1.- A Guide to Bipolar, Dipolar, & Direct-Radiating Surround Speakers

Bernal

See the Forum. I received answers but not solved the doubt. I found the following information on the Internet. Helped me answer the question. I feel complete and whole and for the share.

The best explanation:

http://forum.blu-ray.com/showthread.php?t=66471

OFF-AXIS RESPONSE OF SOUND WAVES
The following diagram demostrates that high frequency sound waves have a narrower off-axis response and low frequency sound waves have a much wider off-axis response. That is why you can stand completely to one side of an 8" diameter woofer and still hear bass frequencies like you were in front, but lose midrange or higher frequencies.
Off-Axis Radiation
It is said that the higher frequencies sound waves behave like headlights or flashlights and low frequency sound waves behave like ripples of water waves in a pool. This is why tweeters are shaped like domes. A curved diaphragm pushes high frequency sound waves in a way that aides dispersion.
INTERFERENCE OF SOUND WAVES
Interference usually refers to the interaction of waves with each other, When similar independent waves are combined, the result can be either constructive or destructive interference, depending on whether the waves are in phase or out of phase. This interference occurs when the waves have the same or nearly the same frequency. Constructive interference will enhance sound. Destructive interference will weaken sound. If two identical waves are 180 degrees out of phase, they will cancel out. Whether the interference is constructive or destructive, the individual waves continue to exist separately. The interference itself is merely the effect of the waves together at one point in space.
Source: Headwize.com
Two waves in phase and two waves 180° out of phase.
Source: Wikipedia
The animation in the following demonstrates two waves traveling in the same direction. The phase difference between the two waves varies with time so that we see constructive interference when maximum points are aligned (peak) and destructive interference when minimum and maximum points are aligned (null).
Animation courtesy of Dr. Dan Russell, Kettering University

THREE TYPES OF SPEAKERS

The direction that speakers radiate their sound depends on how the drivers are lined up in the enclosure, and also whether there are drivers or ports on the back of the speaker. If all the speakers are on the front, this would be called Direct or Monopole. If there are speakers on the front and back, and they are in phase (both speaker cones moving away from the magnet), this is called Bipolar, whereas if the cones are out of phase (one cone moving in towards the magnet, while the other cone is moving outwards, away from the magnet), the speaker design is called Dipolar.

Direct Speaker (Monopole)
A conventional sealed box speaker pushes a small cone back and forth against the air and creates sound. You can see that for a box speaker most of the sound goes one direction (monopole) and that the box construction plays an important part of how this makes sound.

Conventional Box (Monopole)
Source: Integracoustics.com

Bipolar Speaker
Another way to create sound waves is by a bipolar arrangement, where the front and back speakers work against each other (in phase). Bipolar speakers are made by mounting in-phase monopoles on opposite sides of a box as a way of creating omnidirectional radiation patterns. Here is what they look like
Bipolar Speaker
Source: Integracoustics.com and Djsociety.org
Dipolar Speaker
Dipolar speaker arrangement is similar to bipolar. However, the front and the back speakers work in the same direction (out of phase). This can be useful in reducing the stimulation of resonant room modes at low frequencies. It also results in high frequencies being reflected from the rear wall, which can create more diffuse reverberation, though in theory it could reduce stereo localization. Here is how they look

Dipolar Speaker
Source: Integracoustics.com and Djsociety.org

DISPERSION PATTERNS OF THE MAIN DIAPHRAGMS

Direct Radiating (Monopole)
In a direct radiating monopole, a diaphragm compresses the air in front of it and pushes the wave outward. As the wave expands, its energy is distributed, resulting in lower pressure and a reduction in its loudness. This shown in the following diagram (please be patient, the file size of this simulation is 5.6MB and takes a long time to load):


Monopole Dispersion Pattern
Source: Integracoustics.com

Bipolar Speaker Dispersion Pattern
For two sound sources at same frequency, amplitude, and phase, there will be points in space where the overall sound level is high (constructive interference) and other places where the sound level is approximately zero (destructive interference). The constructive interference occurs as their off-axis response (mostly lower frequency waves) reinforce each other. That is why bipolar speakers are known for their strong dispersion.

Bipole Dispersion Pattern
Source: Wikipedia and Integracoustics.com

Dipolar Speaker Dispersion Pattern
The radiation pattern of a dipole is demonstrated in the following diagram. The left waves are equal and opposite to the waves on the right. Because they are firing out of phase, they cancel each other where these waves are superimposed off-axis (top and bottom, mostly lower frequency waves). This is called destructive interference and creates a figure 8 pattern. Dipole speakers are known to have weaker bass response.

Dipole Dispersion Pattern
Source: Integracoustics.com

Comparison of the Dispersion Patterns

Source: Integracoustics.com

In the above diagram, note that the bipole field shows a larger off-axis response (the middle part of the diagram) compared to the monopole. This is because the drivers in the front and rear are in-phase, which constructively reinforce the sound waves.

Dipole speaker arrangement, however, exhibits a smaller off-axis response (middle part of the diagram) compared to the monopole. This is because a dipole arrangement has out-of-phase drivers which behave in a destructive manner and cancel the sound waves with the same frequency.

