Chapter 15

Question 1

What are aspects of sound

Answer 1

1: energy! (there are lots of other kinds of energy)
2: longitudinal waves! (not all longitudinal waves are sound waves)
3: travel by air (also travel in other things, but a medium is required!
4: waves that stimulate the eardrum! (whose eardrum? Humans, animals, deaf people?)

Question 2

World Book Dictionary gives these definitions for sound:

Answer 2

“that which is or can be heard” and “the sensation produced in the organs of hearing by stimulation of the auditory nerve”

Question 3

Dictionary.com for definition of sound:

Answer 3

“the sensation produced by stimulation of the organs of hearing by vibrations transmitted through the air or other medium” and “mechanical vibrations transmitted through an elastic medium, traveling in air at a speed of approximately 331 m per second at sea level”

Question 4

What sound waves consist of:

Answer 4

Compressions and rarefactions. One cycle is one compression and one rarefaction

Question 5

Sound physics definition

Answer 5

• Sound is a longitudinal wave, created by a vibrating object, that can only be transmitted in a medium. -

Question 6

Examples of sounds caused by vibrating objects:

Answer 6

e.g. - Vibrating strings on piano, guitar, violin - Vibrating reed on clarinet, saxophone - Vibrating column of air and vibrating lips on flute, trumpet - Vibrating skins on drums; vibrating metal on cymbals, triangles, etc. - Vibrating vocal cords on humans

Question 7

Frequency of sound

Answer 7

• Since all sound results from vibrations of a source, the frequency of the sound wave will be the same as that of the source.

Question 8

What are sound frequencies below 20Hz called? What are sound frequencies above 20,000 Hz called?

Answer 8

• Sound frequencies below 20 Hz are called infrasonic (“below sound”) and those above 20 000 Hz are called ultrasonic (“beyond sound”).

Question 9

What frequencies do elephants communicate with:

Answer 9

Apparently elephants communicate with frequencies much too low for humans to hear, and rhinoceroses call each other using infrasonic frequencies as low as 5 Hz.

Question 10

Difference between pitch and frequency:

Answer 10

• Another term related to frequency is “pitch”. Whereas “frequency” is a precise term that relates to a specific, measurable quantity, “pitch” is more of a subjective description and is used as a general description. (A flute produces higher-pitched sounds than a tuba does.)

Question 11

The range of sound frequencies that humans can hear:

Answer 11

Humans: 20-20,000
This is called audible sound.
- Think back to our description of what sound is. One of the problems with deciding if something should be called sound is whose ear should be able to hear it. The range of sound frequencies that humans can hear is very different from the range various animals can hear.

Question 12

What does supersonic mean?

Answer 12

“Supersonic” means “faster than sound”.

Question 13

What is the average male and female speaking sound frequency:

Answer 13

We’ll talk about the speed of sound a bit later. (The lowest note on the piano is 27.5 Hz, The “A” above middle C is 440 Hz, the average male-speaking voice is about 120 Hz, and the average female-speaking voice is about 250 Hz.) -

Question 14

What does the speed of a wave depend on?

Answer 14

Speed of sound - As we already mentioned, the speed of a wave depends on the properties of the medium. (See chart on page 405.)

Question 15

How does sound travel in a vacuum, in liquids, and in different temperatures?

Answer 15

Sound does not travel at all in a vacuum, it travels faster in liquids than in gasses, and it travels the fastest in solids. → in air at room temperature, sound travels at about 343 m/s. In water, the speed is about 1500 m/s and in iron it is over 5000 m/s.

Question 16

Are substances in which sounds travel faster tend to be better conductors of sound or worse? How is it changed underwater?

Answer 16

Substances in which sound travels faster also tend to be better conductors of sound. → Although the sound may be muffled, sounds tend to be louder underwater than when they travel through the air.

Question 17

How does knowing the approximate speed of sound in the air allow us to determine how far away lightning is?

Answer 17

Knowing the approximate speed of sound in the air allows us to determine how far away a lightning strike is. Since the light reaches us virtually instantly, the number of seconds between the lightning and the thunder, divided by three, tells us how many kilometers away the lightning struck.

