# On the Sequence of Unmasked Reflections in Shoebox Concert Halls
## 3 Literature Review
### Acoustic Signature  
Humans possess an inherent spatial ability to understand their surroundings by how passive objects modify the acoustics of a space. The frequency content and level of background noises as well as the delays of sounds bouncing off passive objects creates an aural image to a listener. Therefore, to some extent, we can \"see\" with our ears thanks to our ability to decode spatial attributes using acoustic cues [2].  
Shaping these passive objects--that is, the walls, ceilings, and architectural elements of concert halls--is a relatively young science. One of the major advances on the field was the finding of the effect of lateral reflections in 1952, when [PERSON] identified the apparent extension of a sound source with the presence of a sound reflection [3; 4].  
In the late 1960s, [PERSON] and [PERSON] advocated that perceptible early lateral reflections were significant in improving Spatial Impression (SI) [5; 6]. [PERSON] concluded that a sequence of skeletal reflections that are perceptible and unmasked could enhance the room response, providing the room with a _\"premium quality of acoustical experience\"_[7].  
This statement is essential to understand the purpose of this work. While most of the standard acoustic parameters are based on energy integrals, the concept of unmasked reflection sequences provides an innovative approach to the design of concert halls to be used alongside the rest of the acoustic parameters.  
In the 1970s, [PERSON]'s research on the unmasking of reflections related to the loudness levels signalled a connection between SI and Loudness [8]. This research was followed up by [PERSON] et al. in 2017, linking the musical dynamic of a hall to the unique reflection sequence at a particular listening position as a reason why concert halls \"wake up\" differently [9]. A concert hall that \"wakes up\" as the dynamic increases could be interpreted as a concert hall that reveals its acoustic signature as its sequence of early reflections becomes unmasked as the overall reflection level increases.  
Through a set of listening tests in 2011, the Aalto University team found that listeners could recognize a concert hall through its early sound, which is primarily composed of a sequence of early reflections created by the room's shape [10]. This reveals the strong connection between the sequence of early reflections and the recognizability of a concert hall.  
Therefore, designing concert halls with a strong acoustic signature could be a strategy to create spaces that left an aural memory on the audience. Indeed, the mere-repeated-exposure effect describes that an individual exposed to familiar and new stimulus objects shows a preference for the familiar object. This could be one of the reasons why the Grosse Musikvereinssaal has become such an archetypical hall or why Funkhaus Studio 1, with its strong character, has become iconic for classical recordings.  
From the literature, two elements with relevant influence on the signature of a concert hall are:  
1. The reflection sequence and relative reflection levels at the listener position.
2. The perceptibility of the reflections relative to the perception thresholds.