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"?

Time Domain Representation of a Sound

What parameter is common to all types of sounds?

Amplitude/Gain/Level/Magnitude/Loudness, etc.

More on sound amplitude

Human perception of loudness

What are the 2 "main types/families" of sounds?

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:

More on 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

Shannon sampling theorem

Audio bit depth

Digital audio signals range and clipping

x(x) ∈ [-1.0,1.0]