What Is Cardioid in Subwoofers?

Jun 15, 2017

What is Cardioid and what does it do in the context of subwoofers?

Cardioid Mode Deployment involves one or more boxes facing the audience with at least one facing away from the audience.

Cardioid, or heart-shaped, refers to the shape of the coverage, or pickup, of a transducer. A cardioid microphone picks up sounds in front of it more effectively than it picks up sounds behind it. A cardioid subwoofer or subwoofer array produces a heart-shaped coverage pattern in which levels are louder to the front of it and lower behind it.

There are several methods for achieving a cardioid pattern but the principles of the function depend upon signals from multiple sources being aligned in one direction and mis-aligned in the other.

The sound pressure level measured in the direction of cancellation is reduced and the sound pressure level measured in the opposite direction is increased. When these measurements are graphed on a polar plot, the graph appears to be heart-shaped, so it is called a Cardioid Pattern.

Of note, a Polar Plot refers to a graph that shows sound pressure level measurements with the lowest level in the center and the highest level to the outer edge, so louder measurements are graphed farther from the center and quieter measurements are graphed closer to the center. The source, the speaker, is imagined to be at the center with the perspective of the viewer being above, looking down. (Specifically, this would describe a horizontal coverage polar plot.)

The waves of sound radiate away from a subwoofer in a largely spherical pattern unless they are interfered with by a surface or by opposing pressure. Positive pressure waves can be canceled by negative pressure waves, so it’s possible to use a subwoofer to cancel the output of another subwoofer.

If you’ve ever hooked up one subwoofer in reverse polarity to another, you know how effective the cancellation can be. The trick with a cardioid speaker or cardioid array is to use spacing and delay (aka time offset) so that the cancellation is targeted in only one direction rather than cancelling the output everywhere.

Example of Cardioid Polar Plot

Polar plots are measured at individual frequencies or frequency bands because they graph output relative to position rather than output relative to frequency so there would be slightly differently shaped graphs through the operational range of the loudspeaker in question.

The result of a properly deployed Cardioid Array is the ability to focus more bass where you want it and have less go where you don’t. That can mean keeping bass away from neighbors, off stages, away from turntables or keeping it from reflecting off walls and causing destructive interference elsewhere in a room.

A cardioid speaker or array requires more parts, more processing and ultimately more money than an ordinary subwoofer system. Two identical subwoofers used side-by-side in a conventional setup would deliver more output to the front, and also to the rear, than the same two subwoofers used in a Cardioid setup, however, in the Cardioid setup there will be MUCH less output to the rear and only slightly less output to the front.

There are “passive cardioid” loudspeakers that use tuned ports on opposite sides on the enclosure to create the cardioid effect however their operational bandwidth and output is limited relative to active cardioid systems.

The response graph above shows the forward vs rearward radiation of a pair of subs in cardioid configuration. The upper (yellow) line is the output SPL at one meter from the audience side, or what you might call the front of the cabinets, and the lower (pink) line is the output SPL at one meter from the stage side, or back of the cabinets. The graph indicates that the level reduction is over a large range of frequencies. 

The effectiveness does taper off below 30Hz because the wavelengths get too long for the offset distance of the depth of only one box to control. Nevertheless, over the majority of the operational bandwidth of the test boxes, there is a big difference in what you hear and experience behind vs in front.  

One of the advantages to cardioid-configured subwoofers indoors, besides keeping levels down behind them, is virtually eliminating the reflections that come from walls behind the subwoofers. Those reflections can cause big, nasty dips in the frequency response observed in audience and dance floor areas. Those dips can cancel far more energy than what you lose by dedicating one of your subwoofers to backwards-canellation-duty, aka cardioid mode.

Three-dimensional representation of Cardioid pattern:
boxes stacked vertically.

Three-dimensional representation of Cardioid Coverage:
seen from above.

13 Comments

  1. Aaron

    Does a cardioid subwoofer set up only work when they are in blocks and centered on the stage? Can you have 4 subs, two on either side of the stage and have each set be cardioid? Does that help or hurt when working indoors? 🙂

    Reply
  2. David Lee

    Hi Aaron,

    A cardioid setup works indoors or outdoors and works with a minimum of 2 boxes no matter where you put them, unless they are under a stage or backed up against a wall. A combination of three boxes, with two facing forwards and one backwards, is the ideal combination however they can also be used in pairs. When you use cardioid pairs, it’s best to reduce the level of the reversed box by 3dB because one cardiod-mode reversed box at full output can compensate for the rearward radiation of two boxes in normal mode.

