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Because the spacing of the mounting holes is uneven, close attention must be paid when installing equipment. The installer must ensure that all rail assemblies align with the three-hole pattern, otherwise the assembly will not fit correctly, this is known as 'being off a U'.

Acoustic material: Any material considered in terms of its acoustical properties. Commonly and especially, a material designed to absorb sound.

Ambient noise: The composite of airborne sound from many sources near and far associated with a given environment. No particular sound is singled out for interest.

Absorption: In acoustics, the changing of sound energy to heat.

Acoustics: The science of sound. It can also refer to the effect a given environment has on sound.

Audible frequency range: The range of sound frequencies normally heard by the human ear. The audible range spans from 20Hz to 20,000Hz.

A-weighting: A frequency-response adjustment of a sound-level meter that makes its reading conform, very roughly, to human response.

Baffle: A movable barrier used to achieve separation of signals from different sources. The surface or board upon which a loudspeaker is mounted.

Bandwidth: The total frequency range of any system. Usually specified as something like: 20-20,000HZ plus or minus 3 dB(A).

Damp: To cause a loss or dissipation of the oscillatory or vibrational energy of an electrical or mechanical system.

dB(A): A sound-level meter reading with an A-weighting network simulating the human-ear response at a loudness level of 40 phons.

Decibel: dB---the term used to identify ten times the common logarithm of the ratio of two like quantities proportional to power or energy. (See level, sound transmission loss.) Thus, one decibel corresponds to a power ratio of 100.1.

Electronic Industries Alliance (EIA): A trade organization composed as an alliance of trade associations for electronics manufacturers in the United States.

Fletcher-Munson Curve: Our sensitivity to sound depends on its frequency and volume. Human ears are most sensitive to sounds in the midrange. At lower volume levels humans are less sensitive to sounds away from the midrange, bass and treble sounds "seem" reduced in intensity at lower listening levels.

Interference: The combining of two or more signals results in an interaction called interference. This may be constructive or destructive. Another use of the term is to refer to undesired signals.

Logarithm: An exponent of 10 in the common logarithms to the base 10. For example, 10 to the exponent 2=100; the log of 100=2.

Loudness: A subjective term for the sensation of the magnitude of sound.

Noise reduction (NR): The difference in sound pressure level between any two points along the path of sound propagation. As an example, noise reduction is the term used to describe the difference in sound pressure levels between the inside and outside of an enclosure.

Phon: The unit of loudness level of a tone.

Psychoacoustics: The study of the interaction of the auditory system and acoustics.

Sound: Sound is vibrational disturbance, exciting hearing mechanisms, transmitted in a predictable manner determined by the medium through which it propagates. To be audible the disturbance must fall within the frequency range 20Hz to 20,000Hz

Sound Absorption: (1) The process of dissipating sound energy. (2) The property of possessed by materials, objects and structures such as rooms of absorbing sound energy.

Sound level: Of airborne sound, a sound pressure level obtained using a signal to which a standard frequency-weighting has been applied.

Sound power level, Lp: Of airborne sound, ten times the common logarithm of the ratio of the sound power under consideration of the standard reference power of 1 pW. The quantity so obtained is expressed in decibels.

Sound Pressure Level (SPL): Given in decibels (dB) is an expression of loudness or volume. A 10db increase in SPL represents a doubling in volume. Live orchestral music reaches brief peaks in the 105db range and live rock easily goes over 120dB(A).

Sound Waves: Sound waves can be thought of like the waves in water. Frequency determines the length of the waves; amplitude or volume determines the height of the waves. At 20Hz, the wavelength is 56 feet long! These long waves give bass its penetrating ability, (why you can hear car boomers blocks away).