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PHY 1812 Unit 1 review

Terms in this set (46)

What are two different meanings of the word "sound?"
a) an auditory sensation in the ear
b) the disturbance in a medium that can cause this sensation
What is the science of sound generally called?
acoustics
What is the difference between a longitudinal and a transverse wave? Give a example of each.
longitudinal waves flow back and forth in the direction they travel
transverse waves move perpendicular to the motion of travel
What are four different processes that can produce sound? Give an example of each.
vibrating bodies (a drum head)
changing airflow (speaking or singing)
time-dependent heat sources (an electrical spark)
supersonic flow (a plane breaking the sound barrier)
What is the difference between speed and velocity?
Speed is distance travelled over a period of time.
Velocity is the total displacement over a period of time in a direction
The slope of a graph of position versus time is equal to what quantity?
average velocity
What three quantities are related by Newton's second law of motion?
force, mass, and acceleration
Describe the motion of an object when no net force is applied:
The object has no motion, no change in velocity (it is not accelerating).
Arrange the following in order from largest to smallest: 0.004 m, 0.4 mm, 4 x 10^-5 km, 4 x 10^-5 μm.
4 x 10^-5 km; 0.004 m; 0.4 mm; 4 x 10^-5 μm
What unit is used to express energy? What unit is used to express work?
joules (J)
newton-meters
What is kinetic energy? What is potential energy?
kinetic energy is the energy of motion
potential energy is stored motion
Give the formula for the potential energy of a displaced guitar string and explain each symbol:
PE = (2T/L) * y^2, where T is the tension of the string, L is its length, and y is the displacement from the string's midpoint.
What is the difference between power and energy?
Power is the rate of transfer of energy.
When you pay your electric bill, are you paying for power used or for energy used?
Energy
What is meant by the period of a vibration? How is period related to frequency?
The period of a vibration is how long it takes to complete one oscillation
The frequency is the number of oscillations per second
In what units is K (the spring constant of a spring) expressed?
newtons per meter (N/m)
What is meant by simple harmonic motion?
motion with a restoring force proportional to the displacement
Doubling the distance a spring is stretched increases the restoring force by a factor of what?
2
Doubling the distance a spring is stretched increases its potential energy (not the restoring force) by what factor?
4
How does the frequency of a pendulum change when its mass is doubled?
no change
How does the frequency of a Helmholtz resonator change when its volume is doubled?—how about if the radius of its neck is doubled?
multiply by 1/√2
multiply by 2—twice as high
Describe the lowest mode of vibration in a vibrating string:
this is the fundamental frequency
What are nodes? Describe a node in a vibrating string vs. on a vibrating membrane.
Nodes are points at which minimal vibration occurs.
On a vibrating string, this is any "pivot point" where the string does not appear to be moving.
On a vibrating membrane, nodes may form nodal circles near the center or nodal lines across the diameter
In the lowest mode of a bar with two free ends, how many nodes are there?
two nodes
Describe the first two vibrational modes of a tuning fork.
The principal mode occurs at a much lower frequency than the clang mode
Each mode pivots on its own node (one node on each bar)
Additionally, the clang mode has a node near the stem
How many times faster do light waves travel compared to sound waves?
(3 * 10^6 m/s) / (346 m/s) = 880,000 times faster
Doubling the frequency of a sound waves multiplies the wavelength by what factor?
0.5; doubling f cuts λ in half
What must be true of the sizes and shapes of two pulses that meet on a rope in order for them to cancel each other out?
the sizes must be equal in magnitude
the shapes must be opposite in direction
Compare the speed of sound in air, water, and steel.
The speed of sound in steel is faster than water which is faster than air.
How does the speed of sound in air change with temperature?
as air temperature increases, the speed of sound increases
Does the speed of sound change with atmospheric pressure?
No; not significantly
Describe the sound of a car horn as the moving car passes an observer standing at roadside.
the horn sounds louder as it approaches and quieter as it moves away
Why are curved walls sometimes detrimental to concert hall acoustics?
curved walls focus and reflect sound waves back on the audience
Why is it sometimes difficult to be heard when you shout into a strong wind?
the wind causes upward refraction of the sound waves
Why is it possible to hear around a corner but not to see around a corner?
diffraction bends the sound waves around corners, whereas light waves propagate in a straight path
Give a definition of resonance as well as a couple of examples.
Resonance: When a vibrator is driven by a force that oscillates with a frequency at or near the natural frequency of the vibrator, and a relatively large amplitude results.
Examples: Child on a swing, standing waves in a pipe or string
Which amplitude curve would represent a higher Q: a sharper one, or a softer one? Why?
Sharper curves have higher Q values—in Fig. 4.4, the solid curve has a higher Q
Softer curves indicate damping, which makes for a lower Q; high Q means little damping, hence a sharp curve
What does n equal in Figure 4.5(d) (p. 63)? How many wavelengths equal L?
11 nodes
5 wavelengths
Distinguish between partials, harmonics, and overtones.
Partials, harmonics, and overtones all make up a sound.
A partial includes the fundamental plus the overtones
An overtone has a greater frequency than the fundamental
A harmonic is a mode of vibration whose frequency is a whole-number multiple of the fundamental frequency.
Suppose you blow over the ends of two pipes, one with the other end closed and one with it open. Which pipe should give the lower note? How much lower will it be?
The closed end pipe will be an octave lower because it vibrates at half the frequency of the open pipe.
Suppose a pipe with one open and one closed end has a fundamental of 200 Hz. What are the frequencies of its next two resonances? (Hint: odd-number multiples)
The next two resonances will be [3 x 200 Hz] = 600 Hz and [5 x 200 Hz] = 1000 Hz.
What is acoustic impedance?
the ratio of sound pressure to volume velocity
To lower the Helmholtz resonances of a guitar, should you make the sound hole smaller, or larger?
Smaller; a smaller sound hole corresponds to a lesser neck area and will thus yield a lower resonance
To excite a singing rod in its fundamental mode, where should you hold it? Where should you stroke it?
Hold the rod at the center where the node is
Stroke towards the ends
Can a singer break a wineglass by singing loudly?
perhaps
What is the main function of a piano soundboard?
produces sympathetic vibration; enhances the sound output from the piano strings
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