Which one of the following statements is not correct ?

Ultrasonic waves are high-frequency sound waves (above 20,000 Hz) that behave like audible sound. Statement 1 is incorrect because ultrasonic waves undergo reflection, refraction, and absorption when they encounter material boundaries or internal discontinuities. These principles are fundamental to Ultrasonic Testing (UT), where reflected waves (echoes) are analyzed to detect internal defects like cracks, porosity, or inclusions in materials. Statement 2 is correct as UT is a standard non-destructive testing (NDT) method for characterizing material flaws. Statement 3 is correct because ultrasonic machining uses high-frequency vibrations to drill or shape extremely hard materials like diamond. Statement 4 is correct because sound is a mechanical wave requiring an elastic medium (solid, liquid, or gas) for propagation and cannot travel through a vacuum.