April 26, 2016

Intro to Sound and Data Sonification

Definitions

Acoustics: (1) The branch of physics concerned with sound. (2) The properties of a concert hall with respect to the way sound interacts with it.

Psychoacoustics: The branch of psychophysics that studies the sense of hearing. Psychoacoustics defines, qualifies and quantifies sensations in relation to the stimuli (sounds) that cause them.

Electroacoustics: The intersection of acoustics and electronics. Electroacoustics studies the conversion of sound into an electronic signal (called transduction), the manipulation of the electronic signal, and the conversion of the signal back into sound (also transduction).

Sound: a mechanical vibration transmitted through a medium (usually air) to the ear, with an amplitude and frequency capable of being perceived by the auditory system.

IF A TREE FALLS IN THE WOODS with no one around, it does make a sound.

IF A TREE FALLS ON THE MOON, even with someone around, it does not make a sound. (Sound does not travel in a vacuum.)

Bats produce ultra-sound (sound too high for humans to hear). Elephants produce infra-sound (sound too low for humans to hear).

Signal: any other vibration or energy variation that does not fit the definition of sound, even if the vibration or variation represents a sound. We commonly refer to electric and digital signals.

Analog Signal: A smoothly varying signal. In other words, a direct “analog” for sound. An electric signal is an analog signal. The grooves on an LP are also a type of analog signal. A cassette tape stares an analog signal magnetically.

Digital Signal: A signal that varies in discreet steps. A digital signal can be created is created by sampling an analog signal at regular interval, called the sampling rate. A digital signal is like a rasterized image: It is a series of numbers, or each number (or “sample”) representing the intensity of a signal at a given moment in time. (Whereas a raster image is a series of numbers each representing the color of an image at a given point on the screen or page.) A digital signal can be stored in a variety of ways, including magnetically (on a digital audio tape, or DAT), and optically (on a CD). A digital signal can be transmitted electrically (in a computer chip, or on a specially built electric cable) or optically (fiber optics).

Analog-to-Digital Conversion (ADC): The process of sampling an analog signal in order to create a digital signal.

SOUND → ELECTRIC (ANALOG) SIGNAL → DIGITAL SIGNAL

Digital-to-Analog Conversion (DAC): The process of converting a series of samples to a continuously varying (analog) electric signal.

DIGITAL SIGNAL → ELECTRIC (ANALOG) SIGNAL → SOUND

(NOTE: technically, in the diagrams above, the transition from electric to sound is “transduction.”

Sample: (1) An individual number in a digital signal. The sample represents the intensity of a signal at a given time. The sample is to a digital signal as a pixel is to a digital image. (2) The length of time it takes for one sample to go by (depends on the sampling rate, but is usually a small fraction of a millisecond). (3) An entire bit of digitally recorded sound, stored as a series of numbers (A.K.A. a stored digital signal). This is also the popular usage of the term.

Sampling Rate (in Samples per Second): (1) The rate at which an analog signal is sampled in order to create a digital signal. The most common sampling rate is 44,100 samples per second. This is the rate used by CD players. Other common sampling rates are 22,050 samples per second and 48,000 samples per second. Sampling rate is analogous to the resolution of a raster image.

How Acoustic Parameters of Sound Map to Perception (and possible data types)

physical parameter	perceptual parameter	possible data mapping (Q=Quantatiative, O=Ordinal, N=Nominal)
Frequency (Hz)	Pitch, or “height”	QON
Intensity	Loudness	(Q)ON
Waveform (spectrum)	Tone Color	(Q)(O)N
Intensity+Frequency+Waveform in Time	Timbre	(Q)(O)N

Notice, the term “volume” is not used. “Loudness” and Intensity are more precise. “Volume” is used in psychoacoustics to refer to an esoteric characteristic of sound, which could be described as its “fullness.” We will avoid the term “volume” for now.

More Definitions

Noise:

any undesirable, uncomfortable or dangerous sound. Sound pollution refers to this meaning. This is the popular meaning.
The opposite of signal. Parasitic vibrations accompanying a signal that interfere with its clear transmission. The “Signal-Noise ratio” refers to this meaning.
an a-periodic sound (a sound without a definable frequency, hence without a definite pitch). This is the opposite of “Musical Sound.”

Musical sound or pitched sound: a periodic sound (a sound with a definable frequency, hence with a definite pitch).

Unpitched sound: Noise (definition 3.)

Auditory Perception

As graphic perception must be taken into account when designing scales for visualization, auditory perception must be taken into account when designing scales for sonification of data.

One notable example of how auditory perception should influence scale design is in the construction of pitch scales.

For a discussion, see: http://blog.ericmarty.com/7/perceptually-uniform-pitch-loudness-scales-for-data-sonification