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Terms & Definitions (cont)


[L2]---the unit of measure of sound absorption in the inch-pound system.

Sabine: The originator of the Sabine reverberation equation.

Self-extinguishing: A material which will not support combustion when external source of flame is removed.

Self-noise, n: Extraneous non-acoustical signals, generated or induced in a measurement system.

Signal-to-noise (SN) ratio: The range or distance between the noise floor (the noise level of the equipment itself) and the music signal.

Sine wave: A periodic wave related to simple harmonic motion.

Slap back: A discrete reflection from a nearby surface.

Smoke developed: Measure of smoke density developed by a material when compared with red oak, which has a smoke density index (SDI) of 100 in accordance with ASTM E 84. The infill of IAC sound absorptive metal panels has an SDI of 20.

Soft room: Room with highly sound absorptive surfaces.

Sone: The unit of measurement for subjective loudness.

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 possessed by materials, objects and structures such as rooms of absorbing sound energy. (3) A: [L2]; metric sabin---in a specified frequency band, the measure of the magnitude of the absorptive property of a material, an object, or a structure such as a room.

Sound absorption coefficient, *: [dimensionless]; metric sabin/m* ---of a surface, in a specified frequency band, the measure of the absorptive property of a material as approximated by the method of Test Method C423. Ideally, the fraction of the randomly incident sound power absorbed or otherwise not reflected.

Sound attenuation: The reduction of the intensity of sound as it travels from the source to a receiving location. Sound absorption is often involved as, for instance, in a lined duct. Spherical spreading and scattering are other attenuation mechanisms.

Sound energy, E: [ML2T-2]; J-energy added to an elastic medium by the presence of sound, consisting of potential energy in the form of deviations from static pressure and of kinetic energy in the form of particle velocity.

Sound energy density, D: [ML-1T-2]; J/m---the quotient obtained when the sound energy in a region is divided by the volume of the region. The sound energy density at a point is the limit of that quotient as the volume that contains the point approaches zero.

Sound insulation: The capacity of a structure to prevent sound from reaching a receiving location. Sound energy is not necessarily absorbed; impedance mismatch, or reflection back toward the source, is often the principal mechanism.

Sound intensity, I: [MT-3]; W/m2 the quotient obtained when the average rate of energy flow in a specified direction and sense is divided by the area, perpendicular to that direction, through or toward which it flows. The intensity at a point is the limit of that quotient as the area that includes the point approaches zero.

Sound isolation: The degree of acoustical separation between two locations, especially adjacent rooms.

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

Sound power, W: [ML2T-3]; W---in a specified frequency band, the rate at which acoustic energy is radiated from a source. In general, the rate of flow of sound energy, whether from a source, through an area, or into an absorber.

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, p: [ML-1T-2]; Pa-a fluctuating pressure superimposed on the static pressure by the presence of sound. In analogy with alternating voltage its magnitude can be expressed in several ways, such as instantaneous sound pressure or peak sound pressure, but the unqualified term means root-mean-square sound pressure. In air, the static pressure is barometric pressure.

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

Sound receiver: One or more observation points at which sound is evaluated or measured. The effect of sound on an individual receiver is usually evaluated by measurements near the ear or close to the body.

Sound spectrograph: An instrument that displays the time, level, and frequency of a signal.

Soundstage: A listening term that refers to the placement of a stereo image in a fashion that replicates the original performance. A realistic soundstage has proportional width, depth and height.

Sound transmission class, STC: A single-number rating, calculated in accordance with Classification E 413 using values of sound transmission loss. It provides an estimate of the performance of a partition in certain common sound insulation problems. A single number rating that indicates the sound transmission loss of a partition or ceiling system between adjacent closed rooms, abbreviated STC.

Sound transmission coefficient, r: [dimensionless]---of a partition, in a specified frequency band, the fraction of the airborne sound power incident on the partition that is transmitted by the partition and radiated on the other side.

Sound transmission loss, TL: Of a partition, in a specified frequency band, ten times the common logarithm of the ratio of the airborne sound power incident on the partition to the sound power transmitted by the partition and radiated on the other side. The quantity so obtained is expressed in decibels. The reduction in sound level when sound passes through a partition or ceiling system.

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).

Source room: In architectural acoustical measurements, the room that contains the noise source or sources.

Speaker level: Taken from the speaker terminals. This signal has already been amplified.

Specific airflow resistance, r: [ML-2T-1]. Mks rayl (Pa-s/m)---the product of the airflow resistance of a specimen and its area. This is equivalent to the quotient of the air pressure difference across the specimen divided by the linear velocity, measured outside the specimen, of airflow through the specimen.

Spectral balance: Balance across the entire frequency spectrum of the audio range.

Spectrum: the distribution of the energy of a signal with frequency.

Spectrum analyzer: An instrument for measuring, and usually recording, the spectrum of a signal.

