ABSORPTION: A property of materials that allows a reduction in the amount of sound energy reflected. The introduction of an absorbent material into the surfaces of a room will reduce the sound pressure level in that room by not reflecting all of the sound energy striking the room's surfaces. The effect of absorption merely reduces the resultant sound level in the room produced by energy that has already entered the room.

ABSORPTION COEFFICIENT: A measure of the sound-absorbing ability of a surface. It is defined as the fraction of incident sound energy absorbed or otherwise not reflected by a surface. Unless otherwise specified, a diffuse sound field is assumed. The values of the sound-absorption coefficient usually range from 0.01 for marble slate to almost 1.0 for long absorbing wedges often used in anechoic rooms.

ACOUSTICS: 1. The science of sound, including the generation, transmission, and effects of sound waves, both audible and inaudible. 2. The physical qualities of a room or other enclosure (such as size, shape, amount of noise) that determine the audibility and perception of speech and music within the room.

ACOUSTIC TRAUMA: Damage to the hearing mechanism caused by a sudden burst of intense noise, or by a blast. The term usually implies a single traumatic event.

AIRBORNE SOUND: Sound that reaches the point of interest by propagation through the air.

AMBIENT NOISE: The total of all noise in the environment, other than the noise from the source of interest. The term is used interchangeably with the term background noise.

ANSI: The American National Standards Institute.

ATTENUATION: The reduction of sound intensity by various means (e.g., air, humidity, porous materials, etc).

AUDIO FREQUENCY: The frequency of oscillation of an audible sound wave. Any frequency between 20 and 20,000 hertz.

AUDIOMETER: An instrument for measuring individual hearing activity.

BACKGROUND NOISE: The total of all noise in a system or situation, independent of the presence of the desired signal. In acoustical measurements, strictly speaking, the term "background noise" means electrical noise in the measurement system. However, in popular usage the term "background noise" is often used to mean the noise in the environment, other than the noise from the source of interest.

BAND: Any segment of the frequency spectrum.

BAND PASS FILTER: A wave filter that has a single transmission band extending from a lower cutoff frequency greater than zero to a finite upper cutoff frequency spectrum.

BROADBAND NOISE: Noise with components over a wide range of frequencies.

CALIBRATIOR (ACOUSTICAL): A device that produces a known sound pressure on the microphone of a sound level measurement system, and is used to adjust the system to Standard specifications.

CUTOFF FREQUENCIES: The frequencies that mark the ends of a band, or the points at which the characteristic of a filter change from pass to no-pass.

CYCLE: The complete sequence of values of a periodic quantity that occurs during one period.

CYCLES PER SECOND: A measure of frequency numerically equivalent to hertz.

CYLINDRICAL WAVE: A wave in which the surfaces of the constant phase are coaxial cylinders. A line of closely spaced sound sources radiating into an open space produces a free sound field of cylindrical waves.

DAMPING: The dissipation of energy with time or distance. The term is generally applied to attenuation of sound in a structure owing to the internal sound-dissipative properties of the structure or to the addition of sound-dissipative materials.

dBA: Unit of sound level. The weighted sound pressure level by the use of the A metering characteristics and weighted specified in ANSI Specifications for Sound Level Meters. dBA is used as a measure of human response to sound.

DECIBEL: A unit of sound pressure level, abbreviated dB.

DIFFRACTION: A modification, which sound waves undergo in passing by the edges of solid bodies.

DOPPLER EFFECT (DOPPLER SHIFT): The apparent upward shift in frequency of a sound as a noise source approaches the listener or the apparent downward shift when the noise source recedes. The classic example is the change in pitch of a railroad whistle as the locomotive approaches and passes by.

ECHO: A wave that has been reflected or otherwise returned with sufficient magnitude and delay, so as to be detected as a wave distinct from that directly transmitted.

FAR FIELD: Describes a sound source region in the free space where the sound pressure level obeys the inverse-square law (the sound pressure level decreases 6 dB with each doubling of distance from the source). Also, in this region the sound particle velocity is in phase with the sound pressure. Closer to the source where these two conditions do not hold constitutes the 'near field" region.

FILTER: A device for separating components of a signal on the basis of their frequency. It allows components in one or more frequency bands to pass relatively unattenuated, and it attenuates components in other frequency bands.

FREE SOUND FIELD (FREE FIELD): A sound field in which the effects of obstacles or boundaries on the sound propagated in that field is negligible.

FREQUENCY: The number of times per second that the sine wave of the sound repeats itself, or that the sine wave of vibrating object repeats itself. Now expressed in hertz (Hz), formerly in cycles per second (cps).

HEARING: The subjective human response to sound.

HEARING LEVEL: A measured threshold of hearing at a specified frequency, expressed in decibels relative to specified standard of normal hearing. The deviation of an individual's threshold in decibels from the zero reference of the audiometer.

HEARING LOSS: A term denoting an impairment of auditory acuity. The amount of hearing impairment, in decibels, measured as a set of hearing threshold levels at specified frequencies. Types of hearing loss are 1. Conductive: A loss originating in the conductive mechanism of the ear; 2. Sensor-neural: 3 A loss originating in the cochlea or the fibers of the auditory nerve; 3. Noise induced: A sensor-neural loss attributed to the effects of noise.

HERTZ (Hz): Unit of measurement of frequency, numerically equal to cycles per second.

IMPACT INSULATION CLASS (IIC): A single-figure rating that compares the impact sound insulating capabilities of floor-ceiling assemblies to a reference contour.

IMPACT SOUND: The sound produced by the collision of two solid objects. Typical sources are footsteps, dropped objects, etc. on an interior surface (wall, floor, ceiling) of a building.

INFRASONIC: Sounds of a frequency lower than 20 hertz.

INTENSITY: The sound energy flow through a unit area in a unit of time.

ISO: The International Organization for Standardization.

LEVEL: The logarithm of the ratio of a quantity to a reference quantity of the same kind. The base of the logarithm, the reference quantity, and the kind of level must be specified.

LOGARITHM: The exponent that indicates the power to which a number must be raised to produce a given number. For example, for the base 10 logarithm, used in acoustics, 2 is the logarithm of 100.