If you want to record silent speech, where someone is producing sound waves but the audible sound is not present or very faint, neither an infrasound nor an ultrasonic microphone would be suitable for capturing the speech accurately.
Infrasound refers to sound waves with frequencies below the range of human hearing, typically below 20 Hz. Ultrasonic sound waves, on the other hand, have frequencies above the range of human hearing, typically above 20 kHz.
Silent speech, which involves the production of sound waves without audible sound, typically falls within the audible frequency range (20 Hz - 20 kHz). Therefore, using an infrasound or ultrasonic microphone would not capture the intended speech signals accurately.
To record silent speech, specialized technologies like throat microphones or electromagnetic articulography (EMA) systems are often used. Throat microphones detect vibrations directly from the larynx or throat, bypassing the need for audible sound. EMA systems utilize sensors placed on articulatory structures, such as the tongue and lips, to capture movements and positions during speech production.
These specialized techniques focus on capturing the subtle physical movements and vibrations associated with speech production, rather than relying on audible sound.