A sound wave is a periodic disturbance in a medium like air, though sound can also travel through liquids and solids — more on that later.

A. Sound is generated by vibration of something, like your vocal chords, a tuning fork or a piano string. As a vibrating object moves, it periodically pushes on the surrounding air molecules, causing them to move and compress.

B. One such compression zone has been formed by a mechanical vibration on the left, and will now begin to move or propagate to the right. Particles that have received a push from the left will push on particles to the right, and the compression zone moves along.

In panels C and D, the wave continues to propagate. The zones of high density gas are called compression zones and the zones of relatively low density in between are called rarefaction zones.

E. A new compression zone has been formed and will continue to propagate to the right. The distance between compression zones is the wavelength of the wave.

The disturbance in the medium (air) is in the same direction as the direction of the wave, so sound waves are called longitudinal waves. The disturbance is along the direction of travel. This is in contrast to transverse waves like water waves, in which the disturbance in the medium is at right angles to the direction of travel.

The diagram below is another common way of illustrating compression and rarefaction in sound waves.

• and relative phase is important for sound waves. Sounds can interfere, and even cancel, if they are produced out of phase.

The relationship λ · ν = speed also holds for sound waves. The speed of sound in air and several materials are listed in this table:

where ρ (rho) is the Greek symbol for density. Clearly, as the density increases, the speed decreases. Why?

It's because higher density means more mass in a given volume. That means heavier gas particles, which have more inertia and are harder to accelerate as the wave propagates, resulting in a slower wave.

Gases

It's a little different for gases. When a gas is heated at constant pressure (think of atmospheric pressure), the molecules speed up and have harder collisions, creating more space between them and a lower density. We'd expect to see a reduced speed of sound in a hotter gas, but this is what we actually see:

This is where science gets fun — when we don't get the expected results. The reason for this is that the faster speeds of hotter gas particles hasten collisions, and thus hasten the propagation of sound waves.

– the smallest in the body, the incus, malleus and stapes, commonly referred to (because of their shapes) as the hammer, anvil and stirrup, respectively.

The hammer is connected to the inner side of the drum, and thus vibrates when the drum does. Acting like a series of levers, the hammer, anvil and stirrup amplify the vibrations of the drum.

The stirrup is in contact with fluid (mostly water) in the snail-shaped cochlea. The cochlea (the word comes from Greek and Latin and means "spiral.") is an ingenious device for separating frequencies of sound. Lower pitched (long wavelength) sounds travel all the way through the spiral of the cochlea and are picked up by nerve endings there, while higher-frequency sounds don't get as far before being translated into a nerve impulse.

The auditory nerves lead to the brain where sounds are processed and perceived.

The ear is a transducer, a device that converts mechanical sound waves (longitudinal waves that impart kinetic energy to the ear drum) to electrical signals in the brain.

We know (or you will eventually study) that when a coil of wire is moved through a magnetic field, a current is generated in the wire through electromagnetic induction. The two ends of the wire forming the coil are fed to an amplifier circuit which converts the current to a useful signal, say for broadcasting or recording.

Speakers work in a similar way, just in reverse. An amplifier circuit delivers a current through a coil of wire, which produces a small magnetic field that alternatively opposes and aligns with the field of a permanent magnet. This produces a motion in the coil, which in turn vibrates a membrane, a reed or a large paper or fabric cone, depending on the range of pitches that the speaker is designed to produce.

The decibel is used to measure sound levels, but also to measure how much electronic signals are amplified or attenuated. It is a logarithmic unit in this sense:

Imagine that there are two stereo speakers, one playing sound with power P₁ and the other with P₂. Let's say that P₂ > P₁. In decibels, the difference in the sound level between the two speakers is

$$10 \cdot log_{10} \left( \frac{P_2}{P_1} \right) \, dB$$

If P₂ = 2P₁ (speaker 2 uses twice the power of speaker 1), then our difference is:

$$10 \cdot log_{10} \left( \frac{P_2}{P_1} \right) = 10 log_{10} (2) \approx 3 \, dB.$$

If speaker 2 uses ten times the power of speaker 1, then we'd have

$$10 \cdot log_{10} \left( \frac{P_2}{P_1} \right) = 10 log_{10} (10) = 10 \, dB.$$

Here is a table of some decibel values given the ratio P₂/P₁:

From the table you can see that for each factor of ten in the power ratio, we get another ten decibels.