Multi-Sub Optimizer Tutorial

Introduction to Multi-Sub Optimizer

This tutorial will show you how to use Multi-Sub Optimizer (MSO) and briefly introduce you to the theory behind it. Using data from a set of measurements that have already been performed, you'll learn how to set up an MSO project and run the optimization without having to do any measurements beforehand yourself. This will get you familiar with MSO first, so you'll be ready to optimize your system as soon as you perform the necessary measurements on your own setup.

Even if you are an experienced MSO user, you can benefit from at least skimming the tutorial to get familiar with the Measurement Import Wizard and the Configuration Wizard, first introduced in version 1.1.0. Creation of new configurations using the wizard-driven approach is faster than the old approach, even for experienced users.

What is Multi-Sub Optimizer?

Multi-Sub Optimizer is free software that performs multiple subwoofer integration, and can optionally integrate the subs and main speakers. Given a set of individual subwoofer and optional main speaker measurements, it performs the following tasks automatically.

It does this by calculating gain, delay, and equalization (EQ) individually for each subwoofer. This technique is more powerful than the traditional EQ approach described below.

What is the Traditional EQ Approach?

When you place subwoofers and main speakers in a room and the measure their frequency response, room modes in the modal region (typically 200 Hz and below, depending on room dimensions) cause large peaks and dips in the measured frequency response. A typical way to tackle this problem is to use a digital signal processing (DSP) device that provides parametric EQ for the subwoofers, or the built-in "room correction" of an AV receiver. The conventional approach is to apply the same EQ to all subwoofers in order to flatten the measured response at the main listening position (MLP), or to flatten a response curve calculated as the average response over multiple listening positions. After this step, you'd typically adjust the subwoofer distance and level settings in the AVR or preamp/processor for best integration of the subs and main speakers. After each such sub distance or level adjustment, you'd make a new measurement to determine if the result is good enough. In a more sophisticated approach using a DSP device, you might set individual delays for each subwoofer. These delays are often calculated once based on the relative distances of each sub from the main listening position, and left at their calculated value. This step alone can make a significant change in the response of the combined subs but it does not guarantee optimum results.

What are the Problems with the Traditional EQ Approach?

At the specific frequencies of the room modes, the frequency response may have peaks at some listening positions and dips at others. Unless you're very lucky, flattening the response at the main listening position will make it worse at others. Also, when integrating the main speakers and subs, each new adjustment of subwoofer distance or level requires a new measurement to determine if the integration of main speakers and subs is good enough. That can become very time consuming.

How is MSO Better than Traditional EQ?

As mentioned earlier, MSO is capable of optimizing the system response in the modal frequency region at multiple listening positions simultaneously. This is different than, say, taking an average of the responses at multiple listening positions and equalizing that. Using MSO, it's possible to reduce the seat-to-seat variation of the system response in the modal region.

How is the reduction in seat-to-seat response variation possible? Consider the two hypothetical systems described below.

When going from System A to System B, it isn't just the frequency response in the modal region that changes. For a sinusoidal input to System B at a frequency covered by the subs, the spatial distribution of acoustic pressure and particle velocity at that frequency is altered relative to System A, due to the position-dependent changes in the interaction of the subs with one another. System B is a mode manipulation technique. In section 8.2.8 of the third edition of Floyd Toole's book Sound Reproduction, Dr. Toole calls this idea "Mode Manipulation for Any Room Using Multiple Subwoofers and Signal Processing". To a certain extent, this sort of mode manipulation occurs when only individual sub gain or delay is altered, but the effectiveness of the mode manipulation is maximized when the delay, gain and EQ can all be adjusted on a per-subwoofer basis.

It is this alteration of the spatial distribution of acoustic pressure that allows the seat-to-seat frequency response variation of System B to be reduced relative to System A. In a similar way, bad choices of individual subwoofer EQ, gain or delay can actually make the seat-to-seat frequency response variation of System B worse than that of System A, so it's important to choose the relevant parameter values carefully. That's a role for which automatic optimization software is well-suited.

