5.9 Sound

When you hit a tuning fork, why do you hear a note? All a tuning fork is a piece of metal, so what in it makes noise? The regular vibration of the prongs, produced when you hit the fork, is what creates sound. Sound is caused by a vibration of particles in a medium. In the case of the tuning fork, the prongs bang periodically into the air molecules next to it, which in turn bang into the particles next to them. . . and the wave can then be carried through the air. If you can remember this type of wave from unit 5.1, you will know that sound is a longitudinal wave.

Sound is also a mechanical wave, which means it needs a medium to travel though (i.e. compared to light, which can travel through a vacuum). Because it relies on the collisions between particles to be propagated, sound must necessarily travel better through materials with more particles in the same amount of space. That is, the more dense a material, the more easily sound will travel through it. For example, sound travels about 340 m/s in air, about 1500 m/s in water, and about 5000 m/s in steel.

What makes us hear sound? Our ears are very fine receptors to the vibrations of the air molecules, for instance those caused by the tuning fork. The change in air pressure, due to the longitudinal nature of sound waves is picked up on the ear drum, or tympanic membrane. The sounds we hear depend on the frequencies of the sound wave. Humans can normally hear sounds of about 20 Hz to 20,000 Hz. It is interesting to note that in the human ear, smaller frequency changes are more easily noticeable when the frequencies are high to begin with.