Acoustics Glossary
 

Amplitude: Amplitude is the variation or displacement of a wave from its mean value.  With sound waves, it is the extent to which air particles are displaced, and is experienced as the intensity or loudness of a sound.  

Antinode: Antinodes are the points on a wave with the greatest displacement or amplitude.

Bulk modulus: Bulk modulus (K) is the measure of a material's resistance to uniform compression.

Constructive Interference: Constructive Interference occurs where two identical phases meet and combine to create a wave with a greater amplitude.

Destructive Interference: Destructive Interference occurs where two opposite phases meet and combine to create a wave with a lesser amplitude.

Diffraction: The ability of waves to bend or change direction through an opening or around a barrier [3].

Dissipated Energy: Dissipated Energy is energy that is transformed or lost from an action or movement.  Friction and heat are common examples of dissipated energy.

Elastic Energy: The energy of compressing or stretching of a substance. Example: A compressed spring has elastic energy.

Fixed End Reflection: A Fixed End Reflection occurs when a wave meets the end of a medium, or a heavier/denser medium.  Fixed End Reflections cause a phase inversion of a wave. Compressions of longitudinal waves are reflected back as rarefactions, while crests of transverse waves are reflected as troughs [16].

Free End Reflection: A Free End Reflection occurs when a wave meets an open end, or a lighter/less dense medium.   Free End Reflections allow the phase of the wave to remain unchanged.  Compressions of longitudinal waves are reflected back as compressions, while crests of transverse waves are reflected as crests [16].

Frequency: Frequency is simply a mathematical representation of how low or high a pitch is, and is based on the rate at which waves vibrate [15, pg. 8].  It is measured in Hertz, or number of complete wave cycles per second, and is commonly abbreviated as Hz [18]. 

Frequency Ratio: The Frequency Ratio is the mathematical equivalent to a musical interval.  Examples: a major third would be 5/4, a fifth would be 3/2 and an octave would be 2 [11].

Fundamental Principle: The Fundamental Principle states that if a frequency of a tone is n, that of its octave is 2n, that of its fifth is 3/2n, and that of the major third is 5/4n.   Therefore, tube or string length is inversely proportional to frequency, i.e., every time length is halved, frequency is doubled [15, pg 10].

Harmonics/Overtones: All Harmonics are Overtones, but not all Overtones are Harmonics.  Harmonics are defined as a series of musical pitches whose frequencies are multiples of the fundamental frequency, whereas Overtones are defined as all harmonic and non-harmonic frequencies that are produced as sympathetic vibrations above the fundamental.  Therefore, Overtones also include the body of non-harmonic and dissonant frequencies that are produced in membranophones and idiophones.   In terms of classification, Harmonics above the fundamental can also be called overtones; the second harmonic is the first overtone, the third harmonic is the second overtone and so forth [15, pg. 10].

Kinetic Energy: Kinetic energy is the energy possessed by an object because of its motion.  Example: Temperature is a way of measuring the kinetic energy of particles moving about.

Longitudinal Wave: Longitudinal Waves transfer energy parallel to the direction of motion.  To further understand this concept, imagine a slinky lying flat on the floor, stretched out in a straight line.  When you push on it either forward or backward the slinky compresses and expands, sending a pulse down the slinky.  Sound is an example of a longitudinal wave.

Medium: A medium is a substance through which waves propagate. Examples would be air, water, wood, or metal. Sound requires a medium through which to travel, and therefore cannot exist in a vacuum.

Membrane: A membrane is the material that is stretched over a drum, also referred to as a drum head.   

Musical Interval: The distance between two pitches [14, pg. 418]. Examples: a major third, a perfect fifth, or an octave.

Node: Nodes are the points on a wave where the displacement or amplitude is equal to zero.  For example, on a glockenspiel the nodal points would be found at 0.224 x the length of the tube.

Nodal Points of the Fundamental Frequency

Period: The period of a wave is the duration of time it takes for one complete wave cycle to occur. Period is the inverse of frequency.

Reflection: Reflection occurs when a wave reaches a boundary between mediums of different densities, or the end of a medium.  The two most common types are fixed-end and free-end reflection.

Refraction: Refraction is the bending of a wave as it changes mediums [3].

Speed of Sound: The speed of sound changes depending on the medium through which it travels.  At normal atmospheric pressure, and at 20 °C (68 °F), the speed of sound is approximately 343 meters per second.  This speed can be used to determine either frequency or wavelength with the equation: v = fλ, where v is the speed or velocity of sound, f is the frequency, and λ is the wavelength.

Standing Wave: A Standing Wave is a vibrational pattern that results when two waves with the same frequency travel in opposite directions and reinforce each other through constructive and destructive interference.   This wave pattern is characterized by nodal points and anitnodal points that appear stationary, or standing still.  With musical instruments, standing waves occur only at harmonic frequencies. 

Timbre: Timbre (pronounced tam-ber) refers to the quality or tonal color of a sound, and it is determined by which overtones are mixed in with the fundamental.

Torsional Vibration:  Torsional (pronounced tor-shun-al) vibrations are characterized by a twisting of the tube in a diagonal or side-to-side motion [1, pg. 64].

Transverse Waves: Transverse waves transfer energy perpendicular to the direction of motion.  These waves are most commonly expressed as sine waves or sinusoidal waves and are plotted on a x-y coordinate graph.

Velocity: Velocity is the change in position, or displacement, divided by the change in time.  Since it is a vector, both speed and the direction of motion are required to define it.

Wavelength:  The distance between two corresponding points on successive waves, usually crest-to-crest or trough-to-trough.