Chapter 19 Review Questions 3. Frequency is the number of vibrations per second the sound makes. Pitch is the lowest frequency of the sound that we hear. The paper cone vibrates, pushing the air and then pulling away so the air expands again. This makes the air vibrate, causing a sound wave.
Air is a poorer conductor of sound than either solids or liquids.
The speed of sound is faster in water than in air, and faster yet in steel than in water. An echo is reflected sound. If all the feet step at the same time that is a lot of force at set intervals. If this is near a natural or resonant frequency of the bridge this could set up a large oscillation and damage or break the bridge. The moon has no atmosphere so there is no medium to transmit the sound.