If the frequency of a sound wave is below the threshold of human hearing, it is defined as.

• A. Audible
• B. Ultrasonic
• C. Infrasonic
• D. Vibrational

Answer: (C)

The main idea is how we categorize sound by the range of frequencies humans can hear. Humans typically hear from about 20 Hz up to 20 kHz. Frequencies below that threshold are infrasonic, which means they are not audible to us but can sometimes be felt as very low-pitched vibrations. That’s why infrasonic is the correct label here. In contrast, frequencies above 20 kHz are ultrasonic, which are also not heard by humans. The term audible would apply to sounds inside the 20 Hz to 20 kHz range, which isn’t the case for frequencies below threshold. "Vibrational" isn’t a specific standard category for sound frequency bands, so it isn’t used to describe whether something can be heard. Some animals do hear infrasonic frequencies, such as elephants, while others, like bats, use ultrasonic frequencies for echolocation.