ABU-DEFINITION OF TERMS

Absorption Coefficient

the fraction of the incident sound energy absorbed by a surface

Anechoic Chamber

a sealed room in which all the surfaces are designed to completely absorb all sound produced in the room.

Attenuation

a reduction in sound level. Sound ______ in air-conditioning is specified in terms of dB per meter.

Background Noise

ambient noise

Break-in Noise

transfer of noise from a space surrounding the duct into the duct through duct walls.

Break-out Noise

transfer of noise from the interior of a duct through duct walls into a space outside the

duct.

Dead Room

a room containing an unusually large amount of sound absorption.

Decibel (dB)

a unit of measurement for sound pressure level, sound intensity level or sound power level.

Diffraction

a change in the direction of propagation of sound as a result of bending caused by a barrier in the path of a sound wave.

Diffuse Sound (field)

a sound field in which the sound comes in equal intensity from all directions.

Direct Sound

the sound that arrives at a receiver along a direct line from the source without reflection from any surface.

Echo

a sound that has been reflected with sufficient time delay.

Environmental Noise

exterior background noise in a neighborhood (ie. traffic, aircraft).

Fidelity

faithful reproduction of a sound source.

Flutter Echo

a rapid but repetitive succession of sound from a sound source usually occurring as a result of multiple reflections in a space with hard, flat and parallel walls.

Frequency

the number of full cycles per second measured.

Impact Noise

noise caused by the collision of two objects.

Infrasonic

a sound that is below the human audible frequency, below 20 Hz.

Insulation

lack of acoustical connection.

Intermittent sound

a sound which is discontinuous or fluctuates to such an extent that at times its sound pressure level falls below a measurable level.

Inverse square law

a law which states that the sound intensity in a free field varies inversely with the square of the distance from the source.

Isolation

a lack of acoustical connection

Leak

small opening in a barrier that allows airborne sound to pass through.

Live room

a room containing an unusually small amount of sound absorption.

Loudness

an auditory sensation that depends on sound pressure level and the frequency of sound.

Masking

the increase in the threshold of audibility of a sound that is required so that the sound can be heard in the presence of another sound.

Noise Isolation Class (NIC)

a single number rating derived from the measured value of noise reduction between two rooms.

Noise Reduction (NR)

the reduction in sound pressure level of noise.

Noise Reduction Coefficient (NRC)

a single number rating derived from measured values of sound absorption coefficients of a material at 250, 500, 1000 and 2000 Hz.

Outdoor-Indoor Transmission Class (OITC)

a weighted single number rating of the sound reduction effectiveness of a partition that separates an indoor space from the outside.

Pitch

a listener’s perception of the frequency of a pure tone

Reflection Coefficient

a measure of the sound reflective property of a surface.

Resonance

the relatively large amplitude of vibration produced when the frequency of the source of sound is equal to the natural frequency of a room.

Reverberant Sound Field

a sound field created by repeated reflections of sound from the boundaries in an enclosed space.

Reverberation

the continuation of sound in an enclosed space after the initial source has been terminated.

Reverberation Time (RT)

the time it takes for sound intensity to decay by 1 millionth of its steady state value after the sound source has been terminated.

Sabin

a unit of measure of sound absorption.

Scattering

an irregular diffraction of sound in many directions

Sound Insulation

the ability of a barrier to prevent sound from reaching a receiver.

Sound Intensity (SI)

the average rate of sound energy flow through a unit area in a given direction.

Sound Intensity Level (SIL)

a quantity expressed in decibels of airborne sound.

Sound Lock

a small space that works as a buffer between a source room and a receiving room.

Sound Pressure

fluctuating pressure of sound superimposed on the static air pressure.

Sound Pressure Level

a quantity expressed in decibels of airborne sound.

Sound Transmission Class (STC)

a single number rating of the sound insulation rating of a partition.

Structure-Borne Sound

sound propagated through a solid structure.

Transmission Coefficient

the ratio of transmitted sound energy to incident sound energy

Transmission Loss (TL)

is the measure of sound insulation of a partition.

Wavelength

distance between two adjacent compressions or rarefactions in a sound wave.

White Noise

a noise whose energy is uniform over a wide range of frequencies. This is analogous to the term “white light”, which consists of almost equal amount of light of different wavelength (colors). A white noise sounds hissy.