Special Topics: Working With Dissimilar Subwoofers
When the subwoofers you're using aren't identical, extra steps are needed to ensure that the optimization performed by MSO does not degrade maximum output substantially. This might occur if you follow some recommendations to have one large, high-output sub ("super sub"), along with several other smaller, lower output subs ("helper subs"). Unless you take extra steps, MSO might apply too much gain to a helper sub that's not capable of the corresponding output. These steps consist of the following.
- When setting up subwoofer gains before measurement, set them so the helper subs are at the highest permissible level relative to the super sub and no higher.
- Apply the appropriate constraints to the gains on the helper subs to prevent these gains from increasing relative to the super sub gain in the as-measured condition.
These guidelines are based on the assumption that maximum output should be limited only by the super sub, not the helper subs.
The first step of setting up the relative gains of the subs before measurement is straightforward only if you've designed the subs and can produce a plot of the cone excursion vs. frequency for a given voltage amplitude. If you have this information, you can set the gains of the helper subs relative to the "super sub" such that for a sine wave sweep, all subs have the same ratio of maximum peak cone excursion over frequency to maximum allowable cone excursion xmax. Without such information, some guesses must be made to determine the relative gains.
The second step of applying constraints to sub gains should be done as below in Figure 4.
The super sub channel has no gain block, so its gain value is forced to 0 dB. The leftmost gain block adjusts the level of all subs relative to the mains simultaneously. The maximum allowable gain for the helper subs is set to 0 dB using the Properties Window. Since step 1 of the recommendations above sets the as-measured condition such that the helper subs are at their maximum permissible level relative to the super sub, all helper subs are now constrained to be below this level after frequency response optimization.