineering Society paper by Todd Welti and Allan Devantier (Harman International) defined three metrics for characterizing subwoofer–room low-frequency performance: Mean Spatial Variance (MSV), Variance of Spatial Average (VSA), and Mean Output Level (MOL). MSV is the variance of sound level in dB as a function of seating location (typically four to six seats), calculated at each frequency and then averaged across frequency. It is a measure of seat-to-seat consistency in amplitude response.

By analyzing over 100,000 subwoofer configurations in rectangular rooms, for arrangements of one to four subwoofers, the study found that placement at wall midpoints is optimal. Four subwoofers at wall midpoints was the best practical configuration in terms of MSV. Two subwoofers at opposing wall midpoints performed nearly as well and also provided stronger low-frequency output; configurations with more than four subwoofers were not found to be advantageous, particularly once cost is factored in.

Software such as MSO (Multi-Sub Optimizer) computes the delay, gain, and parametric EQ (PEQ) filters needed for each subwoofer from measurements taken with Room EQ Wizard (REW). MSO performs these calculations using the Superposition Principle (a generalization of REW's "A+B" measurement processing function), which assumes a linear system in which subwoofer distortion is negligible.

Comparisons and real-world use

Without bass management (summing correlated bass to a common signal before distributing it to subwoofers), seat-to-seat variations of 40 dB or more at some frequencies can occur, or similarly large variation at one seat across different frequencies. For comparison, in the rectangular rooms Welti and Devantier investigated, the response dip at a modal null with a single subwoofer is typically in the −25 dB range, versus a theoretical negative infinity dB with no damping.

Trinnov describes its WaveForming technology as using multiple front and rear subwoofers (a minimum of four: two front and two rear) to create a single coherent bass wavefront steered toward the listening area while minimizing wall, floor, and ceiling reflections, with the aim of preventing room-mode problems rather than correcting for them after the fact. Trinnov's own materials do not name or directly compare WaveForming against MSO, SFM, or Dirac Live Bass Control (DLBC).

Common confusions

MSO is not the only named implementation of the multi-subwoofer optimization concept. The Welti/Devantier paper describes three methods for reducing seat-to-seat low-frequency variation with multiple subwoofers: standardized positional optimization, adaptive or analytically derived high-order filters, and Sound Field Management (SFM), a method described as patent pending at the time. SFM uses only subwoofer placement and "very simple signal processing" and, unlike some other methods, can be used for any room shape.

Adding more subwoofers does not always help. The Welti/Devantier analysis found configurations beyond four subwoofers were not advantageous, especially considering cost. Multi-sub optimization is also distinct from generic bass management. Bass management refers to summing correlated bass-managed channels (e.g., LFE plus bass-redirected channels) to a common signal before distributing it to subwoofers; virtually all methods of optimizing subwoofer frequency response in rooms, including MSO, assume this single-channel-source condition as a starting point rather than as the goal itself.