What do people love about dipole speakers? One of the most frequent comments we hear is that the musician is “in your room, as large as life.”
Recall that we are discussing the dispersion patterns of two different popular speaker types: the dipole versus the direct radiator in a closed-back cabinet. We saw last week how the output of a typical direct radiator speaker is highly directional in the upper treble but mostly omnidirectional in the bass range.
Now let’s see how dipole speakers differ, then see how that can be used to your advantage in a listening room.
Part 2: Dipole Loudspeakers
The dipoles we’ll look at are the are electrostatic, magnetic-planar and ribbon speakers. These have in common very thin, light membranes for sound reproduction. Below are some examples of each type.
They can range from relatively compact Quad electrostatics…
…and the venerable MartinLogan CLS…
…to huge Magnepans…
…and exotic ribbons (Alsyvox below).
What do they all have in common? To keep it simple, the entire audio output exhibits a figure-8 pattern, wherein the front output is 180 degrees out of phase with the rear output. Why is this called a figure-8? Because of the phase inversion, the output in the plane of the speaker (side to side and vertical) is mostly cancelled. Cool! *note: many open-baffle speakers do not emit upper treble from the rear.
What do we love about these speakers? The fast transients and intricate detail can be entrancing, extracting elements of music you may have never heard before. Sound stage depth is almost as natural as a live music performance. Most of all, the nearly uniform power response provides equal tone to the direct and reverberant/reflected signals.
But, Then Where Does the Sound Go?
Unlike the case with the direct radiator speaker, the rear wave launches to the wall behind the speaker (the front wall) at full strength.
What does this mean to the room interaction? Well, a big bite is taken out of the “head end ringing,” as very little sound energy exists in the plane of the speakers. The side walls, ceiling, and floor do not stimulate the early mid bass reverberation that builds up your running noise floor and destroys transient detail and harmonic overtones along with musical timbre and rhythm. Also, the upper treble range reflection from the front wall can provide great sound stage depth and ambience.
So it’s all roses? Sorry, not quite. The back wave has nearly equal power to the front wave, and this causes a time delayed replica of the direct signal, via reflection off the front wall, that can cause extreme comb filtering and dropped notes depending how far your speakers are set up from the front wall. And sadly, the narrow horizontal pattern can prevent the glorious sound stage width you wished for. This back wave can be a blessing… and a curse.
OK, Now How Do I Control This Beast?
You may notice a pattern about to emerge. Pop quiz: what sort of portable, adjustable device can we think of that we could simply set behind a speaker that would maintain the treble reflection level while controlling bass range cancellations and eliminating comb filtering?
And one more tip: you can manipulate the width of your sound stage by rotating the diffuser portion of the TubeTrap. Rotate them outboard to gain stage width, and rotate them inboard to gain center image clarity and presence.
Behold the glory of the MartinLogan Neolith with a nice array of TubeTraps to optimize the back wave.
Hear an open-baffle Dipole in Action.
Still Not Good Enough?
Your room may be too small to enjoy the full benefits of large dipoles because they are simply too tight against the front wall. More TubeTraps will help, but sometimes you need to face the music and change the type of speaker you spend your special time with.
Hopefully the last few weeks have provided some insight as to which of the two major dispersion pattern types are more suited to your listening preferences and room limitations. Perhaps it is time to give yourself the gift of a new pair of speakers, and a set of TubeTraps to match.