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, direct sound is more important than reflected sound. 4

Types of sounds 5

What determines a sound? • Force: where does the sound energy come from? • Oscillator: what vibrates • Resonator: what cavity reshapes the sound ? 6

Vibration: basic mechanics Total energy = Kinetic energy + Potential energy Kinetic low Kinetic high Kinetic low Potential high Potential low Potential high 𝑢 − ∆𝑢 𝑢 𝑢 + ∆𝑢 7

Air pressure: longitudinal wave high low medium medium low high 𝑢 − ∆𝑢 𝑢 𝑢 + ∆𝑢 At any point in 3D space, the air pressure oscillates over time. 8

Time Snapshot high low high low high 9

The sound that we hear is the sum of many sounds waves. 10

𝑄 𝑌, 𝑍, 𝑎, 𝑢 = 𝐽 𝑏𝑢𝑛 + 𝐽(𝑌, 𝑍, 𝑎, 𝑢) 𝐽 𝑌, 𝑍, 𝑎, 𝑢 is called the sound pressure . 𝐽(𝑌, 𝑍, 𝑎, 𝑢) 𝐽 𝑏𝑢𝑛 𝑢 Atmospheric pressure 𝐽 𝑏𝑢𝑛 varies a bit with temperature, altitude, weather. 11

Absolute threshold of hearing 𝐽 0 𝐽 0 = 10 −9 𝐽 𝑏𝑢𝑛 Pain threshold 𝐽 𝑞𝑏𝑗𝑜 = 10 −3 𝐽 𝑏𝑢𝑛 𝐽 0 and 𝐽 𝑞𝑏𝑗𝑜 refer to a pressure deviation around 𝐽 𝑏𝑢𝑛 12

Physics tells us that energy density per unit volume of 𝐽 𝑌, 𝑍, 𝑎, 𝑢 varies with 𝐽(… ) 2 . (Work done to compress or expand. Units omitted – this isn’t a physics course.) Root mean square (RMS) of sound pressure: 𝑈 1 𝐽(𝑌, 𝑍, 𝑎, 𝑢) 2 𝐽 ≡ 𝑈 𝑢=1 13

“Sound pressure level” (SPL): dB 𝐽 2 Bels (B) 𝑚𝑝𝑕 10 2 𝐽 0 𝐽 2 𝐽 10 𝑚𝑝𝑕 10 = 20 𝑚𝑝𝑕 10 decibels (dB) 2 𝐽 0 𝐽 0 Why a log scale? We are sensitive to ratios of sound pressure, not differences. Why dB and not B ? “Just noticeable difference” (threshold) is typically around 1 dB. 14

Examples of SPL (dB) Jet plane 120 Noisy traffic 90 Voice in conversation 60 Quiet room 30 Recording studio 10 Absolute threshold 0 15

If you double the sound pressure I(X,Y,Z,t) over some time interval, what is the increase in dB ? Doubling 𝐽(𝑌, 𝑍, 𝑎, 𝑢) doubles 𝐽 . 𝐽 2 𝐽 = 20 ( 𝑚𝑝𝑕 10 2 + 𝑚𝑝𝑕 10 ) 20 𝑚𝑝𝑕 10 𝐽 0 𝐽 0 20 𝑚𝑝𝑕 10 2 ≈ 6 𝑒𝐶 So the increase in SPL is 16