Ai’s proprietary waveguides form the backbone of the company’s high sound quality offerings. While trying to overcome the myriad problems presented by high frequency reproduction in horns, one of the key advances made was the development of a new theory of horn design now referred to as waveguides. Acoustic waveguides were first introduced to the industry through the work of Ai’s Dr. Earl Geddes (Acoustic Waveguide Theory. JAES Vol. 37, pp. 554-569 [1989 July/August]).

Waveguides provide well controlled high directivity at high frequencies. High directivity in loudspeakers is desirable because of the way loudspeakers interact with the room and how people perceive sound. The sound coming directly from the loudspeaker and reaching the ear is often considered to be the most critical. While this is partially true, the sound that is reflected around the room before reaching the ear is just as important, since it actually comprises most of what we hear as timbre or coloration. Some reflected sound is better than other. Sound arriving before a time threshold of approximately 10 to 20 milliseconds (depending on frequency) will confuse the human hearing mechanism and affect the quality of sound, typically the perceived image created by the loudspeakers. Sounds arriving beyond this threshold are interpreted by the brain as early reverberation, hich is a desirable aspect of a room’s contribution to the overall sound usually called spaciousness.

In order to lower the amount of reflected sound reaching the ear within this time threshold, it is desirable to have a loudspeaker with narrow directivity. Ai’s loudspeakers utilize waveguides that provide an even virtually flat coverage pattern that extends 22.5° from the loudspeaker’s front axis, forming a 45° arc. Beyond this, extending to a full 90° arc, the sound gradually tapers off in level to -6 db at 45°, and deviations from a flat response only occur outside of this angle and are down significantly in level.

This high directivity allows placement that reduces early reflections (when the loudspeaker is placed close to a side wall, for example). And because the sound gradually tapers off moving off axis, any reflections that do occur will be lower in level, reducing their negative effects. Equally important is that the offaxis sound closely matches the direct sound, hence the reverberant field in the room resembles the direct sound leading to a further increase in clarity.

While horns are normally used to provide high directivity, they usually achieve that directivity by using diffraction. Diffraction can be thought of as another type of (very early) reflection that creates a strong perceptual degradation of the sound quality. Diffraction, and higher order modes, are the main reason traditional horn loudspeakers have an objectionable sound quality. Higher order modes (HOM) are sound waves that propagate down a waveguide or horn non-axially. An easy way to envision this is to picture a two-dimensional horn with sound bouncing from one side to another on its way out. This sound, which travels a complex path, becomes delayed from the sound moving directly out of the waveguide. This delay is very short, falling within that 10 to 20 millisecond threshold, becoming another form of unwanted, very early reflection.

Reducing the level of HOM in Ai’s waveguides is accomplished by the use of a patented Refractive Waveguide PlugTM. This open cell foam plug absorbs some sound in the waveguide, but because the HOM travel a greater distance (side to side as opposed to straight out) through the waveguide, they are attenuated more by the foam than the sound passing straight through. The end result is a waveguide with the desirable characteristics of constant directivity and high sensitivity that does not suffer from the colorations that characterize other horn or waveguide designs, or more simply put, horns that do not sound like horns.