If you apply a correction filter to try to boost the signal at a cancellation frequency, you will also boost the refection that’s causing the interference! Unfortunately, you can’t correct for SBIR using EQ. SBIR causes deep dips in the bass response below a certain frequency, and it can affect your low frequency response even more than room modes.įrequency response measurement at the listening position showing a dip / null caused by the speaker-boundary effect. All these terms boil down to the same thing: boundary-induced comb filtering. This is called speaker-boundary interference response (SBIR), listener-boundary interference response (LBIR) or the boundary effect. So, if your direct and reflected sound waves are similar in strength, and half a wavelength out of phase, the waveforms mostly annihilate each other. Speaker-Boundary Interference Response (SBIR) When two sound waves of equal magnitude are 180 degrees out of phase, phase cancellation occurs - they cancel each other out because they are equal and opposite. This means the reflected and direct sound are 180 degrees out of phase. Why? Because a quarter wavelength from your wall, the total travel difference (for a wave reflecting back on itself) is half a wavelength. Shortly after the reflection in this animation, the direct and reflected waves are superimposed and a standing wave is formed.Ĭancellation always occurs at ¼ the wavelength from your wall, regardless of the phase of the wave hitting the wall. If you place your speaker on an antinode, it will cause a peak or boost in the frequency response at 60 Hz. If you place your speaker on a node, it will cause a null or dip in the frequency response at 60 Hz.ġ/2 wavelength from the wall is an anti-node, where the two waves have added together (constructive interference). The animation shows them as equal strength, which does not actually happen in your room… but your bass reflections are still strong enough to cause serious problems! The amount of cancellation depends on the strength of the reflection relative to the direct sound. These are the places where the direct and reflected sound are always cancelling each other out (destructive interference). After reflection, the sound pressure (charted amplitude) is zero at 1/4, 3/4, 5/4, etc wavelength from the boundary (the points in the wave that don’t move). Courtesy of Dan Russell at Penn State University. This causes a horrid dip, notch or null in the frequency response.Īnimation of a sound wave reflecting off a boundary. If your speaker is one quarter wavelength from the wall for a certain frequency, wave cancellation occurs at that frequency. When the reflected sound wave bouncing off your wall combines with the source sound wave coming from your speaker, it creates acoustic interference. Bass waves radiate backward from your speakers, toward the wall in front of you… and when they hit the wall, they reflect. Speakers are more omnidirectional at low frequencies, meaning bass waves radiate in all directions, causing a rumbling ruckus. Not hearing enough bass through your monitors? The distance between your room boundaries and speakers has a huge impact on your bass performance. Speaker Placement and Reflections from Nearby Walls Is Speaker-Boundary Interference Killing Your Bass? PART: 1 2 3 4
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