Von Löwen Designs

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.

Acoustical: The properties of a material to absorb or reflect sound.

Acoustical Analysis: A review of a space to determine the level of reverberation or reflected sound in the space (in seconds) influenced by the building materials used to construct the space. Also the amount of acoustical absorption required to reduce reverberation and noise.

Acoustical Consultant: A professional usually with an engineering degree who is experienced in providing advice on acoustical requirements, and noise control in a variety of situations.

Acoustical Environment: See Acoustics 2.

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

Ambient Noise/Sound: Noise level in a space from all sources such as HVAC or extraneous sounds from outside the space. Masking sound or low-level background music can contribute to ambient level of sound or noise.

Anechoic Room: A room, in which the boundaries absorb nearly all the incident sound, thereby, effectively creating free field conditions.

ANSI: The American National Standards Institute.

Architectural Acoustics: The control of noise in a building space to adequately support the communications function within the space and its effect on the occupants. The qualities of the building materials used determine its character with respect to distinct hearing.

Area Effect: Acoustical materials spaced apart can have greater absorption than same amount of material butted together. The increase in efficiency is due to absorption by soft exposed edges and also to diffraction of sound energy around panel perimeters.

Articulation Class: A single number rating used for comparing acoustical ceilings and acoustical screens for speech privacy purposes. AC values increase with increasing privacy and range from approximately 100-250. This classification supersedes Speech Privacy Noise Isolation Class (NIC) rating method.

Articulation Index (AI): A measure of speech intelligibility influenced by Acoustical Environment rated from 0.01 to 1.00.The higher the number the higher the intelligibility of words and sentences understood from 0 – 100%.

Assistive Listening Device: An electronic device that provides amplification of sound to a hearing impaired person. Devices include personal hearing aids, magnetic induction loops, FM radio systems and infrared systems. All have advantages and disadvantages and some may be dependent on good acoustical environment for optimal performance.

Attenuation: The reduction of sound intensity or energy as a function of distance traveled or various means (e.g., air, humidity, porous materials, etc). See Inverse Square Law.

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

Audiogram: A graph showing individual hearing acuity.

Audiometer: An instrument for measuring individual hearing activity.

A-Weighted Sound Level: A measure of sound pressure level designed to reflect the acuity of the human ear, which does not respond equally to all frequencies. The ear is less efficient at low and high frequencies than at medium or speech-range frequencies. Therefore, to describe a sound containing a wide range of frequencies in a manner representative of the ear’s response, it is necessary to reduce the effects of the low and high frequencies with respect to the medium frequencies. The resultant sound level is said to be A-weighted, and the units are dBA. The A-weighted sound level.

Background Noise: The sum total of all noise generated from all direct and reflected sound sources in a space that can represent an interface to good listening and speech intelligibility. (Hearing impaired persons are especially victimized by background noise).

Baffle: A free hanging acoustical sound absorbing unit – Normally suspended vertically in a variety of patterns to introduce absorption into a space to reduce reverberation and noise levels.

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.

Barrier: Anything physical or an environment that interferes with communication or listening. A poor acoustical environment can be a barrier to good listening and especially so for persons with a hearing impairment.

BEL: A measurement of sound intensity named in honor of Alexander Graham Bell; first used to relate intensity to a level corresponding to hearing sensation.

Boominess: Low frequency reflections. In small rooms acoustical panels with air space behind can better help control low frequency reflectivity.

Broadband Noise: Noise with components over a wide range of frequencies.

Calibrator (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.

Cloud: In acoustical industry terms, an acoustical panel suspended in a horizontal position from ceiling/roof structure. Similar to a baffle but in a horizontal position.

Cochlea: A spirally coiled organ located within the inner ear that contains the receptor organs essential to hearing.

Cocktail Party Effect: Sound in a noisy crowded room generated mostly by conversation. Levels rise and fall as people compete with one another to be heard. Perception of speech can be nearly impossible in high levels of noise.

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: In acoustics, the cycle is the complete oscillation of pressure above and below the atmospheric static pressure.

