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Hi Paul
Directional audio is not an exact science, so there are many elements to consider: content, space design, proximity to other audio, ambient noise, and desired fidelity to name a few.

The three major suppliers that I have used or at least demo'd are: Brown Innovations, Dakota Audio, and Holosonics. Each has its own method and all are much more expensive than conventional audio. 

The domes have the best fidelity.  The sound bars are good for close proximity at a low volume, and the flat panels are the most aesthetic, but lack full range fidelity/frequencies.  All have some amount of bleed-over outside of the 'focused' target area.  Each company has several options to choose from. 

That's pretty general, but is an overall assessment of current tech. 

Best,
Alan

1. The dispersion patterns on these sound bars is not what you'd expect based on how other speakers behave. Rather that projecting more or less in a cone shape out from the drivers, it's more like a donut that extends to the front, back, top, and bottom. We had tried mounting the bars on the ceiling, but the donut dispersion ended up casting sound all over the galleries. Using the visitors as baffles, in effect, cut down quite a bit on bleed.
2. The sweet spot for these speakers is within a few feet, and aligns with the exact center of the bar. If the visitor is even just outside that sweet spot, intelligibility starts to fall off. For music, this can have a significant impact. Since we can expect that visitors will stand at or near the center of the touchscreen as they use it, we have pretty good luck with this.
3. Keeping the volume levels low is critical. These things are designed to play within a few dB over ambient. After all, they rely heavily on psychoacoustic trickery to make the effect really work! I'm not sure about other manufacturers, but Brown provides software to tweak the settings on the amplifier to really dial this in: how many dB over ambient, output floor and ceiling, how quickly they respond to changes in ambient levels (e.g. how quickly should the amp adjust if a talkative school group walks in right as the audio starts playing?), etc. It's slightly tedious but the right settings make a huge difference.

1. The dispersion patterns on these sound bars is not what you'd expect based on how other speakers behave. Rather that projecting more or less in a cone shape out from the drivers, it's more like a donut that extends to the front, back, top, and bottom. We had tried mounting the bars on the ceiling, but the donut dispersion ended up casting sound all over the galleries. Using the visitors as baffles, in effect, cut down quite a bit on bleed.
2. The sweet spot for these speakers is within a few feet, and aligns with the exact center of the bar. If the visitor is even just outside that sweet spot, intelligibility starts to fall off. For music, this can have a significant impact. Since we can expect that visitors will stand at or near the center of the touchscreen as they use it, we have pretty good luck with this.
3. Keeping the volume levels low is critical. These things are designed to play within a few dB over ambient. After all, they rely heavily on psychoacoustic trickery to make the effect really work! I'm not sure about other manufacturers, but Brown provides software to tweak the settings on the amplifier to really dial this in: how many dB over ambient, output floor and ceiling, how quickly they respond to changes in ambient levels (e.g. how quickly should the amp adjust if a talkative school group walks in right as the audio starts playing?), etc. It's slightly tedious but the right settings make a huge difference.