IAT 340 note 1

IAT 340: Sound Design Notes

Fundamentals of Audio Processing

• Instructor: Philippe Pasquier

• Email: pasquier@sfu.ca

Today's Lecture Outline

• Fundamentals of Acoustics:

  • Sound and Waveforms

  • Frequency

  • Amplitude

  • Phase and Envelope

• Digitalisation of Sound:

  • Sampling Rate and Bit Depth

  • Aliasing and Quantisation Noise

  • Digital Audio Recording and Playback Chain

What is Sound?

• Perception of Sound:

  • For normally constituted humans, true silence does not exist.

  • Surrounding sounds (e.g., monitor buzz at 15 kHz, birds, motors, people) are always present.

  • In utter silence, one may begin to hear bodily sounds (e.g., breath, heartbeat, blood circulation).

• Acoustics:

  • Acoustics is a branch of classical physics that studies and analyzes the properties of sound.

  • It serves as a logical starting point for effective sound design in new media.

• Definition of Sound:

  • Sound is the aural perception of pressure vibrations within a given medium.

  • Production/Generation: An object vibrates transferring mechanical energy into acoustic energy (e.g., a speaker).

  • Propagation: Sound travels from its source to a receptor; the medium in this context is air.

  • Perception: Sound is received and interpreted by a receptor (e.g., human ear).

  • Sound can be defined as variations in atmospheric pressure in the sonic range.

Propagation of Sonic Waves

• Molecular Displacement:

  • Compression and rarefaction involve slight displacement of molecules while the wave propagates.

• Representations:

  • Common representation of sound waves is a waveform, depicted as Time vs. Amplitude.

Waveform Characteristics

• Waveform Definition:

  • A waveform is a visual representation that helps analyze sound characteristics, including wavelength, frequency, amplitude, envelope, and harmonics.

  • The purest waveform is a sine wave, according to the Fourier Theorem—all waveforms can be synthesized from sine waves.

• Types of Waveforms:

  • Periodic Waveform: Regularly repeating.

  • Complex Periodic Waveform: Comprised of multiple frequencies.

  • Random Waveform: Non-repeating, lacks regular rhythm.

Sonic Waves Frequency

• Frequency Definition:

  • Frequency is the rate of cycles of compression and rarefaction occurring in waves, measured in Hertz (Hz), which indicates cycles per second.

  • Corresponds directly to perceived pitch: Higher frequency results in higher pitch.

• Human Hearing Range:

  • Approximately 20 Hz to 20,000 Hz (20 kHz).

  • Frequencies below this range are termed subsonic or infrasonic; above this range are ultrasonic.

• Object Frequency Production:

  • Various objects can produce sonic waves at different frequency ranges.

Amplitude of Sonic Waves

• Amplitude Definition:

  • Amplitude measures deviation from normal atmospheric conditions, indicating the extent of compression or rarefaction relative to sound energy.

• Perceived Loudness:

  • Loudness (as perceived amplitude) is averaged across peak-to-peak values.

  • RMS (Root Mean Square): Calculated as the root of the mean of the squares of sample values, used as a standard measure.

• Intensity Measurement:

  • Intensity is presented in Watts/m², while sound level is measured in decibels (dB).

  • The dB scale is logarithmic to represent load intensity, addressing human sound perception.

  • Logarithmic Relationships:

  • 3 dB corresponds to a 2:1 ratio, 6 dB to 4:1, 10 dB to 10:1, etc.

Sound Propagation and Amplitude

• Speed of Sound: Sound travels at approximately 344 m/s in air (at 20 °C and 70% humidity).

• Intensity Relation to Distance:

  • Sound intensity decreases with distance; for every doubling of distance, sound intensity loses 6 dB.

  • Example:

  • At 5 m from a speaker, intensity is a reference point. At 10 m, intensity is 6 dB softer (4x less), and at 20 m it's 12 dB softer (16x less).

  • This effect is known as the Inverse Square Law.

Sound Wavelength

• Definition:

  • The wavelength is the physical distance of one complete cycle of a sound wave.

  • Longer wavelengths correspond to lower frequencies (e.g., 20 kHz has a wavelength of 2 cm, whereas 20 Hz has a wavelength of 17 m).

  • Period Definition:

  • The period is the time required to complete one cycle.

  • Frequency (in Hz) is calculated using: $ext{Frequency (in Hz)} = rac{1}{ ext{Period (in sec)}}$

Envelope of Sound

• Envelope Definition:

  • The envelope represents the intensity variation of sound over time as perceived by humans.

  • A common model for sound envelope is the ADSR (Attack, Decay, Sustain, Release).

Phase of Sound Waves

• Phase Definition:

  • The phase represents the starting angle within a 360-degree cycle of wave motion.

  • Acoustic phenomena are linear, indicating that sounds from multiple sources combine through addition.

• Constructive and Destructive Interference:

  • Waveforms can interfere constructively (in phase) or destructively (out of phase), affecting sound intensity and perception.

• Phase Reference Points:

  • Displacement of a waveform can be described as:

  • Quarter wave = 90°

  • Half wave = 180°

  • Full cycle = 360°

Psychoacoustics

• Definition:

  • Psychoacoustics investigates human sound perception, addressing psychological aspects such as:

  • Pitch: how high or low a sound is perceived in relation to frequency.

  • Loudness: perceived volume as related to amplitude.

  • Timbre: quality or texture of sound distinguishing it from others.

Digitalisation of Sound

• Analog vs Digital:

  • Sound is an analog phenomenon; its pressure variations are continuous without breaks.

  • Computers require a digital representation, necessitating the conversion of sound into binary values through methods like ADC (Analog to Digital Converter) and DAC (Digital to Analog Converter).

• Sampling:

  • Sampling captures sound amplitude at given intervals to create a series of numerical values.

  • Sampling Rate: frequency of sampling in Hz, affecting sound resolution.

  • Bit Depth: defines sample formats and the number of bits for amplitude representation, common depths include 8, 16, 24, and 32 bits.

Quantisation Error

• Definition:

  • Quantisation error arises from approximating analog values into digital format, leading to potential loss of information.

  • This error presents as quantisation noise in the reconstructed signal.

  • The signal-to-noise ratio (SNR) quantifies this error, approximated by: $ext{SNR} ext{ (dB)} = 6N$ where N is the number of bits.

Jitter

• Definition:

  • Jitter is a digital error caused by instability in the clock signal during the ADC or DAC processes, affecting the timing accuracy of samples.

Digital Audio Recording and Playback Chain

• Overview of digital processes in creating, manipulating, and reproducing sound.

Summary of Key Concepts

• Fundamental understanding of acoustics and their application in digital audio processing:

  • Production, propagation, and perception of sound

  • Representing sonic waves: waveforms, frequency, amplitude, phase, envelope

  • Digitalisation details: ADC, DAC, sampling rate, bit depth, and the implications of aliasing and quantisation noise.

Practical Applications and Forward-Looking Statements

• Continued evolution of technology influences sound design techniques and applications in media.

Active Listening Exercise

• Hearing vs Listening:

  • Hearing is passive, while listening involves actively processing sound information.

  • It requires practice to develop effective listening skills.

Resources for Sound Acquisition

• Freesound.org

• Pro Sound Effects library available at SFU library.

Notable Quotes

• "Since light travels faster than sound, people appear bright until you hear them speak." – Chinese Proverb

• "We have two ears and one mouth so that we can listen twice as much as we speak." – Epictetus