Puzzler Imagine that you have three boxes one

Puzzler Imagine that you have three boxes, one containing two black marbles, one containing two white marbles, and the third, one black marble and one white marble. The boxes were labeled for their contents – BB, WW and BW – but someone has switched the labels so that every box is now incorrectly labeled. You are allowed to take one marble at a time out of any box, without looking inside, and by this process of sampling you are to determine the contents of all three boxes. What is the smallest number of drawings needed to do this? 1

Solution: One !!! The key is that every box is incorrectly labeled. Draw a marble from the box labeled BW Assume it is black. – you now know which box has 2 black marbles. Therefore you know the contents of the box labeled WW must be BW (it can’t be labeled correctly!) Then the third box labeled BB must hold WW marbles 2

Introduction to Audio and Music Engineering Lecture 7 • Sound waves • Sound localization • Sound pressure level • Range of human hearing • Sound intensity and power 3

Period: Waves in Space and Time Frequency: Angular Frequency: Spatial Wavelength: T Seconds Hertz (cycles per second) Radians per second Meters Radians per meter Spatial Wavenumber: On a string the frequency of oscillation and the wavelength are connected through the speed of propagation of a bending wave. c

Sound waves Sound is a Longitudinal Wave: Disturbance varies along the direction of propagation. Transverse wave: (string) Disturbance varies in a direction perpendicular to the direction of propagation. pressure Density of air = 1. 21 kg/m 3 c = 343 m/sec f ≈ 20 Hz 20 k. Hz 5

Question What is the wavelength of a sound wave of frequency 20 Hz? c = 343 m/sec 17. 15 meters Wavelength = 1. 715 cm @ 20 k. Hz 20 Hz ≤ f ≤ 20 k. Hz 17 m ≤ ≤ 1. 7 cm c = 343 m/sec = 1125 ft/sec about 1 foot per millisecond Remember this! 6

Human ability to localize sound Distance between human ears is ≈ 22 – 24 cm f ≈ 1430 Hz 7

Sound localization f < 1500 Hz f > 1500 Hz Wavelength is larger than Wavelength is smaller than distance between ears Humans determine directionality of sound by two basic methods: Interaural Time Difference (ITD) f < 1500 Hz Interaural Intensity Difference (IID) f > 1500 Hz But there is some overlap of methods in the range 800: 1600 Hz 8

IID and ITD: f ≥ 1500 Hz IID: f ≤ 1500 Hz ITD time delay: 22 cm 650 µsec The shape of the outer ear (pinna) plays a significant role in 3 D audio; Head Related Transfer Function: HRTF Head shadows the sound at more distant ear. 9

Sound Pressure Level P is the measured pressure Pref = 20 µPa (micro-Pascals) 1 Pascal = 1 Newton/meter 2 Sound pressure of 20 µPa 0 d. B SPL Sound pressure of 20 Pa 120 d. B SPL 1 Atmosphere = 14. 7 lbs/in 2 = 1. 01 x 105 Pascals 1 Atmosphere = 194 d. B SPL 10

Range of Human Hearing HOG ≈ 143 d. B 11

Adaptation of Human Hearing At any given time we hear over a dynamic range of about 90 d. B. Our auditory system adapts our hearing sensitivity to the average SPL – much like our eye adjusts to different lighting conditions. 90 d. B SPL 140 d. B SPL 12

Sound Intensity and Power Sound Intensity: When a pressure wave propagates through air the air moves slightly. u Dimensional Analysis I=pxu p “Impedance” of air Density of air So … Speed of sound Small air moves a lot Large air moves little Sound Intensity ≈ Pressure 2 13

Inverse Square Law Power = I x 4πr 2 I = Power / 4πr 2 r pop Total Radiated Sound Power of Musical Instruments Entire Orchestra ≈ 75 Watts Trombone ≈ 6 watts Violin ≈ 0. 1 W 14