Dipole Speakers

Speaker… box. These things go together.  Right?  Very familiar, the speaker and its box, making the type of sound you expect to hear from a speaker.  The sound comes from a cone, or a dome, installed over a hole in the box.  Half the sound goes forward, toward you, the listener.  The other half of the sound goes into the box.

But what happens if there is no box?  Sound is created in two directions, forward and backward.  Both sides are the same, except they are mirror opposites.  In fact, if you could add the front and back together, layering one over the other, they would completely cancel each other, leaving silence!

With sound free to radiate both ways from a no-box speaker, the front and back form a pattern.  This pattern resembles a figure-8, or two circles side by side.  The figure-8 is the characteristic output pattern of a Dipole Speaker.  Just a diagram, but what this shows is the front sound as one circle, like in a bubble, plus the rear sound in another bubble.  And what is outside these two bubbles?  The front and back are opposites, like matter and anti-matter.  They erase each other where they mingle, to the left and right sides of the speaker.

If you listen from the side, at 90° off-axis, hearing the front and back equally, then you no longer hear the speaker directly.  In fact what you do hear is surprising, because the sound seems to come not from the speaker but from the room boundary closest to you.

Controlled Directivity

On the one hand, a dipole speaker sends just as much sound energy to the rear as it does to the front.  Sometimes people assume this means it splashes sound everywhere.  But on the other hand, there is this figure-8 pattern where the sound gets erased toward the left and right.  In fact, dipoles are a type of Controlled Directivity speaker design.

Other examples of Controlled Directivity speakers typically have horns or waveguides which shape their output pattern.  These features are effective within a certain frequency range, corresponding to their dimensions and the wavelengths of sound passing through.  Pattern control for this type of speaker is lost below a certain frequency, where wavelengths are too large to be shaped by the speaker’s geometry.  Whatever pattern may be created at midrange or treble frequencies, the bass ends up omnidirectional, like the light radiating from a bare light bulb.

In contrast, a dipole directivity pattern can be maintained continuously from bass to treble.  All a dipole needs in order to form the figure-8 pattern is symmetry (front / back) so the sound at both sides is equal (but opposite) plus a physical size that is not so large it gets in the way of the sound.

Ideal size depends on the frequency being generated, and smaller size is better for higher frequencies due to the shorter wavelengths.  Extending dipole behavior into the treble range requires a very compact design.  At bass frequencies, a larger baffle (in width and depth) helps to improve efficiency.

This explains the tapering shape of the IAQ-1.  The lower cabinet is relatively wide for bass output (where wavelengths are longest) then the upper baffle narrows and drops to 1″ thick for the midrange, then narrows further at the top for the tweeters.

Unlike a planar dipole such as an electrostat or magnetostat, this design avoids excessive directivity at higher frequencies, because each driver is a point source within its range (meaning the diaphragm is small relative to the wavelengths it generates).  We call this a minimalist dipole, or a point source dipole.

How Does It Sound

Enough about how it works—how does it sound?  Three characteristics you can’t miss are the transparent midrange, the bass clarity, and full room tonal balance.

First, the open back midrange offers exceptional clarity and transparency.  Without a box, there’s not a chamber full of reverberating energy behind the drivers, coloring the sound.  But the sense of transparency goes beyond that, because the sound effectively travels through the speaker rather than radiating from it.  Well made boxed speakers offer good transparency compared to lesser models, but this is a different league.  With a great recording and proper setup, you get a sonic hologram, a three-dimensional presentation of sound that seems to come out of thin air.

Next feature to jump out is the bass.  Open baffle speakers can convincingly reproduce the presence of an acoustic instrument, such as a timpani drum.  The bass just sounds more real than a conventional speaker.  That clarity results from the figure-8 directivity pattern, which extends to the sub-bass region.  The sound that is cancelled to the left and right of the speaker effectively reduces the total amount of bass energy in the room, relative to what you hear at the sweet spot.  That means there is less reverb or feedback from the room to wash out detail and texture in the bass.

Finally, another trick with the figure-8 is how it maintains the tonal balance, meaning the relative amounts of bass, midrange and high frequency energy.  The balance remains remarkably consistent even as you walk all the way around the speaker.  From different vantage points the loudness will vary, and the soundstage always comes together best at the sweet spot, but the character of sound stays consistent throughout the room.  This comes in handy for folks who listen with friends, or for playing music in the background while doing other things.