To improve the quality you can pre-distort the output signal. Taking the square root works quite well, but expands the bandwidth significantly (infinitely, in theory). There is a lot of literature on pre-distortion with bandwidth constraints for telecom power amplifier linearisation. You will also need a linear amplifier to power the array.
The ultrasonic transducers used in this post are very narrowband, having a resonance peak of merely a few 100Hz. You can reduce the Q factor with resistive loading but the output power significantly drops. It seemed these transducers quickly start making an audible whining noise when used for continuous transmission at higher powers. I don't know what caused that, apart from this effect they seemed to hold up for essentially infinite duration.
Using a larger wideband ultrasonic transducer instead of an array of small narrowband transducers again increases the sound quality a lot. We did not find a commercial supplier of such transducer for a reasonable cost, but made some improvised custom electrostatic ones with conductive foil. There is a lot of literature on how to construct ultrasonic transducers but this is not my field.
You will not be able to play bass notes due to physics, the power required would be insane.
A "full range" speaker will send the lows in all directions but the highs mainly in the direction of its axis. A listener caught in the beam will hear a shrill sound, whereas someone off axis hears it muffled. Guitar speakers are like this; particularly the 12" ones and particularly in the 4x12 cabinet arrangement. Sometimes musicians use dispersing devices mounted on the speakers, like "beam blockers". Or the speaker is picked up by a microphone close to it, so that the audience hears it fro the PA system (which solves the sound dispersion problems in its own way).
There are situations in which it is desirable for a speaker box to "beam", like when it is mounted far away or high above a target listening area that is relatively small, calling for the speaker to be a kind of spotlight.
Visually the technology for two people to see different video on the same TV has existed for a while, there's just no demand or market for it (either glasses syncing with TV to block certain frames, or there was a technique that depended on the angle you're viewing from).
And for tracking people walking around the room, to then know where to point audio or video at, there's released & integrate-able technology available like head and eye tracking from Tobii.
Is there also some very expensive option for having audio split between people in the same room (without using any devices like earphones) and just equally no general demand from consumers wanting to use or pay for it, or do the laws of physics prevent sound waves from working well this way?
Would this just be due to the fact that the reflecting surface isn't perfectly smooth so the reflections do not reflect back 180° and pretty much scatter up reflection?
With proper mixing you could emit different sounds in different directions, at the same time.
So, right, just thinking from the OP, between mirrors there was some highly favorable line of amplification, and that line meant that the beam out of the laser would be an extension of that line and form a "narrow" beam!!!
Right, if use some voltage on some piezoelectric crystal to make tiny adjustments in the distance between the mirrors, then will make small changes in the frequency of the light, i.e., there is a highly favorable wavelength that fits a whole number of times between the mirrors or some such.
The changes in frequency of the light still have to correspond to the thermally moving gas atoms generating the light. Right, if have the favorable frequency in the middle of the feasable range, will get slightly less power in the beam, a dip, called the Lamb dip. Could that dip be used as a length standard? First job, worked on that, physicist, NIST, then the NBS, US National Bureau of Standards.
That is, at the end of the laser we have a tiny light source that puts out a very narrow beam. How? As above and not from antenna theory.
https://www.youtube.com/watch?v=yVDWrWpaBho
https://www.instructables.com/Acoustic-Levitator/
https://pubs.aip.org/aip/rsi/article/88/8/085105/962938/Tiny...
https://www.youtube.com/results?search_query=acoustic+levita...
There's some interesting applications combining it with projection here (Acoustic holography):
https://www.youtube.com/watch?v=Q9GybXczNAc