Monday, February 8, 2016

following up on questions and/or topics that came up in class #1



Posted by CDJ
Psychoacoustics
 Different authors provide different understandings of psychoacoustics…perhaps reflecting cultural perspectives or dominant ideas at a particular moment in time or combinations of informing factors.

From, Acoustic Communication, by *Barry Truax, 1984:                   
(pdf available on-line)
The Energy Transfer Model

To go more deeply into the sources of the gap between traditional knowledge and contemporary problems, we can begin by examining the model on which most disciplines dealing with sound have been based, namely the energy transfer model. With the advent of electronic technology, the model has become that of signal transfer, but we can see that the same principles are embodied in it as well. The energy transfer model deals with acoustic behav- ior as a series of energy transfers from source to receiver. It examines how these transfers occur, how efficient they are, and what variables affect them. The energy originates with a vibrating object that radiates its energy to the air or through any object with which it is in contact. The most common example quoted is probably that of the tuning fork which sounds faint when struck, but becomes clearly audible when placed on a table or other object with a large surface area. Acoustics tells us that the energy transfer in the first case is inefficient (it's called an impedance mismatch). In the second
4 SOUND, LISTENING, AND SOUNDSCAPE
case where a greater surface area is involved by "coupling" the fork to the table, the energy transfer is more efficient, and since less energy is lost in the transfer to the air, the sound is more clearly heard. Similarly, we discover that the outer parts of the ear (the auricle and pinna) are especially well suited for the transfer of energy from the air to the narrow auditory canal, because they act as a kind of funnel to direct the sound waves in the appro- priate direction.
Once the energy has radiated from the source, it propagates through the "medium," normally air or water, with varying speeds and other charac- teristics. A denser medium where the molecules are closer together, such as water or metal, allows the energy to travel more quickly. In air, the speed of sound through a warmer air mass is greater than through a cooler one, and so on. Environmental acoustics also studies how different frequencies behave during propagation, a subject that will be dealt with as "response" charac- teristics later on. When the sound wave comes into contact with objects, its energy is transmitted through the object, absorbed within it, or reflected from it with varying degrees of efficiency depending on frequency.
On arrival at the ear, the sound becomes the subject of study for psychoacoustics which examines the chain of energy transfers as the sound wave
is transmitted from the outer ear via the eardrum to the bones of the middle ear called the ossicles. This transmission involves the transfer of energy from the air to a solid, a process which the eardrum through the course of its long evolution from the equivalents found in fish and reptiles is remarkably adept at performing. The actual distance moved by the eardrum in response to the slightest vibration which can be heard is less than a wavelength of light, i.e., it can not be seen, even with a microscope! The bones of the middle ear are attached to the oval window of the spiral-shaped cochlea which is filled with a fluid that can transmit the energy from the mechanical vibration of the stirrup, the last of the bones of the middle ear. The sound energy in the cochlea creates a bulging of the basilar membrane located within the cochlea, and the shearing motion of this membrane against the thousands of tiny hair cells in the organ of Corti activates them to produce electrical impulses which travel via the auditory nerve to the brain. It is through the firing of the hair cells that the first level of analysis of the sound wave occurs in terms of frequency and intensity, or more generally, in terms of the energy distribu- tion (or "spectrum") of the sound. Once analysis is involved, it seems appro- priate for psychoacoustics to use a signal processing model to describe the operation of the auditory system.
Psychoacoustics documents the processing of incoming sound waves by the auditory system to extract usable information for the brain, in other words, the process called hearing. To do so, it has relied in the past quite heavily on a model drawn from 19th-century psychophysics, namely the stimulus-response model. The founder of modern psychophysics, Gustav
ACOUSTIC TRADITION AND THE COMMUNICATIONAL APPROACH 5
Fechner, attempted to understand how the brain formed subjective impres- sions based on the magnitude of external stimuli. For instance, how does our concept of "heftiness" relate to objective measures of weight? Fechner dis- covered that there was a systematic relationship between the magnitude of the stimulus and that of the subjective response. In fact, he suspected that there was a universal logarithmic relationship between the two for many, if not all, stimuli, i.e., that larger and larger stimuli were required to produce equal increments in the corresponding subjective sensation. Although mod- ern work has refined the nature of this principle, we can see that Fechner's concept generated several new ideas. First, it allowed subjective reactions to be scaled and therefore made amenable to scientific study. And secondly, it allowed the concept of energy transfer to be extended into the realm of individual experience by treating it as a "stimulus" with dimensions called parameters which transfer to the corresponding dimensions of subjective "response."
Thus came about the modern scientific distinction between the "objec- tive" acoustic parameters, such as intensity, frequency and waveform, and their psychoacoustic, "subjective" counterparts, namely loudness, pitch and timbre, respectively, which describe the brain's response to those parameters (Plomp, 1976; Roederer, 1975; Tobias, 1972; Moore, 1982). This distinction allows us, for instance, to ask what is the smallest change in the objective stimulus that produces a perceptual change, a measure called the "just
nc
noticeable difference" (j 0- Or, we can ask, as did Fechner, whether the relation between stimulus and response is linear (i.e., equal changes in stim- ulus produce equal changes in response), or whether it is logarithmic as described above.
More subtly, psychoacoustics determines what are the physical (i.e., acoustic) characteristics of a stimulus that result in a single sensation or a double one (e.g., one tone or two, based on frequency difference). Masking experiments, for instance, determine the conditions under which one sound, by virtue of its intensity or frequency content, makes it difficult or impossible to hear another sound. The time variable is also considered: how long must a sound last or be separated from another for a certain type of percept to occur? Such data, collected over many subjects, tends to show a statistical unity of response; that is, within certain, fairly narrow statistical limits, individuals perceive sound in generally the same way, according to the psychoacoustic parameters as defined. Just as with the concept of energy transfer, response to stimuli can be rationalized as essentially known, quantifiable behavior.
* (courtesy of Wikipedia)Barry Truax (born 1947) is a Canadian composer who specializes in real-time implementations of granular synthesis, often of sampled sounds, and soundscapes.[1] He developed the first ever implementation of real-time granular synthesis, in 1986, the first to use a sample as the source of a granular composition in 1987's Wings of Nike, and was the first composer to explore the range between synchronic and asynchronic granular synthesis in 1986's Riverrun. The real-time technique suites or emphasizes auditory streams, which, along with soundscapes, inform his aesthetic.
Truax teaches both electroacoustic music and acoustic communication at Simon Fraser University. He was one of the original members of the World Soundscape Project. His students include composers Jean Piché, David Monacchi, Michael Vincent, Paul Dolden, Susan Frykberg, and John Oswald.


No comments:

Post a Comment