18. Waves &amp; Sound

A source emits a sound in the shape of a cone, as shown in the figure below. If you measure the intensity to be 100 W/m² at a distance of 0.5 m, what is the power of the source?

432

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A source emits sound spherically with a power of 2.2 × 10⁴ W. What is the minimum distance away from this sound that would be considered safe (a volume of 150 dB or less)?

Suppose an explosion can just be heard over normal conversation with an intensity of

1.0 \times 10^{- 6} ​ {W/m}^{2}

from 10 km away. At what distance from the explosion will the sound have an intensity that causes pain (

10 ​ {W/m}^{2}

)? Note that this calculation ignores absorption of sound energy by air and objects.

A speaker at a rock concert is causing you pain, so you reason the sound level must be 130 dB. The speaker is 11 m away from you. To what final distance from the speaker should you move for the sound level to reach a less-painful 105 dB? Keep in mind that we are ignoring absorption of sound energy by air and objects.

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A sound wave from a police siren has an intensity of 0.01 W/m² at a certain distance. A second sound wave from an ambulance has a sound intensity level 8 dB greater than the police siren, when measured at the same distance. What is the sound intensity level (in dB) of the sound wave due to the ambulance?

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(a) By what factor must the sound intensity be increased to raise the sound intensity level by 13.0 dB? (b) Explain why you don't need to know the original sound intensity

314

A baby's mouth is 30 cm from her father's ear and 1.50 m from her mother's ear. What is the difference between the sound intensity levels heard by the father and by the mother?

250

For a person with normal hearing, the faintest sound that can be heard at a frequency of 400 Hz has a pressure amplitude of about 6.0 * 10-5 Pa. Calculate the (a) intensity

478

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You live on a busy street, but as a music lover, you want to reduce the traffic noise. (b) If, instead, you reduce the intensity by half, what change (in dB) do you make in the sound intensity level?

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You live on a busy street, but as a music lover, you want to reduce the traffic noise. (a) If you install special soundreflecting windows that reduce the sound intensity level (in dB) by 30 dB, by what fraction have you lowered the sound intensity 1in W>m2 2?

364

A sound wave in air at 20°C has a frequency of 320 Hz and a displacement amplitude of 5.00 * 10-3 mm. For this sound wave calculate the (a) pressure amplitude (in Pa)

556

You are trying to overhear a juicy conversation, but from your distance of 15.0 m, it sounds like only an average whisper of 20.0 dB. How close should you move to the chatterboxes for the sound level to be 60.0 dB?

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Sound is detected when a sound wave causes the tympanic membrane (the eardrum) to vibrate. Typically, the diameter of this membrane is about 8.4 mm in humans. (a) How much energy is delivered to the eardrum each second when someone whispers (20 dB) a secret in your ear?

710

What is the speed of sound in air (a) on a cold winter day in Minnesota when the temperature is -25°F, and (b) on a hot summer day in Death Valley when the temperature is 125°F?

192

A concert loudspeaker suspended high above the ground emits 35 W of sound power. A small microphone with a 1.0 cm² area is 50 m from the speaker. a. What is the sound intensity at the position of the microphone?

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Which of the following statements is explained best using the Chargaff's rule?

115

Assuming that the maximum displacement of the air molecules in a sound wave is about the same as that of the speaker cone that produces the sound (Fig. 16–46), estimate by how much a loudspeaker cone moves for a fairly loud (105 dB) sound of

(b) 35 Hz. <IMAGE>

(I) What are the lowest and highest frequencies that an ear can detect when the sound level is 40 dB? (See Fig. 16–6.)

(II) Expensive amplifier A is rated at 280 W, while the more modest amplifier B is rated at 45 W.

(b) What output wattage amp would sound about twice as loud as the 45-W amp?

(II) Your ears can accommodate a huge range of sound levels. What is the ratio of highest to lowest intensity at (a) 100 Hz, (b) 5000 Hz? (See Fig. 16–6.)