Achieving balanced coverage of bass frequencies in studio control rooms has always been an issue for engineers and producers. While a track may sound great at the desk, those on the couch behind it often hear a very different mix.
The problem is room modes, which are caused by sound waves, particularly bass frequencies, reflecting off of various room surfaces. In small rectangular spaces (less than approximately 25 by 25 feet), room modes can create differences in sound levels. As these waves bounce between surfaces, they can cause areas where low frequencies cannot be heard while, just a few feet away, they can be too loud.
Positioning four subwoofers in the four corners of a control room can increase seat-to-seat consistency and bass efficiency.
Larger rooms have lower frequencies at which room modes exist so, in a theater, for example, room modes might only become an issue below 10–15 Hz, which is below the frequency at which people can easily hear. In small rooms, however, problematic room modes can reach 100 Hz or more and are very audible.
Groundbreaking research by HARMAN Senior Principal Engineer Todd Welti has shown that using a single subwoofer to reproduce bass frequencies in small spaces is often at the heart of the problem. The configuration of two monitors and one subwoofer has become a trend in studio speaker setups. But, according to Todd, using only a single subwoofer intensifies room modes and causes low frequency inconsistencies. Todd discovered that strategically positioning four subwoofers in the corners of a room effectively cancels some of the room modes. He uses the analogy of riders on either side of a teeter-totter, both trying to push up or down at the same time and not moving at all. Similarly, by using multiple subwoofers, some room modes are canceled out.
Although it is possible to minimize room modes by using other numbers of subwoofers (two or five, for example), with four, it is easiest to position them in a way that results in more uniform sound throughout the room.
When using four subwoofers, it’s necessary to either lower each cabinet’s gain or use smaller subwoofers, so they each produce a quarter of the desired output. In many cases, four subwoofers are actually more efficient, meaning they can each provide less than a quarter of the acoustical output of a single subwoofer and create the same sound level at different seats in a small room. Key to the system’s success is making sure the four subwoofers are identical models with identical settings.
For non-rectangular rooms and situations requiring a more powerful solution, Todd and his team have developed the technology to further optimize multiple-subwoofer systems. After making a series of measurements of each subwoofer at each seat in the studio, in just a few minutes, a computer using proprietary JBL Professional software can analyze up to 20 million combinations of subwoofer gains, delay settings and simple filter settings. It continually adjusts the parameters until it lands on the solution that affords the most consistency in all seats. This is called Sound Field Management™, which is currently only available to professional installers.
Learn more by reading Todd Welti’s white paper, Subwoofers: Optimum Number and Locations.
Are you a recording engineer who has experienced challenges with reproducing bass frequencies? Let us know how you solved them in the comments.
