Acoustics is the science of sound. Sound is produced by a series of rapid vibrations.
Vibration Everything in the universe has vibration. It is the back and forth motion of an object, which creates waves.
Frequency, or pitch is determined by the number of vibration cycles per second.
Harmonics When a vibrating object, such as a string is set in motion it vibrates both as a whole with a frequency known as a ‘fundamental’, and also in parts. A string, for example, vibrates in its own length, but also in halves, thirds, fourths, fifths, etc. If the smaller lengths are integer fractions (1/2, 1/3, 1/4…) of the length of the string, their frequencies are called harmonics. Harmonics are also standing waves.
Standing Waves are a wave phenomenon resulting from the interference of waves of the same frequency and kind traveling in opposite directions. They are described by the absence of propagation and the presence of nodes or partial nodes and antinodes, or loops, which have a fixed distribution in space.
Beat Frequencies If two notes are about 15 hz. or less apart, interference will result from the closeness of the frequencies. They will slowly move in and out of phase. It is a form of amplitude modulation.
Transverse Waves oscillate in a direction that is perpendicular to the direction of displacement of the transmitting field or medium. Light waves and ocean waves are transverse.
Longitudinal Waves oscillate back and forth on the same axis as the one along which the wave propagates. Sound waves traveling through the air are longitudinal, and waves traveling through a fluid or gas are always longitudinal.