A very popular misconception among people is "Phase issues are relative, and exist only when two different channels blend together, therefore do not exist in mono recordings."
If you're a recording engineer, I know you'd probably be laughing out loud reading this. So did I, when I first heard this! But over time, I realized, many mixing engineers and music producers think this way! And stay calm, it's totally normal.
Recording Engineers and Acoustic Consultants are naturally more exposed to the technical side of acoustics compared to the in-the-box musicians, so it's okay. To correct this myth, I got your back!
The misconception that phase issues are exclusive to stereo recordings has persisted within the realm of audio, leading to the belief that mono recordings are immune. However, this notion oversimplifies the intricacies of phase interference and fails to recognize its relevance in mono contexts. This article endeavors to demystify the phenomenon, highlighting its occurrence and implications in mono recordings, with a particular focus on the role of comb filtering and the potential remedies offered by acoustic treatment.
Understanding Phase Interference:
Phase interference transpires when sound waves of the same frequency are out of sync, leading to either constructive or destructive interference. While commonly associated with stereo recordings, the fundamental principles underlying phase interference apply to both stereo and mono scenarios.
Phase interference // Source: Soundgirls
Comb filtering occurs due to the interaction between the direct sound and its reflections. When two complex sound waves interfere, different frequencies interfere differently - constructively, destructively, and partially. Accordingly affecting the resulting sound at that interference spot. Some frequencies get boosted, some get suppressed, and some get nulled completely. The resulting frequency response pattern resembles the teeth of a comb, hence the name. In recordings, these comb-filtering effects can distort the intended tonal balance and frequency response of the original sound, leading to undesirable sonic artifacts.
Comb Filtering // Source: Sweetwater
Dispelling the Myth of Mono:
The misconception that phase issues can't exist in mono recordings stems from a lack of awareness regarding the underlying acoustic principles. The number of sound generators or capturing devices involved doesn't determine the presence of phase interference. Instead, it's the acoustic environment itself that plays a significant role.
Consider a scenario where a sound wave is emitted toward a wall. Upon reaching the wall, the wave bounces back towards the mic (or the listener), creating a reflection. Now, this reflection would interfere with the original sound signal, resulting in phase issues. This occurrence becomes especially pertinent in situations where the sound wave's reflection path is delayed by a few ms, essentially acting as an additional sound generator with a distinct timebase.
In the context of mono recordings, a reflective surface (near the path of the source and listener/mic) is akin to a delayed sound generator. Although it does not actively emit sound, its reflection arrives at the capturing device or listener's position after a certain delay. This delay introduces a phase shift, resulting in constructive or destructive interference between the direct and reflected sounds.
If this was a little complex data to process, here's a simpler example:
Place 2 speakers a couple of feet apart emitting a mono sound from the same source in a regular stereo playback setup. Place a directional mic in front of them, and start recording. While recording, move the mic left to right, slowly and vice versa a couple of times. Listen to the recording, the phasey effects generated are due to the two same sound signals from two time-aligned sources arriving at the same single source a few milliseconds apart.
The same example is excellently demonstrated by Kyle from Audio University in his video on
Youtube: https://youtu.be/NaK6GrLg4cE?t=141
The repercussions of phase interference and comb filtering in mono recordings are profound. The altered frequency response can lead to certain frequencies being accentuated or canceled out, leading to a compromised audio quality that lacks the natural timbre of the source.
Tackling Phase Issues:
The solution to mitigating phase interference and comb filtering lies in effective acoustic treatment. By strategically placing sound-absorbing materials and diffusers, the acoustics of a space can be controlled. Acoustic treatment aims to minimize the impact of reflections that contribute to phase interference and comb filtering.
Through the meticulous arrangement of sound absorbers, unwanted reflections can be minimized, ensuring that the direct sound from the source is dominant and reducing the potential for phase interference and comb filtering. Absorption is the simplest and most straightforward method to control reflections, simply evading any possibility of phase issues, at least in the frequency range of the absorber's capability.
Solutions for LIVE rooms / MONO recordings:
Live rooms, where preserving the dynamic character of sound is essential, demand special attention to phase issues. While sound absorbers are proficient at curbing reflections, their overuse can lead to an overly damped acoustic environment. Here, diffusers prove invaluable. These devices scatter reflections, maintaining the room's liveliness while preventing phase-related anomalies and the associated comb-filtering effects. Phase grating diffusers distort the phase of the reflected energy, disperse the soundwaves as if particles, and produce edge diffraction effects, essentially absorbing some of the energy. One can perceive diffusers as narrow band absorbers, but the reflected energy is mostly amorphous, hence won't probably cause troubles when mixed with the original sound. If this wasn't self-explanatory, let me caution you. Diffusers are a complex product of science and require very careful and meticulous planning, calculations, and decisions before you place one in. While they are very powerful devices, they might just destroy the entire room response if not used wisely. Refer to our older blogs for more insights:
**Conclusion:**
In closing, debunking the myth that phase issues and comb filtering are confined to stereo recordings is crucial for achieving optimal sound quality in both mono and stereo contexts. Understanding the intricate relationship between delayed reflections, phase shifts, and comb filtering effects allows audio professionals to take targeted steps in mitigating these issues. By harnessing the potential of acoustic treatment and diffusers, the adverse impact of phase interference and comb filtering in mono recordings can be effectively minimized, ensuring the faithful reproduction of sound and the enhancement of overall audio quality.
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