Further understanding in the quest to deliver LF energy with maximum impact and control...
January 10, 2017, by Craig Leerman
Today’s music and special effects (like explosions in motion picture soundtracks) include a lot of low-frequency content. While larger full-range loudspeakers may have a wide frequency range, it takes subwoofers to really reproduce low end with impact, especially for bass-heavy music like electronic dance (EDM) and reggae.
Deployment of main loudspeakers is usually a relatively straightforward matter, but locating and configuring subs presents numerous options and can be a bit of a challenge. Let’s start with the basics.
Subwoofers are specialized cabinets that reproduce the extreme low end of the frequency range. Studio and home theater models may operate in the range of 20 to 250 Hz, while subs for sound reinforcement usually operate around 30 to 120 Hz, with 80 to 100 Hz being common crossover points. They can be passive, fed by external amplifiers and processors, or active, with onboard amplification and processing.
Smaller systems may send the subs the same main output feed as the full-range mains, while larger setups might receive content only from the instruments with LF content like kick drum, bass guitar or even a bass vocalist, usually sent via an aux send on the console. This can help clean up “muddy” sounding low end because it eliminates the open microphones on stage sending signals to the subs that can be out of phase with each other.
There are several different types of subs, with each design being a trade-off between bandwidth, efficiency, portability and cost. Designs can utilize single or multiple transducers, almost always cone drivers arranged in a variety of configurations, including:
Sealed/Acoustic Suspension. The driver(s) are mounted in a sealed cabinet. While the transient response of this design is good, it’s less power efficient compared to vented enclosures and can be lacking when reproducing very low frequencies, especially at high volume levels. One notable exception is a specific design that utilizes a proprietary electronic processing control system for solid reproduction at extreme low frequencies (under 20 Hz).
Bass-Reflex/Ported/Vented. The most common type in live audio, a bass-reflex design has the driver(s) mounted into a box chamber that also houses one or more vent opening(s). The vent (a.k.a., port) is of a specific size and length to allow sound emanating from the rear of the driver to exit the enclosure, with the driver(s) and porting combining to provide a specific response characteristic. The advantages of bass-reflex over a sealed design are many, including extended LF response, increased power handling and increased output.
However, these advantages come at the cost of larger enclosure size and weight and slower transient response, along with the possibility of needing additional high-pass filtering slightly below the sub’s tuning frequency to avoid over-excursion of the driver(s), which can cause damage at high levels.
Bandpass. This approach places one or more driver(s) in a tuned chamber that can be sealed or vented, with the front of the driver(s) playing into a second tuned vented chamber before exiting the box. By passing sound through tuned ports, the design limits the bandwidth that the system can reproduce, resulting in increased output within a specific frequency range, along with a reduction of upper harmonics.
Schematics of the LAB Sub, a horn-loaded design developed by the PSW Live Audio Board community.
The downside is that placing a second tuned chamber in front of the driver results in a larger enclosure.
Horn Loaded. As the name implies, the driver(s) are located in a sealed (or sometimes vented) chamber whose output is channeled out via a horn. Horns can increase the output of the driver(s) and also can improve directionality, depending on the length and mouth area of the horn (the part that meets the outside air). Because low frequencies require a large horn, designers typically bend, fold or curve the horn in the enclosure, resulting in a more manageable sized cabinet.
While horn-loaded designs offer an increase in gain over a bass-reflex design, they sometimes don’t reproduce the lowest frequencies very well because a substantially sized horn is required to handle those long wavelengths. However, they can be stacked in groups for additional LF extension. The large size and weight of these enclosures usually limit their use to larger events.
Tapped Horn. A driver is placed in the mouth of a horn, with one side firing into the horn and the other side firing into the mouth of the horn. This mounting location reduces the amount of driver excursion required in comparison to a driver located in a sealed chamber at the back of a horn, with the result being lower distortion and greater output.
