A Boundary surface is any large, solid surface (such as a wall, floor, or ceiling) that acts as an obstacle to sound waves.

When a sound wave hits one of these surfaces, it cannot pass through and is reflected. At the point of contact with the surface, a zone of high sound pressure is created.

To our ears, this high pressure manifests as a significant boost of low frequencies (bass). This phenomenon is known as the "Surface Proximity Effect" or "Boundary Effect." This effect occurs with both the sound source and the microphone: Sound Source: Placing a speaker on the floor or near a wall will increase the perception of bass frequencies. Microphone: Placing a microphone near a solid surface (such as the floor or a table) will increase the pickup of low frequencies. PZM (Pressure Zone Microphone) type microphones are specifically designed to take advantage of this effect.