Question 18

How did old Westerns check if a train was coming?

Answer 18

→ Old Westerns: Put the ear to the railway track to hear if a train is coming.

Question 19

What does the speed of sound depend on in gas?

Answer 19

In a gas, the speed of sound depends on the temperature and the molecular mass of the gas, but not the pressure.

Question 20

What happens to speed of sound in higher temperatures?

Answer 20

Speed increases with higher temperature. (e.g., For dry air, the speed of sound is given by v = (331+0.6T) m/s, where T is the temperature in Celsius.) - You don’t have to know this.

Question 21

What determines the speed of sound in liquids and solids?

Answer 21

• In liquids and solids, there are quite a few factors that determine the speed, density being only one of them.

Question 22

What happens to the speed of sound with greater molecular mass?

Answer 22

Speed decreases with greater molecular mass. (e.g., Sound travels more than 3 times as fast in helium as it does in oxygen.)

Question 23

What do waves have to do with the transfer of energy?

Answer 23

• Waves are about energy transfer, and we’ve talked about power as being the rate at which energy is converted or transferred. → A person shouting as loud as possible is transferring energy at a rate of about 1 watt.

Question 24

Is the measurement of frequency subjective?

Answer 24

Just as “pitch” is a subjective term related to the measurable quantity “frequency” ‘, “loudness’ ‘ is a subjective term related to the measurable quantity “sound intensity level’ ‘. (Intensity is power/area)

Question 25

Why is describing the intensity of the amount of energy involved in sound impractical?

Answer 25

The actual amount of energy involved in sound is so little that describing the intensity in W/m’ is impractical. (e.g. A crowd of 50 000 fans at a football game cheering for 1½ hours produces about enough sound energy to heat one cup of coffee.)

Question 25

Do we notice the large difference in sound intensities that are measured in watts/meter^2 ?

Answer 25

• Our ears and brain have an incredible ability to adjust to different sound intensities, and so we do not notice the large difference in sound intensities that are measured in watts/meter^2 would give us.
  → For example, the loudest sounds that our ears can withstand are 10^13 times the intensity of the quietest ones that we can hear.

Question 26

Decibal system:

Answer 26

(“decibel” = 1/10 bel, named after Alexander Graham Bell)

→ O dB is the intensity level of the threshold of hearing, the quietest sounds we can hear. It corresponds to 10^-12 W/m^2.

Question 27

Decibel being a logarithmic system:

Answer 27

→ The dB system is a logarithmic system, so each increase of 10 dB is really 10-times the intensity. * 10 dB is 10 times the intensity of 0 dB, 20 dB is 10 times the intensity of 10 dB and 100 times the intensity of 0 dB, etc. l → There is a complicated logarithmic formula that calculates decibel levels, but you don’t have to worry about it. Just be able to do simple questions based on the above statement.

Question 28

What does an increase of 10 decibels do?

Answer 28

→ Each increase of 10 decibels (i.e., 10 times as much intensity) seems to us to be about twice as loud.

Question 29

Doppler effect:

Answer 29

→ As a train goes by you, its whistle seems to change frequency.
→ As an ambulance, fire truck, or police car goes by, the frequency of its siren seems to drop in pitch.
→ As a car speeds by, the noise it makes seems to change frequency.

Question 30

Why doppler effect happens

Answer 30

As the source moves, it “catches up” to the wavefront it just produced which is now spreading all around it. So the new wavefront the source produces is closer to the previous one in front of the source, and further from the previous one behind the source than it would normally be. Thus although the actual frequency emitted is constant, the wavelength is reduced in front of the moving source and extended behind the moving source. Since the speed at which the sound waves move is the same in both directions, an observer will hear a higher frequency in front of the source and a lower frequency behind the source.

Question 30

• Since the frequency of a wave depends on the source, how could it change?

Answer 30

If the source is moving, the wavelength changes, causing a change in the frequency observed.

Question 31

Defn: - The Doppler effect

Answer 31

is the phenomenon that the frequency of a sound perceived by the observer is higher than the emitted frequency if the source is approaching the observer, and the perceived sound is lower than the emitted frequency if the source is leaving the observer.

