Physics 211 – lecture 28: Sound Waves

1. Sound Spectrum – three classes of sound waves infrasonic audible ultrasonic increasing f decreasing  20Hz 20kHz Physics 211 – lecture 28: Sound Waves Sound Waves -mechanical longitudinal waves ·Sound waves come from periodic pressure variations moving along in a substance. 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 Derivation in book Where

3. Sound Intensity Intensity = power (or energy transfer rate) divided by area Units: W/m2 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 EffectThe 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: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/imgsou/dopp2.gif

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 Doppler Effect Equations: Stationary observer Stationary source: + = getting closer - = moving away + = moving away - = getting closer vo = observer velocity fo = observed frequency vs = source velocity fs = source frequency v = speed of sound

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 ? GivenPathWantConversions/Equations Note: Storm is like observer moving toward storm. Then, it bounces back signals with same frequency it observed.

7. Example: Ch17 # 3Flowerpot 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 # 16Cu bar is at 99.5% of Y=13N/m^2. 500Hz sound wave is then transmitted. • Find displacement amplitude required to break bar • Find max speed of Cu atoms at breaking. • Find sound intensity in bar.

9. Example: Ch17 # 34Firework 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 # 38Fetus 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