Chapter 17: Mechanical Waves & Sound

Mechanical Waves

Disturbances in matter that carry energy from one place to another.

Medium

The material through which waves travel, which can be solids, liquids, or gases.

Transverse Waves

Waves that cause the medium to vibrate at right angles to the direction in which the wave travels.

Longitudinal Waves

Waves where the vibration of the medium is parallel to the direction the wave travels.

Surface Waves

Waves that travel along a surface separating two media.

Crest

The highest point of a wave.

Trough

The lowest point of a wave.

Rest Position

The starting point of the medium before the wave.

Compression

Particles in a medium spaced close together.

Rarefaction

Particles in a medium spread out.

Periodic Motion

Any motion that repeats at regular time intervals.

Period

The time required for one cycle.

Frequency

The number of complete cycles in a given time, measured in hertz (Hz).

Wavelength

The distance between the same point on one wave and the next cycle of the wave.

Wave Speed

Calculated using the formula Speed = wavelength x frequency (v = λ f).

Amplitude

The maximum displacement of the medium from its rest position.

Reflection

Occurs when a wave bounces off a surface that it cannot pass through.

Refraction

The bending of a wave as it enters a new medium at an angle.

Diffraction

The bending of a wave as it moves around an obstacle or passes through a narrow opening.

Wave Frequency Example

A rope shaken with a frequency of 1 vibration/s results in a wave frequency of 1 cycle/s (1 Hz).

Transverse Wavelength Measurement

Measured from crest-to-crest or trough-to-trough.

Longitudinal Wavelength Measurement

Measured by the distance between compressions or rarefactions.

Frequency-Wavelength Relationship

Increasing frequency of a wave decreases its wavelength.

Reflection Characteristics

Does not change the speed or frequency of a wave, but can flip the wave upside down at a fixed boundary.

Refraction Characteristics

Occurs because one side of the wave moves slower than the other side.

Diffraction

Waves diffract more if their wavelength is large compared to the size of the opening/obstacle.

Interference

Interference occurs when two+ waves overlap and combine together.

Constructive Interference

Two+ waves combine to produce a wave with larger displacement.

Destructive Interference

Two+ waves combine to produce a wave with smaller displacement.

Standing Waves

Standing Waves appear to stay in one place - it doesn't appear to move through the medium.

Nodes

Points on the wave with no displacement.

Antinodes

Points on the wave with the most amount of displacement.

Properties of Sound Waves

Sound waves are longitudinal waves, with compressions and refractions.

Speed of Sound

Sound travels fastest in solids, slower in liquids, then slowest in gases.

Intensity

Intensity is the rate at which a wave's energy flows through a given area.

Loudness

A physical response to the intensity of sound; a subjective measurement.

Frequency

As frequency increases, so does speed of vibration.

Pitch

Pitch is the frequency of a sound as you perceive it.

Ultrasound

Sound higher than human hearing range, used for sonar and ultrasound imaging.

Infrasound

Sound lower than human hearing range.

Sonar

Sound navigation and ranging, a technique for determining the distance to an object under water.

Doppler Effect

A change in sound frequency caused by motion of the sound source, listener, or both.

Outer Ear

Gathers and focuses sound into the middle ear.

Middle Ear

Receives and amplifies the vibration.

Inner Ear

Nerve endings sense vibration and send signals to the brain.

Wind Instruments

Use valves to change the air column where waves are produced.

String Instruments

Use pressed fingers to change the length of the strings.

Resonance

The response of a standing wave to another wave of the same frequency, which can increase amplitude significantly.

Sound Frequency Range

People hear the range of 20 Hz - 20,000 Hz.

Amplitude

The maximum extent of a wave's displacement from its rest position.

Decibels (dB)

A unit of measurement for sound intensity, based on powers of ten.

Wavelength

The distance between successive crests of a wave.