MEMTECH ACOUSTICAL’S Glossary of Acoustical Terms

This glossary of terms has been provided with a brief description, for the most part in non-technical terms in an effort to remove some of the mystery surrounding Acoustics. While the explanations may not be totally correct in their literal interpretations, it is hoped that the plain language approach will provide a better understanding of the terminology frequently used in the field of acoustics.

ABSORPTION: The properties of a material composition to convert sound energy into heat thereby reducing the amount of energy that can be reflected.

ABSORPTION COEFFICIENT: The absorption coefficient of a material or the sound absorbing device is the ratio of the sound absorbed to the sound incident on the material or device.

ACOUSTICAL: The properties of a material to absorb or reflect Sound (adjective) Acoustically, (Adverb).

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: The acoustical characteristics of a space or room influenced by the amount of acoustical absorption, or lack of it in the space.

ACOUSTICAL MATERIAL: A material used to alter a sound field. The material may be used to absorb, amp or block acoustical energy.

ACOUSTICS:  The science of Sound. Its production, transmission and effects. The branch of physics that treats the phenomena and laws of sounds as it effects people

AIRBORNE NOISE: Noise that arrives at a point of interest by propagation through the air.

AIRBORNE SOUND: Sound that reaches the point of interest by propagation through the 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 test chamber which has a lining of absorbent acoustical materials to eliminate all sound reflections. It is most often used to determine the sound radiation characteristics of equipment.

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. The 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. Device 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 energy as a function of distance traveled. (See also Inverse Square Law).

AUDIOMETER: An instrument for measuring hearing acuity.

A WEIGHTING: An electronic filtering system in a sound meter that allows meter to largely ignore lower frequency sounds in a similar fashion to the way our ears do.

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.

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 person 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.

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 snail shaped mechanism in the inner ear that contain hair cells of basilar membrane that vibrate to aid in frequency recognition.

COCKTAIL PARTY EFFECT: Sound in a noisy crowed 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.

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. (See FREQUENCY) Low frequency sounds have fewer and longer oscillations.

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.

DEAF: Loss of auditory sensation with or without use of assistive listening device. Loss of hearing more severe than is generally characterized as “Hearing Impaired”.

DECIBEL (dB): Sound level in decibels as a logarithmic ratio. Sound intensity described in decibels, i.e.:

1. Breathing – 5 dB
2. Office Activity – 50 dB
3. 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

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

DISSIPATIVE SILENCER: A device inserted into an air-duct or opening to reduce noise transmitted through the duct or opening. Noise reduction is accomplished through the use of internal sound absorbing materials.

FREQUENCY SPECTRUM: A graph or plot of the sound pressure level in each band from a set of octaves or 1/3 octave bands.

INSERTION LOSS: The reduction of sound power level attained by inserting a silencer muffler in an acoustic transmission system.

MASS LAW: A rule for estimating the transmission loss of a barrier in its mass control region. The rule states that transmission loss increase/decreases 6dB for each doubling/halving of either frequency or barrier surface density.

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 the 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 spaces 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.)

FIELD SOUND TRANSMISSION CLASS (FSTC): A sound transmission rating obtained under “real-life” conditions. The general method to obtain this rating is almost the same as the method used in laboratory conditions. Procedures, however, have been added to take into account the differences between field conditions and laboratory conditions (e.g. flanking paths, absorption, modal distribution, etc.)

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 other wise obtainable sound transmission loss of a partition. Examples of flanking paths within buildings are ceiling plena above partitions; duct-work, piping and electrical conduit penetrations through partitions; back-to-back electrical boxes within partitions, window mullions, etc.

FREE FIELD: 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 make up the overall sound spectrum. i.e.: Higher Frequency Sound or Pitch vs Low Frequency.

HEARING (Hz): Frequency of sound expressed by cycles per second. (See CYCLE)

HEARING IMPAIRMENT: A degree of hearing loss, temporary or permanent due to many causes. Hearing loss can be caused by illness, disease or by exposure to excessively high noise levels. 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 apposed 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 RANGE:

1. 16 – 20000 Hz (Speech Intelligibility)
2. 600 – 4800 Hz (Speech Privacy)
3. 250 – 2500 Hz (Typical Small Table Radio)

