Surround emulation: How does it work?
The GT5051’s systems isn’t entirely new. In 1999, Altec Lansing already introduced speakers to emulate 4.1 sound with the ADA310, which had two speakers and one subwoofer. The GT5051 satellites share the same architecture. On the lower part of the satellite is a 3” speaker which reproduces frontal voices pointing towards the listener. On top is another 3” speaker pointing to the side and back. These two Side-Firing Dipole speakers emulate the surround channels found on 5.1 systems.
How does it work ?
To explain the functioning of this speaker set Altec Lansing sent us a rather simple diagram. We can see that the frontal and central voices work in the normal way sending sound directly to the user. It gets trickier with the Side-Firing speakers, which emulate the surround voices. Sound is supposed to bounce off the side and/or back walls and come back to the listener in what is called the principle of sound reflection. This only works if your walls are located in a certain position, which isn’t necessarily the case for everyone. As you can see the red arrows point to the front and also to the back of the satellite bouncing back on a rear wall.
5.1 sound emulation is based on two principles. First of all, speakers need to be placed with their back to walls with a high reflective capacity (like stone, concrete or metal). And the listener should be right in the middle of the two walls. This may already pose a problem for some people as their office or home environment may differ. The second principle is that sound must be resituated by the Side-Firing Dipole speakers.
Depending on the frequency, sound dispersion of standard speakers has the shape of a globe or flat ellipse (the angle increases with the frequency, the higher the frequency the more elliptic the globe is). In theory it is then possible to rely on high frequency sound directivity at some point. This is the reason why some speakers include diffraction grids for tweeters dedicated to treble. With the GT5051, Altec Lansing uses dipole speakers to reduce sound dispersion and increase directivity. A dipole speaker emits sound to the front and back of the speaker and uses the phase to reduce sound emitted to the side. You may notice that the top satellite, the one for surround sound, has a grid in the back to let sound through.
So with this type of speaker, surround voices don’t point directly to the user but bounce off the walls and come back on the side.
In order to have a surround effect with dipole speakers you need to be meticulous in their positioning. They need to be placed at least 1.2 meters apart and the frontal speakers need to point directly towards the listeners.
