BenLaw
Well-known member
oldric_naubhoff said:
If you just read the bits with 'dipole' subheadings it's all there, but no problem. There are also some decent, simple diagrams in the article. I'll include all the basic setup as an easy reference for all:
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.
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 Monopole[/b]. 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[/b], 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[/b].
Dipolar Speaker[/b]
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 t
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.
In the above diagram, note that the bipole field shows a larger off-axis response (the middle part of the center 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 on the right) 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.
Reflections from the Back Wall for Dipolar Speaker[/b]
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[/b] 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.
Music:[/b]
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 (monopole) models send sound directly toward the listeners’ ears.[/list]
Conventional direct-radiating monopole 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.
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