How do you get sound from a big panel? Continued ..
OK, now we know how static magnetic fields are created from bar magnets. How can we make use of these? Motors, cone drivers and Magnepans all rely on the same basic physical process to accomplish their jobs, they all drive a current through a magnetic field.
Current, force and a magnetic field are all fundamentally related by the following equation for a wire in a magnetic field
F = I L × B
F = Force on the wire
L = length of wire that carries the current subjected to ...
B = the magnetic field
I = the current through the wire
L × B = the Thiele-Small “Bl” product given for raw drivers
the bold faced characters are vectors, and the “×” is a cross product. This just means that if you take a x:y:z coordinate system, if l is along +x, and B is along +y, then the force will be out of the page along +z by this equation.
So it’s, in theory, simple, take a wire that you can send a current, or signal through, put a magnetic field perpendicular to it, then a force will be on that wire in the other direction. This way you can have a mylar sheet with wires embedded in it vibrate back and forth when you send an alternating current (music) through it.
Here is a picture (borrowed from the Magnepan website) that shows the ribbon tweeter.
You can see (after reading the previous page about bar magnets) that the magnetic field goes left to right.
The other design is the quasi-ribbon. This is a ribbon which doesn't quite have the same performance as the true ribbon because of the variation in the magnetic field with excurting distance from the magnets, and because the field strength mayn't be as strong. Its advantage is that it’s cheaper to make, and more importantly you can make a really wide planar-magnetic driver, which you must have to reproduce low frequencies. Also, high frequency planar-magnetic drivers see a single uniform source which precludes interference patterns as rows of true-ribbons could as reproduced wavelengths shorten.
But what if you want the performance of the ribbon (uniform, strong magnetic field) with the large panel size of the quasi-ribbon? Remember the previous lesson, where you could get very uniform and stong magnetic fields by having two magnets facing each other of opposite polarity? Do that, and put the conducting sheet between them, and you have a push pull design (one magnet pushes, the other pulls)
However, under the rule that you don't get something for nothing, you just segmented the output which can introduce interference patterns as the sound from the holes recombine.
How Maggies work, Part III
Now you've got these mylar sheets vibrating back and forth that push against the air and creates sound. Box speakers do the same, here's how they work
A conventionally configured sealed box speaker pushed a small cone back and forth, much like the big mylar plane in a Maggie. You can see that for a box speaker most of the sound goes one direction (monopole), forward, and that the box construction plays a critical part of how this makes sound.
Another way to create sound waves is by a bipole, where the front and back work against each other. Here is what that looks like
A planar speaker just has this big sheet of mylar hanging in the air, so it radiates as a dipole. Here's how that would look (using normal cone drivers in this example)
These create the following radiation patterns
Notice that the dipolar field shows a diminished off-axis response compared to the monopole, as opposed to the bipolar field which sees an increase. This is due to the contributions from the front & rear superimposed together. Our dipole has out-of-phase components which destructively interfere with other & cancel the sound with anti-noise. The position of this null is referred to as a cancellation node. FWIW, the bipolar enhancement is because the sonic components are in-phase which simply adds the 2 strengths together. This is called constructive reinforcement.