Sine Waves and Sound Properties

• A sine wave represents the simplest form of sound, characterized by a single frequency, which is heard as a pitch ranging from high to low.
• Complex sounds, such as those from instruments, have a fundamental frequency that defines the pitch, as well as harmonic frequencies which determine the timbre, or unique identity, of the sound.
• Pitch is the perception of a frequency as high or low, used typically in musical contexts whereas frequency measures the rate at which a sound wave fluctuates.
• Frequency quantifies how often sound waves fluctuate, while amplitude measures the extent of the wave's fluctuation.
• The speed of sound remains constant, regardless of its frequency.
• The speed of sound depends on the medium through which it travels, moving at approximately 340 meters per second in air, four times faster in water, and fifteen times faster in solid iron.
• Hertz (Hz) is the unit for cycles per second, where higher frequency sounds change shape more rapidly than lower frequency sounds.
• Every musical note has a primary, or fundamental, frequency, and voices have specific ranges they can produce.
• Timbre is the quality that differentiates the sound of one instrument from another.
• A barometer measures air pressure, in pascals, by assessing the density of air molecules in a given space.
• Air pressure depends on many factors.
• Decibels measure loudness, with sound pressure level serving as the defining reference point.
• Higher frequency waves have shorter cycles because of more cycles per second.
• Wavelength measures a single cycle of a wave, including its rise, peak, trough, and return to the starting point.
• An oscilloscope graphs sound waves, showing time on the x-axis and air pressure on the y-axis.
• A sine wave oscillates between positive and negative amplitude over time.
• Each oscillation is measured in degrees: 0, 90, 180, 270, 0, indicating the wave’s phase, or its position in the cycle.
• Phase refers to the timing of the sound wave within its cycle.
• Combining two waves creates a new wave whose shape is determined by the timing of the original waves relative to each other, called relative phase.
• Two in-phase sounds, where their ups and downs are synchronized, combine to create a sound twice as loud.
• Out-of-phase sounds, where the ups and downs are perfectly opposite, cancel each other out.
• Combining two different sine waves results in a complex sound that incorporates elements of both.
• Comb filtering occurs when a complex sound combines with a delayed copy of itself, leading to cancellations and reinforcements of certain frequencies.
• Polarity inversion, or flipping the sound wave upside down, causes cancellation.
• The McGurk effect demonstrates how sight can influence the perception of sound.

3 Domains of Sound

• Acoustic: This involves vibrating waves in a medium, not recordable without tools and lacks electronics.
• Analog: This converts acoustical waves into electrical current using devices like microphones, where the current traces the acoustic sound wave, allowing physical manipulation but limitations on altering pitch, speed, and tone.
• Digital: This converts audio into numbers through sampling, allowing manipulation via software.

Transducers

• Transducers convert sound from one form to another, like microphones and speakers.
• A feedback loop occurs when a microphone picks up sound from a speaker, amplifying it and creating a loop and can be reduced by adjusting speaker position, microphone direction, or distance.

Signal Flow

• A typical signal flow is: Acoustic sound to transducer/mic to preamp (boost to line level or recording) to power amp (boost to speaker level) to transducer/speaker to amplified acoustic sound, without reaching the digital domain in this basic path.

Recording Considerations

• A transient is a sound or part of a sound that is short-lived.
• Choosing the right microphone depends on its sound characteristics, the sound being recorded, and other elements in the mix.
• Signal to noise ratio measures the sound, environment and the noise you want where louder wanted noise typically creates a clearer sound

Analog Signal Levels

• Mic level signal is the weakest requiring a preamp to boost it to line level.
• Line level is also weak but requires a power amp to reach speaker level.
• Speaker level signal is strong, with high voltage (up to 100 volts), and can be harmful, typically found in cables carrying live signals while studios use line level.

Gain Staging

• Too strong of a signal going into a gain stage causes distortion, requiring attenuation or turning down of the input signal.
• Input signals that are too low unbalance the signal-to-noise ratio.
• Balance input signal with output to remove noise and distortion throughout the stages.
• Larger speakers are more effective for low sounds, while smaller speakers are better for high sounds, similar to musical instruments.

Digital Domain Path

• Acoustic sound is transduced into analog then, instead of recording to analog tape, it’s sent to an ADC converter, which saves the audio as numbers.
• During playback, the computer reads the stored ADC data using a DAC converter, which then goes into an amp and speaker to produce acoustic sound.

Sampling and Aliasing

• Aliasing occurs when there is insufficient sampling, that inaccurately reproduces waveforms.
• The Nyquist theorem states that there must be a rate, which must be at least twice its frequency. The Nyquist theorem underpins all analog-to-digital conversion and is used in digital audio and video to reduce aliasing.
• Use a good ADC and a sample rate of minimum 44.1khz to cleanly record all freqs in humans hearing.

Quantization

• Quantization maps continuous values to a set of discrete values, thus approximating real-world values with a digital representation.
• Quantization distortion occurs when low gain setting in the digital domain, the output sounds erratic, with sections cutting in and out.
• Quantization distortion occurs when a waveform is ‘squared-off’ or rounded to a bit depth that doesn’t accurately represent its amplitude. Quantization distortion can occur at lower bit depths because there are not enough bits for rounding, sounding similar to square wave distortion.
• Dithering adds noise to a signal to mask higher-order harmonics, making quantization distortion less noticeable and should only be used when decreasing a signal’s bit depth
• Applying an EQ (equalizer) can help remove frequencies that are either audible or not audible.
• Dithering should be turned on during mastering.

Bit Depth

• Bit depth in audio refers to the number of bits used to describe each audio sample.
• Determines the precision and dynamic range of the audio.
• The more bits used, the more accurately the sample can be represented.
• Higher depth signifies more detail in quiet and loud audio.
• A 16-bit audio file can represent 65,536 levels of sound.
• A 24-bit audio file can represent over 16 million levels of sound.
• Compact Disc Digital Audio uses 16 bits per sample.
• DVD-Audio and Blu-ray Disc can support up to 24 bits per sample.

DAWs (Digital Audio Workstations)

• DAWs with MIDI history from the 1980s include Logic, Digital Performer, and Cubase.
• DAWs with audio history from the 1990s include ProTools, SAW Studio, and Reaper.
• DAWs were originally built for live and loop-based music creation like Ableton, Propellerhead Reason, and Bitwig Studio.
• Single-track audio editors (destructive audio editors) include Audacity, iZotope RX, Adobe Audition, and Sound Forge.
• Music notation applications for writing sheet music and playback include Finale, Sibelius, NoteFlight, and MuseScore.

Dynamic Processing

• Compressors and limiters decrease dynamic range.
• Expanders and gates increase dynamic range.
• Compressors and expanders are subtle, while limiters and gates are strict.
• A limiter turns the signal down as much as possible when the signal surpasses a threshold.
• A compressor lowers the signal slightly when the signal surpasses a threshold.
• A gate mutes the signal when it falls below a threshold.
• An expander slightly turns down the signal when it drops below a threshold.