To start, let’s recall that the low frequency energy radiated from a conventional subwoofer is effectively omnidirectional below 200 Hz. This is because the directivity of a subwoofer is related to the ratio of its size to the length of the sound wave it produces. The length, or size, of the sound wave is inversely proportional to its frequency, and therefore when low frequency waves are much longer than the dimensions of the subwoofer transducer and enclosure, the radiation pattern becomes effectively equal in all directions, including to the sides and rear.
The Directional Cardioid Radiation
Now, as we have all witnessed low frequencies omnidirectional radiation can sometimes be problematic, as it may cause feedback, create bass build up in some part of a room or disturb neighbors. Since the late 50’s sound engineers have setup subwoofer’s array using a minimum of two units, and with careful adjustments of delay, polarity, output levels and distance between the units, they managed to force the radiated energy to become directional. So, the term cardioid radiation – or heart-shaped – refers to the shape of the directional coverage in which levels are louder at the front of the unit and quieter behind it.
To implement such a cardioid radiation, two technical solutions can be used. Either design a fully cardioid subwoofer that features both front and rear-firing drivers within the same enclosure (QSC KS212C subwoofer), or employ two separate standard subwoofers together with on-board pre-programmed DSP presets that will provide either forward- or rear-facing radiation for each unit (QSC KS118 model). Working together the two units (more than two can be used, as we will discuss later) will combine to produce a cardioid radiation pattern with up to 15 dB of rearward rejection.
So, if two, or more, KS118 subwoofers can be arranged and configured to produce a cardioid radiation pattern very simply without complex calculations or processing settings required, a question we should answer is how to correctly position such a subwoofer array to get the best possible cardioid performance?
The processing necessary for cardioid operation is already programmed into each KS118 subwoofer’s DSP. On each subwoofer facing forward (toward the audience), the cardioid preset called ‘Forward’ should be selected and on each unit facing away from the audience, the preset ‘Rear’ should be turned on. Then, the same audio signal should be fed into both subwoofers with the same gain setting on each one.
The question is now how to physically position the subwoofers on the stage, or nearby floor surface. There are three possible deployments, and the choice is very dependent on the stage and venue layout, where the audience is in relation to the stage, the possible masking of video projections, etc.
A Good Choice: Side-by-Side
Placing KS118 subwoofers side-by-side (Figure 2) may save space and allow good visibility of the stage for the audience, but creates a cardioid pattern with limited precision.
A Better Solution: Stacked
Stacking two KS118 subwoofers on top of each other (Figure 3) offers similar results to the side-by-side arrangement. Of course, this setup clears the floor space, which is often beneficial in small venues.
The Best Option: Back-to-Back
Placing subwoofers back-to-back (Figure 3) offers the best cardioid performance, with 15 dB of sound attenuation at the rear of the array. The venue, or stage, should allow for such an arrangement, as it requires a little larger footprint.
Three Subwoofers Cardioid Arrangements
A three subwoofers configuration, with two forward facing and one rear-facing units, provides additional acoustic output at the front but with slightly less efficient rear attenuation. Once again, the stacked arrangement is less optimal than the back-to-back option.
Subwoofers’ cardioid deployments are very useful in a myriad of different production events and fixed installation applications. From keeping low frequencies spreading to neighbors’ home, to containing bass in a defined area (trade shows, conferences, seminars, etc), to minimizing bass feedback on stage or allowing multiple audio events to occur in the same large venue or outdoor, cardioid subwoofers’ rear attenuation (2x KS118 and one KS212C provide 15 dB level reduction) widens significantly the way you can use subwoofers.
Deploying multiple QSC subwoofers in a cardioid configuration is rather simple (check also our video on this subject), once you understand the benefits of each setup. Happy bass listening!