Cardioid. This approach delivers more output from the front of the box and less from the rear. It’s usually accomplished by adding drivers to the rear of an enclosure and changing their phase relationship and/or their output arrival time in relation to the front drivers, helping cancel out the sound waves to the rear.
Cardioid designs focus LF energy on the audience while reducing unwanted reflections and noise on stage. These benefits come at the cost of larger and heavier subs, and additional amplification and processing are also required if they’re passive cabinets.
However, the results can be very good, particular in problematic acoustical environments.
Hybrid. These utilize a combination of approaches inside a single enclosure. For example, one configuration I recently ran across has two drivers sharing a common vented chamber. One driver’s frontal radiation is direct, while the second driver is set at 90 degrees and radiates into a second vented chamber.
A variety of cardioid configurations. (Credit: D.A.S. Audio)
Another take is a cardioid passive dual-transducer configuration with an 18-inch driver in a bass-reflex configuration and a 15-inch driver feeding a folded horn.
The Nature Of Waves
Before we look at various ways to deploy subwoofers, we need to spend a little time on sound waves. Sound is a pressure wave through a medium like air or water.
At sea level in dry air at 75 degrees (Fahrenheit), the speed of sound is approximately 775 miles per hour (1136.6 feet per second). Humans can hear these vibrations if they’re in the frequency range of our hearing, usually considered between 20 Hz to 20 kHz. The wavelengths (one cycle of a tone or pitch) for bass frequencies are longer than higher pitched frequencies.
Because subwoofers operate in a general frequency range of 30 to 110 Hz, it means we’re dealing with wavelengths of about 10 feet to about 35 feet long.
Subs can be flown behind arrays without impacting their performance.
Long wavelengths aren’t usually affected by slender items in their path, like a support column in a building, unlike smaller high-frequency wavelengths that can be redirected and reflected off even small obstructions.
Anything within one-quarter (1/4) of the wavelength in distance can affect the output, including floors and walls (a.k.a., boundaries), as well as additional subs stacked next to each other. Most subs radiate energy in an omnidirectional pattern, as shown in Figure 1.
If we suspend a sub above the ground, its output emanates in all directions, and if it’s more than 8.75 feet away from any surface (a one-quarter wavelength of 30 Hz or 35 feet), it does not get any gain in output from a boundary. Place the sub on the floor (called half-space loading) and there’s an additional (theoretical) 3 dB of output because the energy that would have traveled down is now reflected up from the floor.
Placing it on the floor next to a wall (quarter-space loading) adds 6 dB more output, and moving it to a corner (eighth-space loading) adds 9 dB. (While this looks great on paper, in the real world the numbers won’t be that high because of interference from the boundary and the acoustic space.)
Now, place the sub one-quarter wavelength from a rigid wall, and it’s output will bounce off the wall and return to the sub, making it about one-half wavelength, with the phase about 180 degrees from the original signal. This results in destructive cancellation. Depending on distance from a boundary and the pitch of the signal, frequency response is affected.
A common phenomenon is a bass “power alley” where LF output is strong between left and right stacks because the output from each sub combines to add power. However, as you move off center, phase and time delay issues cause cancellations. This effect is most pronounced outdoors where walls and ceilings are not adding reflections and destructive cancellations.
Let’s move on to a variety of ways to deploy subs. Prediction software can be very helpful in this regard, allowing us to model various configurations as well as see the impact of things such as signal delay and high-and low-pass filtering.
The plots we’re presenting here were done with the Subwoofer Array Designer Calculator (www.merlijnvanveen.nl) by the software designer himself, Merlijn van Veen. (Thanks Merlijn!)
Single. Placed in a convenient spot, output will only be affected by the room and not other subs. A corner location can be better as it affords a boost in output. A downside is lack of pattern control, with the possibility of too much LF energy on stage and/or in other areas.
Multiples. This will “move more air” than a single sub, but stacking or lining up the enclosures will effect the pattern because the group produces longer than a one-quarter wavelength, which in turn will narrow the coverage pattern. Placing them near a wall or corner will additionally alter the pattern.
