Acoustic treatmentUsed in architectural acoustics to isolate noise or vibration and to correct acoustical faults in spaces by addition of absorption devices, reflectors or other devices, sometimes including electronic systems.
AcousticalThe properties of a material to absorb or reflect Sound (adjective) Acoustically, (Adverb).
Acoustical AnalysisA 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 EnvironmentThe acoustical characteristics of a space or room influenced by the amount of acoustical absorption, or lack of it in the space.
AcousticsThe 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 SoundSound that reaches the point of interest by propagation through air.
Airborne Sound Insulation Index : Ia''Former name for Weighted Apparent Sound Reduction Index : R'w
Ambient noiseBackground or general noise level characteristic of a location, often used in comparison with a specific noise source. The metric most often used in the United Kingdom to describe this is the sound pressure level in dB(A) exceeded for 90% of the time, i.e L90, although L95, or even L99 are used as the measure of background in some regions.
Anechoic chamberA room designed to suppress internal sound reflections. Used for acoustical measurements. Because there are so few reflections, any sound will come from one direction only, it is used in microphone directivity measurements.
ANSIThe American National Standards Institute. They set USA standards, that in acoustics are usually VERY different to the International (IEC) standards and are often incompatible. The ANSI sound level meter standard is ANSI S1.4-1983 (R2006). ANSI standards can be bought online at http://webstore.ansi.org
Apparent Sound Reduction Index : R'Field measurements of Sound Reduction Index include Flanking and any other 'on-site' acoustic limitations.
R' = D + 10 lg S/A (dB)
D = Level Difference
S = area of the test specimen (m2)
A = Equivalent Sound Absorption area of the receiving room
AttenuateTo reduce the level of an acoustical signal.
Background noiseThe 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).
Coincidence effectMass Law provides a good working rule to predict the airborne Sound Insulation of a partition up to the region of the Critical Frequency and the coincidence effect. The coincidence effect occurs when the Wavelength of the sound in air is the same as the bending waves in the partition.
At a certain frequency and angle of incidence, the bending oscillation of the partition will be amplified and the Acoustic Energy will be transmitted through the partition almost without attenuation.
Critical FrequencyThe lowest frequency at which the Coincidence Effect occurs in a partition is obtained when the incident sound waves graze the partition (parallel with it). This frequency is called the critical frequency in building acoustics.
CycleIn acoustics, the cycle is the complete oscillation of pressure above and below the atmospheric static pressure.
Cycles per secondThe number of oscillations that occur in the time frame of one second. (See FREQUENCY.) Low frequency sounds have fewer and longer oscillations.
DampingThe dissipation of vibratory energy in solid media and structures with time or distance. It is analogous to the absorption of sound in air.
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
DeflectionThe distance an elastic body or spring moves when subjected to a static or dynamic force. Typical units are inches or mm.
DiffusionThe scattering or random reflection of a sound wave from a surface. The directions of reflected sound are changed so that listeners may have sensation of sound coming from all directions at equal levels.
EarAn 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.
EchoReflected 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 FlutterShort 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.
Flanking transmissionFlanking is the transmission of sound from a source room to a receiving room by paths other than through the separating partition. For example, impact sound may be transmitted from one room to another through a common timber floor. Other common mechanisms for flanking transmission include suspended ceilings, pipework, ducting, etc.
Flanking sound is always present, except in the 'ideal' acoustics laboratory. In practice the sound insulation is often limited by the flanking transmission.
FrequencyThe 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.
Helmholtz resonator – A reactive, tuned, sound absorber. A bottle is such a resonator. Many good sound calibrators incorporate a Helmholtz resonator, to increase their equivalent volume. Named after Hermann von Helmholtz a German physicist.
Hearing range16-20000 Hz (Speech Intelligibility)
600-4800 Hz (Speech Privacy)
250-2500 Hz (Typical Small Table Radio)
Hertz (Hz)Frequency of sound expressed by cycles per second.
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 E 492 or ISO 140/6 standards. For field measurements, refer to ASTM E 1007 or 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 the floor surface from a 40 mm 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:
An absorption equal to 10 metric Sabins, or
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 E 989 (ISO 717/2) standards, are met:
The sum of the deviations above the normalizing curve should not exceed 32 dB.