IMPACT ISOLATION CLASS (IIC): The methods to measure the degree of impact noise isolation provided by a floor/ceiling assembly, in laboratory conditions are described in the ASTM E492 or ISO 140/6 standards. For field measurements refer to ASTM E1007 of ISO 140/7. The impacts for these measurements are produced by the “Standard Tapping Machine” an electrically operated mechanism consisting of five 0.5 kg hammers which fall regularly and freely onto floor surface from 40mm height at a rate of 10 impacts/second. The sound pressure levels generated in the room directly below the floor/ceiling assembly undergoing testing are then measured for each of the 16 third-octave-bands between 100 Hz and 3150 Hz and they are normalized according to:

1. An absorption equal to 10 metric Sabins or
2. A reverberation time of 0.5 seconds (ISO 140/7)

The Normalized Impact Sound Pressure Levels (NISPL) are then plotted on a standard graph

The IIC rating of the tested floor/ceiling assemblers determined by sliding the classification curve on the graph representing the normalized sound pressure levels, until the following conditions described in the ASTM E989 (ISO 717/2) standards are met:

1. The sum of the deviations above the normalizing curve should not exceed 32 dB
2. The maximum deviation above the normalizing curve should not exceed 8 dB (see previous note on the classification of the isolation of airborne noise according to the ISO standard)

When the IIC contour is positioned in such a way that these two requirements are satisfied the Impact Isolation Class (IIC) can be obtained by reading the normalized impact sound pressure level at the intersection of the IIC contour frequencies of 500 Hz and by subtracting this value from the number 110.

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, or ceiling) of a building.

INTENSITY: Another word for loudness

INVERSE SQUARE LAW: Sound levels fall off with distance traveled. Sound level drops off 6 dB from source point for every doubling of distance.

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.

LOUDNESS: The average deviation above and below the static value due to sound wave is called 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.

MASKING: The process by which the threshold of hearing of one sound is raised due to the presence of another.

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.

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.

NOISE: Unwanted sound that is annoying or interferes with listening. Not all noise needs to be excessively loud to represent an annoyance or interference.

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.

1. RT = Reverberation Time in the Receiving Room

NOISE REDUCTION (NR): The amount of noise that is reduced through the introduction of sound absorbing materials. The level (in decibels) of sound reduced on a logarithmic basis.

NOISE REDUCTION COEFFICIENT (NRC): The NRC of an acoustical material is the arithmetic average to the nearest multiple of 0.05 of its absorption coefficients at 4 one third octave bands with center frequencies of 250, 500, 1000, 2000 Hertz.

NORMALIZED NOISE ISOLATION CLASS (NNIC): The Normalized Noise Isolation Class (NNIC) rating is obtained by applying the ASTM E413 classification standards to the Normalized Noise Reduction (NNR) values measured on a partition.

NORMALIZED NOISE REDUCTION (NNR): Noise Reduction Normalized as a function of a 0.5 second reverberation time in the receiving room. Where:

2. NNR = NR + 10 log (RT/0.5)
3. NNR = Normalized Noise Reduction
4. NR = Noise Reduction

OCTAVE: A pitch interval of 2 to 1. The tone whose frequency is twice that of a given tone.

OCTAVE BANDS: Sounds that contain energy over a wide range of frequencies are divided into sections called bands. A common standard division is in 10 octave bands identified by their center frequencies 31.5, 63, 125, 250, 500, 1000, 2000, 4000 Hz.

OTO: Pertaining to the ear.

OTOLOGIST: A doctor specializing in the structure, disorders and treatments of the ear.

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

PHON: Loudness contours. A subjective impression of equal loudness by listeners as a function of frequency and sound level (dB). An increase in low frequency sound will be perceived as being much louder than an equivalent high frequency increase.

PITCH: The perceived auditory sensation of sounds expressed in terms of high or low frequency stimulus of the sound.

PRESBYCUSIS: The loss of hearing due primarily to the aging process. High frequency loss is frequently a result of early hearing loss.

REFLECTION: The amount of sound wave energy (sound) that is reflected off a surface. Hard non-porous surfaces reflect more sound than soft-porous surfaces. Some sound reflection can enhance the quality of signal of speech and music. (See Echo).

RESONANCE: The emphasis of sound at a particular frequency.

RESONANT FREQUENCY: A frequency at which resonance exists.

RADIATION: The process whereby structure borne vibration is converted into airborne sound.

REVERBERATION ROOM: A test chamber so designed that the reverberant sound filed within the room has an intensity that is approximately the same in all directions and every point. It is commonly used to measure sound absorption.

REVERBERATION: The time taken for sound to decay 60 dB to 1/1,000,000 of its original sound level after the sound source has stopped. Sound after it has ended will continue to reflect off surfaces until the wave loses enough energy by absorption to eventually die out. Reverberation time is the basic acoustical property of a room which depends only on its dimensions and the absorptive properties of its surfaces and contents. Reverberation has an important impact on speech intelligibility.

