ITMA08 Introduction to Digital Audio I

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ITMA08 Introduction to Digital Audio I

  1. 1. Acoustics (or physics of sound) ITMA 08 • What is sound? Introduction to Audio I – Rapid vibration in the pressure of air molecules producing sound waves. – Sound waves cause our eardrums to vibrate. Krates Ng Fall 2008 11/17/2008 9:54 AM Krates Ng 2 “Ruben’s Tube” http://hk.youtube.com/watch?v=VEiEBEadZFI 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 3 4 How fast does sound travel? Characteristics of sound waves • V = 331.4 + 0.6 T • V = velocity (m/s), T = temperature in C • 344 meters per sec. at 20C • Speed of light in vacuum  299,792,458 m/s or 870,000 times the speed of sound. • Period (T) = time required to complete a full cycle • Frequency (f) = number of cycles per sec. or Hertz • Sound travels faster in dense medium, e.g. (Hz) = 1/T 6,420 m/s in Aluminum. • Amplitude (A) = max. displacement. 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 5 6 1
  2. 2. Frequency Power of sound (frequency) http://hk.youtube.com/watch?v=17tqXgvCN0E Natural Frequency (resonance frequency) • Most of us can hear from 20Hz to 20kHz Tuning Fork • Most sensitive at 2kHz to 4kHz http://hk.youtube.com/watch?v=C5LS6scAL3E • Normal voice range: 500 Hz to 2kHz • How does 1kHz sound? Collapsing Bridge due to a no. of reasons: • How does 100Hz sound? http://hk.youtube.com/watch?v=j-zczJXSxnw 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 7 8 How do we make sound? Measuring sound intensity • Sound Pressure Level (SPL) in Decibel (dB) • Intensity Level = 10 Log (P1/P2) dB http://hk.youtube.com/watch?v=rET_6pLBHxE • P2, reference intensity = threshold of hearing, 10-12 W/m2 (watts per square meter) • 130 dB SPL : threshold of pain! • 110 dB SPL: mid-loud rock concert • 70 dB SPL: vacuum cleaner from 10 ft. • 20 dB SPL: average household silence 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 9 10 Noise Pollution Effects from Noise From: 經濟日報 30 March 2006 From: 經濟日報 25 Nov 2005 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 11 12 2
  3. 3. Phasing Common Connectors RCA ¼” Mono ¼” Stereo XLR (Cannon) 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 13 14 Microphone Dynamic Mic. • Convert vibration to electric signals. • Two main types: – Dynamic Mic. – Condenser Mic. 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 15 16 Condenser Mic. Directional Properties • Flatter freq. response. • Omni-directional microphone • More responsive and sensitive. – Equal output/sensitivity at all angles. • Requires power (electret condenser mic. does not). – Good for picking ambient sound 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 17 18 3
  4. 4. Directional Properties Directional Properties Hypercardioid Bidirectional • Cardioid microphone 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 19 20 Frequency Response Impedance • Low impedance (600) • Medium impedance (600  - 10,000 ) • High impedance (> 10,000 ) • Matching impedance. • Generally speaking, low impedance is preferred. • The way a microphone (or system) responds to different frequencies. 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 21 22 Why U87? A Good Condenser Mic. • Georg Neumann U87ai • Omni, cardioid, figure-8 • 200 impedance • It is cheap…. HK$20,000 only! • www.neumann.com 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 23 24 4
  5. 5. Neumann U87ai Dolby Digital 5.1 – Cinema Center, Left, Right, Left Surround, Right Surround, LFE (Low-freq. effects) 11/17/2008 9:54 AM Krates Ng 11/17/2008 9:54 AM Krates Ng 25 26 Dolby Digital 5.1 – Home 11/17/2008 9:54 AM Krates Ng 27 5

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