Left/Right (Ground). In Figure 2 we see the result of an L/R placement at 40 Hz. The power alley is clearly in evidence, and also note that on each side of the power alley, there are power valleys or null zones where destructive interference has reduced the output level.
Compare this to Figure 3, which shows the same configuration at 80 Hz. Note the destructive interference has resulting in multiple null zones in the output.
Left/Right (Flown). Particularly popular at larger events, subs can fly above a loudspeaker array, as a part of a horizontal array, next to an array, and behind an array. The behind configuration minimizes the overall width of the loudspeaker hangs and doesn’t intrude on audience sightlines. Flown subs can be configured in directional arrays as well as be joined by additional subs on the ground.
Vertical Array. Subs stacked vertically (usually flown beside or behind an array) will develop some vertical pattern control, with the length of the array determining how much. Longer = greater control.
Center (Ground & Flown). A center location with one or more subs can be ideal for providing a smoother coverage pattern over an L/R placement, but this method may result in putting too much bass back onto the stage unless cardioid cabinets or configurations are implemented. Center flown subwoofer arrays are popular for installs, especially in theaters, but not so much for live events.
Horizontal. This arrangement places loudspeakers next to each other (or with small intervals between boxes) in a row, usually across the front of the stage.
The effect of using a large line of cabinets will narrow the horizontal pattern, but unless the array is made up of cardioid cabinets, the stage will still be awash in low frequencies.
Curving the array instead of placing the cabinets in a straight array can offer more constant directivity. Figure 4 shows a horizontal array at 40 Hz, while Figure 5 shows the same configuration using cardioid cabinets.
Distributed/Delay. Uses multiple subs in various locations (such as hidden beneath a runway stage along its length), and may employ time delay to achieve a coherent arrival time at various audience locations.
This approach is popular for corporate-type events where a large stack or wall of subs is not desired for aesthetic reasons, while a number of subs spaced apart down the walls, flown, or hidden under a stage is better received by event planners.
Distributed-type systems can also be found installed in venues where a single subwoofer placement would not cover the intended audience area.
Making It Directional
As previously noted, a primary goal is keeping excessive LF energy out of areas where we don’t wish it to be. With select placement/arrangement of subs and the application of processing such as signal delay, various radiation patterns can be attained.
Some of the more popular directional array techniques include:
Delay Shaded Array. This reduces the output of the boxes at or near the ends of an array (like a horizontal array), with the intent to make the coverage pattern more regular and less frequency dependent.
Cardioid Array. Two popular cardioid approaches are stacking and side by side. Stacking places the boxes on top of each other, with one cabinet (usually the middle cabinet in a stack of three) reversed to point to the rear. Side by side places the boxes next to each other and reverses one (again normally the middle one of three).
Further pattern control can be attained by applying delay, either by delaying the output of the front-facing cabinets to arrive at the same time as the output of the rear-facing cabinet, or by delaying the rear-facing cabinet to the output of the front-facing cabinets while also reversing the polarity of the rear-facing cabinet so that it’s signal is 180 degrees out of phase with the front-facing cabinets.
End Fire Array. Multiple cabinets are aligned in a row, arranged one in front of the other with all pointing forward. All cabinets are delayed in relation to the rear-most cabinet. This approach makes it possible to project powerful, directional bass over long distances. Figure 6 shows how an end fire array provides very good pattern control at 80 Hz.
Obviously, there’s a lot to discuss when it comes to subwoofers, and what I’ve presented here is intended as a primer, really just scratching the surface.
I recommend further research to enhance your understanding—for example, ProSoundWeb offers dozens of articles on subwoofers and related topics, while the LAB Subwoofer Forum on PSW is an excellent resource for both information and getting questions answered.
It’s well worth your time, as delivering LF energy with maximum impact and control is one of the defining factors of a successful sound reinforcement experience.
Senior contributing editor Craig Leerman is the owner of Tech Works, a production company based in Las Vegas.