I searched for information on digital 40kHz ultrasonic parametric speakers. While most open sources do mention that you need some modulation, what type of modulation and how it does so is hardly ever discussed.
Since I want to understand how it works and make something not completely from a existing design I wondered What kind of modulation does it use and how does it need to be modulated?
I found people saying it is demodulated by the surface it hits, so my guess is it works kind of like digital audio RC filter based ultrasonic PWM for volume regulation (so it would be digital AM (1 and 0)).
For simplicity I will now assume digital audio with no difference in volume.
To be exact, how I estimate it works right now is that there is a main frequency on 40kHz and that that 40kHz signal would be turned on and off by the lower frequency audio signal. Then this will generate a virtual sound wave at the audio signals frequency.
Is this understanding of it right or does it use another principle, and if so would the method I described be realistically seen possible and why/why not?
My idea was to make the world's simplest ultrasonic parametric speaker which would use a +-40kHz square wave generator (1), and then that signal is connected to a transistor which is either on or off, this transistor will turn on and off depending on the values it gets from the audio. would this work. My other idea to add to it was to use a 3 component which would add volume control by playing a PWM at a frequency far above the 40kHz, for example at something like 400kHz, this is on the assumption that this will work like a volume control since the ultrasonic transducer is rated at 40kHz+-2kHz, so adding this higher frequency won't be picked up by the transducer and instead will be like the voltage is changed.
Are my estimations on how it works around right? If not, how do they work or what would be a better aproach?
1->(microprocessor or something like a 555 or Schmitt trigger inverter, but probably first a microprocessor (Arduino) to make it more available to others without tweaking.)