Standing acoustic wave

Professional Definition

A standing sound wave is a stable pattern of vibrations created by the interference of two waves of the same frequency and amplitude, moving in opposite directions (most often the incident wave and reflected wave).

This phenomenon is characterized by a time-invariant spatial distribution of vibration amplitudes. In a standing wave field, we distinguish:

• Nodes: Points where the amplitude of the acoustic pressure is always minimal (theoretically zero).

• Antinodes: Points where the pressure amplitude reaches maximum values.

Unlike a traveling wave, a standing wave does not transport energy over a distance, but stores it in specific areas of space.

Acoustics in Simple Words

The best analogy for a standing wave is a jump rope or string that is moved by two people standing opposite each other. If they synchronize their movements, the rope takes on a shape that appears to be stationary horizontally – it does not move from one person to another but merely "pulses" up and down.

On such a rope, you will notice places that do not move at all (nodes) and those that swing out the most (antinodes).

In your room, the "rope" is the air, and the people shaking it are the speaker and the opposite wall reflecting the sound. If the wavelength of the sound perfectly matches the distance between the walls, the sound gets "trapped" in the room. As a result, a strange phenomenon occurs: in one spot of the room (antinodes), you can hear a powerful, unpleasant bass thumping, while just a meter away (nodes), the same sound may be nearly completely inaudible.

Summary

• No energy transport: Sound does not "flow" through the room but resonates within it.

• Room modes: Standing waves are responsible for the so-called eigenfrequencies of spaces, which can drastically distort the sound of music.

• Low-frequency problems: This phenomenon is the most troublesome in the low-frequency range (bass), where the waves are long enough to easily fit the dimensions of typical rooms.