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« KEF's Z-Flex Surround: Smoother HF Through Science | Main | Better HF Through KEF's Stiffened Dome Technology »
Jul 30 2014

Why So Called "Coaxial" Speakers Aren't Uni-Q

First, a few definitions:

  • Coaxial: Geometrically speaking, two (or more) three-dimensional forms that share a common axis. Triaxial would specifically refer to three three-dimensional shapes sharing the same axis
  • Paraxial: Refers to two (or more) three-dimensional forms that lie in close approximation to each other and that form a small angle between each other
  • Point Source: A single localized source not perceptibly distinguishable from other sources
  • Directivity: Referred to as 'Q,' the measure of the radiation pattern from a speaker. A loudspeaker with a high degree of directivity (narrow dispersion pattern) is said to have a high Q.

Now that we've got that out of the way, back in the day, the Holy Grail of audio enthusiasts was the coaxial speaker, and if you were really advanced, the triaxial speaker.

The photograph at left shows a very cool "triaxial" speaker from a 1955 Pilot Console Hi-Fi Set . The speaker is from University Sound which was an early division of the Altec Company. The advertising copy listed the speaker as "diffaxial," with three drivers mounted concentrically to each other. The size of the HF driver is pretty substantial, as the LF driver looks to be about 12" in diameter.

As coaxial speakers evolved in the years since this behemoth was first introduced, one of the main problems to developing a truly coaxial speaker was the sheer size required to make a decent HF unit. One look at the magnet in the Pilot/University Sound speaker and we can see that even with all of that space, the speaker was still not truly coaxial. Just sticking an HF unit in the center of an MF unit doesn't solve the problem of point sourcing and phase coherency.

Right about now you might be wondering why you should even care about any of this.

Right Now I'm Wondering Why I Should Even Care About Any of This

To truly replicate a musical performance, the concept of acoustical point source needs to be taken into account. When you speak, regardless of whether you use your lowest tone or your highest tone, all of the sound (basically) comes from one point: your mouth. To take this one step further, a recording engineer will generally use only one microphone to record your voice. A sound generated from a single source point is recorded by an instrument (the microphone) that is a single point receiver: All of the frequency components of the sound (in this case your voice), come from one source and are then captured by one receiver.

A recorded speaking voice isn't a particular challenge for most speakers as the human voice is limited to a rather small range of frequencies (think about the speaker in your phone).

That is unless you want the playback to sound as much like the original as is technically possible.

  • Male singing voice typically ranges from about 100Hz to about 800Hz
  • Female singing voice typically ranges from about 250Hz to a little over 1kHz
  • Our speaking voices are far more limited. My regular speaking voice sits right around 200Hz but when I say words with T s and S s in them I can measure components that go well into the 4kHz range

To reproduce sounds as faithfully as possible, two (or more) drive units are required. Large, heavy speakers do a great job reproducing deep bass but just can't cut it for higher range instruments and sounds (like flutes and cymbals). Conversely, drivers that are quick and light enough to reproduce high frequencies generally begin to distort as soon as they are asked to reproduce bass sounds.

Let's take a listen to Yo-Yo Ma and Bobby McFerrin (along with guests Marc O'Connor on violin and Edgar Meyer on contra bass) performing Hush Little Baby. Not only is this a fabulous performance of a simple song, but it will also help illustrate the concept of single point sourcing.

There are several ways to look at single point sourcing.

From the audience's perspective, the stage is an array of single point sources (the cello, McFerrin's voice, the contra-bass and the violin) that all work together to produce a single, cohesive musical performance. Add the reverberation of the room and the ambiance of the audience and you get the full experience of a live performance.

From a recording engineer's point-of-view, there are also multiple single-point sources. At the 2:31 mark we can see that there are four separate microphones used to record the performers on stage, (there are three stage mics on boom stands and one handheld microphone). There are also mics placed in other strategic spots on the stage and around the theater to capture the ambiance of the space. These sounds, all captured by single-point receivers (the microphones) are then mixed together resulting in a recording that replicates (if everything is done right) what the audience heard. The tricky part is when you take the recording home and try to replicate it on your home stereo, and the purest way to do so is with a loudspeaker that is in and of itself a point source.

