The correct answer is: D. none of the above
The noise produced by a machine is due to the vibration of the machine’s components. The vibration of the components causes the air molecules around the machine to vibrate, which in turn causes sound waves to be produced. The density of the air affects the speed of sound waves. The speed of sound is higher in denser media, so the sound waves will travel faster in air than in hydrogen. However, the noise produced by a machine is not affected by the speed of sound. The noise produced by a machine is affected by the amplitude of the sound waves, which is the distance between the maximum and minimum pressure of the sound waves. The amplitude of the sound waves is affected by the vibration of the machine’s components. The vibration of the machine’s components is affected by the machine’s design and construction. The noise produced by a machine can be reduced by using sound-absorbing materials around the machine.