The dispersion characteristics exhibited by our waveguide geometry causes the emanating sound wave to be radiated into the environment with a 120° horizontal angle, from approximately 1,000 to 10,000Hz. This means that as the frequency varies, the area that is "illuminated" by the driver remains fairly much the same over a 120° arc, symmetrically and directly in front of the waveguide. Without some form of wave "directing" device this simply is not possible.
It should be pointed out here though that in our waveguide design we make no claims to what is commonly referred to as “Constant Directivity.” Much ado has been made about the advantages of CD performance in recent years, but in more near-field applications, it is our belief that such advantages are over rated. To be sure, in professional sound system engineering for live concert venues, CD performance is a highly desirable attribute. But then, the engineering criteria for such an environment is considerably different than that of other smaller, commercial or domestic uses. It is Aether Audio’s position that if one focuses almost to the point of exclusion on optimizing the CD performance of a waveguide device and/or completed system, they do so at the expense of other, more important performance characteristics.
Specifically, any waveguide device designed to optimize CD performance down to the 1KHz region will be quite deep with regards to the distance between its entrance (the throat) and its exit (the mouth). In addition, the mouth of such a device would also be required to exhibit a considerably wide diameter (assuming a spherical geometry). When these two aspects are combined, the result is a device of considerable size and difficulty to integrate into a product of acceptable market proportions.
Another drawback that such a device would tend to exhibit is what, in engineering terms, often referred to as Higher Order Modes. "HOMs" are primarily the result of the relatively great depth such a device would exhibit. They are essentially manifest as standing-wave reflections that take place down the length of the device, and which propagate both parallel to the axis of radiation as well as orthogonally with respect to it. Parallel HOMs are invariably the result of insufficient mouth termination causing impedance reflections as the wave meets the invisible boundary presented by the abrupt transition to the air load (please reference the discussion above regarding acoustic impedance). Due to the large mouth diameter such a CD device would require, it is all but impossible from a practical standpoint to provide the needed additional area that a smooth transition to the outer baffle would incur. This is because as the aspect ratio (depth to mouth diameter) of the device increases, the required smoothing area increases almost exponentially.
Conversely, the orthogonal HOMs result from reflections that take place across the infinitely varying diameters of the waveguide surface that the radiation encounters as it travels down the length of the device. Seeing that such a device is so deep, both parallel and orthogonal modes have far more opportunity to develop as the wave makes its progression from beginning to end of the device.
Heroic attempts have been made to dampen and thereby minimize the amplitude of such HOMs by the use of semi-acoustically transparent foam plugs (and what-not) for use in devices developed by others, but in the end the whole effort seems to be an exercise in the development of a “Rube Goldberg” contraption – at least in our view.
In addition to the above, in order to “push” CD performance down to the lowest possible frequency in a system, the designer is then highly motivated to use a woofer that exhibits matching dispersion characteristics to that of the waveguide. The idea is to select a woofer that exhibits a dispersion pattern that complements the total system by continuing to extend the dispersion angle of the waveguide to as low of a frequency as possible below the chosen crossover frequency. Now, a given piston membrane will exhibit a narrowing of its radiation pattern as operating frequency increases, and this characteristic is directly proportional to its increasing diameter. That being the case, the designer of a true CD system would naturally choose a woofer of relatively large diameter, as such a larger diameter device would then extend the narrowing effect to a lower frequency.
The primary problem with the entire approach is that this larger diameter woofer is, with all thins being equal, required to also exhibit a sensitivity rating that is high enough to yield a total system sensitivity that is commensurate with market expectations. That being the case, a larger woofer must posses a relatively light diaphragm. Well, large woofer diaphragms of lightweight design are relatively free to flex and generate “break-up” modes as discussed in the WOOFER section of our Technology page. At this point we are almost embarrassed to point out that break-up modes are essentially the same thing as… HOMs!!! Once again these nasty things have reared their ugly heads. Much to the chagrin of the CD system designer, even if he has managed to greatly eliminate them in his waveguide by using special treatments within it, he has now painted himself in the corner again by selecting such a large woofer. In essence, he has traded one devil for another just to force one performance issue. As desirable as true CD performance may seem to be, focusing on developing a system that exhibits that one characteristic above all others leads down a dead-end street to an unbalanced design.