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 SoundThe 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.
Impact Sound InsulationIs expressed by a single value Ln,w or L' n,w
ISOThe International Organization for Standardization. They are a similar organisation to IEC, but ISO sets standards for measurements methods NOT for the instrument. They are available from www.iso.org/
Level difference : DAirborne sound insulation - field measurements. The difference in the space and time averaged Sound Pressure Levels.
D = L1 - L2
L1 = average Sound Pressure Level in the source room
L2 = average Sound Pressure Level in the receiving room
Ln : Normalized Impact Sound Pressure Level : laboratory measurement.
L'n : Normalized Impact Sound Pressure Level : field measurement.
LnT : Standardized Impact Sound Pressure Level : laboratory measurement.
L'nT : Standardized Impact Sound Pressure Level : field measurement.
Ln,w : Weighted Normalized Impact Sound Pressure Level : laboratory measurement.
LnT,w : Weighted Standardized Impact Sound Pressure Level : based on laboratory measurement of LnT.
L'nT,w : Weighted Standardized Impact Sound Pressure Level : based on field measurement of L'nT.
Mass LawA doubling in Mass or Frequency results in a 6 dB increase in the sound insulation of a single leaf partition over a defined frequency range.
Mass Law provides a good working rule to predict the airborne sound insulation of a partition up to the region of the Critical Frequency and the Coincidence Effect
ModeA room resonance. Axial modes are associated with pairs of parallel walls. Tangential modes involve four room surfaces and oblique modes all six surfaces. Their effect is greatest at low frequencies and for small rooms.
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 impact sound pressure level : L'nField measurement.
The Normalization formulae for Ln directly above also applies for L'n.
Normalized impact sound pressure level : LnLaboratory measurement.
Ln = Li + 10 lg A/A0 dB
A = measured Equivalent Sound Absorption area in the receiving room
A0 = Reference Absorption area.
In all cases where it is uncertain whether results are obtained without flanking transmission the normalized impact sound pressure level should be denoted by L'n.
Normalized level difference : DnAirborne sound transmission. The sound insulation index measured under field conditions, between 'real' rooms and therefore includes effects due to Flanking, different room sizes and other on-site considerations.
Dn = D - 10 lg A/A0
D = level difference in dB
A = Equivalent Sound Absorption area of the receiving room in square meters
A0 = Reference Absorption area in square meters (10 m²)
R : Sound Reduction Index : laboratory measurement.
R' : Apparent Sound Reduction Index : field measurement.
Rw : Weighted Sound Reduction Index : laboratory measurement.
R'w : Weighted Apparent Sound Reduction Index : field measurement.
Certified Sound Insulation Test Equipment
Pink noiseA noise signal whose spectrum level decreases at 3dB per octave rate. This gives the noise equal energy per octave and is used to test many acoustic devices.
Porous absorber – Sound absorbing finish where the sound energy falling on it is dissipated by viscous losses within the pores of the material and converted to heat.
ReflectionThe 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 quality of signal of speech and music. (See Echo).
ResonanceThe emphasis of sound at a particular frequency.
Resonant frequencyA frequency at which resonance exists.
ReverberationThe 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 timeSound after it is ended at the source will continue to reflect off surfaces until the sound wave loses energy by absorption to eventually die out.
Sound absorption coefficient – Of a surface or material at a given frequency and under specified conditions : the complement of the sound energy reflection coefficient under those conditions, i.e., it is equal to 1 minus the sound energy reflection coefficient of the surface or material.
Sound insulationIs the ability of building elements or structures to reduce sound transmission
To compare sound insulation properties you need to take into account the area of the dividing partition/wall, as well as the volume and sound absorption properties of the receiving room. To do this, measurements are Normalized to a Reference Absorption value or Standardized Reverberation Time.
Absorption and Reverberation Time are mathematically related so if the reverberation time is measured in the receiving room then both procedures are catered for.
A single number to present the results and compare products would be useful, this is where the Weighted term comes in.
The sound insulation is measured at different frequencies, normally 100-3150 Hz.
