Acoustic Arenas and the Spatial Formation of Aural Spaces

Auditory spatial awareness is a neurological reaction (conscious and unconscious) to spatial acoustics and is one information channel through which the auditory organ receives stimuli. This cognitive process transforms raw sensation into awareness by triggering an elevated state of mental and physical awareness. The response associated with auditory spatial awareness has three stages: detection, recognition and consciousness.
Detection is a raw biological and physiological response. Recognition and consciousness are conditioned by environmental exposure and are learnt associations. The individual perceiving the sound (i.e., sonic event) is the receiver and the resulting vibrations are transformed into neural signals. If detection is established, the receiver is considered within the aural space of the sonic event, also known as the ‘acoustic arena.’ Architects and designer may influence the programmatic aspects of the space directly, but occupants have control over the dynamics of the aural space design. Activities within these spaces create sonic events as the size of the acoustic arena changes (BLESSER, Barry and Salter, Linda-Ruth, 2006). This post discusses the principles of acoustic arenas and the spatial formation of these aural spaces.

Form and Delineation

A superimposed diagram on Pierre Auguste Renoir, Luncheon of the Boating Party (1880-81) [Underlay] The diagram shows different scales of acoustic arenas.

Aural architectural space design follows the principles of acoustic arenas. If a sonic event is sufficiently powerful and a group of receivers detects it, an acoustic arena is formed, which is the volume centred on the sonic event. Anyone who cannot hear this sound, even if unable to detect the source visually, is considered beyond the boundary of the arena. Such sound is beyond the receiver’s ‘acoustical horizon,’ a dynamic auditory space centred on the receiver.

Arena volumes are as dynamic as the activities that generate sonic events. At any given time, the receiver can exist in multiple arenas, intermittently or simultaneously (BLESSER, Barry and Salter, Linda- Ruth, 2006). A successful acoustical design can exploit this interplay by enhancing the auditory connection between a particular sonic event and the listener. This connection is known as and defined by the sonic properties of the ‘auditory channel’.

The view from the whispering gallery in St Paul’s Cathedral in London. Image by Aiwok

The properties and the background ambient sounds of a sonic event define the form of the aural space. Acoustic arenas are in constant interplay, and one arena can encroach upon, substitute, or entirely engulf another. Like a visual partition, the auditory demarcation delineates the boundaries of the arena or horizon. This threshold depends on the physical qualities of the sound and the perceptual response of the listener, which blurs the perimeter. The interplay between arenas creates intermediate zones of acoustical interference where detection, recognition and consciousness do not occur.

Acoustic arenas have unusual configurations compared to other known physical spaces. For example, in a whispering gallery, one arena can exist in two non-contiguous physical areas, simultaneously [1]. If one sonic event dominates a room, the aural boundary may align with or
extend beyond its physical boundaries. Alternatively, the activities could create a matrix of aural arenas with interference zones that act as virtual cubicles within the physical boundaries (BLESSER, Barry and Salter, Linda-Ruth, 2006).

This set of basic acoustic arena principles is the foundation of the algorithm and paradigm used in this study. The volume and shape of an arena are highly dependent on the sensory modality and physical attributes of the sound and surrounding space. In the abstract, an arena exists in free space as a spherical form. The reflectivity and form of the adjacent surfaces have a direct effect on the shape and volume of the aural space and its location (BLESSER, Barry and Salter, Linda-Ruth, 2006). For example, Bill Fontana’s technique of targeting sound energy toward a specific wall manipulates the shape of his designed aural domains (DUFF, Simon, 2011).

Physical and perceptual: Scale and Volume

The scale of the three-dimensional aural space is related directly to loudness (i.e., volume in m3). Designers can use sound intensity and the surrounding absorption coefficient of the material to manipulate volume. Aural sensory design is a hybrid of two juxtaposing concepts, the physics of sound and the human perceptual response. Volume depends on the perceptual response to the frequency of the sonic arena; low-frequency sonic events create small arenas (TURNER, John and Pretlove, A.J., 1991). A physical space with highly reflective surfaces creates significantly large acoustic spaces and vice versa. An aural designer can increase the volume of an arena without increasing reverberation time by creating strong reflections that reach the listener shortly after the direct sound, as they are detected as one strong aural channel (BLESSER, Barry and Salter, Linda-Ruth, 2006).

In the abstract, the volume of an arena centred on one sonic event with no interference is considered to propagate uniformly in all directions to create a spherical space. Energy loss defines the distance from the sonic event to the edge of the aural space (i.e., radius of a spherical arena). Sound energy is absorbed as waves that travel through the air and reflect off surfaces. Sound is measured by different units; the perception of sound is measured along a logarithmic decibel scale (dB) and is often referred to as sound pressure levels (SPL). The smallest change the human ear can detect is 1-3 dB (TURNER, John and Pretlove, A.J., 1991).

