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Chapter 15 Sound

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Chapter 15 Sound

Quiz 15

- Demonstrate knowledge of the nature of sound waves and the properties sound shares with other waves
- Solve problems relating the frequency, wavelength, and velocity of sound
- Relate the physical properties of sound waves to the way we perceive sound

- Define the Doppler shift and identify some of its applications
- Describe the origin of sound
- Demonstrate an understanding of resonance, especially as applied to air columns

- Explain why there is a variation among instruments and among voices using the terms timbre, resonance, fundamental, and harmonic
- Determine why beats occur

- Earthquakes
- Intensity of waves and energy transfer

- What is reflection, refraction, and diffraction?
- Why does light not diffract very well around corners?

- S- Waves are transverse waves and are not transmitted through fluids.
- P- Waves are longitudinal waves and can be transmitted through fluids
- S-Waves travel slower than P-Waves. Geologists are able to pinpoint the epicenter by recording when the waves arrive (P waves travel 4-8 km/s and S waves 2-5 km/s)

- The Richter Scale is logarithmic.
- An earthquake of 3.0 is 10x stronger (more powerful than a 2.0 earthquake and 100x a 1.0 earthquake.

- How much more energy (by ratio) is released by a 7.6 earthquake on the Richter scale than a 4.2?
- Calculator Instructions: Find the difference (7.6-4.2 = 3.4) and use (2nd Log or 10x of 3.4 to get your answer)
- How much more energy (by ratio) is released by a 8.0 earthquake compared to 4.0? Don’t use your calculator

- The danger of earthquakes is the transfer of energy. The ground particles of the earth rarely travel more than 1.5m themselves (same with thunder, the air particles travel less than a meter), but the transfer of energy still occurs.

- For a wave that travels in 3 dimensions (which is most waves), the intensity of the wave is a measure of the average power per area. The units are Watts/meters2.

- As you get farther away from the source of the wave, the intensity drops because of energy absorbed by the medium being traveled through and mostly because the wave spreads out.
- Air does not absorb much energy from sound waves so they can be heard from great distances.

- As the distance from the sound increases, the arc length of the angle increases and the power is spread out over a much larger area.

- Since the waves propagate in all dimensions, the propagate as a sphere.
- Intensity (J/s) = Power (of source)/radius squared (and constant)

- 10,000 J of energy are released from a point source and radiates out as a wave in 3 dimensions. What is the intensity of the wave at:
- a) 2.0 m
- b) 4.0 m
- c) 8.0 m

- Your ear has an area of 1 E-4m2. If sound at an intensity of 1 E-5W/m2 is reaching your ear, how much energy will your ear absorb in an hour?

- Shock waves
- Echolocation and other real life uses of waves

- Moving Source
- Moving Observer
- Both

- Fo = Observed frequency
- Fs = Frequency produced by noise maker
- Vo = Velocity of Observer
- V = Velocity of sound wave normally (331m/s at 0 C)
- Vs = Velocity of Source of sound

- If observer is moving towards the wave, then Vo is a negative speed
- If observer is moving away from the wave, then Vo is positive
- If source is moving towards observer, then Vs is positive
- If source is moving away from observer, then Vs is negative

- There is also a Doppler effect for light. Instead of a sound source, there is a light source. An increase in frequency is called a blue shift. The frequency is shifted towards the ultraviolet (higher energy, higher frequency).
- A decrease in frequency is called a red shift. The frequency is shifted towards the infra red (a lower frequency, lower energy).

- Far stars and galaxies, as we observe them, show these shifts as they move closer and farther from us. It is this shift (a measurement of it) that allows astronomers to calculate the speed at which they are moving relative to us. A star that spins fast enough has both a red and blue shift. The side turning away from us has a red shift and a blue shift on the side turning toward us. This allows us to calculate the speed of the spinning star.

- Police radar is able to tell our speed by doing calculations of frequency. The antenna of the police car sends out a signal (with a certain frequency). Then when the wave gets reflected back, it has a frequency shift based off of how fast the car was moving. That is how radar works.
- Illegal devices send out the same wave out of phase…

- A fire truck turns on its sirens and starts driving at 20 m/s. The siren produces a sound with a frequency of 100 Hz. What is the perceived frequency of an observer (standing still) when:
- a) The fire truck is approaching?
- b) The fire truck has moved past?
- Speed of Sound: 340 m/s

- If the source is standing still, and you travel away from the source at the speed of sound, do you ever hear the source?