Question 32

What is the difference between the wavelength between a stationary and moving source with the Doppler effect?

Answer 32

A Doppler shift also happens if the source is stationary but the observer is the one that moves toward or away from the source. In this case, the wavelengths don’t change in the air, but the wavelengths do change relative to the observer’s ears since the observer is always catching up to the next wavefront if he is moving toward the source, and if he is moving away from the source, each wavefront has to go further to reach his ears.

Question 33

How is a shock wave produced?

Answer 33

if the source moves faster than the speed of the wave, a shock wave is produced.

Question 34

What is breaking the sound barrier?

Answer 34

If the source moves at the speed of sound (i.e., “Mach 1”), then the sound compressions pile up right in front of the object. This super-compressed air is effectively a wall that it is difficult for a jet to break through. In fact, when a jet or other object does break through this “wall” by going faster than the speed of sound (i.e., traveling at a supersonic speed), it is referred to as “breaking the sound barrier”. The result is a cone of highly compressed air trailing behind the jet, as illustrated on page 491, and when this cone passes a person’s ears, the person hears a “sonic boom”.

Question 35

What is a sonic boom?

Answer 35

• The crack of a whip is actually a sonic boom, resulting from the tip traveling faster than the speed of sound.

Question 36

• Other applications of the Doppler shift (other than sound):

Answer 36

→ radar to check the speed of a car or a baseball
- the frequencies of the emitted and reflected waves are compared and the speed is computed.

→ bats use it to navigate and catch insects
- similar to above, the bat’s brain compares the frequency of the reflected signal to the ultrasonic signal it emits

Question 37

How can scientists calculate how fast the stars are moving from redshift?

Answer 37

→ You probably know that scientists believe that the universe is expanding. Light from distant stars does not reach us as pure white light, but tends to be shifted toward the red end of the spectrum (longer wavelength than blue light). This “red-shift” indicates that the sources of the light (i.e., the stars) are how fast the stars are moving. moving away from us. Based on the amount of shift, scientists can calculate how fast the stars are moving.

Question 38

Stringed instruments (how they work)

Answer 38

A Standing Wave is formed when 2 waves of the same amplitude and wavelength, traveling in opposite directions, interfere. Often it is the result of a source wave meeting its reflection. For example, when a string is attached to a fixed end and the other end is shaken, the reflected wave will travel back in the opposite direction to the original wave and interfere with it. If the timing is right, a standing wave is formed, which is a wave that does not appear to travel along the string at all. Rather, the initial and reflected waves alternately result in constructive and destructive interference so that a vibration is formed which has an amplitude twice that of the original waves.

Question 39

What can maintain a standing wave pattern?

Answer 39

→ In a given medium, only certain wavelengths can maintain a standing wave pattern.

A standing wave can be formed on a string for any frequency such that the length of the string is a multiple of half the wavelength.

Question 40

Antinodes

Answer 40

→ Points that experience maximum displacement are called antinodes. These occur where double crests or double troughs occur.

Question 41

Nodes

Answer 41

→ Points that remain at rest at all times are called nodes

Question 42

Loop

Answer 42

→ The whole section between two adjacent nodes is called a loop.

Question 43

What is the wavelength of a standing wave in comparison to the distance between adjacent nodes?

Answer 43

→ The wavelength of the standing wave is twice the distance between adjacent nodes.

Question 44

What is the amplitude of standing waves?

Answer 44

→ As already mentioned, the amplitude of the standing wave is twice that of the 2 interfering waves.

Question 45

Fundamental frequency

Answer 45

→ The lowest frequency/longest wavelength is the one in which half a wave fits on the string. This is called the fundamental frequency, or Is harmonic (since it has nodes at both ends and 1 loop). * This is the natural frequency for the string, and is the one that will sound when a string of a stringed instrument is played normally.

Question 46

Resonance

Answer 46

• In fact, every object has a natural frequency at which it will vibrate if stimulated. When the object vibrates at its natural frequency, it is said to be in resonance, and is vibrating at its resonant frequency. (That is why a tuning fork produces the note that it does.)

