by Daniel Tay
The focus of my research was on the physics behind the propagation of sound in a small room setting, and how the same sound is perceived by the user. This setting best describes the home environment that our group is designing ‘invisible sound’ for. Such a specification of the room setting is vital, as the acoustics of small spaces defer greatly from that of larger venues such as concert halls.
An important aspect of the sound quality is based on perception. Acousticians then conduct experiments to quantify such qualities scientifically. An acoustically good room sounds ‘live’, and the converse, ‘dead’. The most dated method of quantifying this is the Reverberation Time, where a high RT number indicates better acoustic quality. Acousticians such as D’Antonio and Eger (1986); Geddes (2002); Jones (2003); Kuttruff (1998) have questioned this given that smaller rooms typically have smaller RT numbers. They propose that sound reflection takes on even greater importance in the overall listening experience, which brings us to the next section.
Toole (2008) talks about the need to delve deeper beyond simple measurements. He too concurs with the former group of acousticians about the influences of early reflected sounds. He further proposes that the knowledge of ‘directivity and off-axis frequency responses of loudspeakers and the directional reflective, diffusive, and absorptive characteristics of materials at the points of first reflections are essential’.
Toole (2008) writes about a cognitive effect called the ‘precedence effect’ of the brain localizing in the presence of multiple different sound sources. The results gathered from different research is presented as follows- Firstly, stronger reflections shift the position of the source and make it seem larger. Secondly, reflections and delayed reflections change the timbre of the perceived sound, adding coloration and resonance that may affect the listening experience in both ways. Humans perceive such effects most if the reflections are in the median plane.
The timing in between reflections too affect the listening experience. Haas, Gardner, Lochner and Burger all found that the first reflection dominates this experience. However, the timings between subsequent reflections determine if these are heard as ‘spatially separated auditory images’ (long delay) or as a single image (short delay). Also, the volume of the delayed sound determines how ‘spacious’ the room sounds.
In relation to our project, it is clear that for an even perception of sound across the entire living space, it is necessary to consider the acoustics of the individual spaces such as the kitchen, bathroom and hallway that make up the entire living space. Factors such as the volume of the room, the presence of strongly sound-absorbing curtains and furniture in the living room in comparison to hard table tops in the kitchen do affect the listening experience. Sound sources should therefore cater for and smoothen out these localized irregularities to create an excellent, across-the-board listening experience. If possible, the sound system could even take advantage of reflections and utilize them to enhance the listening experience.
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