Theoretical Underpinnings of Psychoacoustics in Mixing

The field of psychoacoustics explores how the human auditory system interprets sound. Psychoacoustics seeks to elucidate how the human brain and auditory system perceive, process, and analyze vibrations as sound. These insights are pivotal in mixing, allowing engineers to unlock new dimensions of sound and create engaging, perceptually significant auditory experiences.

This article provides a shallow but broad overview of the psychoacoustic theories and concepts underlying modern mixing techniques. Please use it as a springboard for further reading and research.

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Frequency and Place Theories

The Frequency Theory suggests a direct correlation between the frequency of an incoming sound wave and the frequency of the neural impulses sent to the brain. This theory provides a basis for understanding human pitch perception. Place Theory adds to this understanding, positing that different frequencies stimulate different locations on the basilar membrane within the cochlea, with each location corresponding to a specific frequency. This theory is foundational in understanding how humans discern different frequencies, guiding mix engineers in making informed decisions concerning frequency allocation and balance.

In practice, these theories predict that the human auditory system will struggle to differentiate two mixed elements from one another if they occupy the same frequency space. Mixers are well-advised to pick which sound—the more important of the two—should “win” in any overlapping areas. For example, if a song has a lead vocal and a guitar that are prominent at 1 kHz, the mixer should attenuate the guitar at 1 kHz to ensure the vocal remains the focal point. That way, the vocal’s midrange corresponds cleanly to the neural impulses sent to the listener’s brain, unobscured by the guitar.

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Masking and Critical Bands

Masking, a phenomenon where the perception of one sound is affected by the presence of another sound, has significant implications in mixing. By understanding masking, engineers can clean up a mix by removing frequencies that are being masked, thereby improving clarity and separation. Closely tied to masking is the concept of critical bands, which explains that the human ear perceives frequencies within specific bandwidths, and sounds within the same critical band are more likely to mask each other. This concept informs decisions concerning frequency separation and arrangement in a mix, aiding in achieving a clean, well-defined sound.

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Haas or Precedence Effect

The Haas or Precedence Effect is a cardinal psychoacoustic principle that describes the effect of the same sound arriving at the ear from different directions at slightly different times—in which case the brain interprets the sound as coming from the direction of the first arriving sound. This principle is employed in mixing to create a sense of space and width in the stereo field without creating a lopsided balance.

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Loudness Perception and Fletcher-Munson Curves

The Fletcher-Munson curves, also called equal-loudness contours, illustrate how the human ear is more sensitive to midrange frequencies than low and high frequencies. For this reason, humans perceive midrange frequencies as louder than neighboring frequencies, even when they are played at the same volume. Mixing engineers compensate for this nonlinearity by using proportionally more aggressive moves to affect the lows and highs during level balancing, dynamic processing, and mastering. This ensures a balanced and well-polished mix across a variety of playback systems. My article, ‘Understanding and Applying Fletcher-Munson Curves,’ explores this crucial topic in depth.

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Temporal Perception and Binaural Hearing

Temporal perception, including echo, reverb, and temporal masking, bears significant implications in mixing. Similarly, binaural perception and spatial hearing are crucial in understanding how humans localize sound in three-dimensional space, guiding decisions concerning panning, spatial processing, and immersive audio. See my article, ‘Psychoacoustic Phenomena: Temporal Perception and Binaural Audio,’ for more information on this fascinating new frontier.

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Conclusion

The theoretical realm of psychoacoustics is vast, and its application in mixing is transformative, offering a wellspring of knowledge for crafting sonically pleasing and emotionally evocative mixes. Through a deep understanding of these psychoacoustic principles and their practical implications, mix engineers are well-equipped to traverse the sonic landscape, creating auditory experiences that resonate profoundly with listeners.