The incorrect statement among the given options is option 2: "Sound waves having frequency below 20 Hz are known as ultrasonic waves."

To understand why this statement is incorrect, let`s analyze each option:

Option 1: Sound waves in gases are longitudinal in nature.

This statement is correct. Sound waves are mechanical waves that travel in a medium by compressing and rarefying particles. In gases, the particles move back and forth in the same direction as the wave propagates, making sound waves longitudinal.

Option 2: Sound waves having frequency below 20 Hz are known as ultrasonic waves.

This statement is incorrect. Ultrasonic waves refer to waves with frequencies above the range of human hearing, typically above 20,000 Hz. Sound waves below 20 Hz are called infrasonic waves, and they are also outside the range of human hearing.

Option 3: Sound waves having higher amplitudes are louder.

This statement is correct. In general, the amplitude of a sound wave determines its loudness. Higher amplitude means greater variation in pressure and leads to a louder sound.

Option 4: Sound waves with high audible frequencies are sharp.

This statement is correct. High-frequency sound waves correspond to a higher pitch or "sharp" perception of sound. Low