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COMP 546
Lecture 18
Sound 1
- Thurs. March 22, 2018
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I spent the first hour today finishing off the previous
- lecture. See lecture 17
Sound 1 Thurs. March 22, 2018 1 I spent the first hour today - - PowerPoint PPT Presentation
Sound 1 Thurs. March 22, 2018 1 I spent the first hour today - - PowerPoint PPT Presentation
COMP 546 Lecture 18 Sound 1 Thurs. March 22, 2018 1 I spent the first hour today finishing off the previous lecture. See lecture 17 slides and notes. 2 For vision, reflected light is more important than direct light. 3 For audition,
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For vision, reflected light is more important than direct light.
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For audition, direct sound is more important than reflected sound.
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Types of sounds
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What determines a sound?
- Force: where does the sound energy come from?
- Oscillator: what vibrates
- Resonator: what cavity reshapes the sound ?
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Vibration: basic mechanics
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Total energy = Kinetic energy + Potential energy
Kinetic low Potential high Kinetic high Potential low Kinetic low Potential high
π’ β βπ’ π’ + βπ’ π’
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Air pressure: longitudinal wave
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high low medium medium low high π’ β βπ’ π’ + βπ’ π’ At any point in 3D space, the air pressure oscillates over time.
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Time Snapshot
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high low high low high
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The sound that we hear is the sum of many sounds waves.
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π π, π, π, π’ = π½ππ’π + π½(π, π, π, π’)
π½ π, π, π, π’ is called the sound pressure. π’
π½ππ’π
Atmospheric pressure π½ππ’π varies a bit with temperature, altitude, weather.
π½(π, π, π, π’)
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Absolute threshold of hearing π½0
π½0 π½ππ’π = 10β9
Pain threshold
π½ππππ π½ππ’π = 10β3
π½0 and π½ππππ refer to a pressure deviation around π½ππ’π
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Physics tells us that energy density per unit volume of π½ π, π, π, π’ varies with π½(β¦ )2 . (Work done to compress or expand. Units omitted β this isnβt a physics course.) Root mean square (RMS) of sound pressure:
π½ β‘ 1 π
π’=1 π
π½(π, π, π, π’)2
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βSound pressure levelβ (SPL): dB
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πππ10
π½ 2
π½0
2
Bels (B) 10 πππ10
π½ 2
π½0
2
= 20 πππ10
π½
π½0 decibels (dB)
Why a log scale? We are sensitive to ratios of sound pressure, not differences. Why dB and not B ? βJust noticeable differenceβ (threshold) is typically around 1 dB.
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Examples of SPL (dB)
Jet plane Noisy traffic Voice in conversation Quiet room Recording studio Absolute threshold 120 90 60 30 10
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