# Sound Waves

1. Picture a long row of standing dominos, one a little behind the other so that knocking over the first will knock over the second and so on. Now imagine an idealized tube an inch or two in diameter extending from New York to California. Suppose this tube is filled with greased, essentially frictionless marbles and you push one more in at the East Coast end only to have one pop out at the West Coast end. What's happening, and how do these arrangements of dominos and marbles related to sound waves?

2. In December 1916, a major diaster befell a contingent of the Austrian army in the Alps fighting in Wourld War I. Almost as soon as they began a cannon barrage, an avalanche of snow came roaring down, burying thousands of soldiers. What does that suggest about sound?

Multiple Choice:
1. The speed of sound in air at 40 deg C by comparison tot eh speed at 0 deg C is: (a) much lower (b) a little lower (c) the same (d) higher (e) none of these. Explain your choice.

2. A 60-dB sound-level corresponds to an intensity of : (a) 10^-10W/m2 (b) 10^6 W/m2 (c) 10^-12 W/m2 (d) 10^12W/m2 (e) none of these. Expalin your choice.

3. On a day when you are listening to music at a sound level of 20 dB above the threshold of audibility, an average person would say that it was: (a) very loud (b) of normal listening level (c) very soft (d) slightly loud (e) none of these. Explain your choice.

Problem

1. A detector placed perpendicular to a streaming sound wave has an active area of 10.0 cm2 and recoreds energy incident at at rate of 25.0 uJ/s. What is the intensity of the wave?

2. The intensity of the acoustic wave from an underground explosion measured 5.0 km away is 1.6 x 10^4W/m2. What intensity will be recorded at a site 50 km away? Assume no losses.

3. A cannon produces a 90-dB sound-level at a certain distance from a detector. What will the device read when two such cannons at the same distance are fired?

4. Two sounds have intensities of 10^-2W/m2 and 10^-10W/m2. What will be the difference in their intensity levels as read by a sound-level meter?

5. The intensity-level of a large engine is measured to be 130 dB at a distance of 10.0 m. Approximately what intensity will exist at a point 100 m away?

6. Two sound waves of angular frequencies 900.0 rad/s and 896 rad/s overlap. What is the resulting beat frequency?

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#### Solution Preview

...) (d) it is higher, actually here is the approximate formula for the speed of sound as a function of air temperature:
<br>v = 331 + 0.6 T
<br>where velcoities are in m/s and temperature are celsius.
<br>(2)(e) I guess because dB is a relative measure of the sound intensity and no reference (Io) is given here.
<br>(3) (c) it will correspond to a soft voice (20 db)
<br>
<br>PROBLEMS
<br>(1) Intensity is power divide by area:
<br>
<br>I = P/A = 25 j/s / (10 10^(-4) m^2)= 25000 W m^2
<br>(2)If there's no loss and we assume a unifrom distribution of sound the sound intensity will ...