1 Physics 211 – lecture 28: Sound Waves Sound Waves - mechanical longitudinal waves  Sound waves come from periodic pressure variations moving along in a substance. increasing f decreasing infrasonic audibleultrasonic 20Hz 20kHz Sound Spectrum – three classes of sound waves Sound Speed  Sound speed IN AIR at room temperature (20  C) is : _____________  Sound speed equation (IN AIR only): Note – speed  as density  and speed  as elasticity (stiffness) 

2 Wave Equation for Sound Recall For transverse, we now have longitudinal Max longitudinal displacement Or in terms of pressure Where Derivation in book

3 Sound Intensity Intensity = power (or energy transfer rate) divided by area Units: W /m 2 Inverse Square Law: Decibels = measure intensity relative to the minimum intensity we can hear. The decibel is a __________ scale. Our hearing works on this scale. 10 dB increase  increase by factor of 10 in intensity 20 dB increase  increase by factor of 100 in intensity 30 dB increase  increase by factor of 1000 in intensity and so on… Decibel Equation:

4 Doppler Effect Doppler Effect The Doppler effect describes a change in frequency (pitch) of sound waves due to a moving source or moving observer. Example: train approaches with high pitched whistle, passes by, and pitch decreases. Source moves: toward observer ________away from observer________ Observer moves: toward source _______away from source _________ Source:

5 Doppler Effect in Light Red Shift - light from objects receding (moving away) from us is shifted to the red side of the spectrum Blue Shift - light from objects approaching (moving toward) us is shifted to the blue side of the spectrum v o = observer velocityf o = observed frequency v s = source velocityf s = source frequency v = speed of sound + = moving away - = getting closer + = getting closer - = moving away Doppler Effect Equations: Stationary observerStationary source:

6 Example (Doppler Effect): A storm is formulating with winds of up to 150km/hr. A Doppler radar device is monitoring the storm by sending out a 35MHz signal? What frequency will bounce back to the station if the storm winds are A) approaching? B) receding ? Given Path Want Conversions/Equations Note: Storm is like observer moving toward storm. Then, it bounces back signals with same frequency it observed.

7 Example: Ch17 # 3 Flowerpot 20m up falls towards 1.75m tall person. Find max time can wait before shouting from top if person below needs 0.3s to move.

8 Example: Ch17 # 16 Cu bar is at 99.5% of Y=13N/m^2. 500Hz sound wave is then transmitted. a)Find displacement amplitude required to break bar b)Find max speed of Cu atoms at breaking. c)Find sound intensity in bar.

9 Example: Ch17 # 34 Firework explodes 100m up. Observer directly under explosion hears average intensity of 0.07W/m^2 for 0.2s. a) Find total sound energy of explosion b) Find decibels measured by observer

10 Example: Ch17 # 38 Fetus ventricular wall moves in simple harmonic motion with amplitude 1.8mm at 115 beats per minute. Detector on mother procudes sound at 2x10^6Hz which travels through tissue at 1.5km/s. Find a) Max linear speed of heart wall b) Max frequency arriving at wall of heart c) Max frequency of reflected sound detected