System B above, which is what MSO requires, would be very difficult and time-consuming to optimize by hand because of the complex interaction of the subs with one another. Both the magnitude and phase of the sub responses must be taken into account in the calculations. For each new guess of EQ, delay or gain parameters, a new measurement would need to be taken at each of the listening positions. By contrast, MSO will do millions of iterations if needed. Once the original measurements have been taken, no more measurements are necessary until it's time to verify the new DSP configuration calculated by MSO and loaded into the hardware.

Stated differently, if you had four subs to be optimized at four listening positions and wanted to evaluate a million combinations of EQ, gain and delay parameters, manual optimization would require four million measurements (a million EQ settings with combined sub responses measured at four listening positons), while MSO would require only sixteen. Obviously, such a manual optimization would be impractical, but it's very typical of what an MSO optimization does.

What is the History of Multiple Subwoofer Optimization?

The history of multiple subwoofer optimization is discussed in a separate topic.

How Does MSO Perform its Calculations?

MSO uses optimization code to adjust the parameters of simple IIR filters for each sub and optionally one or more main speakers. In addition, it can adjust delay and gain parameters (but only for subs, not main speakers). These adjustments try to minimize a calculated error value that's a measure of the deviation of the computed frequency responses at multiple listening positions from some ideal condition. It provides three modes of optimization.

  1. A mode for which the calculated error represents the combined deviations of the computed responses over frequency from a desired target curve at multiple listening positions. If no target curve is specified, a flat response is assumed. This has been historically referred to as the "flatness error", even though the target curve might not necessarily be flat.
  2. A second mode that combines two components of error together to form a composite error that it tries to minimize. These error components are:
    • The RMS error of the main listening position's response from the desired target curve.
    • An error that represents only how much the computed responses vary from one another from seat to seat.
  3. A third mode introduced in version 1.0.46 that only tries to minimize the seat-to-seat response variation without trying to fit any target curve at all. This is useful for users who e.g. might only be using per-sub delays because of a desire to maximize SPL.

For a detailed description of how the errors are calculated, see the error calculations section in the reference manual.

General Considerations

MSO supports configurations consisting only of subs, and also configurations consisting of subs plus satellites. When a "sub-only" configuration is used, only the computed response error of the subs at multiple listening positions is minimized. For a configuration consisting of both subs and satellites, the composite error includes both the subs and main speakers. In this case, MSO optimizes the integration of subs and main speakers at multiple listening positions. Regardless of the optimization method chosen, when a configuration includes both main speakers and subs, the optimization of the subs and their integration with the main speakers are all done in one step in order to maximize the usage of the limited EQ resources available in low-cost DSP units.

What Measurements Do I Need to Take?

The measurements described below are what you need to perform an optimization using your own data. If you're just running the tutorial, you won't need to perform any measurements first, as the measurement text files needed for import into MSO are contained in the tutorial download.

For each listening position you wish to measure, you need to measure the frequency response of each sub individually at that position. If you are using MSO to integrate the mains and subs, you must also measure the main speaker(s) at each listening position. When using Room EQ Wizard (REW) with an HDMI interface and a USB microphone, you must use the REW acoustic timing reference to obtain time-synchronized measuremnts. The description on the measurements page gives suggestions for measuring both left and right main speakers and subs or just a center speaker and subs. You'll need to keep track of which measurement is which for later reference. When using MSO to optimize both the subs and their integration with the main speakers, HDMI channel 4 must not be used when energizing the subs for the measurements. When using MSO to optimize subs only, using HDMI channel 4 to energize the subs for the measurements is okay.

After you have performed the measurements, you'll need to export your measurements as text files. In REW, this is most easily done using File, Export, Export all measurements as text from the REW main menu. For more details, see these instructions. If you have e.g. four subs, and you're integrating them with left and right main speakers, you'll have six measurements per listening position. You'll need to keep track of which speaker or sub was being measured, and what the listening position was via careful choice of file names.

Running the Tutorial

The tutorial is designed so you won't need to do any measurements first. Just download the sample files. They contain all the measurement data you'll need, exported from REW into text files that are ready to import into MSO.

The present tutorial is new as of version 1.1.0. Starting with version 1.1.0, many ease-of-use improvements and productivity enhancements have been incorporated into MSO. This includes a wizard-driven approach for importing measurements and creating configurations. The tutorial has been completely rewritten to illustrate the use of these techniques. The original tutorial has been preserved elsewhere.