MUS 1360 Unit 1: Sound and Audio Fundamentals

Fundamentals of Sound and Vibration

  • Nature of Sound: Sound is defined as vibrations that travel through a medium. These vibrations can occur in several states of matter:

    • Gas: Most commonly air, which is the primary medium for human communication and music.

    • Solid: Sound can travel through building materials, such as walls or floors.

    • Liquid: An example would be sound traveling through sea water.

  • Visualizing Sound: Waveforms and slow-motion videos can be used to demonstrate how musical instruments or other sources create physical vibrations that disturb the medium.

  • Properties of Sound:

    • Frequency: The number of cycles a sound wave completes in one second, measured in Hertz (Hz), which determines the pitch of the sound.

    • Amplitude: The height of the wave, indicating the loudness of the sound, often measured in decibels (dB).

    • Wavelength: The distance between consecutive peaks of a sound wave, which affects how we perceive sound in different environments.

    • Timbre: The quality or color of a sound that distinguishes different types of sound production, such as voices and musical instruments, which results from the combination of harmonics and the fundamental frequency.

    • Pitch: The perceived frequency of a sound, which determines how high or low it sounds to the listener.

  • Speed of Sound: The speed of sound is approximately 1100 ft/sec1100\,ft/sec. There are several variables that can affect this speed (such as temperature and humidity).

    • Practical Implications:

      • Timing: In large venues like concerts or sporting events, the distance between the source and the listener results in a noticeable delay.

      • Recording Phase: In studio environments, when microphones are placed at different distances from the same source, the speed of sound affects the arrival time at each mic, which can lead to phase-cancellation issues.

Human Hearing and Psychoacoustics

  • Frequency Response: The standard range of human hearing is from 20 Hz20\text{ Hz} to 20 kHz20\text{ kHz}.

    • Hertz (Hz): A unit used to measure frequency, representing cycles per second.

    • Logarithmic Scale: Audio frequencies are typically displayed on a log scale rather than a linear scale because humans perceive frequency changes (octaves) as equal intervals.

  • Hearing Sensitivity: Measured in Decibels (dB).

    • Threshold of Audibility: 0 dB0\text{ dB}, the quietest sound a healthy human ear can detect.

    • Threshold of Pain: 120+ dB120+\text{ dB}, the level at which sound causes physical pain and potential immediate damage.

    • Comparisons: Typical levels include normal conversation (lower dB) and loud music (higher dB).

  • Localization: Humans use two ears to determine the direction of a sound source. This is the foundation for stereo sound.

    • Mono: Single-channel audio where sound comes from one source or is identical in both ears.

    • Stereo: Two-channel audio that provides depth and localization.

  • Pitch Perception: Pitch is the subjective interpretation of highness or lowness of a note based on frequency.

    • Perfect Pitch: The ability to identify or recreate a given musical note without a reference tone.

    • Relative Pitch: The ability to identify the relationship between two notes (part of aural theory or solfge).

    • Octave Relationship: An octave represents a doubling or halving of a frequency. For example:

      • A3 = 220 Hz

      • A4 = 440 Hz

      • A5 = 880 Hz

      • xt{ Hz}

  • Fletcher-Munson Curves: Human hearing is not "flat." We do not hear all frequencies at the same volume. These curves show how the ear's sensitivity changes across the spectrum, specifically requiring more low and high-frequency energy to perceive them as equal in volume to mid-range frequencies.

Audio Technology and Signal Path

  • Historical Progression: Recording materials have evolved from metal and cylinders to the modern digital era.

  • The Acoustic Energy Cycle:

    • Sound begins as acoustic energy, hits a microphone (transducer), and is converted into electricity.

    • The signal moves through a mixer to speakers (transducers), which convert electricity back into acoustic energy.

  • Digital Signal Path:

    • Microphone $\rightarrow$ Audio Interface (Analog-to-Digital Conversion) $\rightarrow$ Computer $\rightarrow$ Audio Interface (Digital-to-Analog Conversion) $\rightarrow$ Speakers.

  • Electronic Characteristics:

    • AC Voltage: Audio signals are alternating current voltages.

    • Inputs and Outputs: Directionality is key; identifying which component sends the voltage (Output) and which receives it (Input).

    • Measurements: Amplitude is measured in dB, and Frequency is measured in Hz.

  • Monitoring:

    • Speakers vs. Headphones: Each has distinct positives and negatives regarding spatial accuracy, comfort, and frequency response.

Digital Audio Principles

  • Concept: Digital audio is a numerical representation of sound vibrations. While analog recording (records, tape) captures continuous waves, digital audio uses levels of discrete data.

  • Binary: The fundamental storage method using 0sands and1$$s.

  • Converters:

    • ADC: Analog-to-Digital Converter.

    • DAC: Digital-to-Analog Converter.

  • Quality Factors:

    • Sample Rate: Determines the frequency range being captured. Digital audio is not "stair-stepped" when reconstructed correctly via a DAC.

    • Bit Depth: Determines the dynamic range and resolution of the amplitude levels.

    • Best Practices: It is critical to choose the correct sample rate and bit depth at the start of a project; changing these later can significantly degrade sound quality.

  • Digital Audio Workstations (DAW): DAWs do not use compressed formats like MP3 for their internal processing and storage, though they do allow users to import them.

Audio File Types

  • Uncompressed / Lossless: High-quality files that retain all original audio data.

    • WAV: Standard uncompressed audio format for Windows/General usage.

    • AIFF: Standard uncompressed audio format for Apple/Mac.

  • Compressed / Lossy: Smaller files that remove data deemed inaudible to save space.

    • MP3: Most common lossy format.

    • OGG: An alternative lossy format often used in gaming and streaming.

Assignments and Activities

  • Fletcher-Munson Curves Activity: Complete an online hearing test and submit a screenshot of the results to see how hearing sensitivity varies.

  • Mobile Hardware Exploration: Identify exactly where the sound output originates on a personal smartphone.

  • Visual Waveform Identification: Identify characteristics of audio by sight (e.g., distinguishing between a tall/short waveform, or a kick drum vs. a violin waveform).

  • Conversion Lab: Select a file and convert it into various bit depths and sample rates (using software like Audacity) to observe changes.