Sound is the vibration of the individual molecules of any substance. Sound vibrates in the air and moves through the substance like a wave, hence the term "sound wave". Three characteristics that define a sound wave are frequency, wavelength, and amplitude. We will look at these in depth in the following paragraphs. The only place in which sound can't travel is in a vacuum. Noise and sound are often used to mean the same thing and are measured in Decibels.

Decibel

A decibel (dB) is a unit of measurement of the loudness or strength of a sound. The decibel was named after Alexander Graham Bell, and was originally created to measure cable and equipment performance. The lowest decibel level a human can hear is the zero point. A difference of 1 decibel is the minimum perceptible change in volume. An increase of 6 dB is a doubling of the volume. An increase of another 6 dB would again double the volume, so 12 dB is a four-fold volume increase. 18 dB is an eight-fold increase, etc. Below is a list of examples of decibels.
0 dB : Threshold of hearing, 20 dB: Buzzing insect at 1 metre, 30 dB: Public library, whispering, 40 dB: Household living room, 50 dB: Light traffic, refrigerator, 60 dB: Normal conversation, air conditioner, 70 dB: Heavy traffic, busy restaurant, 80 dB: Subway, noisy factory, 90 dB: Lawnmower, large truck, 100 dB: Chainsaw, jack hammer, 120 dB: Rock concert, 140 dB: Artillery fire, jet engine, 180 dB:  Rocket launch.

Frequency

Frequency of sound means the number of vibrations or cycles per second a sound wave generates. A cycle is one complete rise and fall of pressure in the air through which a sound is passing. It is measured in hertz (Hz). The average human ear can detect a frequency between 20 and 20,000 Hz, or vibrations per second. Anything below 20 Hz is called infrasound, and above 20,000 is known as ultrasound. In general, younger people can hear lower frequencies of sound than older people. The pitch of sound, i.e. how high the note is, depends on the frequency of the wave. The higher the frequency, the higher the pitch, and vice versa. Dogs can detect frequencies as low as 67 Hz and as high as 45,000 Hz. Bats can detect frequencies as high as 200,000 Hz.

Wavelength

The wavelength is the horizontal distance between two successive points of a sound wave. The upper portion of the wave is called the crest, and the bottom is the trough.

Amplitude

The amplitude is the measure of the amount of energy in the sound wave, measured from the fixed point of the wave to the crest ( positive amplitude ) or to the trough ( negative amplitude ). With a loud sound there is more energy, hence the wave is higher. A softer sound produces a shorter wave.