Sound Travels Through The Air In Waves From A Central Source Much As GMAT Reading Comprehension

Reading Passage Question

Sound travels through the air in waves from a central source much as ripples from a pebble dropped into a pond travel across the surface of the pond, diminishing in intensity as they move away from the source. The speed at which sound waves travel in the air is affected by the air temperature, but for most purposes we can consider the speed of sound to be relatively constant at 1,100 feet per second. The distance between the peaks of the waves is the wavelength of the sound just as the distance between the ripples in the pond is the wavelength of the water. If we continue with the ripple-in-the-pond analogy and imagine a cork floating on the surface of the water, we can think of the frequency of sound waves as the number of times the cork bobs up and down during a given interval as the waves of water pass it in two dimensions.

The frequency is simply the speed of propagation of the wave divided by its wavelength. Therefore, if a sound is created at a given point, a system of spherical waves propagates from that point outward through the air at a speed of 1,100 feet per second, with the first wave making an ever-increasing sphere with time. On that sphere, the sound energy remains essentially constant in an ideal case. As the wave spreads, the height of the wave or the intensity of the sound at any given point must diminish as the fixed amount of energy is spread over the increasing surface area of the sphere. This phenomenon is known as the geometric attenuation of the sound. If we placed monitoring stations along the path of propagation of the sound, we would find that the intensity of the sound near the source would be much higher than the intensity of the sound at a great distance due to this phenomenon. Mathematical relationships have been derived to describe this geometric attenuation, according to which for every doubling of distance the sound level will decrease by 6 decibels (dB). In other words, if station I were at a distance of 50 feet from the point source, and if station 2 were 100 feet from the point source, the sound level measured at station 2 would be 6 dB less than the sound level measured at station 1.

This kind of relationship holds true when the sound source is a single vehicle or an aircraft and when sound is propagating in free air, either from an airplane to the ground in completely spherical propagation or, in the case of an automobile on the ground, when the propagation field is only half a sphere. When a number of vehicles are lined up and constitute a continuous stream of noise sources, the situation is no longer characterized by a spherical or hemispherical spreading of the sound. Instead, the reinforcement by the line of point sources makes the propagation field more like a cylinder or half-cylinder. In this case, the decrease in sound for each doubling of the distance from the line source is only 3 decibels.

“Sound travels through the air in waves from a central source much as”- is a passage for the GMAT that addresses reading comprehension. Candidates must have a firm understanding of GMAT reading comprehension in English. This GMAT reading comprehension section consists of four comprehension questions. The purpose of the GMAT Reading Comprehension questions is to assess a candidate's capacity to understand, evaluate, and apply knowledge or ideas. By responding to the GMAT Reading Comprehension Practice Questions section, candidates can actively practice.

Solutions and Explanation

1. In the analogy of a ripple in a pond to a sound in the air drawn in the first paragraph of the passage, the floating cork is analogous to:

1. a solid medium that retards sound propagation
2. a sound-frequency monitoring device
3. an object on which a sound is likely to echo
4. the source of a particular sound
5. a sound wave moving through space

2. According to the passage, sound waves and water waves are similar in all of the following ways EXCEPT:

1. Each forms a sphere of ever-increasing size.
2. Each is characterized by a particular wavelength.
3. Each is propagated outward from a central source.
4. Each is characterized by a particular wave frequency.
5. Each is characterized by a diminishing average strength at greater distances from the source.

3. The intensity of a sound is lower farther from the source because:

1. The amount of sound energy present decreases rapidly with outward movement of the sound waves.
2. The frequency of the sound waves decreases as the sound spreads.
3. The same amount of sound energy is spread over a larger area.
4. Most of the sound energy is lost within a few feet of the source.
5. The wavelength of the sound increases with its distance from the source.

4. The propagation field of a sound produced by an automobile on the ground is half a sphere because:

1. the surface of most highways reflects sounds upward
2. the engine of an automobile is generally located in the front half of the vehicle
3. the metal body of an automobile conducts sound relatively well
4. most highways are surrounded by embankments on either side
5. the sound is not propagated through the ground below the automobile

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