REFLECTION FROM THE BOUNDARIES

Reflection from the Back Wall for Bipolar Speaker
The following diagrams demonstrate two waves when the speaker drivers are operating in phase. The reflection form the rear driver can interfere with the wave coming from the front driver and create constructive reinforcement (called lobing, light areas) or destructive interference (called null, dark areas).

2 Dimensional and 3 Dimensional
Source: Integracoustics.com

Reflections from the Back Wall for Dipolar Speaker
When the back-wave starts off from the rear of the speaker and reflects off the back-wall and travels back to the speaker where it recombines with the new wave just being started from the front driver. Because of timing difference, at some frequencies the two waves destroy each other and at other times they reinforce one another. This effect is called comb filtering A comb filter adds a delayed version of a signal to itself, causing constructive and destructive interference. Some dipolar users treat the back-wall with absorbent material that absorbs all frequencies except the long bass frequencies. That is one way to prevent comb filtering. Another way is to turn the dipolar speakers inward so that they are not parallel to the rear wall.

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Bernal

FRONT SPEAKERS SETUP

For front speakers, put the speakers at least 2 to 3 feet away from the rear wall.
The distance between the two speakers should be between 6 to 10 feet.
The distance between your main listening chair and the distance between the two speakers should be approximately the same. They should form approximately an equilateral triangle.

The tweeters should be about the same level as your ears when you are seated. For smaller speakers, use a stand. Slight tilting backward or forward of the speakers until the tweeters point toward your head may also work.
You can slightly angle (toe-in) the speakers toward the listening chair. You can use a string to make sure both speakers are angled exactly the same. Angling the speakers too much may have two disadvantages:
It may ruin the sound stage.
It will not be very effective for others who may be sitting elsewhere in the room. Don't be selfish.
Play a CD with good soundstaging and a singer in the middle.
If the soundstage is good and wide but the center stage is blurred, move the speakers closer together.
If the center stage is focused and sounds great, but the soundstage isn't very wide, angle the speakers away from the listening position a little at a time until the soundstage becomes wider without losing the center stage.
Many speakers may sound better if they are pointed at your shoulders rather than directly at your head.
Always use a string to make sure both speakers are angled the same way.
Don't be afraid to experiment. Rooms are not the same and interaction with room boundaries, furniture, carpets, etc. play an important role in the sound that you hear.
Many experts believe that in a small home theater room, you hear the room and not the speakers.

DIPOLAR & BIPOLAR SURROUND SPEAKERS SETUP

Normally, surround speakers are placed at or above ear level. If they are placed behind the seating area, bipolar speakers are best. If the speakers are placed to the sides of the listener, dipolar speakers are the best. When doing 7.x, the additional speakers are placed behind the listener, again at or above ear level.

Source: Polk Audio

Dipole surround speakers are typically labeled Left and Right so that, when you place them on the sides, the in-phase drivers are pointing towards the front of the room. This allows them to be in phase with your front speakers and avoid cancellation problems that can occur when out of phase drivers are facing each other.

If, in a 7.x setup, you decide to use another pair of dipole surround speakers in the rear, you will have to place the left speaker on the right side and the right speaker on the left side as in the diagram below. This is to avoid cancellation problem when the speakers are out of phase. If you decide to use direct radiating speakers in the rear, you will not need any special consideration.

Diagram created by Big Daddy

For dipole speakers, if there are arrows on the speakers they should face the front of the room for the side speakers, and toward the center of the wall for the rear speakers.

KEY CONSIDERATION WHEN CHOOSING DIRECT OR BIPOLE/DIPOLE SURROUND SPEAKERS

Arguments will vary from person to person, based upon the preference of the individual.
Movies:
The surround speakers should not distract your attention away from the movie.
They should help to draw you into the film. This is what surround speakers should do.
Surround speakers are intended to reproduce ambient effects like explosions, gunfights, the hum of a spaceship, or crickets at night so they sound like they are coming from everywhere in the room — not just where your speakers are located.
Bipoles/Dipoles have a bigger surround sweet spot than direct radiating surrounds have.
Bipoles /Dipoles are a closer match to the surround speaker arrays found in movie theaters.
Music:
With stereo music, you are essentially creating images in space with two speakers.
You can move instruments around in space, and expect the listener to perceive more than two speakers.
Add two more speakers in the rear and you now create the ability to pinpoint the source of the music between all four speakers, so in theory you can place an instrument anywhere in 360 degrees around your head.
By introducing dipoles/bipoles, you destroy that image. You are more at the mercy of the room. The room will most certainly color the sound and add its own signature.
Some may actually like this effect, but the music is less accurate. Your room's acoustics will affect bipole/dipole designs more than conventional direct-radiating.
Direct-radiating models send sound directly toward the listeners’ ears.
Conventional direct-radiating speakers place the listeners in a sound field in which the direct sound is more prominent. It is possible that the majority of people find stereo to be more attractive if the room reflections are strong. The sound tends to be open and spacious, with a good sense of depth like a real live concert. It has the advantage of making stereo listening region more enlarged. However, the specific images can be rather vague.
On the other hand, there are some listeners who do not like this kind of music reproduction, and prefer to have a very specific, almost pinpoint, sense of image position. Many recording engineers prefer this because they need to be able to hear, very precisely, the results of their manipulations. As a result, recording studios are often acoustically rather dead, and the loudspeakers are directional and identical. They use identical speakers so that they are perfectly timbre matched so that the sound is not affected by the differences between the speakers. However, these same people usually prefer the more spacious sound field at home.