Question 47

2nd harmonic

Answer 47

→ The next frequency, or second longest wavelength, is the one in which one whole wave fits on the string. This is called the first overtone, or 2nd harmonic (since it has 2 loops).

Question 48

3rd Harmonic

Answer 48

→ The next frequency is the one in which 1½ wavelengths fit on the string. This is the second overtone, or 3rd harmonic (3 loops), → This does not double the frequency, so it is a different note of the scale.

Question 49

b) Wind instruments

Answer 49

Just as transverse standing waves can be produced in a string or a spring, longitudinal waves can also form standing wave patterns. - The explanation in the text shows why there must be a pressure node at the open end of a tube. However, for our understanding of longitudinal standing waves, it is easier to focus on the location of displacement nodes and antinodes, and that is what “node” and “antinode” will refer to in the rest of our studies.

Question 50

How is frequency different in the first harmonic and fundamental?

Answer 50

• Note: Since the wavelength is half that of the lst harmonic, it is twice the frequency, which means it is one octave higher than the fundamental frequency.

Question 51

What does a metal rod produce?

Answer 51

• Both transverse and longitudinal waves can be produced in an object such as a metal rod.

Question 52

How do wind instruments produce their sounds?

Answer 52

• All wind instruments produce their sounds by means of longitudinal standing sound waves in tubes or air columns.

Question 53

What happens at the resonant frequency of an air column

Answer 53

• Every air column has a natural resonant frequency at which a standing wave will be produced

Question 54

There are three types of tubes: 2 nodes

Answer 54

That means that the lowest frequency/longest wavelength that will produce a standing wave is one in which half a wavelength fits in the tube. The next frequency is the one in which a whole wavelength fits in the tube. Next, 1½ wavelengths, and so on.

→ In this way, it is just like waves on a string

Question 55

There are three types of tubes: Both open-ends

Answer 55

This is the type that many musical wind instruments are. Since air is free to move at both ends, an antinode will be formed at both ends. As with the tube closed at both ends, this means half a wavelength fits in the tube at the lowest frequency that can be produced. The next frequency is the one in which a whole wavelength fits, then 1½ wavelengths, etc.

→ Although the relationship between the column length and the wavelengths/frequencies produced is the same as for tubes closed at both ends, remember that it is an antinode at each end rather than a node.

Question 56

There are three types of tubes: A tube with one end open and one end closed (like, an antinode blowing over a pop bottle)

Answer 56

In this case there will be a node at the closed end, since the air is not free to move there, and there will be an antinode at the open end. That means the longest wavelength that will form a standing wave is one in which ¼ wavelength fits in the tube (i.e., the wavelength is twice as long as that formed node in a tube open at both ends → compare the difference between open and closed boomwhackers).
The next frequency is the one in which ¾ wavelength fits, then 1¼ wavelengths, etc.

Question 57

Do sound waves come from tubes with closed ends?

Answer 57

** is rare for sound waves to be produced in columns that are closed on both ends, except in the videos we saw. (Not much sound would get out, so what’s the point?) Most musical instruments have either both ends of the air column open,s or else one open end and one closed end.

Question 58

How do wind instruments play notes?

Answer 58

• wind instruments play notes by directly producing standing longitudinal waves in some sort of pipe or tube. In some cases, one end is open and the other is effectively closed by the mouth of the person playing the instrument. (clarinets, trumpets, etc.). In other cases, such as a flute, both ends are open.

Question 59

Beats interference

Answer 59

Just like other waves, two sound waves can interfere with each other, producing constructive interference at some points and destructive interference at other points. This is not normally noticeable unless the sound waves coming from two sources are identical. - However, the interference that occurs is quite different when two slightly different frequencies are sounded together.

Question 60

Beats

Answer 60

Beats are periodic variations in the loudness of sound caused by the interference of two slightly different frequencies

Question 61

Beats Frequency

Answer 61

• The beat frequency is simply the difference between the two frequencies.
• Musicians can make use of beats to tune their instruments very accurately.