Understanding Sound Wave Frequency and Classification

Sound waves are mechanical waves that travel through a medium, such as air, water, or solids. They are characterized by properties like amplitude, wavelength, speed, and frequency. Frequency is a crucial characteristic of a sound wave as it determines the pitch we perceive. Frequency is measured in Hertz (Hz), which represents the number of wave cycles per second.

The human ear can typically detect sound waves within a specific frequency range. This range varies slightly from person to person, but it is generally accepted to be from about 20 Hz to 20,000 Hz (or 20 kHz).

• Sounds with frequencies within this range are called audible sound.

However, sound waves exist with frequencies outside this range. These sounds are not typically heard by humans but can be detected and used by other animals or specialized equipment. We classify sound waves based on their frequency relative to the human hearing range:

• Infrasonic waves: These are sound waves with frequencies below the lower limit of human hearing, i.e., less than 20 Hz.
• Ultrasonic waves: These are sound waves with frequencies above the upper limit of human hearing, i.e., greater than 20,000 Hz.

Analyzing the Options

The question asks about sound waves having a frequency less than 20 Hz. Let's look at the provided options:

• Intelligent: This is not a term used to classify sound waves based on frequency.
• Inferior: This term is unrelated to sound wave classification based on frequency.
• Infrasonic: This term is used to describe sound waves with frequencies below 20 Hz. Examples include sounds produced by earthquakes, volcanoes, and large animals like elephants.
• Instrumental: This term refers to music played by instruments, not a classification of sound waves by frequency.

Based on the definition, sound waves with a frequency less than 20 Hz are known as infrasonic waves.

Frequency Ranges of Sound Waves

Here's a simple table summarizing the different classifications based on frequency:

The question specifically asks about waves with frequency less than 20 Hz. Referring to the table and definitions, these are called infrasonic waves.

Conclusion

Sound waves with frequency less than 20 Hz are called infrasonic waves. Therefore, the correct option is Infrasonic.

Revision Table: Sound Wave Classification

Additional Information on Infrasonic Waves

Infrasonic waves are interesting because although humans cannot consciously hear them, they can sometimes be felt as vibrations at very high intensities. They can travel long distances because they are less affected by scattering than higher-frequency sound waves.

Sources of infrasonic waves include:

• Natural phenomena: Earthquakes, volcanic eruptions, large ocean waves, lightning, aurora borealis, meteors.
• Man-made sources: Explosions, machinery, wind turbines, aircraft turbulence.

Scientists study infrasonic waves to monitor these phenomena. For example, detecting infrasound can help predict volcanic eruptions or track atmospheric events.