While the CD concept may look good on paper, in real-world applications for the home and studio, such a device is excessively cumbersome to produce and then place in a domestic or commercial studio environment… and essentially unnecessary. We have found that as long as a given waveguide provides a smoothly changing dispersion pattern and behaves in a quasi-CD manner over a large part of its operating range, it provides totally sufficient dispersion characteristics in the typical environment for which it was designed. It then takes on a much more manageable form-factor as well.
Specifically, the necessary aspect ratio is greatly reduced as compared to an equivalent “true CD” design, and the generation of significant HOMs are almost completely avoided do to the device’s relatively shallow depth and fast flare rate. Along with this, smaller diameter woofers can now be selected that exhibit slightly higher mass and therefore a more rigid construction that avoids the break-up modes (a.k.a. HOMs) as discussed above. The combination of such a woofer and waveguide system then offers the potential to virtually eliminate distortion in the form of HOMs across the entire audio spectrum, and this is the path we have taken at Aether Audio in the development of our cornerstone device – the waveguide.
In retrospect of our above arguments, we feel we may have diminished the importance of what dispersion control our waveguide does provide. Quite to the contrary, we feel that it is equally important to point out that typical drivers mounted on a flat baffle produce a coverage pattern that often varies considerably. This is not good, as many times each individual driver will be found to exhibit a very broad dispersion pattern at the lower end of their operating range, and then begin to narrow as the frequency is increased. This effect is then repeated for each successive driver in the system. Such erratic and unpredictable behavior makes system placement difficult and the optimal listening area generally narrower than would be preferred. The problem is highly multiplied when multi-way (3+) driver arrays are employed.
The advantage that moderately controlled dispersion offers is quickly realized when one is faced with "not so perfect" room placement options. In more ideal settings, the reduction of reflected sound from nearby walls permits the natural ambiance in a given recording to be heard clearly, with less of the masking effect caused by early room reflections. A 120° dispersion angle is ideal for achieving this while still providing a very wide listening area.
Professional recording engineers will most certainly appreciate such controlled dispersion characteristics. The ability to monitor recording sessions with near-field accuracy and mid-field freedom of movement is a combination seldom experienced and highly sought after.
In addition, with the recent advent of home theater, designers have sought to find ways to control the vertical dispersion of their designs in order to comply with the THX specification. A common technique is to employ two or more drivers operating over the same frequency range that are spaced some fixed distance apart. This method causes destructive interference at angles off-axis vertically from a pre-determined listening position. While this does effectively limit vertical dispersion to some degree, it most frequently does so in a rather erratic manner. Upon close analysis it can be found to produce a vertical pattern that varies considerably more with frequency than would be ideal. A stable, controlled pattern over the broadest possible range of frequencies is much more preferred and is a natural by product of waveguide technology. Hence, you will find that Aether Audio products are inherently more "home theater ready" than many products advertised as such and without their inherent musical limitations.
In our final analysis regarding dispersion, we wish to make clear that while controlling it to the highest possible degree is a very desirable thing to do, it is not the “be-all and end-all” of either waveguide or total system design. Rather, it should be incorporated into system design from a balanced approach that emphasizes the reduction of system distortion as the highest priority.
This is mainly because it has been shown scientifically that human hearing has a very acute ability to differentiate between direct sounds and those originating from reflections that take place approximately outside of a 10 to 20-millisecond time window. Inside this window the brain tends to merge direct and reflected sound, so artifacts such as diffraction and early room reflections should be avoided if at all possible. Outside of this time window though the brain has the ability to separate the direct sound coming from the loudspeaker and other reflections originating from elsewhere within the environment.
That being the case, diffraction, early room reflections (particularly at high frequencies) and other more traditional forms of distortion should be avoided whenever possible, and should remain the primary criteria for domestic and professional studio loudspeaker design. At Aether Audio, this is our philosophy on the matter… and this we have done.