Earthquake Waves and Sound: Properties and Behavior
Earthquake Waves and Their Properties
Waves generated by an earthquake are classified into two main types: P-waves and S-waves. P-waves are longitudinal and can travel through both solids and fluids. S-waves are transverse and can only propagate through solid materials. The reflections and refractions of these waves provide valuable information about the Earth’s interior.
Understanding Longitudinal Waves
The wavelength of a longitudinal wave is the distance between successive compressions or rarefactions. For example, when sound waves pass through air, air molecules vibrate back and forth around an equilibrium position.
Wave Interference
While solid objects, such as stones, cannot occupy the same space simultaneously, multiple vibrations or waves can coexist in the same place and time. If two stones are dropped into water, the waves produced by each may overlap, forming an interference pattern. The effects of this ripple can be an increase, decrease, or even cancellation of the waves.
Principle of Superposition
When multiple waves occupy the same space simultaneously, their displacements at each point add together. This is known as the principle of superposition.
- Constructive Interference: When the crest of one wave overlaps the crest of another, their individual effects combine, resulting in a wave of greater amplitude.
- Destructive Interference: When the crest of one wave overlaps the trough of another, their effects are reduced. The crest of one wave essentially fills the trough of another, leading to a decrease in amplitude.
Sound: Source and Characteristics
Most sounds are waves produced by the vibrations of material objects.
- In a piano, violin, or guitar, sound is produced by vibrating strings.
- In a saxophone, a vibrating reed produces sound.
- In a flute, a wavering column of air within the embouchure creates sound.
- In the human voice, sound is generated by the vibration of the vocal cords.
In each of these cases, the initial vibration stimulates the vibration of something larger or more massive.
Frequency and Pitch
We describe the frequency of sound using the term pitch.
- A high-pitched sound, like that from a flute, has a high frequency of vibration.
- A low-pitched sound, such as that from a foghorn, has a low frequency of vibration.
The human ear can typically detect sounds within the frequency range of approximately 20 to 20,000 Hz. This range tends to shrink with age.
- Infrasonic Waves: Sound waves with frequencies below 20 Hz are considered infrasonic.
- Ultrasonic Waves: Sound waves with frequencies above 20,000 Hz are considered ultrasonic.
Humans cannot hear either infrasonic or ultrasonic waves.
Nature of Sound in Air
When we clap our hands, the sound produced is not periodic. It consists of an impulse or pulse wave that propagates in all directions. This impulse disturbs the air similarly to how an impulse would disrupt a slinky spring. Each particle moves along the direction of the expanding wave.
Example: Sound Propagation in a Room
Imagine a long room with a window and a curtain at one end and a door at the other. If the door is pushed open, the air molecules next to it are pushed and move from their initial positions to the positions of neighboring molecules. These neighboring molecules, in turn, push their neighbors, and so on, like a compression traveling along a spring. This compression eventually reaches the curtain, causing it to move in and out of the window. This compressed air pulse is called a compression.
When the door is closed, it pulls some air molecules out of the room, creating a low-pressure area behind it. Neighboring molecules move into this area, leaving behind another low-pressure region. This area of low-pressure air is rarefied. Other molecules further from the door move into these regions, and the disturbance propagates through the room. This is observed as the curtain moving inward. This disturbance is a rarefaction.
In both cases, the impulse travels from the door to the curtain. We know this because the curtain moves after the door is opened or closed. If you continuously open and close the door in a periodic motion, you can create a series of compressions and rarefactions, which will cause the curtain to move in and out of the window.