The size of an acoustic arena and the enveloping shape of the auditory demarcation are directly related to the decibel scale. This phenomenon exists at many spatial scales, including intimate, personal, conversational, public and urban. The volume of an intimate arena is approximately half a meter in diameter. The most obvious example is an arena of two people whispering, which creates a domain with an SPL of 20 dB (BLESSER, Barry and Salter, Linda-Ruth, 2006). Less researched examples are the intimate spheres that form within highly noisy environments, such as speaking loudly at a nightclub or on a cliff during a strong gust of wind. The difference is that the threshold in the first example is entirely dependent on energy loss and the later arena shrinks because of high interference (TURNER, John and Pretlove, A.J., 1991). A normal conversation creates a ‘personal’ scale arena, approximately one meter in diameter, and the conversational arena may extend up to four meters. Similar to the intimate arena, these diameters contract in high-interference environments (BLESSER, Barry and Salter, Linda-Ruth, 2006).

Spatial Perception and Navigation

Humans perceive sound binaurally through two receiving organs (i.e., ears) separated by the radius of the head. The ears receive two independent channels and convert spatial attributes into one spatial ‘image.’ The acoustic behaviour of adjacent objects and geometry result in aural navigational cues, such as time difference, amplitude and spectrum. Close surfaces in small spaces amplify low-frequency (heartbeats) and strengthen the resonance of high-frequency sounds.

Aural cues also support spatial navigation. For example, one experiment prompts blindfolded subjects to walk down a corridor. When subjects start at the centre, their ears detect similar tonal coloration reflections on both sides. If subjects deviate to one side, they detect a change in low-frequency tonal coloration in the ear corresponding to that side. Findings reveal that upon perceiving this change, subjects, consciously or subconsciously, correct their courses back toward the centre (BLESSER, Barry and Salter, Linda-Ruth, 2006).

 

A diagram illustrating navigational aspects. A Sonic Navigation cue in corridors

 

Normally, low-level sonic reflections occur in any space. The brain creates spatial mental maps that allow the individual to determine the location and direction of a sonic event. The decoding process distinguishes sonic reflections from direct sound waves. If the receiver is located between a wall and a sonic event, the ear closer to the surface detects tonal coloration and the other ear detects direct waves. A sonic shadow occurs if the sonic event and the receiver are on opposite sides of the wall. The type of sonic shadow depends on the physical aspects of the sound wave and the wall. For example, a low-frequency sound casts blurred and diffused shadows, while a high-frequency sound casts sharp shadows. Open door frames also create discernible acoustic shadows cues. In this instance, the listener registers two cues, the absence of tonal coloration at the gap in the surface and the sonic shadow that results from the sound beyond the opening (BLESSER, Barry and Salter, Linda-Ruth, 2006).

Cue fidelity also changes in unnatural environments. Humans are not as adept in recognising directionality cues within high resonant, small or anechoic spaces. Anechoic spaces, considered ‘aurally dark,’ are typologies where sound waves do not reflect and are especially uncommon. Spaces can also be perceived as acoustically ‘opaque’ or ‘transparent’. An opaque space refers to one where the sounds inside are reflected back and no external sonic event permeates through the envelope. An acoustically transparent space allows the internal sonic events to propagate to the external environment while external sounds infiltrate the interior space (BLESSER, Barry and Salter, Linda-Ruth, 2006).

 A diagram comparing transparent and opaque acoustical spaces. [Left] Acoustically opaque. [Middle] Visually transparent but acoustically opaque. [Right] Acoustically transparent and visually opaque.

A diagram comparing transparent and opaque acoustical spaces. [Left] Acoustically opaque. [Middle] Visually transparent but acoustically opaque. [Right] Acoustically transparent and visually opaque.

[1] Acoustical mirrors or “listening ears” were used during World War I until they were replaced by radar technology. In fact, in the 1930’s there were attempts to establish a sonic connection between England and France across the channel.

Acoustic arenas have unusual configurations compared to other known physical spaces. For example, in a whispering gallery, one arena can exist in two non-contiguous physical areas, simultaneously [1]. If one sonic event dominates a room, the aural boundary may align with or
extend beyond its physical boundaries. Alternatively, the activities could create a matrix of aural arenas with interference zones that act as virtual cubicles within the physical boundaries (BLESSER, Barry and Salter, Linda-Ruth, 2006).

Humans perceive sound binaurally through two receiving organs (i.e., ears) separated by the radius of the head. The ears receive two independent channels and convert spatial attributes into one spatial ‘image.’ The acoustic behaviour of adjacent objects and geometry result in aural navigational cues, such as time difference, amplitude and spectrum. Close surfaces in small spaces amplify low-frequency (heartbeats) and strengthen the resonance of high-frequency sounds.

Aural cues also support spatial navigation. For example, one experiment prompts blindfolded subjects to walk down a corridor. When subjects start at the centre, their ears detect similar tonal coloration reflections on both sides. If subjects deviate to one side, they detect a change in low-frequency tonal coloration in the ear corresponding to that side. Findings reveal that upon perceiving this change, subjects, consciously or subconsciously, correct their courses back toward the centre (BLESSER, Barry and Salter, Linda-Ruth, 2006).