Ch 7 - Waves and Sounds

Question 1

What type of oscillations to transverse waves have?

Answer 1

oscillations of wave particles perpendicular to the direction of wave propagation

Question 2

What type of oscillations to longitudinal waves have?

Answer 2

oscillations of wave particles parallel to the direction of wave propagation (e.g. sound waves)

Question 3

What is displacement in a wave?

Answer 3

how far a point is from the equilibrium position, expressed as a vector quantity

Question 4

What is amplitude (A)?

Answer 4

the magnitude of a wave’s maximum displacement

Question 5

What is a crest?

Answer 5

the maximum point of a wave (point of most positive displacement)

Question 6

What is a trough?

Answer 6

the minimum point of a wave (point of most negative displacement)

Question 7

What is the wavelength?

Answer 7

the distance between 2 crests or 2 troughs

Question 8

What is the frequency?

Answer 8

the number of cycles a wave makes per second (Hz)

Question 9

What is angular frequency (w)?

Answer 9

another way of expressing frequency and is expressed in radians per second
w = 2 x pi x frequency = (2 x pi)/T

Question 10

What is a period?

Answer 10

the number of seconds it takes to complete a cycle (inverse of frequency) –> T = 1/f

Question 11

What is interference?

Answer 11

describes the way in which waves interact in space to form a resultant wave

Question 12

What is the difference between constructive, destructive, and partially constructive/destructive interference?

Answer 12

• constructive: occurs when waves are exactly in phase with each other (Aresultant = Ainterference1 + Ainterference2)
• destructive: occurs when waves are exactly out of phase with each other (Aresultant = Ainterference2 - Ainterference1) - exactly 180 degrees out of phase, Aresultant = 0
• partially: occurs when 2 waves are not quite perfectly in or out of phase with each other (Xresultant = Xinterference1 + Xinterference2)

Question 13

What are traveling waves?

Answer 13

have continuously shifting points of maximum and minimum displacement
- have nodes and antinodes that move with wave propogation

Question 14

How are standing waves produced?

Answer 14

by the constructive and destructive interference of 2 waves of the same frequency traveling in opposite directions in the same space
- have defined nodes and antinodes that do not move with wave propagation

Question 15

What is the difference between antinodes and nodes?

Answer 15

• anti: points of maximum collision

- node: points where there is no collision

Question 16

What is resonance?

Answer 16

the increase of amplitude that occurs when a periodic force is applied at the natural (resonant) frequency of an object

Question 17

What is damping?

Answer 17

a decrease in amplitude caused by an applied or nonconservative force

Question 18

How is sound produced?

Answer 18

by mechanical disturbance of a material that creates an oscillation of the molecules in the material
sound (v) = square root (bulk modulus (B) / density)

Question 19

Where does sound propagate and when does is propagate fastest?

Answer 19

• through all forms of matter (but not a vacuum)

- fastest through solids, followed by liquids, and slowes through gases

Question 20

How are density and speed of sound related within a medium?

Answer 20

as density increases, the speed of sound decreases

Question 21

What is pitch?

Answer 21

related to frequency of sound

Question 22

What is the Doppler effect?

Answer 22

a shift in the perceived frequency of a sound compared to the actual frequency of the emitted sound when the source of the sound and its detector are moving relative to one another

Question 23

When is the apparent frequency higher than the emitted frequency?

Answer 23

when the source and detector are moving toward each other

Question 24

When is the apparent frequency lower than the emitted frequency?

Answer 24

when the source and detector are moving away from each other

Question 25

What determines the relationship between the apparent and emitted frequency?

Answer 25

the apparent frequency can be higher, lower, or equal to the emitted frequency when the 2 objects are moving in the same direction, depending on the relative speeds

Question 26

When can shock waves be formed?

Answer 26

when the source is moving at or above the speed of sound

• shock waves are the buildup of wave fronts as the distance between those wave fronts decrease which occurs maximally when an object is travelling at exactly the same speed as the wave is travelling
• once an object moves faster than the speed of sound, some of the effects of the shock wave are mitigated because all of the wave fronts will trail behind the object, destructively interfering with them

Question 27

What is intensity and what is it related to?

Answer 27

• I = P/A
• loudness or volume of sound is related to its intensity
• intensity is related to wave’s amplitude

Question 28

When does intensity decrease?

Answer 28

over distance and some energy is lost to attenuation from (damping) from frictional forces

Question 29

How does an open pipe support standing waves?

Answer 29

the length of the string or pipe is equal to some multiple of half wavelengths

Question 30

How does a closed pipe support standing waves?

Answer 30

the length of the pipe is equal to some odd multiple of quarter wavelengths

Question 31

How is sound used medically?

Answer 31

in ultrasound machines for both imaging (diagnostic) and treatment (therapeutic) purposes

Question 32

What is wave speed (v)?

Answer 32

the rate at which a wave transmits the energy or matter it is carrying
- product of frequency and wavelength

Question 33

What is the equilibrium position?

Answer 33

the point with 0 displacement in an oscillating system

Question 34

How does applying a force at the natural frequency of a system change the system?

Answer 34

• the object will resonate because the force frequency equals the natural (resonant) frequency
• the amplitude of the oscillation will increase

Question 35

What is the equation that relates wavelength of a standing wave and the length of the string that supports it?

Answer 35

wavelength = 2L/n
n = positive integer called harmonic which corresponds to the number of half wavelengths supported by the string
- frequency = nv/2L

Question 36

How is harmonic determined when strings are attached to both ends of the pipe?

Answer 36

the number of antinodes present will tell you which harmonic it is

Question 37

How is harmonic determined for open pipes?

Answer 37

the number of nodes present will tell you which harmonic it is

Question 38

How is the harmonic determined for closed pipes?

Answer 38

• unlike strings and open pipes, one cannot simply count number of nodes or antinodes
• for closed pipes, have to actually count the number of quarter wavelengths contained in the pipe to determine harmonic
  wavelength = 4L/n

Question 39

How do ultrasound machines calculate distance?

Answer 39

based upon travel time of the reflected sound

Question 40

What equation relates sound level to intensity?

Answer 40

Bf = Bi + 10log (If/Ii)

Question 41

What does it mean for the difference between emitted and perceived frequencies to be 0?

Answer 41

implies that the source of the sound is not moving relative to the other object