Demonstrations of Sound Waves in Air

Properties of Air

Sound travels from its source through a medium that is usually air. This simulation shows particles in still air.

Two properties of air that are important for sound are its temperature and density. Air temperature measures the average amount of this random motion due to collisions. These simulations demonstrate the effects of changing the air temperature on the motion of the particles.

Air density measures the average number of particles in a unit volume. These simulations demonstrate the effects of changing the air density on the motion of the particles.

Air pressure measures the average force of particle collisions on a unit surface area and depends on both the density and temperature of air. How do you expect the pressure to change if the temperature increases or if the density increases?

Sound Travelling Through Air

Sound travelling through air is a repetitive disturbance of the air particles. Click here to see an example.
Period and Frequency
Measure the time between regions of dense particles at any point on the screen. This time is the "period" of the sound. The frequency of the sound is defined as frequency = 1/period.

Compare these simulations of high-frequency (small period) and low-frequency (large period) sound travelling through air.

Higher-frequencies correspond to high pitched notes. Lower-frequencies sound like low bass notes.

The Speed of Sound
You can measure the speed of sound by timing how long a region of dense particles takes to cover the length of ruler held up to the screen.

How does the speed of sound vary with frequency? Measure it in these simulations of high-frequency and low-frequency sound.

How Far Does One Particle Go?
Do particles have to travel all the way from a sound source for you to be able to hear it? Follow the highlighted particle near the middle of this simulation for the answer.
Amplitude
The sound amplitude measures the amount of back and forth motion along the direction the sound is travelling. A smaller amplitude corresponds to a quieter sound. A larger amplitude corresponds to a louder sound.

Compare these simulations to see the effect of changing amplitude on the motion of the particles.

Sound Production and Detection

What do all sound sources and detectors have in common? A sound source must be able to create a repetitive disturbance of the air immediately around it. Most sources do this with something solid that moves back and forth with the same frequency and amplitude as the sound they produce.

This simulation is an example of a vibrating solid producing sound waves. The solid in this case could almost anything: your vocal folds, the body of a violin, the surface of a drum, the cone of a speaker or even a mosquito's wings. Sound detectors also have something in common: they must be able to respond to repetitive disturbances in the air around them. Most detectors also do this with something solid that reacts to the changing force of many particle collisions to track the sound.

This simulation shows both a source and a detector. The solid of the detector could be your eardrum or the diaphram in a microphone.

About These Simulations

The simulations used for this demonstration exaggerate the features of real sound waves for clarity. For example, the frequencies used here are much lower than those we can hear (about 20-20,000 Hz). The speed of sound in air is also much faster than the speed used here, and the amplitudes of typical sounds are much smaller.

Try experimenting with the controls available from the "Setup Particles" and "Setup Fields" tabs to explore these simulations further.