Cycles Per Second: The number of oscillations that occur in the time frame of one second numerically equivalent to hertz. See Frequency. Low frequency sounds have fewer and longer oscillations.

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 vibratory energy in solid media and structures with time or distance. It is analogous to the absorption of sound in air.

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 (dB): Sound level in decibels as a logarithmic ratio. Sound intensity described in decibels. i.e.: Breathing 5 dB, office activity 50 dB, Jet Aircraft during takeoff at 300′ distance 130 dB.

Deflection: The distance an elastic body or spring moves when subjected to a static or dynamic force. Typical units are inches or mm.

Deaf: Loss of auditory sensation with or without use of an assistive listening device. Loss of hearing more severe is generally characterized as ‘Hearing Impaired.’

Diffraction/Diffusion: The scattering or random reflection of a sound wave from a surface. The direction of reflected sound is changed so that listeners may have sensation of sound coming from all directions at equal levels.

Directivity Index: In a given direction from a sound source, the difference in decibels between (a) the sound pressure levels of that source, measured at the same distance.

Doppler Effect/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.

Dosimeter: A device worn by a worker for determining the worker’s accumulated noise exposure with regard to level and time according to a pre-determined integration formula.

Ear: An incredible hearing mechanism consisting of outer, middle and inner ear segments that cause sound pressures to be picked up by the ear that are transmitted through auditory nerves where signals are interpreted by brain as sound.

Echo: Reflected sound producing a distinct repetition of the original sound. Echo in mountains is distinct by reason of distance of travel after original signal has ceased.

Echo Flutter: Short echoes in a small reverberative space that produce a clicking, ringing or hissing sound after the original sound signal has ceased. Flutter echoes may be present in long narrow spaces with parallel walls.

Equal Loudness Contours: Curves represented in graph form as a function of sound level and frequency which listeners perceive as being equally loud. High frequency sounds above 2000 Hz are more annoying. Human hearing is less sensitive to low frequency sound. See also Phon.

Equivalent A-Weighted Sound Level (Leq): The constant sound level that, in a given time period, would convey the same sound energy as the actual time-varying A-weighted sound level.

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.

Flame Spread: Classification indicating propagation of flame across a sample compared to flame propagation across concrete panels and red oak. Results are obtained through an ASTM E84 or UL723 test.

Flanking: The transmission of sound around the perimeter or through holes within partitions (or barriers) that reduces the otherwise obtainable sound transmission loss of a partition. Examples of flanking paths within buildings are ceiling plena above partitions; ductwork, piping, and electrical conduit penetrations through partitions; back-to-back electrical boxes within partitions, window mullions, etc.

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. Sound waves from a source outdoors where there are no obstructions.

Frequency: The number of oscillations or cycles per unit of time. Acoustical frequency is usually expressed in units of Hertz (Hz) where one Hz is equal to one cycle per second.

Frequency Analysis: An analysis of sound to determine the character of the sound by determining the amount of sounds at various frequencies that makes up the overall sound spectrum. i.e.: higher frequency sound or pitch vs. low frequency.

Hair Cell: Sensory cells in the cochlea that transforms the mechanical energy of sound into nerve impulses.

Harmonic: A sinusoidal (pure-tone) component whose frequency is a whole-number multiple of the fundamental frequency of the wave. If a component has a frequency twice that of the fundamental it is called the second harmonic.

Hearing: The subjective human response to sound.

Hearing Impairment: A degree of temporary or permanent hearing loss due to varying causes – Hearing loss can be caused by illness, disease, or by exposure to excessively high noise levels and affects 25 – 50 million people in USA of all ages. Hearing impairment as generally used means a hearing loss of a mild, moderate, or severe degree as opposed to ‘deafness’ which is generally described as little or no residual hearing with or without the aid of an assistive listening device. Hearing Impaired persons are particularly victimized by long reverberation times.

Hearing Level: A measured threshold of hearing at a specified frequency expressed in decibels relative to a specified standard of normal hearing. The deviation in decibels of an individual’s threshold 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 – 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.

