Understanding Infrasound Detection Frequencies

This solution explains the frequency range associated with infrasound detection, particularly for phenomena like aircraft wake vortices, clear air turbulence, tornadoes, and seismic events, using specialized microphones.

What is Infrasound?

Infrasound refers to sound waves with frequencies lower than the threshold of human hearing. Typically, this range is considered to be below 20 Hz (Hertz).

How Infrasound Microphones Work

Infrasound microphones are designed to capture these very low-frequency sound waves. Unlike standard microphones that focus on the audible range (20 Hz to 20,000 Hz), infrasound microphones are sensitive to frequencies much lower than 20 Hz. This sensitivity allows them to detect specific natural and man-made phenomena that generate these low-frequency acoustic signals.

Phenomena Detected by Infrasound Microphones

The question highlights several key applications for infrasound detection:

• Aircraft Wake Vortices: The turbulent air generated behind aircraft wings can produce low-frequency acoustic signatures.
• Clear Air Turbulence (CAT): While CAT itself might not be directly audible, the atmospheric disturbances associated with it can sometimes generate infrasound.
• Tornadoes: The violent air movements and pressure changes within tornadoes are significant sources of infrasound.
• Seismic Events: Earthquakes, volcanic eruptions, and even large meteor impacts generate powerful infrasound waves that can travel long distances through the atmosphere.

Determining the Frequency Threshold

The defining characteristic of infrasound is its frequency. By definition, infrasound exists at frequencies below the lower limit of human hearing. The standard convention sets this lower limit at approximately 20 Hz.

Therefore, microphones designed for detecting phenomena that produce infrasound must be sensitive to frequencies below this 20 Hz threshold.

Considering the options provided:

• 30 Hz
• 10 Hz
• 5 Hz
• 20 Hz

The question asks for the frequency range *below* which these events are detected using an infrasound microphone. The standard definition of infrasound centers around the 20 Hz mark. While specific events might generate frequencies as low as 10 Hz or 5 Hz, the general category and the operational boundary for infrasound detection capability is frequencies below 20 Hz.

Conclusion on Detection Frequency

An infrasound microphone enables the remote detection of various significant events, including atmospheric disturbances and geological activity, by sensing acoustic energy in the infrasonic spectrum. This spectrum is defined as frequencies below 20 Hz.