Listener envelopment

What is the listener envelopment?

The listener envelopment describes the temporal evolution of sound energy in a room after a sound is emitted.

It’s characterized by the density and distribution of early reflections on surfaces, notably the EDT (Early Decay Time), which measures the initial decay of sound level and directly influences the clarity of the sound. A shorter EDT generally corresponds to greater clarity.

In other words, the Listener envelopment determines how sound propagates and reflects in a space, impacting our perception of the spatialization or extent of a place.

It’s crucial to distinguish this from the overall dynamics, which is the difference between the loudest and quietest sound of an audio signal.
The listener envelopment describes the temporal behavior of the sound energy, while dynamics describe the level variations.

Listener envelopment measurements and studies are relatively new, and this article can be updated.

Factors influencing the listener envelopment

• Early reflections (EDT): Early reflections, which occur on the room’s surfaces, play a crucial role in the perception of spatialization.

  These reflections arrive shortly after the direct sound. EDT (Early Decay Time) refers to the time it takes for the sound level in the room to decrease significantly after the sound source has stopped emitting.

  A short EDT corresponds to a rapid reduction in sound energy, which contributes to better clarity, as the early reflections are absorbed or diffused quickly.
  A long EDT creates a more enveloping sensation, as the lingering reflections help enrich the perception of the space.

  More details : Clarity, EDT and ITDG

  The pattern and timing of these early reflections are critical for envelopment. Too few reflections can sound sparse, while too many can blur the sound.

  The Haas effect (or precedence effect) describes how our auditory system integrates these early reflections, often perceiving them as a single fused sound event.

• Room proportions: The dimensions of a room influence the formation of resonance modes (standing waves) and the distribution of reflections.

• Materials: The materials used for the walls, floor, and ceiling determine the amount of sound absorbed or reflected. Absorbent materials reduce reverberation, diffusing materials scatter sound waves in multiple directions. A balanced acoustic design often uses a combination of absorption and well placed diffusion. Too much absorption can make a room sound “dead,” while too little can make it sound overly reverberant.

• Directivity: The characteristics of the sound source, including its directivity, play a significant role in listener envelopment. This directivity must be adapted to the environment, ensuring it is constant, with coverage suited to the listening distance in order to maintain a balanced 50/50 ratio between the direct and reverberated fields taking account of our sensitivy. As we will see, with early reflections and diffusers, the horizontal axis becomes the most important in achieving this balance.

The critical distance

The critical distance refers to the distance from a sound source at which the level of direct sound equals the level of reverberated sound.

Beyond this point, reverberant sound dominates the sound field. The critical distance depends on both the characteristics of the sound source (such as its directivity) and the acoustics of the room.

While being at the critical distance influences the balance of direct and reflected sound, achieving a sense of good envelopment also requires careful management of early reflections and the distribution of sound throughout the space.

Lateral treatment, directivity and over-absorption

The treatment of lateral walls must always be considered in relation to the horizontal directivity of the loudspeaker. With wide horizontal radiation patterns, lateral walls are strongly excited and the first priority is often to control the amount of lateral energy reaching the listener, typically through significant absorption in order to limit early dominant reflections.

When the horizontal directivity is adapted to the listening distance (90° in a regular room), lateral excitation is naturally reduced. In typical domestic listening rooms, this places the listener close to the critical distance, or slightly beyond it, resulting in a more balanced relationship between direct sound and early reflected energy.

Under these conditions, rather than further reducing the quantity of lateral energy, the focus can shift to the quality of its redistribution. Because the lateral sound field is already sufficiently moderated, treatments can primarily aim at shaping the nature of reflections, minimizing specular components while preserving the lateral energy that contributes to listener envelopment and apparent source width.

This is where well-designed diffusers become particularly relevant. Quadratic or pseudo-random diffusers allow early lateral reflections to be redistributed over a wider angular and temporal range, reducing specular effects while maintaining the energy required for spatial perception.

When lateral excitation is higher, the need to reduce excess energy tends to dominate the treatment strategy, leaving less freedom to optimize the spatial quality of lateral reflections. When directivity is adapted to the listening distance, diffusion becomes an effective tool to improve the perceptual quality of early lateral reflections without collapsing the horizontal soundstage.

