Waves and Sound
wavelength
the distance between successive identical parts of the wave
amplitude
the distance from the midpoint to the crest of the wave
A nurse counts 76 hearbeats in one minute. What are the period and frequency of the heart's oscillations?
76 heartbeats/60 seconds = 1 heartbeat/ 0.79 seconds frequency = 1/period = 1/0.79 s = 1.27 The period is 0.79 seconds and the frequency is 1.27 Hz.
How much more intense is (a) a close whisper than the threshold of hearing? (b) a close whisper than normal breathing?
A close whisper is 100 times more intense than the threshold of breathing. A close whisper is 10 times more intense that normal breathing.
How does pitch relate to frequency?
A high-pitched sound has a high vibration frequency, while a low-pitched sound has a low vibration frequency.
As the frequency of sound is increased, does the wavelength increase or decrease? Give an example.
As the frequency of a sound is increased, the wavelength decreases. For example, during a concert, you do not hear the high notes in a chord before you hear the low ones; the sounds of all instruments reach you at the same time. Wavelength and frequency vary inversely to produce the same wave speed for all sounds.
What two physics mistakes occur in a science fiction movie when you see and hear at the same time a distant explosion in outer space?
First of all, sound cannot travel through space, and second, the sound made by a distant explosion would take longer to reach an observer than the light of the explosion, so the sound would not occur at the same time that you see the explosion.
If a wave vibrates up and down twice each second and travels a distance of 20 m each second, what is its frequency? Its wave speed?
If a wave vibrates up and down twice each second and travels a distance of 20 m each second, its frequency is 2 Hz, and its wave speed is 20 m/s. This question is better answered by carefully reading the question rather than searching for a formula because all of the information needed is given in the question.
If you triple the frequency of a vibrating object, what will happen to its period?
If you triple the frequency of a vibrating object, its period will decrease by a factor of 3.
Why can a tuning fork or bell be set into resonance, while a tissue paper cannot?
In order for something to resonate, it needs a force to pull it back to its starting position and enough energy to keep it vibrating. Tissue paper is too limp to resonate.
Suppose that a pendulum has a period of 1.5 seconds. How long does it take to make a complete back-and-forth vibration? Is this 1.5 second period pendulum longer or shorter in length than a 1-second period pendulum?
It takes 1.5 seconds to make a complete back-and-forth vibration. This 1.5-second pendulum is longer in length than a 1-second pendulum.
If you encounter a sonic boom, is that evidence that an aircraft of some sort exceeded the speed of sound moments ago to become supersonic? Defend your answer.
No, a shock wave and its resulting sonic boom are swept continuously behind an aircraft traveling faster than sound. The aircraft that has generated this shockwave may have broken through the sound barrier hours ago.
Would it be correct to say that the Doppler effect is the apparent change in the speed of a wave due to motion of the source?
No, the Doppler effect is the apparent change in frequency and pitch of a wave due to motion of the source. This question is a test of reading comprehension as well as a test of physics knowledge because it requires you to know the difference between frequency and speed and the definition of the Doppler effect.
What is the relationship between forced vibration and resonance?
Resonance occurs when the frequency of a forced vibration on an object matches the object's natural frequency and an increase in amplitude occurs.
Suppose a piano tuner hears 2 beats per second when listening to the combined sound from her tuning fork and the piano note being tuned. After slightly tightening the string, she hears 1 beat per second. Should she loosen or should she further tighten the string?
She should further tighten the string.
Light can travel through a vacuum, as is evidenced when you see the sun or the moon. Can sound travel through a vacuum also? Explain why or why not.
Sound cannot travel in a vacuum. The transmission of sound requires a medium. If there is nothing to compress and expand, there can be no sound.
Why is a sound louder when a vibrating source is held to a sounding board?
Sound is louder when a vibrating source is held to a sounding board because the sounding board is forced to vibrate and its larger surface area sets more air in motion.
Why does sound travel faster in solids and liquids than in gases?
Sound travels faster in solids than in liquids and gases because solids have higher elasticity than liquids and gases.
What causes a standing wave?
Standing waves are the result of interference. When two waves of equal amplitudes pass each other in opposite directions, the waves are always out of phase at the nodes. The nodes are stable regions of destructive interference.
How does interference of sound relate to beats?
The interference of two sound sources of slightly different frequencies produces beats.
