We all know that pushing your subwoofers together gives you more SPL and less power alley. But what if you want to pull them apart to make the line longer for more narrow coverage? Imagine a long, narrow room. You’ve got 4 subs. Each of those subs is 3.8 feet wide, giving you a line of 15.2 ft total. That’s going to give you directional control down to 74Hz. Not bad. But what if you are crossing over at 80Hz? You need more control. Prediction @ 30Hz In that case, you could push the subs apart 8 feet, giving you 24 extra feet of line length, for a total of 39.2 feet. That gives you directional control down to 30Hz without breaking the pattern. Woot! 30Hz with 8 ft spacing How did I figure that out? You will always have some amount of subtraction when two frequencies meet at equal level between 120º and 240º of phase offset. What you need to do is make sure that your speakers are within 2/3 wavelength of the highest frequency at which they could combine. In our case, we are crossing over at 80Hz, but to be safe I used 90Hz for the calculation since the Sub and Main won’t be level isolated right at 80Hz. Then I found the wavelength of 90Hz, which is 12.5 feet. After that, all I needed was 2/3 of that, for 8.3 feet, which I rounded down to 8 feet. Those steps again: 1. F = the critical operational range of your subs 2. max distance from center to center = (speed of sound / F) * (2/3) Warning: the pattern will narrow as frequency rises. The tradeoff of pattern control down to 30Hz is that you may now be too narrow at 80Hz. The solution? Create a physical or delayed arc. Further questions? Click here to download my new eBook, 105 Questions about Sound System Tuning, free until Nov 20th. It’s everything you wanted to know about live sound system setup, but were afraid to ask.
Words of wisdom from those in the know.
When doing live shows, there are two things that can make setting up and patching the stage go smoothly and efficiently. They are an input list and stage plot.
There is such a huge disparity between digital surfaces and systems from one console to another- it can be overwhelming to know where to start on a board you haven’t used before.