Comprehensive Study Guide: Fundamentals of Digital Audio and Perception

Definition of Sound: Sound is defined as the vibration of matter manifesting as alternating fluctuations in air pressure. These fluctuations occur in a wave-like (wellenförmig) manner.
Prerequisites for Sound Propagation:
- Sound Source (Sender): The originating object that creates the vibration.
- Transport Medium: The material (e.g., air, water) through which the sound waves travel.
- Ear: The physiological receiver that reacts to air pressure fluctuations.
The Process of Sound Creation:
- Speed of Sound: Subject to a velocity of $330\,\text{m/s}$ .
- Periodic Vibrations (Klänge): These are repeating waveforms typical of musical instruments and singing (Gesang).
- Non-periodic Vibrations (Schwingungen): These lack a repeating pattern and include speech (Sprache) and general noise (Geräusche).

Sound Pressure (Schalldruck):
- This is an objective physical measurement.
- It is primarily dependent on differences in air pressure.
Volume (Lautstärke):
- This is a subjective assessment of the strength of the sound.
- Perception of volume varies with frequency; different frequencies may be perceived as having different loudness levels despite equal physical intensity.
Pitch (Tonhöhe):
- Corresponds to the physical frequency measured in Hertz ( $\text{Hz}$ ) or Kilohertz ( $\text{kHz}$ ).
- Audible Range: The human hearing range spans from $20\,\text{Hz}$ to $20,000\,\text{Hz}$ .
- Pure Tone (Reiner Ton): Defined as a sine wave (Sinusschwingung) consisting of exactly one frequency.

Tone Characteristics:
- A tone is the sole harmonic vibration of a specific tone frequency.
- It propagates as a longitudinal wave (Longitudinalwelle).
Quality of Tone:
- Determined by Frequency (the pitch) and Amplitude (the volume).
Definition of Audio:
- Audio pertains to the audible frequency range ( $20 - 20,000\,\text{Hz}$ ).
- Infrasound (Infraschall): Frequencies located below this range.
- Ultrasound (Ultraschall): Frequencies located above this range.
- Scope includes tones, speech, noises, and music.
- Quality measurement is based on subjective assessment.

Decibel ( $\text{dB}$ ):
- Used as a unit of measure for sound intensity.
- It exists in relation to the human hearing threshold, which is defined as $0\,\text{dB}$ .
- The scale is logarithmic: the increase from $80$ to $81\,\text{dB}$ is physically larger than the increase from $6$ to $7\,\text{dB}$ .
- $0\,\text{dB}$ does not represent absolute silence, but rather the limit of human audibility.
Sone:
- This is a psychological unit for measuring volume.
- Equivalency: $1\,\text{Sone} = 40\,\text{Phon}$ .
- Scaling: Based on the transcript, $4\,\text{Sone}$ is twice as loud as $2\,\text{Phon}$ .
Audibility Constraints:
- Audibility is a function of both volume ( $\text{dB}$ ) and frequency ( $\text{Hz}$ ).
- Humans possess a non-linear frequency response (frequenzgang).

Typical Sound Levels:
- Whispering (Flüstern): $20\,\text{dB}$ .
- Conversation (Gespräch): $40\,\text{dB}$ .
- Jackhammer (Presslufthammer): $100\,\text{dB}$ .
Health Thresholds:
- Hearing Damage: Risks begin at exposure to $85\,\text{dB}$ .
- Pain Threshold (Schmerzschwelle): Occurs at $130\,\text{dB}$ .
Psychoacoustics:
- Focuses on masking effects (Maskierungseffekte/Verdeckung).
- Logically, louder tones mask or hide quieter tones. This principle is utilized in the development of the MP3 format.

The Signal Chain:
1. Air pressure fluctuations.
2. Microphone.
3. Voltage fluctuations (Spannungsschwankungen).
4. A/D Converter (Analog-Digital-Converter).
5. Data production.
Microphone Mechanism:
- Sound waves strike and move a diaphragm (Membran).
- The diaphragm imparts movement to a coil (Spule).
- The coil is positioned between magnets.
- Movement within the magnetic field induces electrical voltage.
Microphone Polar Patterns/Characteristics:
- Stereo.
- Lobar/Shotgun (Keule).
- Cardioid (Niere).
- Omnidirectional (Kugel).
Headphone Specifications:
- Frequency Response: Indicates which frequencies the headphones are capable of reproducing.
- Impedance: Measured as electrical resistance in Ohms ( $\text{Ohm}$ ). For optimal performance, this value should be as small as possible.
- Total Harmonic Distortion (Klirrfaktor): Measures the purity of the signal output by the headphones, expressed as a percentage ( $\%$ ).
- Active Noise Cancelling (ANC): Technology designed to eliminate background noise.

Image Digitalization:
- Uses Pixels.
- Resolution measured in dots per inch ( $\text{dpi}$ ).
- Formats: TIFF (lossless) and JPEG (lossy/verlustbehaftet).
Video Digitalization:
- Uses Frames.
- Measured by frame rate in frames per second ( $\text{fps}$ ).
- Utilizes compressors and formats such as Avi and MPG.
Audio Digitalization:
- Uses Samples.
- Defined by Sampling Rate ( $\text{Hz}$ ) and Sampling Depth (bit).
- Formats: WAV (lossless) and MP3 (lossy).
- The process requires two main components: the Analog-Digital-Converter (ADC) and the Digital-Analog-Converter (DAC) to transform the electrical voltage from the microphone into data and back.

Sampling Process:
- Involves measuring the frequency at regular intervals.
- The frequency of these measurements is the Sampling Frequency.
Typical Sampling Frequencies:
- $8\,\text{kHz}$ : Standard for speech (Sprache).
- $44\,\text{kHz}$ : Standard for CD-Audio.
- $48\dots\,\text{kHz}$ : Used in professional applications.
Sampling Depth (Sampling Tiefe):
- Determines the detail and fidelity of the recording.
- It represents the measurement of the amplitude.
- Common depths are $8$ or $16\,\text{Bit}$ .
Recording Techniques:
- Stereo: Standard two-channel audio.
- Surround Technique: Multitrack technology used in studio environments to achieve spatial reproduction (räumliche Wiedergabe).

WAV (Waveform): A format for lossless storage.
FLAC (Free Lossless Audio Codec): Currently the most popular lossless audio format.
MP3: An international standard characterized by extensive hardware support.
AAC (Advanced Audio Coding): Designated as the successor to the MP3 format.

Quality Enhancement Tools:
- Equalizer: Used to target specific frequencies to raise or lower their volume.
- Normalization: Adjusts the peak amplitude to a specific target value.
- Noise Reduction (Rauschunterdrückung): Removes unwanted background sounds.
- Effects: General processing to alter sound characteristics.
Editing Operations:
- Cutting (Schneiden).
- Fading.
- Mixing (Mischen).
Conversion Tasks:
- Format Change: Moving between different file extensions.
- Resampling: Altering the sampling rate of the audio file.