Music 256a/CS 476a | Fall 2016
Lecture 2: Acoustics and Digital Audio Basics
Center for Computer Research in Music and Acoustics
Stanford University
What is "Sound"?
https://en.wikipedia.org/wiki/Sound#Physics_of_soundTime Domain Representation of a Sound
What parameter is common to all types of sounds?
Amplitude/Gain/Level/Magnitude/Loudness, etc.
What are the 2 "main types/families" of sounds?
Sounds with a pitch (sine wave and harmonic/inharmonic sounds)
Sounds without a pitch (noise and impulse)
The "simplest kind" of pitched sound: the sine tone
More on the concept of frequency, period and wavelength
Frequency and notes
Harmonic Sounds and Fourier Series
More on harmonics: https://en.wikipedia.org/wiki/Harmonic
More on Fourier series: https://en.wikipedia.org/wiki/Fourier_series
A microphone can be used to convert mechanical pressure waves to an electric analog signal and a speaker can be used to do the opposite.
Analog signals storage, processing and synthesis
From analog to digital: sampling
Waveform of an analog signal to digitize
Sampled waveform
What are the parameters of sampling?
- Sample rate/sampling frequency
- Bit depth
Sampling rate, Nyquist and aliasing
- Typical sampling rate (fs) for audio is 44.1KHz
- Nyquist frequency is at fs/2
Why is Aliasing a Problem?
Signal reconstruction
Audio Bit Depth and Signal to Noise Ratio (SNR)
Typical audio bit depths are 16, 24 or 32 bits
More on Sampling
Digital audio signals range and clipping
x(x) ∈ [-1.0,1.0]