Bernal

SUMMARY

Advantages of Bipole Speakers
The bipolar speakers are wired in phase and push out more air, resulting in a bit more bass.
Bipolar speakers can be used as the main speakers.
This design allows a bipolar speaker to provide the best of both worlds: a direct-speaker’s clarity and focus, and a dipole’s spaciousness.
Bipolar speakers work in roughly the same way as direct-radiating speakers. They feature two sets of drivers pointed in different directions and wired in-phase, both pushing and pulling air at the same time. That gets sound moving toward the walls, ceiling, furniture, etc., introducing more sonic energy in more directions than you would with a direct-radiating speaker. In other words, you will got more sound coming at you from more directions. However, most of the sound is coming from the speakers themselves.
Bipolar surround speakers use speaker drivers aimed towards the front and back of the room to achieve a diffuse sound-field like that created by the multiple surround speakers in movie theaters.
Your room’s acoustics will affect these designs more than conventional direct-radiating speakers.
Disadvantages of Bipole Speakers
Bipolar speakers are affected by your room’s acoustics more than conventional direct-radiating speakers. You are at the mercy of the room. Some may actually like this effect, but it is less accurate.
Some argue that the midrange response of bipolar speakers are not as good, particularly as the main speakers. It is probably more suitable for surrounds.
Bipole speakers do not have the sound-field of the dipole (not as open sounding).
Advantages of Dipole Speakers
Dipolar create a more open space of sound without the listener being able to pinpoint the source of the sound.
Unlike bipolar speakers, the drivers in dipoles aren’t moving in and out at the same time. One driver pushes air while the other pulls. So when the dipoles are placed properly, at 90° from the screen, directly to the left and right of the listener, they create a null zone—an area in which the sound coming from each speaker effectively cancels itself out, usually in the off-axis middle area facing the listener. The sound coming straight toward the listener’s ears is effectively dampened, and instead the listener hears virtually nothing but reflections from the room boundaries. So instead of perceiving sounds as coming from the speaker itself, the result is a diffuse sound-field.
This design offers a more diffuse, spacious sound than a direct-radiating model. This is what surround speakers should do, after all. They are intended to reproduce ambient effects.
Dipolar models are favored for THX-certified designs specifically because of their diffuse sound, which more accurately resembles what you would hear in a real movie theater.
According to Dolby Laboratories: “Surround speaker placement, room acoustics, and personal preference are as important as the speakers’ radiating characteristic. These factors vary greatly, so Dolby Laboratories cannot recommend a particular speaker for home theater use.”
Dipoles are a closer match to the surround speaker arrays found in movie theaters.
Ambience and envelopment are the goals of surround speakers, not the 3D holographic rear stereo images. Dipoles do a better job at envelopment due to the sound being directed away from the listener. Having the forward and backwards firing sounds out-of-phase makes the speaker harder to pinpoint.
Dipoles have a bigger surround sweet spot than direct radiating surrounds have.
Disadvantages of Dipole Speakers
Dipoles are said to provide the most room filling ambient effect, but sacrifice a little bass as the drivers are wired out of phase.
Some critics claim that dipolar speakers were invented in the old analog and matrixed surround sound days. Nowadays, most digital surround audio codecs are discrete with five or more channels. The diffuse sound-field of dipole speakers destroy this advantage.
Others claim that dipoles were originally designed to create a diffuse sound-field in rooms that were acoustically treated to duplicate the effect of multiple direct radiating speakers in commercial theaters. However, even in many high-end home theaters, they do not treat the rooms that much. So the advantage of dipoles is limited.
Advantages of Direct-Radiating Speakers
Direct-radiating models fire sound directly out toward the listeners’ ears. These are a good all-purpose speakers as their clear, focused dispersion pattern can accurately convey the directional sound effect in soundtracks.
Conventional direct-firing speakers place the listener in a sound-field in which the direct sound is more prominent.
Some experts would argue that the use of a monopole, or “direct radiating” surround speakers would provide a more direct sound, which can be of some benefit in listening to multi-channel music.
Disadvantages of Direct-Radiating Speakers
Direct-radiating speakers draw attention away from the movies and to the speakers.
Surround effects lose their three dimensional and diffuse impact with direct speakers.
Direct-radiating speakers are not quite as good in home theater use. They may be advantageous in music listening.
In most cases, the room you are using will have a direct correlation to the choice of loudspeaker, as a speakers placement is just as important as its makeup or construction.

quadzilla

Thanks for pulling all this together into one place.

Bernal

quadzilla wrote: »

Thanks for pulling all this together into one place.

OK. Thank you. It is very helpful