Simple Explanation

The effect of sound reflections off walls or surfaces that causes interference with direct sound waves, leading to frequency dips or nulls, particularly in the bass range.

Concise Technical Definition

SBIR or LBIR refers to the interference that occurs when sound waves reflect off boundaries like walls, causing phase cancellation with the direct sound wave. This results in deep dips in the frequency response, particularly in the bass. When direct and reflected sound waves are of similar strength and 180 degrees out of phase, they cancel each other out, creating a frequency response dip or null. This effect can cause significant low-frequency issues in a room, even more so than room modes. While EQ corrections are ineffective, adjustments to speaker placement and acoustic treatment are the most effective ways to minimize its impact.

Layman-Friendly Analogy

It’s like a soundwave bouncing off a wall and canceling itself out when it returns to you. Imagine two waves coming from the same source, one directly to your ear, and the other reflected from the wall. If the reflected sound is out of phase with the original, it will reduce or cancel out the sound you hear, particularly in the low frequencies, creating a "dead" spot in the sound.

Industry Usage Summary

SBIR and LBIR are critical concepts in acoustic treatment and speaker placement, particularly in home studios, professional recording environments, and other audio-sensitive spaces. This interference is most noticeable in the bass frequencies, where reflections off walls can lead to significant coloration and dips in frequency response. The effect is especially problematic when speakers are placed too close to walls, as this enhances boundary interference. To combat this, acoustic professionals typically recommend speaker placement strategies, such as flush mounting or adjusting the distance from the wall, as well as applying acoustic treatment to address the problematic frequency bands.

Engineering Shortcut

A form of boundary-induced comb filtering caused by sound reflections from walls, resulting in frequency dips, particularly in bass.

Full Technical Explanation

SBIR (Speaker-Boundary Interference Response) and LBIR (Listener-Boundary Interference Response) refer to the interference effects that occur when sound reflections off boundaries like walls interact with the direct sound from the speaker. This interaction causes phase cancellation, especially at low frequencies, leading to dips or nulls in the frequency response. The cancellation typically occurs at a quarter wavelength from the wall, resulting in a frequency dip due to the reflected sound being 180 degrees out of phase with the direct sound. This interference effect is most prominent in the bass frequencies and is often more noticeable than room modes. SBIR cannot be corrected with EQ because boosting the cancellation frequency will also amplify the reflection causing the interference. The best ways to mitigate SBIR are through strategic speaker placement and acoustic treatment. Effective options include placing speakers as close to the front wall as possible (flush mounting being the ideal solution) or moving them far enough away from the wall to move the cancellation frequency outside the range of the speakers. Small speakers may exacerbate low-mid interference due to their less directional nature.