17.4 Normal modes of a standing sound wave

Interference is the hallmark of waves, all of which exhibit constructive and destructive interference exactly analogous to that seen for water waves. In fact, one way to prove something “is a wave” is to observe interference effects. Since sound is a wave, we expect it to exhibit interference.

Interference of sound waves

In Waves , we discussed the interference of wave functions that differ only in a phase shift. We found that the wave function resulting from the superposition of $y_1\left(x,t\right)=A\text{sin}\left(kx-\omega t+\varphi \right)$ and $y_2\left(x,t\right)=A\text{sin}\left(kx-\omega t\right)$ is

One way for two identical waves that are initially in phase to become out of phase with one another is to have the waves travel different distances; that is, they have different path lengths. Sound waves provide an excellent example of a phase shift due to a path difference. As we have discussed, sound waves can basically be modeled as longitudinal waves, where the molecules of the medium oscillate around an equilibrium position, or as pressure waves.

When the waves leave the speakers, they move out as spherical waves ( [link] ). The waves interfere; constructive inference is produced by the combination of two crests or two troughs, as shown. Destructive interference is produced by the combination of a trough and a crest.