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Moving Air: Inside Subwoofer Designs & Configurations
Further understanding in the quest to deliver LF energy with maximum impact and control...
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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.

Figure 1.

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.

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