Reflection and absorption: what they are and how they affect the recording and the mix

Introductory note

This article provides a general overview of sound reflection and sound absorption, fundamental phenomena for recording and mixing. In the following sections, we will look in detail at practical applications and specific solutions to optimise acoustics in audio work environments.

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Reflection and absorption: what they are and how they affect the recording and the mix

When a sound wave encounters an obstacle, it can be transmitted through it, absorbed or reflected. These phenomena significantly influence the quality of audio recording and mixing, determining how sound is captured and perceived in listening environments. Understanding the behaviour of sound in relation to obstacles is crucial to achieving clear recordings and balanced mixes.

Sound Crossing and Absorption

Sound can pass through an obstacle, be absorbed or reflected, depending on the characteristics of the obstacle and the frequency of the sound. An obstacle with low mass and a low sound frequency favours the passage of sound, while denser materials absorb sound more, especially at high frequencies. This principle is the basis of sound insulation, which is essential to avoid unwanted sound leakage.

Reflection of Sound

To effectively reflect a sound, the obstacle must be rigid, smooth and elastic. High frequencies can also be reflected by thin surfaces such as plastic or metal, while very dense materials such as concrete or stone are required to reflect low frequencies.

Incidence and Refraction Angles

Sound follows the same laws as light and the movement of a billiard ball: it bounces off a reflective surface at the same angle as it hit it. If the obstacle is perfectly flat, the reflection will be predictable and directed. By tilting the plane of reflection, it is possible to direct the reflection where one wishes, according to the same geometry with which a ball is handled in the game of billiards.

Concave and Convex Surfaces

Concave surfaces tend to concentrate sound at a specific point, creating unwanted accumulations of sound energy. In contrast, convex surfaces spread sound more evenly, reducing sound focussing problems. This concept underlies the use of diffuser panels, which are designed to disperse the received sound energy, improving the acoustics of a room.

In a closed environment, each propagation line of a source encounters obstacles that determine as many reflections. S+P indicates a sound source whose producer is also a listener of himself (e.g. a singer or violinist): we note the shortest of the 3 highlighted reflections, which determines the pre-delay time as a function of the length of the shortest path (shortest). For the external listener or pick-up microphone (Only P), the predelay will also be a function of the shortest reflection path (shortest). Both will also hear the direct sound, which for S+P is immediate, while for Only P it will be somewhat more delayed, depending on distance, but still more immediate than any other reflection.

Effects of Refraction: Echo and Reverberation

Reflection is the main cause of phenomena such as echo and reverberation, which affect the perception of sound in a room.

• EcoThis occurs when a reflection is delayed enough to be perceived as a separate sound. The delay between original sound and echo depends on the distance of the reflecting surface. The greater this distance, the greater the delay.
• Flutter EchoThis is a repeated, close echo caused by reflections between parallel, non-acoustically treated surfaces. It is a serious problem in recording studios, which can be solved by tilting walls or inserting absorbent panels.
• ReverberationIt is the result of the superposition of multiple echoes, which create a persistent sound tail. The amount of reverberation in a room depends on the number of reflective surfaces and their distance from the sound source.

The sound emitted by a source, after a few moments, will be perceived by the listener as 'direct sound', a little later (after a latency time defined as 'predelay') the listener will perceive the 'first reflections', which will gradually become closer and denser, until they merge into a single perception of 'reverberation'. Note how all events follow one another with a progressive loss of sound energy, until the resonance is completely extinguished.

Practical Implications for Recording and Mixing

Recording:

• Checking First Refractions: Sound waves bouncing off nearby walls return to the microphone and can create phase interference, altering the recorded sound. Treating the walls with absorbent panels or diffusers helps prevent this problem.
• Avoiding unwanted reflectionsExcessive reflection makes the sound muddy, while excessive absorption makes it sound unnatural and lacking in depth. The right balance between absorption and reflection is essential for clear recordings.

Mixing:

• Managing Reverberation: A key parameter is pre-delay, i.e. the time between the original sound and the reverberated response. Setting an appropriate pre-delay helps separate the direct sound from its reverberated tail, maintaining clarity and intelligibility.
• Studio Monitors: Using studio monitors instead of normal audio speakers is essential to evaluate the mix accurately. Professional monitors have a linear frequency response, which avoids unwanted colouration of the sound.
• Headphone Usage and Acoustic Correction: Although mixing is preferably done with monitors, headphones can be useful for checking details and balance. Acoustic correction software exists to improve the neutrality of both monitors and headphones, compensating for imperfections in the room or listening system.
• Panning and Sound SpatialityPanning allows sounds to be distributed between the left and right channels of the stereo mix. Combined with reverb and pre-delay, it creates a well-balanced, three-dimensional sound image.

Conclusion

The way sound behaves in the environment directly affects the recording and the mix. By understanding and controlling reflections, absorption and echoes, audio quality can be improved, preventing phase problems and sound clutter.