In domestic rooms, where asymmetries such as windows or openings are common, this approach also proves more tolerant. A moderate asymmetry in reflection character, for example diffusive on one side and partially specular on the other, is generally less detrimental to envelopment than strong asymmetries created by excessive lateral absorption.

This is why lateral wall treatment strategies cannot be generalized and must always be evaluated in conjunction with loudspeaker directivity, listening distance and room geometry.

Optimization of the listener envelopment

To obtain an optimal listener envelopment, it is necessary to:

• Control early reflections: By using absorbent or diffusing materials strategically, it is possible to reduce unwanted early reflections (e.g., those that arrive too quickly or from unfavorable angles). This might involve using absorption at specific points on the walls or ceiling to tame strong reflections, while using diffusers to scatter sound and create a more spacious feel.

• Create a density of lateral reflections: A sufficient number of lateral reflections (reflections from the side walls) contributes significantly to a feeling of envelopment and better spatialization. These reflections should arrive slightly later than the direct sound and contribute to a sense of width and spaciousness.

• Eliminate specular reflections: Specular reflections, which are like mirror-like reflections, create echoes and comb filtering, degrading intelligibility. It is important to diffuse or absorb them. Convex or 2D diffusers are often used to diffuse specular reflections.

• Room proportions: The dimensions of the room influence the resonance modes and the distribution of reflections. In a corridor-type room, it will be very difficult to have a good envelope due to the lateral reflection and their management.

In some cases, it may be tempting to over-reduce the opening of the horns below what we recommend but this may create a gap in the envelope between low and high frequencies. This can result in a situation where the high frequencies are well-distributed, but the low frequencies are more focused, creating an uneven envelope and impacting the variation of intelligibility and frequency balance.

Early reflections are essential for creating a sense of envelopment in a room. These reflections contribute to the spatial perception of the sound, but their timing and pattern are crucial.

If reflections arrive too quickly or are too sparse, they can negatively impact the sense of space, leading to a feeling of emptiness or a lack of definition.

On the other hand, if there are too many reflections, they can blur the sound, reducing clarity and spatial precision.

The perception of envelopment is influenced by the interaction of these reflections with the direct sound, and the brain tends to integrate reflections that arrive shortly after the direct sound. The ideal balance of reflections helps create a rich and immersive experience without compromising clarity.

This explains why achieving a good listener envelopment is particularly challenging in small, narrow rooms, where reflections tend to occur too closely in time to create a sense of spaciousness. Modal issues are also more pronounced in smaller spaces, further complicating the creation of an optimal sound envelope.

Measurability

Several parameters can be measured to characterize the listener envelopment:

• IACC (Inter-aural Cross-Correlation Coefficient): a measure used in acoustics to evaluate the similarity of sound signals that reach our two ears. Lower IACC values generally correlate with a wider, more enveloping sound, as the signals reaching the two ears are less similar.

• Lateral Fraction (LF): a measure used in acoustics to quantify the proportion of sound energy that reaches a receiver after being reflected by the side walls of a room. Higher LF values suggest a greater contribution from lateral reflections, which are associated with a wider, more enveloping sound.

  However LF is typically measured in concert halls, its application in small rooms is less common.

These measurements make it possible to characterize the envelope, but they generally require the intervention of a professional specialized in the field.

Conclusion

The listener envelopment plays a fundamental role in listening quality. By understanding the mechanisms that govern it and using the appropriate measurement tools, it is possible to optimize the acoustics of a space and create an immersive and realistic listening experience.

Source:

Objective measures of listener envelopment, 1995: https://nrc-publications.canada.ca/eng/view/accepted/?id=be12bb70-20ce-4d9e-ab16-99b48af4ef6c

LF:

http://www.winmls.com/2004/help/lflateralfraction.htm

http://www.winmls.com/2004/help/lglatelateralstrength.htm

http://www.winmls.com/2004/help/lfclateralfractioncosine.htm

IACC:

https://iaem.at/ambisonics/symposium2009/proceedings/ambisym09-avnirafaely-iaccspatcorrsh.pdf