GETTING THE SOUND TO REACH WHERE YOU WANT IT (THE AUDIENCE), AND NOT WHERE YOU DON'T (THE STAGE / THE WALLS / THE CEILING / THE NEIGHBOURS YARD)
Speakers and microphones all have "directional patterns". Microphones used in live sound are typically cardioid or hypercardioid, meaning they are more sensitive to the sounds in front of them than behind them, so we can place the mic at the right angle to pick up what we want it to hear, and reject the sounds we don't want it to hear. Speakers on the other hand aren't quite as directional as we'd like to believe. Most speaker systems are comprised of a number of sub-systems, such as subwoofers, mid-range woofers, and high-frequency horns, each a specialist for the frequencies it's tasked with reproducing. The electronics in the speaker system separate the frequencies and send them to the drivers that can most effectively amplify them. A high frequency horn can very efficiently reproduce high frequencies, but would not be able to reproduce low frequencies very loudly before damaging themselves.
In order to "point" sound the direction we want to, we need to look at each subsystem individually. High frequency horns (flaring ducts or curved cabinet faces used to passively amplify a source - think megaphone) are fairly directional, as the wave lengths of high frequencies are very short (in distance) and therefore can be controlled with a small horn lens. The lower the frequency, the longer the wavelength, and the larger the device must be to control its direction of propagation. Therefore a larger horn will maintain its specified pattern control to a lower frequency than that of a smaller horn. With human hearing picking up sounds as low as 20hz, and as high as 20,000hz, we need to be able to control wavelengths between 56.4 feet long and 0.06 feet long.
This creates some very unique challenges for the loudspeaker designer, and begs into question any speaker manufacturer who claims their speaker has a consistent horizontal or vertical pattern, regardless of frequency. Most "pattern angles" that are specified are within a certain range of frequencies, but will not be throughout the entire frequency range of the speaker, so it's important to understand what happens to the sound coming off the back of the speaker. Also, this pattern specified only means that the sound is 6dB quieter than directly on-axis, not that there's no sound emanating whatsoever. So if a speaker is listed as being 90 degrees horizontal, it doesn't mean that standing 50 degrees to one side would be completely silent, but rather than it would be quieter by 6dB or more, but that's still not silent. This affects stage monitors (lows being omnidirectional and being heard behind them in the audience) as well as the main house system (performers hearing the lows of the mix from the stage even with the speakers pointed away).
Other than horns, we can also use multiple drivers to create a directional pattern through sound wave interference, but you can't get "good" interference without accepting some "bad" or "destructive" interference (known as comb filtering, or large peaks and dips in frequency response in different parts of the coverage area). The best sounding system will always have the least number of speakers involved, reducing interactions that degrade sound quality and consistency throughout the audience area. Adding more speakers to get higher volume levels or better pattern control will always be a trade off for clarity and frequency response.
For two sources of sound to combine constructively they need to be spaced apart within a 1/2 of the wavelength of interest. So for a speaker reproducing 500hz, they would need to be less than a foot apart before the benefits of the second source become a disadvantage due to comb filtering. With subwoofers, their wavelengths are long, and it's fairly easy to add more and more subwoofers to gain sound level without comb filtering. With high frequencies, the drivers can only be so close to each other physically before it becomes impossible to achieve complete summation without comb filtering.
Line Array speaker systems have become quite popular in recent years, since it allows the sound provider to stock a number of small cabinets that can be scaled up or down for smaller or larger vertical audience coverage as needed. The idea is that Line Arrays utilize a number of speaker cabinets arranged in a long vertical array (shaped like a J typically) hung from motorized lifts above the stage. These systems increase their directional control vertically down to a fairly low frequency (based on it's length). Directional control is important for live sound, so it's obvious why this would be so intriguing. Line arrays that are shorter ("dash" arrays as some call them, since it's not a full line) due to site-line concerns, smaller budgets or limited trim height are not able to control the directional pattern of the lower frequencies as well, since the physical size of the array is smaller. At frequencies below 100 Hz (wavelength of 11.3 ft) the line array which is less than approximately 3 meters long will start to become omnidirectional, so the system will not conform to line array theory across all frequencies. Above about 400 Hz the directly-radiating (non-horn-loaded) driver cones themselves become directional, again violating the theory’s assumptions, and at high frequencies, many practical systems use directional waveguides whose behavior cannot be described using classical line array theory. In short, the geometry of real-world audio line arrays as used in public address systems can only be modeled approximately by line array theory, and only in the 100–400 Hz range.
Our Synergy Horn point source loudspeakers designed by Tom Danley allow a single loudspeaker to be deployed per side to maximum fidelity and uniform coverage of the audience area without all the interactions of a multi-speaker approach that degrade the sound in different locations in the venue and would be more suscepible to wind and destructive interference over distance. The SM80 from Danley Sound Labs can easily be heard at the other side of a football field while maintaining its coherence and intelligibility, using less equipment, power, and lifting infrastructure than a "dash" array while maintaining higher fidelity and consistency. They cannot replace a full-scale line array concert system, but for smaller events they are a much preferred solution to a scaled-down line array, allowing a single person to set up a concert-grade loudspeaker system with incredible fidelity and punch.