< img SRC = "/ Uploads/Editor / 2020-05-18/5 ec243f303843. Jpeg" Alt = "" / > < / span > < / p > < p > < br / > < / p > < p > < span style="color:#222222; font-family:Arial, Verdana, sans-serif; background-color:#FFFFFF; font-size:16px;" > understanding of ultrasonic frequency we all know that sound waves above 20,000 Hz have the best directionality, the strongest penetration, and it is also easier to obtain concentrated acoustic energy. Sound waves less than 20Hz are called infrasound waves. Ultrasonic infrasound