Architecture

# When A Sound Wave Moves Past A Point In Air, What Happens To The Density Of Air At This Point?

When a sound wave moves past a point in air, are there changes in the density of air at this point? Explain.

When sound passes a particular point in the air, the air is first compressed and then rarefied as the sound passes. So its density is increased and then decreased as the wave passes.
If the speed of sound were dependent on its frequency, would you enjoy a concert sitting in the second balcony? Explain.
If the speed of sound were different for different frequencies, say, faster for higher frequencies, then the farther a listener is from the music source, the more jumbled the sound would be. In that case, higher-frequency notes would reach the ear of the listener first. The fact that this jumbling doesn”t occur is evidence that sound of all frequencies travel at the same speed. (Be glad this is so, particularly if you sit far from the stage, or if you like outdoor concerts.)
If a single disturbance at an unknown distance emits both transverse and longitudinal waves that travel with distinctly different speeds in the medium, such as in the ground during an earthquake, how can the distance to the disturbance be determined?
If a single disturbance at some unknown distance sends longitudinal waves at one known speed, and transverse waves at a lesser known speed, and you measure the difference in time of wave arrival, you can calculate the distance. The wider the gap in time, the greater the distance—which could be in any direction. If you use this distance as the radius of a circle on a map, you know the disturbance occurred somewhere on that circle. If you call two friends who have made similar measurements of the same event from different locations, you can transfer their circles to your map, and the point where the three circles intersect is the location of the disturbance.

What two physics mistakes occur in a science fiction movie that shows a distant explosion in outer space, where you see and hear the explosion at the same time?
First, in outer space there is no air or other material to carry sound. Second, if there were, the faster-moving light would reach you before the sound.
Apartment dwellers will testify that bass notes are more distinctly heard from music played in nearby apartments. Why do you suppose lower-frequency sounds travel through walls, floors, and ceilings more easily?
There are two principal reasons why bass notes are more distinctly heard through walls than higher-frequency notes. One is that waves that vibrate more often per second transfer sound energy into heat more rapidly than waves of lower frequency. The higher-frequency waves are thermally “eaten up” by the material in the walls, while the lower-frequency vibrations pass with less loss through the material. Another reason is that the natural frequency of large walls, floors, and ceilings, is lower than the natural frequency of smaller surfaces. The large surfaces are more easily set into forced vibrations and resonance.

Two sound waves of the same frequency can interfere, but, in order to produce beats, the two sound waves must be of different frequencies. Why?
Waves of the same frequency can interfere destructively or constructively, depending on their relative phase, but to alternate between constructive and destructive interference, two waves have to have different frequencies. Beats arise from such alternation between constructive and destructive interference.