Distinguish between the period and the frequency of a vibration or a wave. How do they relate to one another?
The period of a wave or a vibration is the time it takes for one complete back and forth vibration. The frequency of a wave or a vibration specifies the number of back-and-forth vibrations it makes in a given time. If the frequency of a vibrating object is known, its period can be calculates, and vice versa. As you can see below, frequency and period are inverses of each other: Frequency = 1/period Period = 1/ Frequency
How does the speed of a wave relate to its wavelength and frequency?
The speed of a wave is equal to its wavelength multiplied by its frequency.
How fast must a bug swim to keep up with the waves it is producing? How fast must a boat move to produce a bow wave?
To keep up with the waves it is producing, the bug must swim as fast as the waves it is producing. To produce a bow wave, the boat would need to be moving faster than the waves that it is producing.
Suppose three tuning forks of frequencies 260 Hz, 262 Hz, and 266 Hz are available. What beat frequencies are possible for pairs of these forks sounded together?
Two, four, and six Hz are the possible beat frequencies for pairs of these forks sounded together.
An oceanic depth-sounding vessel surveys the ocean bottom with ultrasonic sound that travels 1530 m/s in seawater. Find the depth of the water if the time delay of the echo to the ocean floor and back is 8 seconds.
V = d/t 1530 m/s = d/4 6120 m = d
Sound waves travel at approximately 340 m/s. What is the wavelength of a sound with a frequency of 20 Hz? What is the the wavelength of a sound with a frequency of 20 kHz?
V = λƒ 340 m/s = (20Hz)λ λ = 17 m 340 m/s = (20000Hz)λ λ = 0.017 m
Red light has a longer wavelength than violet light. Which has the greater frequency?
Violet light has the greater frequency.
When watching a baseball game, we often hear the bat hitting the ball after we actually see the hit. Why?
We hear the bat hitting the ball before after we actually see the hit because light travels faster than sound.
When a sound wave propagates past a point in the air, what are the changes that occur in the pressure of air at this point?
When a sound wave propagates past a point in the air, the air pressure of this point decreases.
When a wave source moves toward a receiver, does the receiver encounter an increase in wave frequency, wave speed, or both?
When a wave source moves toward a receiver, the receiver encounters an increase in wave frequency, but not in wave speed.
rarefaction
a disturbance in air in which pressure is lowered; opposite of compression
natural frequency
a frequency at which an elastic object, once energized, will vibrate
resonance
a phenomenon that occurs when the frequency of forced vibrations on an object matches the object's natural frequency, and a dramatic increase in amplitude results
compression
a pulse of compressed air; opposite of rarefaction
beats
a throbbing variation in the loudness of sound caused by interference when two tones of slightly different frequencies are sounded together
wave
a wiggle in space
vibration
a wiggle in time
a. How fast does sound travel in dry air at room temperature? b. How does air temperature affect the speed of sound?
a. In dry air at room temperature, sound travels at about 340 m/s. b. For each degree increase in air temperature above 0 degrees C, the speed of sound in air decreases by 60 m/s.
destructive interference
crest overlaps a trough
constructive interference
crest overlaps crest or a trough overlaps a trough
While watching ocean waves at the dock of the bay, Otis notices that 10 waves pass beneath him in 30 seconds. He also notices that the crests of successive waves exactly coincide with the posts that are 5 meters apart. What are the period, frequency, wavelength, and speed of the ocean waves?
period= s/wave = 30/10 = 3s frequency = wave/s = 10/30 = 0.3 Hz wavelength = 5m wave speed = wavelength x frequency = 5m x 0.3s = 1.5 m/s
ultrasonic
term applied to sound frequencies above 20,000 Hz, the normal upper limit of human hearing
infrasonic
term applied to sound pitch too low to be heard by the human ear, that is, below 20 Hz
pitch
term that refers to how high or low sound frequencies appear to be
transverse wave
the medium moves at right angles to the direction in which the wave travels ex. electromagnetic waves, such as light and radio waves
longitudinal wave
the medium moves back and forth parallel to the direction in which the wave travels ex. sound waves
forced vibration
the vibration of an object that is made to vibrate by another vibrating object that is nearby
period
time it takes for one complete back and forth vibration
Calculate the speed of waves in a puddle that are 0.15 m apart and made by tapping the water surface twice each second.
wave speed = wavelength x frequency = 0.15 m x 2 Hz = 0.3 ms/s