Specular reflections: Mirrorlike reflections of sound (angle of incidence equals angle of reflection) from a flat surface. Reflections that do not spread out.

Speech intelligibility: A measure of sound clarity that indicates the ease of understanding speech. It is a complex function of psychoacoustics, signal-to-noise ratio of the sound source, and direct-to-reverberant energy within the listening environment.

Speech transmission index, STI: A single number that indicates the effect of a transmission system on speech intelligibility.

Spherical divergence: Sound diverges spherically from a point source in free space.

Splaying: Walls are splayed when they are constructed somewhat "off square," i.e., a few degrees from the normal rectilinear form.

Standing wave: A resonance condition in an enclosed space in which sound waves traveling in one direction interact with those traveling in the opposite direction, resulting in a stable condition.

Steady-state: A condition devoid of transient effects.

Stereo: From the Greek meaning solid. The purpose of stereo is not to give you separate right and left channels, but to provide the illusion of a three-dimensional, holographic image between the speakers.

Structureborne noise: Generation and propagation of time-dependent motions and forces in solid materials which result in unwanted radiated sound.

Stud: An upright 2 x 4 or 2 x 6 wall framing member.

Subwoofer: A speaker designed exclusively for low-frequency reproduction. A true subwoofer should be able to at least reach into the bottom octave (20-40Hz). There are many "subwoofers" on the market that would be more accurately termed "woofers".

Superposition: Many sound waves may transverse the same point in space, the air molecules responding to the vector sum of the demands of the different waves.

T60: See RT60.

Tangential mode: A room mode produced by reflections off four of the six surfaces of the room.

TDS: Time-delay spectrometry.

TEF: Time, energy, frequency.

Threshold of feeling (pain): The sound pressure level that makes the ears tickle, located about 120 dB above the threshold of hearing.

Threshold of hearing: The lowest level sound that can be perceived by the human auditory system. This is close to the standard reference level of sound pressure, 20uPA.

THX: Refers to a series of specifications for surround sound systems. Professional THX is used in commercial movie theaters. Home THX specifications are not published and manufacturers must sign non-disclosure waivers before submitting their products for THX certification. Manufacturers that receive certification for their products must pay a royalty on units sold.

Timbral: Refers to the overall frequency balance of a system. In a perfect world, all systems would have complete tonal neutrality. With current technology, this ideal is approached but not met. Listening to many equally "good" speakers will reveal that some sound warmer than others, some sound brighter etc. In a surround sound system it is important that all speakers have a close timbral match for the highest degree of sonic realism.

Timbre: The quality of a sound that distinguishes it from other sounds of the same pitch and volume. The distinctive tone of an instrument or a singing voice.

Time-delay spectrometry: A sophisticated method for obtaining anechoic results in echoic spaces.

Tonal:, adj.: In reference to audible sound, capable of exciting an auditory sensation having pitch.

Tone: A tone results in an auditory sensation of pitch.

Tone burst: A short signal used in acoustical measurements to make possible differentiating desired signals from spurious reflections.

Total harmonic distortion (THD): Refers to a device adding harmonics that were not in the original signal. For example: a device that is fed a 20Hz sine wave that is also putting out 40Hz, 80Hz, etc. Not usually a factor in most modern electronics, but still a significant design problem in loudspeakers.

Transient response: The ability of a component to respond quickly and accurately to transients. Transient response affects reproduction of the attack and decay characteristics of a sound.

Transients: Instantaneous changes in dynamics, producing steep wave fronts.

Transparency: Listening term. An analog that can be best "pictured" in photography. The more "transparent" the sound, the clearer the auditory picture.

Treble: The higher frequencies of the audible spectrum.

Tuning frequency: The helmholtz resonant frequency of a box. Also refers to the resonant frequency of other types of systems.

Vibration isolation: A reduction, attained by the use of a resilient coupling, in the capacity of a system to vibrate in response to mechanical excitation.

Volume: Colloquial equivalent of sound level.

Warmth: A listening term. The opposite of cool or cold. In terms of frequency, generally considered the range from approx. 150Hz-400Hz. A system with the "proper" warmth will sound natural within this range.

Watt: The unit of electrical or acoustical power.

Wattage: Is the unit of power used to rate the output of audio amplifiers. For a wattage number to have meaning the distortion level and impedance must also be specified.

Wave: A regular variation of an electrical signal or acoustical pressure.

Wavelength: The distance the sound wave travels to complete one cycle. The distance between one peak or crest of a sine wave and the next corresponding peak or crest. The wavelenth of any frequency may be found by dividing the speed of sound by the frequency. (speed of sound at sea level is 331.4 meters/second or 1087.42 feet/second).

Weighting: Adjustment of sound-level meter response to achieve a desired measurement.

White noise (ANS): Noise with a continuous frequency spectrum and with equal power per unit bandwidth. For example, equal power in any band of 100-Hz width.

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