Understanding Sound and its Properties

What is Sound?

• Definition:
  • Sound is a form of energy, similar to electricity and light.
  • It involves the vibration of air molecules, creating a pattern known as sound waves.
  • Medium: Sound requires a medium (such as air or water) to travel through.

How Does Sound Work?

• Basis: Sound is fundamentally about vibration.
• Sources of Vibration:
  • Vocal cords, strings of instruments (violin, piano), drum heads.
• Mechanics of Vibration:
  • Sound travels through air by displacing air molecules, which results in variations in air density.
  • Molecules are in constant motion and, when displaced (e.g., by a voice), they transmit vibrations through collisions with adjacent molecules.
• Pressure Dynamics:
  • Vibrations create pressure waves that reach the ear, exerting pressure on the tympanic membrane (eardrum).

Compression and Rarefaction

• Displacement of Molecules:
  • The initial layer of air molecules is displaced and collides with neighboring molecules, causing a chain reaction.
• Refraction:
  • Once the pressure normalizes, the first layer of molecules returns to their resting state, which can cause a sharp energy drop.
• Energy in Sound:
  • Energy is a significant factor; it is required to move air molecules as they vibrate.
  • Positive Phase: Refers to the compression of air molecules, indicating a positive energy expenditure.
  • Negative Phase: Refers to the return of air molecules to their initial positions after being displaced.

Acoustic Theory: Sound and Waveform Characteristics

1. Amplitude (Loudness):

   • Distance above or below the waveform's centerline, reflecting the number of vibrating molecules.
   • The quantity of energy influences molecular displacement, hence determining loudness.
   • Sound Pressure Level (SPL) measures amplitude changes due to sound waves.
   • Decibel (dB) is the measurement unit for amplitude:
     • Dynamic range of human hearing: 120 dB.
     • Threshold of pain: 120 dB can cause permanent hearing damage.

2. Frequency (Pitch):

   • Defined as the rate of cycles of compression and refraction in sound waves, influencing pitch.
   • Measured in Hertz (Hz), indicating how many wave cycles occur per second.
   • Wave characteristics:
     • Low Bass: 20-80 Hz (felt sensation).
     • Midrange: 320-2560 Hz (fundamental frequencies of sounds).
     • Treble: 5120-20000 Hz (brilliance and sparkle).

3. Velocity (Speed):

   • Sound's travel speed through a medium (e.g., air): approximately 1130 ft/sec at 68°F.
   • Speed increases by 1.1 ft/sec for each Fahrenheit degree rise in temperature.

4. Wavelength (Duration):

   • Distance for a wave to complete one full cycle, from peak to peak (represented by the Greek letter lambda, λ).
   • Calculated using λ = \frac{v}{f} where:
     • v = speed of sound in the medium,
     • f = frequency in Hz.

5. Phase:

   • Refers to the timing relationship between two or more signals, critical for recording sound clearly.
   • Particularly important when using multiple microphones for recordings, such as in a band or instrument recording situation.

6. Harmonic Content (Dynamic Characteristic):

   • Most natural sounds include overtones and harmonics that give them their unique timbre.
   • Overtones result from frequencies combining (e.g., 440 Hz and 220 Hz may create a 660 Hz overtone).
   • Octave: A frequency that is either half or double another frequency.
   • Sine Wave: A pure tone, includes only the fundamental frequency.
   • Complex Waves: (sawtooth, square, triangle) also encompass harmonic and overtone frequencies.

7. Envelope (Attack, Decay, Sustain):

   • Describes the change in sound amplitude over time, affecting how a sound is perceived during its onset, sustain, and release.