Reflection, transmission of sound The bigger change in acoustic impedance z of sound between the two materials, the more sound is reflected.

Why do we use ultrasound instead of normal sound for this?

Energy in sound Intensity and power

Energy in sound Intensity vs distance (spherical emission)

A spherical speaker puts out an intensity of 10 W/m2 at a distance of 1.5 meters. P1. The intensity at 3 meters away is _________W/m2 A: 1.0 B: 2.5 C: 3.3 D: 5.0 E: 10 The total power the speaker puts out is _______ W.

Decibel intensity scale We hear over a huge range of intensities So use logarithmic scale (powers of 10)

Gun blast (arm’s length) With pain 100 140 Damage W/m2 dB Jet engine at 1 ft Instant 106 180 Gun blast (arm’s length) With pain 100 140 Rock concert (best seats) If 10 min/day 1­ 120 Power mower at 3 ft. If all day 10-2 Vacuum cleaner, 3 ft. 10-4 80 Conversation, 3 ft.   10-6 60 Whisper 10-9 30 Softest sound heard 10-12

Intensity depends on distance from source!   OSHA regulations: No more than 90 dB averaged over 8 hour day.

Properties of logs “Log(x) is the power you raise 10 to, to get x.” or x= 10log(x) log(100) = log(106) = log(ab) = log(a) + log(b) If log(3) = 0.477, log(300) = log(3) + log(100)=

Properties of logs Compare two intensities: If you increase I by a factor of X, add ___________ dB If you increase I by a factor of 10, add ___________ dB If you increase I by a factor of 100, add ___________ dB If you increase I by a factor of 1000, add ___________ dB If you increase I by a factor of 2, add ___________ dB [log (2) = 0.301]

P2. If you increase intensity I by a factor of 8, add ___________ to the decibel level (Hint: do it with 2’s) A: 6 B: 8 C: 9 D: 12 E: 80

Doppler Shift—“Race Car Effect” Applications Doppler ultrasound: blood flow imaging in heart Radar and laser speed tests

Doppler effect Frequency observed _______________when the source and observer approach each other, and ______________when they go away from each other. stokes.byu.edu/teaching_resources/doppler_flash.html

Shock waves When the source moves at or above the speed of the waves (vsource > vwave­) Shock wave of a bullet in flight. http://stokes.byu.edu/teaching_resources/boom_flash.html

P3. 10-5 W/m2 has a dB level of _______ dB P3. 10-5 W/m2 has a dB level of _______ dB. A: 5 B: 50 C: 70 D: 80 E: 90 P4. A siren emitting at 200 Hz is on a car going very fast toward you at ½ the speed of sound (take the speed of sound to be 300 m/s here). You travel away from the car on your bike at 1/3 the speed of sound. The frequency you hear is_­­­_________ Hz A: 150 B: 200 C: 250 D: 267 E: 330

From two sounds with slightly different f’s. Beats From two sounds with slightly different f’s. Applications of beats: Tuning Measuring Doppler shifts in radar, ultrasound