Hearing Range: 16-20,000 Hz (Speech Intelligibility) – 600-4800 Hz (Speech Privacy) – 250-2500 Hz (Typical small table radio)

Hearing Threshold Level (HTL): Amount (in decibels) by which an individual’s threshold of audibility differs from a standard audiometric threshold.

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

Impact Insulation Class (IC): 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.

Impulsive Noise: 1. Either a single sound pressure peak (with either a rise time less than 200 milliseconds or total duration less than 200 milliseconds) or multiple sound pressure peaks (with either rise time less than 200 milliseconds or total duration less than 200 milliseconds) spaced at least by 200 millisecond pauses. 2. A sharp sound pressure peak occurring in a short interval of time.

Infrasonic: Sounds of a frequency lower than 20 hertz.

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

Inverse Square Law: A description of the acoustic wave behavior in which the mean-square pressure varies inversely with the square of the distance from the source. The behavior occurs in free field situations, where the sound pressure level decreases 6 dB with each doubling of distance from the source.

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.

Live End/Dead End: An acoustical treatment plan for rooms in which one end is highly absorbent and the other end is reflective and diffusive.

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.

Loudness: The average deviation above and below the static value due to sound wave is called sound pressure. The energy expended during the sound wave vibration is called intensity and is measured in intensity units. Loudness is the physical resonance to sound pressure and intensity.

Loudness Level: Measured in phons it is numerically equal to the median sound pressure level (dB) of a free progressive 1000 Hz wave presented to listeners facing the source, which in a number of trials is judged by the listeners to be equally loud.

Masking: 1. The process by which the threshold of audibility for a sound is raised by the presence of another (masking) sound. 2. The amount by which the threshold of audibility of a sound is raised by the presence of another (masking) sound.

Masking Noise: A noise that is intense enough to render inaudible or unintelligible another sound that is also present.

Mass: The fundamental property of a material relevant to sound transmission loss through that material; generally, the more massive the material, the greater the sound transmission loss.

Medium: A substance carrying a sound wave.

Mounting: Standards established by ASTM to represent typical installation for purpose of testing materials. i.e. a mounting test specimen mounted directly to test room surface. D mounting furred out to produce air space behind.

Near Field: The sound field very near to the source, where the sound pressure does not obey the inverse-square law and the particle velocity is not in phase with the sound pressure.

NIOSH: The National Institute for Occupational Safety and Health.

Noise: 1. Unwanted sound. 2. Any sound not occurring in the natural environment, such as sounds emanating from aircraft, highways, industrial, commercial and residential sources. 3. An erratic, intermittent, or statistically random oscillation.

Noise Criteria (NC): Noise criteria curves used to evaluate existing listening conditions at ear level by measuring sound levels at loudest locations in a room. NC criteria can be referred to equivalent dBA levels. NC curves are critical to persons with hearing loss.

Noise Isolation Class (NIC): A Single number rating of the degree of speech privacy achieved through the use of an Acoustical Ceiling and sound absorbing screens in an open office. NIC has been replaced by the Articulation Class (AC) rating method.

Noise Level: For airborne sound, unless specified to the contrary, it is the A-weighted sound level.

Noise Reduction (NR): The numerical difference, in decibels, of the average sound pressure levels in two areas or rooms after treatment. A measurement of ‘noise reduction’ combines the effect of the sound transmission loss performance of structures separating the two areas or rooms, plus the effect of acoustic absorption present in the receiving room.

Noise Reduction Coefficient (NRC): A measure of the acoustical absorption performance of a material, calculated by averaging its sound absorption coefficients at 250, 500, 1000 and 2000 Hz, expressed to the nearest multiple of 0.05.

Non- Impulsive Noise: All noise not included in the definition of impulsive noise.

Octave: The interval between tow sounds having a frequency ration of two. There are 8 octaves on the keyboard of a standard piano.

Octave Band: A segment of the frequency spectrum separated by an octave. A common standard division is in 10 octave bands identified by their center frequencies 31.5, 63, 125, 250, 500, 1000, 2000, 4000 Hz.

