### Concept Items

### 14.1 Speed of Sound, Frequency, and Wavelength

What is the amplitude of a sound wave perceived by the human ear?

- loudness
- pitch
- intensity
- timbre

The compressibility of air and hydrogen is almost the same. Which factor is the reason that sound travels faster in hydrogen than in air?

- Hydrogen is more dense than air.
- Hydrogen is less dense than air.
- Hydrogen atoms are heavier than air molecules.
- Hydrogen atoms are lighter than air molecules.

### 14.2 Sound Intensity and Sound Level

What is the mathematical relationship between intensity, power, and area?

- $I=\frac{P}{{A}^{2}}$
- $I=PA$
- $I=\frac{A}{P}$
- $I=\frac{P}{A}$

How does the "decibel" get its name?

- The meaning of deci is “hundred” and the number of decibels is one-hundredth of the logarithm to base 10 of the ratio of two sound intensities.
- The meaning of deci is "ten" and the number of decibels is one-tenth of the logarithm to base 10 of the ratio of two sound intensities.
- The meaning of deci is “one-hundredth” and the number of decibels is hundred times the logarithm to base 10 of the ratio of two sound intensities.
- The meaning of deci is “one-tenth” and the number of decibels is ten times the logarithm to base 10 of the ratio of two sound intensities.

- Timbre is the quality of the sound that distinguishes it from other sound
- Timbre is the loudness of the sound that distinguishes it from other sound.
- Timbre is the pitch of the sound that distinguishes it from other sound.
- Timbre is the wavelength of the sound that distinguishes it from other sound.

### 14.3 Doppler Effect and Sonic Booms

Two sources of sound producing the same frequency are moving towards you at different speeds. Which one would sound more high-pitched?

- the one moving slower
- the one moving faster

- It approaches zero. This is because all wave crests are superimposed on one another through constructive interference.
- It approaches infinity. This is because all wave crests are superimposed on one another through constructive interference.
- It approaches zero, because all wave crests are superimposed on one another through destructive interference.
- It approaches infinity, because all wave crests are superimposed on one another through destructive interference.

- ${f}_{obs}={f}_{s}\left(\frac{{v}_{w}}{{v}_{s}\phantom{\rule{thinmathspace}{0ex}}\pm \phantom{\rule{thinmathspace}{0ex}}{v}_{w}}\right)$
- ${f}_{obs}={f}_{s}\left(\frac{{v}_{w}\phantom{\rule{thinmathspace}{0ex}}\pm \phantom{\rule{thinmathspace}{0ex}}{v}_{s}}{{v}_{w}}\right)$
- ${f}_{obs}={f}_{s}\left(\frac{{v}_{s}\phantom{\rule{thinmathspace}{0ex}}\pm \phantom{\rule{thinmathspace}{0ex}}{v}_{w}}{{v}_{w}}\right)$
- ${f}_{obs}={f}_{s}\left(\frac{{v}_{w}}{{v}_{w}\phantom{\rule{thinmathspace}{0ex}}\pm \phantom{\rule{thinmathspace}{0ex}}{v}_{s}}\right)$

### 14.4 Sound Interference and Resonance

When does a yo-yo travel the farthest from the finger?

- when the amplitude of the finger moving up and down is greater than the amplitude of the yo-yo
- when the amplitude of the finger moving up and down is less than the amplitude of the yo-yo
- when the frequency of the finger moving up and down is equal to the resonant frequency of the yo-yo
- when the frequency of the finger moving up and down is different from the resonant frequency of the yo-yo

- Harmonics are all multiples of the fundamental frequency. The first overtone is actually the first harmonic.
- Harmonics are all multiples of the fundamental frequency. The first overtone is actually the second harmonic.
- Harmonics are all multiples of the fundamental frequency. The second overtone is actually the first harmonic.
- Harmonics are all multiples of the fundamental frequency. The third overtone is actually the second harmonic.

What kind of waves form in pipe resonators?

- damped waves
- propagating waves
- high-frequency waves
- standing waves

- The natural frequency is the frequency at which a system oscillates when it undergoes forced vibration.
- The natural frequency is the frequency at which a system oscillates when it undergoes damped oscillation.
- The natural frequency is the frequency at which a system oscillates when it undergoes free vibration without a driving force or damping.
- The natural frequency is the frequency at which a system oscillates when it undergoes forced vibration with damping.