Sound interacts with actual environments in complex ways that are affected by nearly every aspect of the physical environment. The propagation of a sound wave in air, for instance, is influenced by all atmospheric conditions, and also by the processes of reflection, absorption and transmission at every surface with which it comes into contact. Such information is useful in predicting and describing how sound behaves in any given environment, and also in controlling or manipulating its behaviour in environments where acoustic communication has a special role.

Psychoacoustics offers related information explaining how the human auditory system localizes sounds and extracts spatial cues from the environment. In this century, electroacoustic devices interact with sound environments in the way in which they record and reproduce spatial information, whether in imitation of the natural environment, or completely artificially. Finally, soundscape studies is concerned with the notation of spatial aspects of the acoustic environment, and the information extracted by listeners from the spatial behaviour of sounds.

The specific sub-topics presented here are:

A) Sound propagation characteristics influenced by environmental factors;

B) Incidence and reflection phenomena;

C) Absorption phenomena;

D) Complex propagation phenomena;

E) Resonance and standing waves in enclosed spaces;

F) Binaural hearing and the perception of acoustic space;

G) Electroacoustic recording and reproduction of acoustic space;

H) Soundscape notation and the information contained in the spatial behaviour of sounds.

A) Sound Propagation: sound propagating in outdoor environments is subject to a variety of influences which modify the normal spreading of the sound wave. These are classified broadly as "atmospheric effects" and "surface effects".

Free Field
Simple Sound Source
Sound Propagation

B) Incidence & Reflection: a sound wave approaching (or "incident" to) a surface will be to some degree "reflected" away from it, giving rise to the phenomena of echo and reverberation, among others.

Reflection Coefficient
Echo Chamber
Canyon Effect
Diffuse Sound Field

C) Absorption: sound that is not reflected from a surface will either be absorbed by it, or partially absorbed by it and partially transmitted through it. Absorption is highly dependent on the material and structure of the surface or barrier involved, a knowledge of which allows sound levels to be attenuated and the amount of reverberation to be controlled in both indoor and outdoor environments.

Absorption Coefficient
Anechoic Chamber
Dead Room
Sound Insulation
Sound Shadow

D) Complex Propagation Phenomena: like other wave phenomena, sound waves show special behaviour when changing media, when confronting obstacles, when dispersing through spaces of complex shape, and when a moving source or observer is involved. The terms which explain these types of behaviour are refraction, diffraction, diffusion, and Doppler shift, respectively.

Doppler Shift

E) Resonance and Standing Waves: enclosed spaces have natural modes of vibration corresponding to their spatial dimensions and shape. These natural modes of vibration, which can be advantageous in some cases and not in others, have specific frequencies associated with them, called "resonance" frequencies, or in the case of rooms, "eigentones". The spatial behaviour of waves corresponding to the resonant frequencies in the space or enclosure is called "standing waves".

Sympathetic Vibration
Standing Waves

F) Binaural Hearing and the Perception of Acoustic Space: the process by which the human auditory system recognizes the spatial orientation of a sound source is called "localization", a process having great survival importance. Many localization cues come from comparing the signals arriving at each ear, a process called "binaural hearing". Other cues exist for determining the size and nature of the space, and the relative distances of sound sources.

Binaural Hearing
Monaural Hearing
Minimum Audible Angle

G) Electroacoustic Recording and Reproduction of Acoustic Space: a wide range of recording techniques exist for registering spatial information in specific ways, based on microphone type and placement, as well as the number of separate tracks or channels of information being recorded. Headphone and loudspeaker reproduction possibilities add other variables to the process of both simulating and synthesizing spatial environments. We present here a brief survey of only the basic recording and reproduction terminology and techniques.

Binaural Recording

H) Soundscape Studies: one aspect of soundscape studies is the notation of spatial environments, called "sonography", by analogy to "geography". All sounds can be heard over a certain space, thus each has a "profile"; each listener can hear sounds from varying distances, thus each location has an "acoustic horizon".

Acoustic Horizon
Acoustic Space