Octave Band Level: The integrated sound pressure level of only those sine-wave components in a specified octave band.

Oscillation: The variation with the time, alternately increasing and decreasing, of (a) some feature of an audible sound, such as the sound pressure, or (b) some feature of a vibrating solid object, such as the displacement of its surface.

OSHA: The Occupational Safety and Health Administration.

Oto: Pertaining to the ear.

Otologist: A doctor specializing in the structure, disorders and treatment of the ear.

Otolaryngolist: A doctor specializing in disorders and treatment of the ear nose and throat disorders.

Peak Sound Pressure: The maximum absolute value of the instantaneous sound pressure in a specific time interval. Note: in case of a periodic wave, if the time interval considered is a complete period, the peak sound pressure becomes identical with the maximum sound pressure.

Period: The duration of time it takes for a periodic wave form (like a sine wave) to repeat itself.

Permanent Threshold Shift (PTS): A permanent decrease of the acuity of the ear at a specified frequency as compared to a previously established reference level. The amount of permanent threshold shift is customarily expressed in decibels.

Phon: The unit of measurement for loudness level.

Pink Noise: Noise with constant energy per octave band width.

Pitch: The attributes of auditory sensation that orders sounds on a scale extending from low to high. Pitch depends primarily upon the frequency of the sound stimulus, but it also depends upon the sound pressure and waveform of the stimulus.

Plane Wave: A wave whose wave fronts are parallel and perpendicular to the direction in which the wave is traveling.

Presbycusis: The decline in hearing acuity that is attributed to the aging process.

Pure Tone: A sound for which the sound pressure is a simple sinusoidal function of the time and characterized by its singleness of pitch.

Random Noise: An oscillation whose instantaneous magnitude is not specified for any given instant of time. It can be described statistically by probability distribution functions giving the traction of the total time that the magnitude of the noise lies within a specified range.

Reflection: The return of a sound wave from a surface.

Refraction: The bending of a sound wave from its original path, either because it is passing from one medium to another or by changes in the physical properties of the medium, e.g. a temperature or wind gradient in the air.

Resonance: The relatively large amplitude of vibration produced when the frequency of some source of sound or vibration ‘matches’ the natural frequency of vibration of some object, component, or system.

Resonator: A device that resounds or vibrates in sympathy with a source of sound or vibration.

Reverberant Field: The region in a room where the reflected sound dominates, as opposed to the region close to the noise source where the direct sound dominates.

Reverberation: The persistence of sound in an enclosed space, as a result of multiple reflections, after the sound source has stopped.

Reverberation Room: A room having a long reverberation time, especially designed to make the sound field inside it as diffuse (homogenous) as possible.

Reverberation Time (RT): The reverberation time of a room is the time taken for the sound pressure level to decrease 60 dB from its steady-state value when the source of sound energy, is suddenly interrupted. It is a measure of the persistence of an impulsive sound in a room as well as of the amount of acoustical absorption present inside the room. Rooms with long reverberation times are called live rooms.

RMS Sound Pressure: The square root of the time-averaged square of the sound pressure.

Root-Mean-Square (RMS): 1. The root-mean-square value of a time-varying quantity is obtained by squaring the function at each instant, obtaining the average of the squared values over the interval of interest, and then taking the square root of this average. For a sine wave, if you multiply the RMS value by the square root of 2, or about 1.41, you get the peak value of the wave. The RMS value, also called the effective value of the sound pressure, is the best measure of ordinary continuous sound, but the peak value is necessary for assessment of impulsive noises. 2. A term describing the mathematical process of determining an ‘average’ value of a complex signal.

Sabin: A unit of sound absorption based of one square foot of material. Baffles are frequently described as providing X number of sabins of absorption based on the size of the panel tested, through the standard range of frequencies 125 – 4000 Hz. The number of sabins developed by other acoustical materials is determined by the amount of material used and its absorption coefficients.