REVERBERATION TIME: Sound after it is ended at the source will continue to reflect off surfaces into the sound wave loses energy by absorption to eventually die out.

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 125-4000Hz. The number of sabins developed by other acoustical materials are determined by the amount of material used and its absorption coefficients.

SABIN 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.

SIGNAL TO NOISE RATIO: is the sound level at the listeners 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 is a particular material compared to concrete panels and red oak. Results are obtained through an ASTM E84 or UL723 test.

SOUND: Sound is an oscillation in pressure, stress particle displacement, particle velocity in a medium – in room temperature. (In air speed of sound is 1125’/second or one mile in 5 seconds.) Sound produces an auditory sensation caused by the oscillation.

SOUND ABSORPTION: The property possessed by materials, objects and air to convert sound energy in to heat. Sound waves reflected by a surface causes 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 it surface, then its Sound Absorption Coefficient would be 0.30.

SOUND BARRIER: A material that when placed around a source of noise inhibits the transmission of that noise beyond the barrier. Also, 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: A subjective measure of sound expressed in decibels as a comparison corresponding to familiar sounds experienced in a variety of situations.

SOUND LEVEL METER: A device that converts sound pressure variations in air into corresponding electronic signals. The signals are filtered to exclude signals outside frequencies desired.

SOUND POWER LEVEL (Lw): A measure of the total airborne acoustic power generated by a noise source, expressed on a decibel scale referenced to some standard (usually 10-12 watts)

SOUND PRESSURE: The total instantaneous pressure at a point in space, in the presence of a sound wave, minus the static pressure at that point.

SOUND PRESSURE LEVEL: The sound pressure level, in decibels, of a sound is 20 times the logarithm to the base 10 of the ratio of the sound pressure to the reference pressure shall be explicitly stated and is defined by standards.

SOUNDPROOFING: Building materials that make structures impervious to sound or insulates against sound.

SOUND TRANSMISSION CLASS (STC): A single-number rating obtained by classifying the measured values of Sound Transmission Loss in accordance with ASTM Standard E413, “Classification for Sound Rating Insulations”. It provides a quick indication of the performance of a partition for certain common sound insulation problems.

SPEECH: The act of speaking. Communication of thoughts and feelings by spoken words.

SPEECH PRIVACY: The degree to which speech is unintelligible between offices. Three ratings are used, Confidential, Normal (Non obtrusive), and Minimal.

SPL: SOUND PRESSURE LEVEL: Quantity used to describe the loudness of a sound. The sound pressure level is expressed in decibels and is measured with a sound level meter. For example, a conversation between two people inside an average size room will produce an average “A” weighted sound pressure level of 50 to 55 lb.

SPEECH INTELLIGIBILITY: The ability of a listener to hear and correctly interpret verbal messages. In a classroom high ceilings and hard parallel surfaces such as glass and tile, speech intelligibility is a particular problem. Sound bounces off walls, ceilings and floors, distorting the teacher’s instructions and interfering with student’s ability to comprehend. Centers between 250 and 4000 hertz.

SPECTRUM: The description of a sound waves components of frequency and amplitude.

STRUCTURE-BORNE NOISE: Mechanical vibration in a structure which can ultimately become audible sound. Until such time as radiation occurs, these vibrations are inaudible and of little concern.

TIME WEIGHTED AVERAGE (TWA): The yardstick used by 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.

TRANSMISSION LOSS (TL): The reduction in sound power that is caused by placing a wall or barrier between the source and receiver. Transmission loss is expresses in decibels.

ULTRASOUNDS: Sounds of a frequency higher than 20KHz. The frequency region containing these frequencies is called the ultrasonic region.

VIBRATION: Vibration is a force which oscillates about some specified reference point. Vibration is commonly expressed in terms of frequency such as cycles per second (cps), Hertz (Hz), cycles per minute (cpm) or rpm and strokes per minutes (spm). This is the number of oscillations which occur in that time period. The amplitude is the magnitude or distance traveled of the force.

VIBRATION ISOLATION: A resilient support that tends to isolate a mechanical system from a steady state excitatory.

VOLUME: The cubic spec of a room bounded by walls, floors, and ceilings determined by the Volume = length x width x height of space. Volume influences time.

WAVELENGTH: Sound that passes through air produces a wavelike motion of compression and refraction. Wavelength is the distance between two identical positions in the cycle or wave. Similar ripples or waves produced by dropping a stone in water. Low frequency equals longer wavelengths.