Last year we explored the heart-shaped bass – shaping low frequency sound produced by an array of subwoofers from a sphere to a cardioid or heart shape. In celebration of National Cream-Filled Donut Day, let us add a little more sweetness to your plate by presenting the donut-shaped bass. Read on to learn more about this delicious phenomenon.
The heart-shaped bass works great for venues like rectangular auditoriums where the stage and audience are divisible by a straight line. But there are many other venue shapes that cannot benefit from this typical setup including arenas or drama theaters with a stage in the center and audience seating 360 degrees around it. In this configuration, subwoofers are typically mounted directly above the stage, and the preferred coverage pattern is a donut shape. This shape distributes the sound energy outward to the audience creating a “hole” over the stage that reduces the likelihood of microphone feedback issues.
Like the heart-shaped bass, the recipe for the low frequency donut takes advantage of wave interference. For the heart-shaped bass, one subwoofer is placed behind another in a horizontal plane. For the donut-shaped bass, the subwoofers are stacked vertically and suspended over the center of the stage. A time delay in milliseconds is applied to each cabinet at regular intervals resulting in the effect of low frequency sound distribution to the audience without excessive sound energy on the stage. An example of this setup is shown here.
The graphic below compares the polar responses of an omnidirectional sound source (ideal subwoofers) with and without the shaping. The “sphere” has the same sound level in all directions, but the “donut” is up to 20 dB lower in the center.
Formally, this type of coverage manipulation is called electrical steering. The models below compare the sound level plot of a subwoofer array in a venue. Without any steering, as shown below in the image on the left, the sound level on stage is several dB higher than the audience area, which is undesirable; but with the electrical steering technique in action, the sound level on stage drops significantly while the audience sound levels stay the same.
Besides creating a coverage hole over the stage like a flat donut, electrical steering can also create a tilted donut shape to better suit real venues. For example, in a large venue with sloped audience seats, the subwoofer array is likely mounted at or near the height of the top row. To address the audience at lower levels, a tilted donut shape could push the sound energy outward and downward at the same time. Based on the height and rake of the audience seating and the placement of the subwoofers, the tilt angle can be determined and the corresponding delay time interval can be calculated. Tilt angles of 10 to 30 degrees are common for most applications. A tilted donut also reduces the sound energy striking the upper walls or roof, which could create undesirable reflections. Additionally, for a stadium with an open roof, this technique reduces the noise pollution to the neighborhood by minimizing sound propagation.
Next time you walk into a performance venue, think about what shape of the bass would work the best in there. Heart? Donut? Jelly bean? No matter what, we can cook up the sweet spot for you!