Now let's look at the individual components for a moment to illustrate just how hard it is for a speaker system to replicate this performance:

  • The contra bass is producing frequencies from around 50Hz to around 130Hz while McFerrin's whistle is producing frequencies around 2kHz
  • When McFerrin scats (vamps), his voice is around 600Hz and when he sings he's between 300 and 500Hz
  • The cello is around 500Hz and the violin is up and over 2kHz in some spots
  • Add harmonics and the ambiance that makes a performance "real" and you've got frequencies around 3.8kHZ (voice harmonics), 5kHz (violin), and 7kHz and 9kHz (McFerrin's whistling),

Even in a very simply orchestrated piece, that's a lot of work for a loudspeaker!

In the audio industry it is not new knowledge that to achieve single point source reproduction the acoustic centers of the MF and HF drive units should be in the same place. But as our 1955 hi-fi showed earlier, sheer physical size was an issue, and developing the technology to achieve single point source reproduction was not easy.

To get around this, loudspeakers are designed with the HF mounted above the MF, or with both drive units mounted side-by-side (paraxial). The problem with this is that the sounds come from two distinct locations. Even though McFerrin's whistle, scat singing and regular singing all originally came from the same place, they would be handled by different drive units in a loudspeaker. It's easy to see that if one drive unit is mounted to the left or right, or above or below, another drive unit trying to replicate the same performance, the sound comes from two places, and inherently will not arrive at the same time at the listener's ear.

With one drive mounted slightly ahead of the other as seen in the picture of a conventional coaxial speaker to the left, the sound may come from almost the same physical location on one axis, but phasing becomes an issue if the drive units are not aligned in space properly on all axes.

Both of these issues can cause phase incoherency or confusion that gets processed by our brains unnaturally, resulting in performance reproduction that isn't life-like.

The answer to this problem was found in a magnet made from a combination of neodymium, iron and boron that is ten times stronger than a standard ferrite magnet. The strength of this new magnet material allowed us to build a HF unit that was small enough to fit inside a standard-sized MF drive unit, thus allowing our engineers to find the precise placement point of the two drivers so that they were completely coincident with each other. This 'coincidence' allows all of the upper and lower frequencies of a single performance to actually emanate from the same place in space.

Put technically, the bass and treble units are time-aligned in all directions in the three-dimensional plane, not just in one axis as with vertically separated units like the speaker shown above. This mis-alignment, known as the vertical interference pattern , can result in a very tight 'sweet-spot' ( high Q ) or area of high-quality sound reproduction that is often less than 10 degrees above or below the principle axis.

Translation: Only one person gets to hear the high-quality sound and everybody else gets to hear a mish-mosh of phase incoherency.

Also, in a conventional design in addition to the unwanted distortion and audio confusion, there is an energy dip at the HF/MF crossover point which results in further resonant distortion in the listening area. By being able to precisely time-align our drivers in all directions from the axis, Uni-Q technology completely eliminates both of those problems.

Because it is mounted at the center of the MF unit, the HF unit's directivity is also governed by the angle of the MF unit's cone – the HF directivity is the same as the MF directivity. This means that as the listener moves away from the main axis, the output of both the HF and the MF reduce at about the same rate, increasing the tonal balance and the stereo imaging throughout the listening area.

To quote the engineers who figured this all out: " From a listener's perspective, the combination of the matched directivity and precise time alignment in all directions gives significantly improved stereo imaging over a wide listening area, the realism of which is enhanced by the even balance of the reverberant energy within the listening room ."

The picture at right shows the time-aligned and concentric single point source Uni-Q used in our Blade speaker. Hit the link for more detailed information on our Uni-Q technology .

Jack Sharkey for KEF .

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