So you've decided to turn your promising massless speaker design into a product? Here's a few things you need to consider beyond does it work - and a few things for potential investors to ask the hard questions about before considering funding. It is surprisingly easy to make sound with one of these devices and far harder to make them sound good. Beyond that they are difficult to turn into a product due to current electronic product rules and laws. These rules have become much stricter over time so the earlier designs sold in the 50's to the 80's could not be sold today without some modification.
Massless speakers promise so much sound quality but achieving high fidelity can be hard. The first 95% is not usually a huge issue but getting that last part to create something that actually sounds good is really hard. Beyond frequency response and distortion the hardest issues are typically background noise and volume. Can it go loud enough? If you haven't solved these don't assume that you will.
Background hiss from high voltage conductors is in their nature. Ionic speakers that use higher voltages and more electrodes - in the form of wires or points - naturally produce more hiss. Plasma speakers with smaller and fewer electrodes produce less.
It is often said that as there is no mass in the driver there are no resonances and therefore a flat frequency response and no distortion. While these factors are no longer affected by the mass they can still occur. The speaker still exists in a real space and will have a cavity, electrode spacing and electrode design that can all affect the resonances, frequency response and distortion. You still need to pay attention to these design details.
Having exposed high voltage electrodes is usually a requirement of massless speakers. This can make them very dangerous and proper measures need to be taken to isolate the electrodes from the user. At the same time they can't be completely enclosed otherwise you can't hear them, which is kind of the point! So however this problem is solved, typically Faraday cage mesh is your friend here, it needs to be an integral part of the design from the start. Most countries require your design to be tested for electrical safety before it can be sold, this can cost a lot to get done properly.
Any high voltage electrode exposed in air will generate ozone and NOx, in varying amounts depending on the structure and voltage. These are toxic gases and can be produced in dangerous quantities quite effectively, especially in a closed room. You can't sell a product that emits these gases into peoples homes. There are many ways to address this but most are not perfect and will require some compromises and often some consumables.
EMI - interference
Exposed high voltage electrodes can be very good RF transmitters, even if your design doesn't use RF. Creating a product that wipes out local radio communications is not realistic and again your product needs to be tested that it complies with the countries EMC standards' you are selling into. As an example, you should consider that even your one off hobby design should not be blocking the local emergency services radios, your fun project could mean life or death for someone else.