PT101 - Lesson 1a

Audio Basics

• The three critical parts of audio are:
  • Waveform: the shape of a sound
  • Frequency: the pitch of a sound
  • Amplitude: the energy level of a sound

Waveform

• A waveform represents the tone or timbre of a sound
• Waveforms have a compression phase and a rarefaction phase
• A compression phase is when the energy increases, and a rarefaction phase is when the energy decreases
• The combination of compression and rarefaction phases creates a cycle
• A waveform with a shorter compression phase and a longer rarefaction phase will have a different tone than a waveform with a longer compression phase and a shorter rarefaction phase
• Examples of waveforms include:
  • Sine wave: a simple and pure sound
  • Square wave: a sound with a more complex waveform
  • Triangle wave: a sound with a triangular waveform

Frequency

• Frequency refers to the pitch of a sound
• It is measured in Hertz (Hz) and represents the number of cycles per second
• A higher frequency means a higher pitch, while a lower frequency means a lower pitch
• The human range of hearing is from 20 Hz to 20,000 Hz
• Different animals can hear different frequencies
• The frequency range of human hearing can vary depending on age and listening habits

Amplitude

• Amplitude refers to the energy level of a sound
• It is visually represented in a waveform by its height
• A sound with a greater amplitude will be a taller waveform, while a sound with a lower amplitude will be a shorter waveform
• Amplitude can be affected by the force of a sound, such as hitting a drum harder or softer
• The unit of measurement for amplitude is Decibel (dB)
• The human range of hearing is from 0 dB (silent) to 120-130 dB (painful)

Audio in the Digital Domain

• Audio can be recorded and stored on a digital medium
• The process of converting analog audio to digital audio is called analog-to-digital (A/D) conversion
• The process of converting digital audio back to analog audio is called digital-to-analog (D/A) conversion

Samples and Sample Rates

• A sample is an instantaneous measurement of an audio signal
• A sample rate is the number of samples taken per second
• Common sample rates include:
  • 44.1 kHz (44,100 samples per second)
  • 48 kHz (48,000 samples per second)
  • 88.2 kHz (88,200 samples per second)
  • 96 kHz (96,000 samples per second)
• The sample rate must be at least twice the highest frequency of the sound to accurately record or reproduce it
• The Nyquist-Shannon sampling theorem states that the sample rate must be at least twice the highest frequency of the sound

Bit Depth

• Bit depth refers to the number of bits used to measure the amplitude of a sound
• Common bit depths include:
  • 16-bit: 65,536 possible amplitude values
  • 24-bit: 16,777,216 possible amplitude values
• A higher bit depth allows for more accurate measurements of amplitude and a greater dynamic range
• The formula for calculating the dynamic range of a bit depth is: 6 dB per bit
• 16-bit audio has a maximum dynamic range of 96 dB, while 24-bit audio has a maximum dynamic range of 144 dB

The Impact of Sample Rate and Bit Depth on File Size

• The choices of sample rate and bit depth will influence the file size of a digital audio file
• A higher sample rate and bit depth will result in a larger file size
• The following chart lists sample rates, bit depths, and their corresponding file sizes for a mono, one-minute audio file:
  • 44.1 kHz, 16-bit: 5.3 MB
  • 48 kHz, 16-bit: 5.8 MB
  • 88.2 kHz, 16-bit: 10.6 MB
  • 96 kHz, 16-bit: 11.5 MB
  • 176.4 kHz, 16-bit: 21.1 MB
  • 192 kHz, 16-bit: 23.0 MB
  • 44.1 kHz, 24-bit: 7.9 MB
  • 48 kHz, 24-bit: 8.6 MB
  • 88.2 kHz, 24-bit: 15.9 MB
  • 96 kHz, 24-bit: 17.2 MB
  • 176.4 kHz, 24-bit: 31.8 MB
  • 192 kHz, 24-bit: 34.6 MB