Sabine Formula: A formula developed by Wallace Clement Sabine that allows designers to plan reverberation time in a room in advance of construction and occupancy. Defined and improved empirically the Sabine Formula is T=0.049(V/A) where T = reverberation time or time required (for sound to decay 60 dB after source has stopped) in seconds. V = Volume of room in cubic feet. A = Total square footage of absorption in sabins.

Septum: A thin layer of material between 2 layers of absorptive material. i.e.: foil, lead, steel, etc. that prevents sound wave from piercing through absorptive material.

Shielding: The attenuation of a sound achieved by placing barriers between a sound source and the receiver.

Signal to Noise Ratio: Is the sound level at the listener’s ear of a speaker above the background noise level. The inverse square law impacts on the S/N ratio. Signal to Noise Ratios are important in classrooms and should be in range of 15 to 20 dB.

Smoke Developed Index: Classification that relates to a comparison of smoke development of a particular material compared to concrete panels and red oak. Results are obtained through an ASTM E84 or UL723 test.

Sone: The unit of measurement for loudness. One sone is the loudness of a sound whose loudness level is 40 phons. Loudness is proportional to the sound’s loudness rating, e.g. two sones are twice as loud as one sone.

Sociocusis: Loss of hearing caused by noise exposures that are part of the social environment, exclusive of occupational-noise exposure, physiological changes with age, and disease.

Sound: 1. An oscillation in pressure, stress, particle displacement, particle velocity, etc. in an elastic or partially elastic medium, or the superposition of such propagated alterations. 2. An auditory sensation evoked by the oscillation described above. Not all sound waves can evoke an auditory sensation; e.g. ultrasound.

Sound Absorption: The property possessed by materials, objects and air to convert sound energy into heat. Sound waves reflected by a surface cause a loss of energy, that energy not reflected is called its absorption coefficient.

Sound Absorption Coefficient: The fraction of energy striking a material or object that is not reflected. For instance if a material reflects 70% of the sound energy incident upon its surface, then its Sound Absorption Coefficient would be 0.30.

Sound Barrier: 1. Material that when placed around a source of noise inhibits the transmission of that noise beyond the barrier. 2. Anything physical or an environment that interferes with communication or listening. For example, a poor acoustical environment can be a barrier to good listening and especially so for persons with a hearing impairment.

Sound Level: The weighted sound pressure level obtained by the use of a sound level meter and frequency weighting network, such as A, B, or C as specified in ANSI specifications for sound level meters (ANSI SI.4-1971, or the latest revision). If the frequency weighting employed is not indicated, the A-weighted is implied.

Sound Level Meter: An instrument comprised of a microphone, amplifier, output meter, and frequency weighting networks which is used for the measurement of noise and sound levels.

Sound Power: The total sound energy radiated by a source per unit time. The unit of measurement is the watt.

Sound Pressure: The instantaneous difference between the actual pressure produced by a sound wave and the average or barometric pressure at a given point in space.

Sound Pressure Level (SPL): 20 times the logarithm, to the base 10, of the ratio of the pressure of the sound measure to the reference pressure, which is 20 micronewtons per square meter. In equation form, sound pressure level in units of decibels is expressed as SPL (dB)=20 log p/p.

Soundproofing: Building materials that makes structures impervious to sound or insulates against sound.

Sound Transmission Class (STC): The preferred single figure rating system designed to give an estimate of the sound isolation properties of a structure or a rank order series of structures. This is a rating for doors, windows, enclosures, noise barriers, partitions and other acoustical products. The rating is in terms of their relative ability to provide privacy against intrusion of speech sounds. This is a one number rating system, heavily weighted in the 500Hz to 2000Hz frequency range where speech intelligibility largely occurs.

Sound Transmission Coefficient (STC): The average amount of decibels of sound reflected away from a partition by a particular reflective barrier material.

Sound Transmission Loss (STL): A measure of sound insulation provided by a structural configuration. Expressed in decibels, it is 10 times the logarithm to the base 10 of the reciprocal of the sound transmission coefficient of the configuration.

Spectrum: The description of a sound wave’s resolution into its components of frequency and amplitude.

