## 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_sound

### What parameter is common to all types of sounds?

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

More on sound amplitude

Human perception of loudness

### 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

Shannon sampling theorem

Audio bit depth

Digital audio signals range and clipping

x(x) ∈ [-1.0,1.0]