Background

The last characteristic of sound stimuli we consider is phase. Remember that sound is a change in sound pressure over time and space. Think about that clap again. It creates a wave of high-pressure peaks and low-pressure troughs that propagate across space. In each cycle, the position in the cycle is called phase. There are 360 deg of phase in each cycle. So waves that different by 0 or 360 deg of phase and are the same frequency will always have their peaks and troughs aligned and the sound becomes more intense. When the sounds are the same frequency, but the peaks and troughs are opposite, they are 180 deg out of phase and you will not hear the sound.

In this illustration, there will be two sounds playing and you can manipulate the phase and frequency of the second tone to see the effects of phase on sound.

Instructions

Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab

Settings

Below is a list of the ways that you can alter the illustration. The settings include the following:

Play: start the sound. The button will then change to Pause and pressing it again will stop the sound. Phase (deg): change starting phase for the second tone.
Frequency (Hz): change the frequency of the second tone.
Show 1 sec of Sound: if checked, plot a whole second of the sound waves on the graphs. If not, 50 msec of time is shown.
Show Intensity: If checked, draw a vertical line from the 0 value to the first peak. The height of this line is the intensity or amplitude of that wave.
Show Frequency: If checked, draw a line connecting the first two peaks of the wave. The longer the line, the lower the frequency.

Reset

Pressing this button restores the settings to their default values.

Alter Parameters of the Sound Wave Here

Sound 1: Frequency: 880 Hz
Sound 2


Show 1 sec of Sound
Show Intensity Show Frequency

The top graph shows both sound waves. The bottom graph plots the combined wave that reaches your each from the combination of the two sound waves. Change the phase of the second tone and see what happens to the sound. See if you can make the sound disappear. What happens to the sound if you change the frequency so that the second tone has a frequency only 1 Hz different from the first tone? What about a 2 Hz difference?