(sonicArts) properties of audio – fall 2014 edition

As you review leading up to the quiz on Thursday, here are the properties of audio (physical and perceptual) that I want you to know.

Physical properties are things we can measure. Perceptual properties describe how humans perceive specific physical properties. In general, human perception is exponentially related to physical properties.

frequency and pitch

Frequency is the measurement of the rate of sound vibrations, measure in cycles per second (cps) or Hertz (Hz)Pitch is how we  perceive frequency. For example, the pitch A that an orchestra tunes to has a fundamental frequency of 440 Hz. For every octave increase in pitch you have a doubling of frequency. So the octave above that is A has a fundamental frequency of 880 Hz (2 x 440). And the next octave increase takes you to 1760 Hz (2 x 880).

amplitude and loudness

Amplitude measures the deviation in air pressure of sound. Amplitude can be an instantaneous measurement (peak amplitude), or it can refer to an average over time. Humans perceive amplitude asloudness. Amplitude can be measured in a few ways. One way to measure is in terms of a percentage of maximum, with 100% being maximum amplitude. Another was to measure amplitude is with decibels (dB). Decibels are designed to help map amplitude in a linear fashion to loudness. If you double the amplitude, you increase the decibel level by 6 dB. From 25% to 50% is a 6 dB increase. From 50% t0 100% is also a 6 dB increase. Humans perceive both increases as the same increase in loudness.

Humans perceive loudness differently depending on the frequency of the tone. Low frequencies (below 200 Hz, and even more below 100 Hz) especially require much greater amplitude to perceive the same loudness as higher frequencies (1 kHz to 4kHz).

spectrum and timbre

Spectrum is the frequency content of a tone – specifically the frequencies present and their relevant amplitudes. Almost all tones have more than one frequency component present. We hear spectrum astimbre, or tone color.

envelop and articulation 

Articulation refers to the change of loudness over time, and it is another perceptual property of sound. Articulation relates to the physical property of the amplitude envelope.