    When outdoors, a cardiod setup will reduce the sound pressure level of the low frequencies on stage and in the area behind the stage. When indoors, if there is a wall behind the stage, cardiod setups will also reduce the amount of energy that is reflected off that back wall and back out to the audience/dance floor area.

    Without a cardiod setup, the sound that would ordinarily reflect off the wall behind the stage or DJ would arrive in the audience area out-of-phase with the direct sound from the subwoofers and it often causes loss of level and loss of impact. By running your system in cardiod mode and minimizing those reflections, you can achieve a better result in both level and clarity for the audience and the band or DJ on stage.

    Whether to run left-right or centered is a question all of its own. Sometimes the available space dictates where you can set up, so you have to be able to do either. The primary advantage to center placement is the avoidance of nulls caused by interference between the two separated sub locations. The disadvantage is that it can concentrate the sub energy in the center, where there may not be a balance of enough highs to keep up, and at the same time leave the left and right sides out of balance with too much mid and high energy relative to the sub level there. For instance, on a smaller scale, if the tops are way far apart and the subs are center stacked, the system won’t deliver an even and balanced sound throughout the audience area. When center-stacked subs are done on a festival scale stage, the center area mid and high frequency content is supplemented with center fills. One has to adjust the solution to suit the venue and the event.

    if you’d like a more detailed reference check out the updated manual with additional info on cardioid setup: https://www.bassboss.com/wp-content/uploads/2017/11/BASSBOSS_ZV28_Powered_Subwoofer_Manual.pdf

    thanks!
    best
    David Lee

    Reply
  3. Owen Blevins

    Fascinating topic, huge baseball span. Looking to see you again a DJ expo this year. Can you explain the delay in a bit more depth? In other words if our mixtures are capable of sending a delay to the each sub individually and we reverse one of the two subs, could we duplicate the cardioid affect built into several manufacturers subs on the market currently?

    Reply
    • Owen Blevins

      That was supposed to read, “Huge Bass Boss fan” damn Siri 🙂

      Reply
      • David Lee

        Hi Owen,

        If your mixer is capable of sending a separately processed output to each subwoofer, (or group of subs) you can configure your subwoofers to produce cardioid output. The specific processing required to produce a cardioid effect depends upon the placement of the subwoofers relative to each other. The processing can be relatively simple, such as the delay required for an end-fire configuration where the subs are one-behind-the-other, or it can be more complicated, requiring delay, polarity inversion and other filters when boxes are side-by-side, or stacked on each other, with one facing backwards.

        The simplest and easiest cardioid configuration to implement is a two-box end-fire, with both boxes facing the same way, one behind the other. The front one is delayed to wait for the arrival of the sound from the back one. All you need to make this configuration work reasonably well is a delay function and a tape measure. The biggest down-side is the real estate it requires. The effect is far from absolute but it does help!

        Setting up a cardioid pair of subs properly requires measurement equipment but sometimes a simulation tool can provide you information about the required settings that will produce good results if you provide it with accurate performance characteristics and dimensions. Provided you have enough individual amplifier channels and you enter the right settings into the right channels, a mixer that offers delay and polarity inversion on each output can get you to a functional cardioid subwoofer system.

        Enjoy!
        David Lee
        BASSBOSS

        Reply
  4. Paul

    Does a cardioid effect occur simply by a single driver (designed for infinite baffle) placed on a small baffle without an enclosure? Thanks!

    Reply
    • David Lee

      Hi Paul,

      No, that’s not a cardioid effect. That’s often referred to as a dipole. The back wave is opposite polarity of the front wave, so cancellation occurs 90 degrees off axis of the woofer. You get sound in front of AND behind the woofer, but very little off to the side. Whether the phase of the signal is positive or negative, you will experience the pressure waves as sound, so if you’re in front of the speaker or behind it, the level will be pretty much the same. When the positive waves and the negative waves meet, such as where they travel the same distance to reach the edge of the baffle, they cancel each other out, so you hear almost no sound along the edge of the baffle. Let me know if there are any other questions I can answer. Thanks!

      Reply
  5. Felipe Velazquez

    If you have only one sub, is it still better to play it in cardioid mode, or normal? Placing in the center of course. When playing with 2 subs, does the forward-facing sub play in normal mode and the rear-facing sub in cardioid mode? Thanks David, I’m a big fan of BASSBOSS, I’m saving up for one (to start).

    Reply
    • David Lee

      Hi Felipe,

      If you have only one sub it should always be used in Normal mode. In Normal mode, the sub is phase-coherent with the tops when they are facing the same way.