- How much more energy is released by an earthquake of magnitude 8.7 on the Richter scale compared to an earthquake of magnitude 3.5 on the Richter scale?
- 20,000 J of energy are released from a point source and radiates out as a wave in 3 dimensions. What is the intensity of the wave at 5.0 m away from the source?

- Scenario 1: What happens if the observer moves away from the sound at a speed equal to the speed of the sound wave?
- Scenario 2: As the speed of the source of sound approaches the speed at the wave, what does the observed frequency approach?
- Scenario 3: What if the speed of the source is greater than the speed of the sound wave movement?

- Scenario 1: The sound wave never reaches the observer
- Scenario 2: The frequency approaches infinity
- Scenario 3: A sonic boom is produced as crests pile up on crests.

- Sonic Boom: Objects moving faster than the wave speed
- When an object (usually planes) flies faster than the speed of sound, the crests pile up and produce a cone shaped shock wave. There are two shock waves, one at the front of the object, one at the back of the object.

- It produces an “N” shaped wave, called this because of the N shaped graph of pressure. Initially, the pressure builds up at the front, at the nose of the plane, where all the molecules are crunched together. Then it decreases linearly across the plane, and finally jumps back up as the molecules fly back to fill the hole left by the plane.

- The pressure change is not very large at all, just 50 – 500 Pascals.
- The danger and damage is caused by how fast this occurs. For military planes, this change occurs in 0.1 s or less and for the space shuttle in about 0.5 s. Due to the small interval, only one sonic boom is heard from small military fighters, but two can be heard by the space shuttle. The pressure changes can even cause physical damage to buildings.

- Mach
- Rate of the speed of the plane to the speed of sound. A plane flying at Mach 3.3 is flying at a speed of 1,100 m/s compared to the speed of air (3.3 x 331 m/s)

- Echolocation
- Many animals rely on waves to detect their surroundings. Bats, dolphins, whales, and even some birds find their way around using waves. The emit their own wave and then listen for the echo.

- Bats and Dolphins when hunting also take advantage of the Doppler effect to determine the speed of the object. Some bats can detect frequency changes as small as 0.1 Hz.

- Prey fight back as well. Moths (as well as some other prey) can also detect these emitted waves. Upon hearing the waves, they drop to the ground and collapse their wings (to minimize their surface area). Also moths tend to be slightly furry, this helps absorb some of the energy of the wave and reflect the wave back at smaller intensities.

- The tiger moth sends out its own wave that mixes with the bats waves and makes the wave more incoherent to the bat.

- Sonar Used to detect how far away the ocean bottom is. Also for detecting enemy vessels and distance away.
- Question: If the speed of sound in the ocean is 1500 m/s and it takes 0.24 seconds for a sound wave emitted by the boat to return, how far away is the bottom?

- Cameras Auto focusing cameras send out a wave to see how far the object is away, and adjust appropriately (not all do this, but some do)
- Radar: Relies on reflected waves to see the location of storm and wind velocity. They use EM waves to detect this, but the idea is similar.
- Ultrasound: Waves are reflected back at different mediums, or when the type of tissue changes. This is how babies can be seen while they are developing.

- Sound intensity as perceived by humans. Is log 10 based again (2 Bels is 100 x more intense than 0 Bels)
- 0 dB = 1 E-12 W/m2
- Threshold of hearing

- 10 dB = 1E-11 W/m2

- Since Log scale, 70 dB + 70 dB does not equal 140 dB
- Instead it is around 73 dB
- Add intensity, not dB

- 7 Bels = 1E-5, so 2 x 1E-5 = 2E-5
- Log (I / 1E-12) = Bels

- Double distance = ¼ intensity
- Increase distance by a factor of 10 = drop in dB of 20 (2 Bels)

- Timbre
- Fork = 1 pitch
- Most = lots
- Overtones are multiples of lowest frequency