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.