Speech-Interference Level (SIL): A calculated quantity providing a guide to the interference of a noise with the reception of speech. The speech-interference level is the arithmetic average of the octave band levels of the interfering noise in the most important part of the speech frequency range. The levels in the octave bands centered at 500, 1000 and 2000 Hz are commonly averaged to determine the speech-interference level.

Speed (Velocity) of Sound in Air: 344 m/sec (128 ft/sec) at 70 degrees Fahrenheit in air at sea level.

Spherical Divergence: The condition of propagation of spherical waves relates to the regular decrease in intensity of a spherical sound wave at progressively greater distances from the source. Under this condition the sound pressure level decreases 6 decibels with each doubling of distance from source.

Spherical Wave: A sound wave in which the surfaces of constant phase are concentric spheres. A small (point) source radiating into an open space produces a free sound field of spherical waves.

Steady-State Sounds: Sounds whose average characteristics remain relatively constant in time. A practical example of a steady-state sound source is an air conditioning unit.

Speech Privacy: The degree to which speech is unintelligible between offices. Three ratings are used, Confidential, Normal, (Non obtrusive), Minimal.
Temporary Threshold Shift (TTS): A temporary threshold impairment of hearing acuity as indicated by a change in the threshold of audibility.

Third-Octave Band: A frequency band whose cutoff frequencies have a ratio of 2 to the one-third power, which is approximately 1.26. The cutoff frequencies of 891 Hz and 1112 Hz define the 1000 Hz third-octave band in common use.

Threshold of Audibility/Detectability: The minimum sound pressure level at which a person can hear a specified frequency of sound over a specified number of trials.

Threshold of Pain: The minimum sound pressure level of a sound outside the ear that will produce a transition from discomfort to definite pain.

Threshold Shift: A change in the threshold audibility at a specified frequency from a threshold previously established. The amount of threshold shift is customarily expressed in decibels.

Timbre: An attribute of auditory sensation allowing a subject to judge that two sounds similarly presented and having the same loudness and pitch are dissimilar, e.g. trumpet vs. violin.

Time Weighted Average (TWA): The yardstick used by the Occupational Safety and Health Administration (OSHA) to measure noise levels in the workplace. It is equal to a constant sound level lasting eight hours that would cause the same hearing damage as the variable noises that a worker is actually exposed to. (This hearing loss, of course, occurs over long-term exposures.) Same as LOSHA.

Tinnitus: Ringing in the ears or noise sensed in the head. Onset may be due to an acoustic trauma and persist in the absence of acoustical stimulation (in which case it may indicate a lesion of the auditory system).

Transducer: A device capable of being actuated by waves from one or more transmission systems or media and supplying related waves to one or more other transmission systems or media. Examples are microphones, accelerometers, and loudspeakers.

Ultrasounds/Ultrasonic: Sounds of a frequency higher than 20,000 Hz. The frequency region containing these frequencies is called the ultrasonic region.

Vibration: An oscillatory motion of solid bodies described by displacement, velocity, or acceleration with respect to a given reference point.

Vibration Isolator: A resilient support for vibrating equipment designed to reduce the amount of vibration transmitted to the other structures.

Volume: The cubic space of a room bounded by walls, floors, and ceilings determined by Volume = Length x Width x Height of space. Volume influences reverberation time.

Wave: A disturbance that travels through a medium by virtue of the elastic properties of that medium.

Wavelength: For a periodic wave (such as a sound in air), the distance between analogous points on any two successive waves. The wavelength of sound in air or in water is inversely proportional to the frequency of the sound; thus, the lower the frequency, the longer the wavelength.

Weighting: Prescribed frequency filtering provided in a sound level meter.

White Noise: Noise with energy is uniform over wide range of frequencies, being analogous in spectrum characteristics to white light.

Windscreen: A porous device used to cover the microphone of a sound level measurement system that is designed to minimize the effects of winds and wind gusts on the sound levels being measured. Typically made of open cell polyurethane foam and spherically shaped.

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