You might be looking at the title above and thinking that somebody accidentally combined two different articles. Cardioid is a word we usually use to talk about microphones, not speakers.
Both speakers and microphones are transducers (devices that convert between electricity and air pressure) that just happen to be at the far ends of the audio chain. They are really doing the same thing in opposite directions. So while we talk about cardioid when we discuss microphone pickup patterns and show that pretty heart-shaped diagram, we can apply the same idea to speakers and talk about dispersion patterns.
Almost all speakers are directional to some degree, so we don’t bother to call them cardioid, but subwoofers are a different story. They are all generally omnidirectional because of the laws of physics. To make an audio frequency directional, we need to have a box that is close in size to the length of the frequency we are trying to impact. Since 100Hz is about 11 feet long, it is unlikely your sub box is that big. This means that a sub is putting out roughly the same level in every direction.
This omnidirectionality can cause us some headaches. When we set up a stage, we are generally careful to keep the top boxes in front of the microphones so that we don’t have to fight with feedback, and usually we put the subs right under the top boxes. This keeps them time aligned and in phase with our top boxes. Unfortunately, while our tops are good at focusing their energy forward, the subs are shooting as much energy straight at those open microphones as they are at the crowd. This is why sound engineers are so fond of those low-cut, or highpass filter (HPF), buttons on their consoles for microphones. It helps them control feedback that could easily run rampant at lower frequencies.
If you have more than one sub onstage because you want more bass, they will also tend to interfere with each other. There will be places in the room that the bass cancels, or gets lots louder, because low frequencies have combined. This can happen as a result of timing differences between direct low frequency or from reflections that are more common because the low frequency is going out every which way. Additionally, that energy adds up and sort of rolls around onstage. It can make a hollow stage resonate, make drum hardware rattle, and make life generally miserable.
A Directional Solution
Wouldn’t it be great if we could get the energy of a sub to go mostly forward just like our full-range speakers do? It would solve feedback and buildup problems and make the sub more efficient since it is pushing more of its energy at the audience. That is exactly the idea behind cardioid subs. The concept is not new. It was actually proposed in a paper in the late ’50s, and enterprising sound engineers have been assembling subwoofer arrays for a couple of decades to make the low-frequency material more directional.
They combine several different subs and adjust the timing by either shifting the position forward and back or using electronic delay. By carefully calculating the position, they can make the bass more directional by a combination of phase and sub placement. You can take a couple of subs and move them around onstage yourself to play with the idea. If you stacked them on top of each other with one facing the wrong way, you would be about halfway there.
The basic idea is that you have two sources creating the same low-frequency material, with the polarity flipped on one of the sources. You then shift the timing so that in the front of this sub array the frequencies are in phase and combine to increase the output by 6dB. The signal coming out of the back of the sub is calculated so that it is out of phase and tends to cancel. Since the delays have to be consistent, they will have the most effect at the tuned frequency in the front of the sub. If we chose 55Hz, which is about 20 feet long, we would need to delay the back speaker by 4.5 milliseconds to shift it one-quarter wavelength to get the timing to line up and combine effectively. If we moved the subs or were trying to impact a different frequency, these numbers would change. Since we are generally dealing with a pretty small range of frequencies in a sub (around 30Hz–120Hz), this has more impact than it would in a full-range speaker.
If your eyes are rolling back in your head already, don’t worry. Manufacturers are starting to come to market with cardioid subwoofers in a single, reasonably sized package that do all that work for you. This allows you to get the benefits of keeping all that low end off the stage without having to move multiple heavy boxes around every time. Given that the idea is new to a lot of people, they are trying to make their heart-shaped dispersion diagrams prettier than those of the microphone companies! One example of a cardioid subwoofer is the QSC K212C, which packages up two power amps and two 12-inch drivers in a single box. By playing with the timing between the two drivers, they are able to get the bass to cancel itself at the back of the box — to the tune of 15dB lower output! The beautiful part is that they have done all the work for you, so you don’t need to do the math to get it right. Just turn it on and point it in the right direction!