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
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“Ruben’s Tube”
http://hk.youtube.com/watch?v=VEiEBEadZFI
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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.
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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
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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
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Noise Pollution Effects from Noise
From: 經濟日報 30 March 2006
From: 經濟日報 25 Nov 2005
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3. Phasing Common Connectors
RCA
¼” Mono
¼” Stereo
XLR (Cannon)
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Microphone Dynamic Mic.
• Convert vibration to electric signals.
• Two main types:
– Dynamic Mic.
– Condenser Mic.
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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
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4. Directional Properties Directional Properties
Hypercardioid Bidirectional
• Cardioid microphone
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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.
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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
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5. Neumann U87ai Dolby Digital 5.1 – Cinema
Center, Left, Right, Left Surround, Right
Surround, LFE (Low-freq. effects)
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Dolby Digital 5.1 – Home
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