Sound waves in a crystal lattice

Mechanical waves propagate through a material medium at a speed which depends on the elastic and inertial properties of that medium.

A wave is a disturbance traveling through a medium, a function varying both in time and position. Mechanical waves propagate by virtue of the elastic and inertial properties of their medium. Light is a form of wave which propagates by virtue of the electric and magnetic properties of space. Keep in mind that as a wave passes, it is energy that is moving. After the wave has passed, displacements in the medium return to zero.

In one form or another, the topic of Waves comes up on nearly every MCAT. There are usually going to be a few uncomplicated waves problems. The test-writers will make sure you know how to move between wavelength, wave speed, and frequency, for example. Usually these simple questions are nested within a discussion of a more advanced scenario from wave optics or modern physics, for example. Occassionaly, one might run into a waves passage that represents a more advanced take on a familiar topic, such as two dimensional standing waves on a drumhead or a complex seismic wave. With waves as with most topics,the MCAT likes to give you unfamiliar, difficult-seeming phenomena and then ask questions which are pretty easy if you know the fundamentals.

WikiPremed Resources

Waves Cards
Chapter from the Wisebridge Learning System for Physics

Waves Concepts
Concept chapter for Waves in PDF format

Waves Practice Items
Problem set for Waves in PDF format

Answer Key
Answers and explanations

Waves Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for Waves
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals


Be prepared to distinguish transverse and longitudinal waves and categorize different types of waves along these lines.

Be prepared to solve basic wave problems involving wavelength, wave speed, period and frequency.

Understand how mechanical waves propagate due to the elastic properties of their particular medium and how the speed of mechanical waves is determined.

Be able to describe sound waves both in terms of pressure changes and particle displacements within the medium.

Be able to distinguish sound intensity and loudness and solve basic problems involving the decibel scale.

Be prepared to explain the Doppler effect in clear, simple language.

Understand the basic principles underlying the most important techniques of medical ultrasonography.

Comprehend how the principle of superposition determines the interference of waves sharing a medium and how beats result from interference.

Be able to conceptualize how standing waves are created and describe how the boundary conditions play a role in determining the properties of standing waves.

Suggested Assignments

Review the basic terms for waves using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Study the waves conceptual chapter. Perform the practice items. Here is the answer key for the problem set.

Study the physics cards for waves.

   In ExamKrackers Physics, read pp. 109-127 and perform practice items 97-104 on pg. 128.

Take a review tour of the waves web resources.

Conceptual Vocabulary for Waves

A wave is a mode of energy transfer from one place to another, often with little or no permanent displacement of the particles of the medium, but through oscillations around nearly fixed positions.
Transverse wave
A transverse wave is a wave that causes vibration in the medium in a perpendicular direction to its own motion.
Longitudinal wave
Longitudinal waves are waves that have vibrations along or parallel to their direction of travel.
Frequency is the measurement of the number of occurrences of a repeated event per unit of time.
Sound is a disturbance consisting of vibrations traveling through matter as a longitudinal wave.
Wavelength is the distance between repeating units of a propagating wave of a given frequency.
Electromagnetic radiation
Electromagnetic radiation, or light, is a self-propagating wave in space with electric and magnetic components.
Standing wave
A standing or stationary wave is a wave that remains in a constant position.
A crest is the point on a wave with the greatest positive value or upward displacement in a cycle.
Reflection is the change in direction of a wave front at an interface between two dissimilar media so that the wave front returns into the medium from which it originated.
Refraction is the change in direction of a wave due to a change in its speed when a wave passes from one medium to another.
Interference is the superposition of two or more waves that results in a new wave pattern.
Acoustics is the branch of physics concerned with the study of sound
The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0.
Speed of sound
The speed of sound describes how much distance a sound wave travels in a given amount of time.
The decibel is a logarithmic unit of measurement that expresses the magnitude of a physical quantity relative to a specified or implied reference level.
Sound intensity
The sound intensity is defined as the sound power per unit area.
Fundamental frequency
The fundamental tone is the lowest frequency in a harmonic series.
Doppler effect
The Doppler effect is the change in frequency and wavelength of a wave as perceived by an observer moving relative to the source of the waves.
In acoustics, a beat is an interference between two sounds of slightly different frequencies, perceived as periodic variations in volume.
Diffraction refers to various phenomena associated with the bending, spreading and interference of waves passing by an object or aperture that disrupts the wave.
Seismic wave
A seismic wave is a wave that travels through the Earth, most often as the result of a tectonic earthquake, sometimes from an explosion.
Harmonic series
The harmonic seriew refers to the natural frequencies of an oscillator, limited to integer multiples of the lowest possible frequency.
An overtone or harmonic is a natural resonance or vibration frequency of a system.
Shock wave
A shock wave is a type of propagating disturbance characterized by an abrupt, nearly discontinuous, change in the characteristics of the medium.
A harmonic or overtone of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency.
Blueshift refers to a shortening of a transmitted signal's wavelength.
Sound pressure
Sound pressure is the pressure deviation from the local ambient pressure caused by an acoustic wave.
Huygens-Fresnel principle
The Huygens-Fresnel principle recognizes that each point of an advancing wave front is the center of a fresh disturbance and the source of a new train of waves.
Capillary wave
A capillary wave or ripple is a wave travelling along the interface between two fluids, whose dynamics are dominated by the effects of surface tension.
Ripple tank
A ripple tank is a shallow glass tank of water used in schools to demonstrate the basic properties of waves. It is a specialized form of a wave tank.
Wavenumber is a wave property inversely related to wavelength, having SI units of reciprocal meters.
Redshift occurs when the electromagnetic radiation emitted from or reflected off an object is shifted toward the red end of the electromagnetic spectrum.
Coherence is the property of wave-like states that enables them to exhibit interference. It is also the parameter that quantifies the quality of the interference.
Rayleigh wave
Rayleigh waves are a type of surface wave associated on the Earth with earthquakes and also with ocean waves.
Phase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space.
Group velocity
The group velocity of a wave is the velocity with which the variations in the shape of the wave's amplitude propagate through space.