Acoustic Space MP3 Lecture
Acoustic Space MP3 Lecture, 2003.
This lecture is an introduction to the concept of acoustic space:
Space created with and in sound, how we use (perceive) sound to locate its source in space (location perception), and how we can use different technologies to create a sense of space.
- What is space?
- How do we perceive space?
- Media production using (simulated) 3—SPACE
1. What is space?
Space is a complex idea, not easily described.
OED: 1.a. a continuous unlimited area or expanse which may or may not contain objects. b. an interval between one, two or three-dimensional points or objects ...4. an interval of time (as in in the space of an hour)
These definitions tend to refer to Physical Space but we should also note that we also tend to refer to other types of space such as
- Conceptional (Idea space): 1-D, 2D, 4D, n—D
- Place (physical space with character)
Types of physical space
- 2D (planar)
- 3D (volumetric)
- Virtual Space
Sound in Spaces
Space in Sounds
I use the term 3—space to refer to 3—dimensional physical space.: the space outside of and around us.
2. How do we perceive 3—SPACE?
Spatial perception is defined relative to the perceiver and evolution has determined the characteristic of each organism's spatial perception as it has evolved in it's environment. A worm's 'sense of space' is probably different to a mouse's which is probably different to an elephant's. If space is thought of as a sense, i.e. something perceived, a general parameterisation of 3—space relative to an abstract point perceiver might be azimuth (flat plane radial location), elevation (height) and vergence (nearness or distance), relative to the listener.
NB This general terminology is also usable for other senses, eg. visual, thermal etc. Other terminology might include radial location, height and distance relative to the auditor.
Animals (and at least some plants) of many varieties are sensitive to the direction and distance relative to themselves, of sound sources as well as physical objects and other sense data (such as heat). Though not as acute as other animals, humans have the psychophysical ability to recognise the relative source of some sounds. Different animals have eyes and ears of different shapes and sizes and located on different parts of the head (in land animals, always the head)
Human acoustic space perception is complex (a perceptual gestalt) because it relies not just on our observational of acoustic principles (low frequency sounds bend around objects etc) but on our sense of space and how ‘things’ behave in space
- pigs might fly but elephants are less likely to
our sense of gravity and momentum
- big things move more slowly than small things
- our neck muscles ‘tell’ us when our head is turned to one side
Our skeletal system
- bones transmit tremors throughout our entire body our simultaneous visual perception
Distance and Loudness
All else being equal, the louder a sound is the closer we tend to think is it's source to us.
Distance and Pitch change
A sound source approachs us, the pressure wave is compressed causing its pitch to appear higher to us. A sound source approachs us, the pressure wave is rearfacted causing its pitch to appear lower to us. This effect is know as Doppler—shift after the Austrian physicist C.J. Doppler (d. 1853) who first described its cause. It also applies to light and is used to measure the age of stars.
Distance and Timbre
Timbre is a musical word (we also talk about 'timbre spaces') which refers to a the spectral characteristics of sounds and usually described as colour, brightness, rugosity etc) and the spectral characteristics of a heard sound can give us important clues as to
- how far away a sound source is
- the type of environment (hard—surfaced and echoic, soft—surfaced and absorptive) in which the sound source is produced and through which the sound travels to us
3. Media production using (simulated) 3—SPACE
Because our perception of 3—Space is multimodal, we can use different tools to create virtual spaces (impressions of 3—spaces as well as creating new types of spaces):
- A single channel (monaural) recording still contains sounds whose characteristics have been affected by the space in which they were produced and so give an impression of that space and how the sound source might have changed during the recording. (changes in timbre, pitch, loudness)
- Stereo (co-incident 2-channel) recordings give an enhanced sense of a spatial field because they simulate the inter-aural time differences we perceive as a result of having spatially—separated ears.
- Binaural recordings (using dummy-head or ear-emplaced microphones) further enhance this sense but have certain restrictions due to no two people having the same HRTF (Head-Related-Transfer Function) and sound-field is not affected by head-movement.
- Surround Sound (Dolby 5.1, 7.1 10.2 etc) are attempts at creating a 2D sound around the (cinema) audience. Front Left and Front Right for Foley effects. Front Centre for Dialogue. Back Left and Right for Atmosphere.
- Ambisonics for (re-)creating 3D—sound fields which are not perceiver—location oriented. Encodes a sound-field into its X,Y, Z and O components for recording and permits decoding into multiple speaker arrays. See my domes as examples.
- Multimodal techniques for creating a fuller sense of immersion. Outside the scope of this lecture. Those interrested could read Worrall, D. (1998b).
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http://www.dform.com/inquiry/spataudio.html (last updated 1996)