Airborne sound insulation is expressed by a single value, Dn,t,w, Rw or R'w.
Impact sound insulation is expressed by a single value Ln,w or L' n,w
Sound reduction index : RThe measured quantity which characterizes the sound insulating properties of a material or building element in a stated frequency band - laboratory measurement.
R = L1 - L2 + 10 lg S/A (dB)
L1: average Sound Pressure Level in the source room
L2: average sound pressure level in the receiving room
S: area of the test specimen (m2)
A: Equivalent Sound Absorption area of the receiving room
Sound transmission class (STC)A single-number rating obtained by classifying the measured values of Sound Transmission Loss in accordance with ASTM Standard E 413, “Classification for Sound Rating Insulations”. It provides a quick indication of the performance of a partition for certain common sound insulation problems.
To determine the Sound Transmission Class (STC) in conformance to the ASTM E 413 (lSO 71 7/1) one must slide the STC contour along its Y-axis of the graph on which the transmission loss curve is plotted until the following conditions are met:
The sum of the deviation below the STC contour does not exceed 32 dB.
No deviation below the STC contour exceeds 8 dB.
Note: The ISO standard excludes this last requirement. One should indicate however in the test report, the frequencies at which a difference of 8 dB or more occurs between the noise reduction curve and the STC contour.
When the STC contour is positioned in such a way that these two requirements are satisfied the sound transmission class can be obtained by reading the transmission loss value at the intersection of the STC contour at the frequency of 500 Hz. This value corresponds to the STC of the partition.
Sound transmission class (STC)American single number rating of a partition's isolation value based on laboratory measurement. Results may not be compatible with Rw as a different range of frequencies is used.
SoundproofingBuilding materials that make structures impervious to sound or insulate against sound.
Spatial averagingTaking measurements at various positions and averaging the results. Mandatory in sound insulation measurements and recommended anywhere multiple reflections are present. See other types of Averaging.
Spectrum Adaptation Terms : C and Ctr :The single number rating method defined in BS EN ISO 717 uses a standard reference curve to determine the weighted value of airborne sound insulation.
The spectrum adaptation terms C and Ctr may be used to take into account different source spectra as indicated in the standard.
C is an A-weighted Pink Noise spectrum.
Ctr is an A-weighted urban traffic noise spectrum.
Ctr can also be added to DnT,w or Rw to take into account low frequency noise
SPL: sound pressure levelQuantity 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.
Standardized Impact Sound Pressure Level : LnTThe impact Sound Pressure Level in a stated frequency band, corrected for the standardized reverberation time of 0.5 seconds. Laboratory measurement.
LnT = Li - 10 lg T/T0 dB
T = measured Reverberation Time in seconds
T0 for dwellings = 0.5 seconds.
Standardized Impact Sound Pressure Level : L'nTThe impact Sound Pressure Level in a stated frequency band, corrected for the standardized reverberation time of 0.5 seconds. Field measurement, written L'nT to differentiate between LnT
Standardized Level Difference : DnTAirborne sound transmission. Similar to the Dn, but this index corrects the measured difference to a standardized reverberation time of 0.5 seconds. This RT value is often cited as approximately average for a medium sized, carpeted and furnished living room. It does not require detailed and accurate knowledge of the dimensions of the test rooms.
DnT = D + 10 lg T/T0
D = level difference
T = reverberation time in the receiving room
T0 = reference Reverberation Time, 0.5 seconds for dwellings.
Structure born noiseNoise that arrives at a point of interest by propagation through a solid structure.
VibrationA 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 Minute (spm). This is the number of oscillations which occur in that time period. The amplitude is the magnitude or distance of travel of the force.
Vibration isolatorA resilient support that tends to isolate a mechanical system from steady state excitation.
VolumeThe Cubic space of a room bounded by walls, floors, and ceilings determined by Volume = Length × Width × Height of space. Volume influences reverberation time.
WavelengthSound that passes through air it produces a wavelike motion of compression and Parefaction. Wavelength is the distance between two identical positions in the cycle or wave. Similar to ripples or waves produced by dropping two stones in water. Length of sound wave varies with frequency. Low frequency equals longer wavelengths.
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