      When you add the second sub (that’s necessary for cardioid mode) it does face the rear and it does go in Cardiod mode. The forward-facing sub remains in Normal mode.

      The output of the rearward-firing subwoofer in Cardioid mode is used to cancel the wrap-around energy coming from the Normally positioned, Normal-mode sub.

      When it’s in Cardioid mode, the forward output of the subwoofer is not phase-coherent with the tops, so if you put the sub in its normal, forward-firing position and switch it to Cardioid mode there will be a significant gap in their combined response.

      Best,
      David Lee

      Reply
  6. pfg

    Hi,
    I’m eager to try the three subs setup. How should they be placed? Two facing the listener and one facing the first two? Like this:
    O O
    O
    How can one determine how much delay does the single (rear) sub need?
    Thank you,
    P

    Reply
    • David Lee

      Hey pfg,

      In the case of the BASSBOSS boxes, they should be placed with two boxes facing the audience and one box, usually placed between the other two, facing the stage. All the boxes should be side-by-side. The box facing the stage should be in Cardiod mode. The two facing the audience should be in Normal mode.

      The alternate setup would be to have the 3 boxes stacked on each other, with the bottom one facing the audience, the middle one facing the stage and the top one facing the audience. Again, the one facing the stage would be in Cardiod mode with the other two in Normal mode.

      The delay in the rear-facing sub in the BASSBOSS systems is included in the Cardioid mode preset.

      ++++Additional Information++++

      The setup in your illustration wouldn’t work with the BASSBOSS cardioid mode presets. What you drew is more similar to an end-fire configuration.

      One thing to note is that if you want to build an end-fire configuration, the front speakers are delayed, not the rear speakers.

      An end-fire configuration would be set up like this:

      []—> []—> []—> []—> Audience side
      D=0 D=X D=2X D=3X

      You can do an end-fire configuration with just two boxes but the effectiveness increases with additional tiers. Note that even with just two tiers, the end-fire configuration takes up more space front-to-back.

      The method for determining how much delay is required in an end-fire configuration can be as simple as measuring the distance from the front of one speaker to the front of the next speaker and converting that distance to time at the speed of sound.

      Some processors let you enter delay in distance, so all you need to do is enter the distance between the front of the D=0 box to the front of the D=X box. The appropriate amount of delay is applied to the D=X box, for instance, 42”.

      If there is a third box and it’s spaced the same distance forward from the D=X box, the delay applied to that box (D=2X) would be 84”. The fourth box in the sequence would be delayed 126”. (D=3X)

      Using a measurement system to indicate and align the phase traces of the successive boxes using delay and additional signal processing would provide more accurate control but the general principle is the same and you can do this with a tape measure.

      Best,
      David Lee

      Reply
  7. Hayden

    Hi – I have been looking into this setup as an option for indoor use to try cancel any sound exiting through the rear of a stage and into an area behind the venue with neighbouring occupants. It has so far seemed viable until I recently was told by a sound tech that although sound-cancellation occurs directly behind the setup, the distance behind the setup that is occurs is not endless and at times you can get a secondary point of sound some distance beyond the cancellation zone (i.e. if you stand 5-ft behind the setup you don’t experience sound, although at 20-ft behind you may experience sound again). Does this actually occur or have you experienced this in the past?

    Reply
    • David Lee

      Hi Hayden,

      Sound occurs as pressure waves in a medium, in this case in air. If the medium moves or flows, such as with wind, the sound pressure waves move within the medium. If the medium is still and the source is singular and in open space, the cancellation zone is effectively endless in that direction.

      If, on the other hand, sound is reflected into the cancellation zone, sound will occur in the cancellation zone. Similarly, if sound is blown into the zone by wind or if sound from another source enters the zone, there will be sound in the cancellation zone. That sound could be coming from the other side of the stage, from stage monitors or from reflections off of walls or ceilings.

      The presence of sound in the cancellation zone doesn’t mean that the cardioid effect changes over distance, rather it means that the further you are from the setup, the more other possible sources of noise can be heard in that area.

      All cardioid setups provide a reduction in sound level behind the system relative to the sound level in front of the system. Some can achieve extremely big differences in SPL from front to rear but none are absolute and/or perfect. It’s definitely better to utilize these techniques to reduce the level of sound on stage and in neighboring areas even if there is still some sound audible in the desired cancellation zone.

      Naturally you will want to use the best solution for your specific situation and within your system’s physical size and budget limitations. We’re happy to help you determine which is the right approach for your needs.

      Thanks!
      David Lee

      Reply

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