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ACOUSTIC DIPOLES
WHAT IT IS
An acoustic dipole can be done with two speakers.
Two speakers, that's not an unusual setup ! Here is the standard stereo setup :
The acoustic dipole is what happens when one puts two such speakers back to back :
Here is a dipole prototype in "real life" :
WHAT IT DOES - DIRECTIVITY
It depends on what's fed to the speakers.
From a scientific pont of view :
Now from a perceptive point of view, the "figure of eight" radiation pattern gives very nice results when the listener stands in the middle axis of the speakers :
It gives an extremely strong "out of phase" sensation, and, if the room around is a bit reverberant, the sound seems to come from an often random point quite far from the speakers.
Also, the result in term of timbre is very soft, a bit remote, with not much edge.
WHAT IT DOES - SOUND LOCALIZATION
Sound localisation depends on the 'dipole mode' :
( +/+, omnidirectional) = Mode 1
( +/-1/2, cardioid) = Mode 2
( +/-, figure-of-eight) = Mode 3
MODE 1 :
sound comes from the dipole itself
MODE 2 :
sound comes from between the dipole and a reflective surface, if any
otherwise (no reflective surface), it comes from the dipole
MODE 3 :
sound comes from a random place depending on the listener
generally, sound is heard near a reflective surface
if there are no reflective surfaces, localization is even more random, but it never comes from the dipole itself
MODE 3, listener moves a bit:
localization moves as the listener does
CONSEQUENCES ON STEREO MIXES : RE-IMAGING
In a stereo music mix, there are in-phase and out-of-phase informations.
An in-phase information corresponds to Mode 1 - an out-of-phase information corresponds to Mode 3.
'Slightly out-of-phase' informations correspond roughly to variations of Mode 2.Thus, a stereo mix fed to a dipole is re-imaged.
Out-of phase informations - typically keyboard pads, for instance - are de-localized and heard coming from away from the dipole.
In-phase informations - typically kick drum - are heard coming from the dipole itself.
Interesting effects happen, such as :
Whereas the lead vocal is heard coming from the dipole, the reverberation associated to it is heard coming from the room's wall !
2D DIPOLES
Two dipoles can be mounted together :
This leads to the same kind of results, except that the dipole has now a variable axis :
3D DIPOLES
A Cube made of 3 dipoles.
The goal here is not to have 3 separate dipoles - the Cube actually emulates a single dipole which would be able to move in the 3 directions of space.
Of course, it would be possible to mount two speakers on a rotating device and get the same result, but that's not very practical ;-)
HOW TO MAKE A 3D DIPOLE MOVE VIRTUALLY :
A single dipole can be characterized by one parameter : the ratio between speaker 1 and speaker 2.
This parameter can be represented as a vector.
"r" ranges between -1 and 1.
With the cube, this vector can evolve in the three dimensions of space.
... with r - ie the directivity, varying as if it was a single dipole.
From an "instrumental control" point of view, in Max :
- the direction the vector takes is controlled by the Trackball object :
- the dipole directivity is controlled by a linear slider :
This "moving" dipole enables several insteresting effects derived from the already quite interesting effects that can be obtained with a static dipole.
For instance this phenomenon :
now can happen even when the listener doesn't move : it's the dipole itself that moves.
PSEUDO DIPOLES
It can be of great interest to put out of phase speakers next to each other in different positions.
For instance, this setup :
With small speakers, it gives the result that the listener standing in front of the speakers doesnt have the sound in the face, though the speakers are really in front of the listener's face
From a scientific point of view, it adds an orthogonal directivity component at wavelengths near and above the distance between the speaker's centers.
Other speaker cluster can be tried, such as :
... and different and interesting results will be obtained.