Physics 1251 The Science and Technology of Musical Sound Unit 1 Session 3 Sound Waves Unit 1 Session 3 Sound Waves

Physics 1251 Unit 1 Session 3 Sound Waves Foolscap Quiz: If a person can only distinguish an echo if it occurs more than sec after the initial sound, then how far from a wall must he be to hear a distinct echo of his clapping hands? Round trip time = 0.1 sec. One way = (0.1)/2 sec= 0.05 sec d = v‧ t = (343 m/s)‧ (0.05 sec) = 17.1 m ~ 57 feet

Physics 1251 Unit 1 Session 3 Sound Waves 1′ Lecture: Sound is a pressure/displacement wave that propagates in air with a velocity that increases by 0.6 m/s for every degree C above room temperature.Sound is a pressure/displacement wave that propagates in air with a velocity that increases by 0.6 m/s for every degree C above room temperature. v = 343 m/s (T-20 C)v = 343 m/s (T-20 C) Waves are characterized by frequency f, period P, wavelength λ, velocity v.Waves are characterized by frequency f, period P, wavelength λ, velocity v. f = 1/ Pf = 1/ P f ⋅ λ = vf ⋅ λ = v

Physics 1251 Unit 1 Session 3 Sound Waves 80/20 Pressure = Force per unit Area p = F /A

Physics 1251 Unit 1 Session 3 Sound Waves 80/20 The unit of pressure in SI units is the Pascal [Pa] = 1 Newton/ m 2 1 Newton force 1 square meter area 1 Pascal pressure:

Physics 1251 Unit 1 Session 3 Sound Waves 80/20 Standard atmosphere = 1.0 atm =101 kPa =101 kPa =14.7 psi Tire pressure A few 100 kPa

Physics 1251 Unit 1 Session 3 Sound Waves Which applies the greatest pressure on their foot?

Physics 1251 Unit 1 Session 3 Sound Waves Force = 200 lb = 891 N Force = 4,000 lb = 17,800 N Force = 100 lb = 445 N Foot Area = 36.2 in 2 = m 2 Foot Area = 43.5 in 2 = m 2 Foot Area = 12.7 in 2 =.0082 m 2 p = F/A =200/(2 x 36.2) = 2.8 psi p = 891/(2 x 0.023) = 19 kPa p = F/A =4000/(4 x 43.5) = 23. psi p = 17800/(4 x 0.028) = 159 kPa p = F/A =100/(2 x 12.7) = 3.9 psi p = 445/(2 x ) = 27 kPa

Physics 1251 Unit 1 Session 3 Sound Waves Force = 100 lb = 445 N Heel Area = 0.16 in 2 =.0001 m 2 p = F/A =100/(2 x 0.16) = 313. psi p = 445/(2 x ) = kPa p = 19 kPa p = 159 kPa p = 27 kPa

Physics 1251 Unit 1 Session 3 Sound Waves The Mysterious Swami Seat

Physics 1251 Unit 1 Session 3 Sound Waves 80/20 Pressure = Force per unit Area p = F /A Same Force Different Area ∴Different Pressure

Physics 1251 Unit 1 Session 3 Sound Waves 80/20 Sound is a pressure wave.

Physics 1251 Unit 1 Session 3 Sound Waves Talking About of Waves

Physics 1251 Unit 1 Session 3 Sound Waves 80/20 Frequency f is the number of cycles or pulses per unit time. [ f ] = Hertz = Hz = 1/sec

Physics 1251 Unit 1 Session 3 Sound Waves 80/20 Period is the time it takes to make one cycle, oscillation or pulse. [P] = second 80/20 P = 1/ f

Physics 1251 Unit 1 Session 3 Sound Waves 80/20 Wavelength λ is the distance from one point on a wave to a corresponding point (point of the same phase) on the next cycle. [ λ ]= m λ λ λ

Physics 1251 Unit 1 Session 3 Sound Waves ⃒⃖ λ ⃒⃗ Wavelength ⃗ ⃖ ⃖ ⃗ V ⃗ ⃗ ⃗

Physics 1251 Unit 1 Session 3 Sound Waves Chain of Inference: 1. The distance a wave moves in one cycle = λ 2. Time for one cycle = period P 3. v = distance moved / time to move 4. Therefore, v = λ / P = λ ⋅ f

Physics 1251Unit 1 Session 3 Sound Waves 80/20 Fact: v = λ ⋅ f Always!

Physics 1251Unit 1 Session 3 Sound Waves 80/20 Fact: Sound travels faster in warm air than in cold.

Physics 1251 Unit 1 Session 3 Sound Waves Most accurate relationship of sound velocity on temperature is v = 343 √{(T +293)/293} m/s

Physics 1251 Unit 1 Session 3 Sound Waves 80/20 Most convenient approximation of the relationship of sound velocity and temperature v = (T - 20 C) m/s What is the speed of sound on a hot day when it is 40 C? v = (40-20) = (20) = = 355 m/s v = (40-20) = (20) = = 355 m/s

Physics 1251 Unit 1 Session 3 Sound Waves

© Pat Brady and United Media

Physics 1251 Unit 1 Session 3 Sound Waves © Pat Brady and United Media

Physics 1251 Unit 1 Session 3 Sound Waves Summary: p = F / A p = F / A Standard atmospheric pressure = 101 kPa Standard atmospheric pressure = 101 kPa Frequency = number of cycles per second Frequency = number of cycles per second Hz = Hertz = cycle/second Hz = Hertz = cycle/second Period P = 1/ f Period P = 1/ f v = λ ⋅ f v = λ ⋅ f v = (T – 20 C) m/s v = (T – 20 C) m/s

Physics 1251 Unit 1 Session 3 Sound Waves Credits: Photographs & Graphics Photographs & Graphics Ross Clarke-Jones Ross Clarke-Joneswww. “Rose is Rose” by Pat Brady “Rose is Rose” by Pat Brady © United Media