Airborne Sound

Sound in the air is generated by a material vibrating which in turn causes air molecules to vibrate and create a sound wave. The table below illustrates common sound sources with typical levels in order of increasing sound levels in decibels.

The sound levels shown for occupied rooms are only example activity levels and do not represent criteria for design. Note also that thresholds vary among individuals. All of the ratings are taken while standing near the sound unless specified.

Sound Pressure Level in dB(A) Description
140 Jet Aircraft 50m away
130 Threshold of Pain
120 Threshold of Discomfort
110 Rock Concert
100 Jackhammer
90 Heavy Truck
80 Medium Truck
70 Passenger Car
60 Normal Conversation
50 Suburban Residential Neighborhood
40 Quiet Living Room
30 Quiet Rural Setting
20 Whisper
10 Rustling Leaves in the distance
0 Threshold of Hearing



Science of sound including its production, transmission, and effects.

Airborne Sound Insulation

Airborne sound insulation is the ability of a material or room to contain sound within it, or exclude sound from it. This is commonly measured in terms of sound reduction index (in dB) being the ratio of sound transmitted by the material to that incident upon it. Airborne sound insulation can be measured using the procedures set out in, for example, ISO 16283 (supersede ISO 140 series) and ASTM E 336.

Acoustical Barrier

Contiguous objects such as solid walls, buildings, or earthen berms that substantially block the direct path of sound between a source and receiver, and which, if they have an open edge or edges allowing diffraction around them, are sufficiently wide and high to cause significant reduction of the sound traveling from the source to the receiver.

Acoustical Material Any material considered in terms of its acoustical properties. Commonly and especially, a material designed to absorb sound. Ambient Noise The composite of airborne sound from many sources n ear and far associated with a given environment. No particular sound is singled out for interest.

Apparent Sound Reduction Index, R’, dB

A measurement of sound reduction index but in the presence of flanking sound transmission. Apparent sound reduction index can be established for laboratory or field measurements in accordance with ISO ISO 10140-2 (supersede ISO 140-3) or ISO 16283-1 (supersede ISO 140-1).

A-weighted decibel, dB(A)

A modification of the measured sound pressure level that approximates more closely to the response of the ear over the normal range of sound levels heard and thus correlates reasonably well to the subjective reaction of loudness to sound. A-weighted sound levels are often denoted dB(A) or dBA or dB L A .

Background Noise

Noise from all sources unrelated to a particular sound that is the object of interest. Background noise may include airborne, structure borne, and instrument noise. The A-weighted sound pressure level of the residual noise at the assessment position that is exceeded for 90 % of a given time interval, T, measured using time weighting, F, and quoted to the nearest whole number of decibels. The background noise level is often denoted dB L A90, T .

Ceiling Attenuation Class, CAC

A US-based single-number rating system expressed in decibels, of the laboratory-measured frequency- dependent room-to-room sound attenuation of a suspended ceiling sharing a common ceiling plenum above adjacent rooms.

Decibel, dB A unit of magnitude for sound pressure, sound intensity, sound power and, in relation to sound insulation, the measurement of level reduction. The dB is a logarithmic unit that compresses a wide range of values into a smaller scale. As the decibel is 10 times the logarithmic ratio, the laws of logarithmic addition and subtraction apply. Diffraction A change in the direction of propagation of sound energy in the neighborhood of a boundary discontinuity, such as the edge of a reflective or absorptive surface. Diffuse Sound Field The sound in a region where the intensity is the same in all directions and at every point. Direct Sound Field The sound that arrives directly from a source without reflection.

Element Normalized Level Difference Dn,e, dB

From ISO 10140-2 – the difference in the space and time average sound pressure level produced in two rooms by a source in one, where sound transmission is only due to a small technical element – for example, transfer air devices, electrical cable duc ts, transit sealing systems. Dn,e is normalized to the reference equivalent sound absorption area.

Equivalent Continuous A-Weighted Sound Level (LAeq )

Value of the A-weighted sound pressure level of a continuous, steady sound that, within a specified ti me interval T starting at t1 and ending at t2 and meas ured in decibels, has the same mean square sound pressure as the sound under consideration whose lev el varies with time.

Field Sound Transmission Class, FSTC

Sound transmission class calculated in accordance with Standard standard classification ASTM E 413 using values of field transmission loss FTL. These results should theoretically approach the actual sound insulation of a partition or partition element as would be measured in a laboratory, but in practice they often do not.

Field Transmission Loss, FTL

Sound transmission loss measured in accordance with Annex A1 of Test Method ASTM E336. This requires shielding all surfaces in both rooms except the specimen under investigation.

Flanking Sound Transmission

In sound transmission measurement, the transmission of sound from the sound source room to the receiving room by paths other than through the part ition under test. Flanking transmission may include structure-borne sound transmission. Such transmission paths may include side walls, floor slabs and ceiling slabs. The effect of flanking sound is to l ower the achieved sound insulation between adjacent areas below that which would be expected from the Known performance of the identified dividing barriers.

Floating Floor

A floating floor is part of a composite floor construction whereby the upper surface membrane (possibly a concrete screed or timber deck) is independently isolated (floated) from the lower structural floor by the use of a resilient underlay or an array of flexible pads or spring isolators. This separation results in better air-borne and impact sound insulation than would be achieved by an equivalent solid floor of the same overall mass, provided the ‘isolation’ is maintained throughout with no rigid connections between the floating and structural floor, including around the edges of the floating floor.

Free Field

A free sound field is a field in a homogeneous, isotropic medium free from boundaries. BS 4142 suggest s that free-field environmental noise measurements ne ed to be made at least 3.5 m from any reflecting structure Frequency, f The rate at which sound or vibration is generated b y a source. The human audible frequency scale extends from about 20Hz to 20,000Hz. With such a potentially wide range of frequencies it is necessary , for practical use, to break them down into manageable groups or bands. In building acoustics, octave bands and one third octave bands are generally used .

Hertz, Hz

The unit of frequency for a repetitive occurrence. One cycle per second is termed one hertz. A thousand hertz is often denoted kHz, e.g. 2 kHz = 2000 Hz. Impact Sound Produced when short duration sources such as footsteps, wheeled trolleys or door slams, have a direct impact on a structure. The sound will be heard as surface-radiating air-borne sound within the area containing the source, but it may also be transmitted as structure-borne sound to re-radiate as air-borne sound in more remote locations.

Impact Sound Pressure Level, Li, dB

This is the measurement of sound pressure levels in a receiving room, when the floor/ceiling assembly under test, including the presence of a floor covering if applicable, is excited by a standardized tap ping machine in the room above.

Impact Insulation C lass, IIC

A single-number rating derived from measured values of normalized impact sound pressure levels in accordance with Annex A1 of Test Method ASTM E 492. It provides an estimate of the impact sound insulating performance of a floor-ceiling assembly.

Impulsive Sound

A brief, intrusive sound, such as that associated w ith a tire blowout, operation of a punch press, the discharge of a firearm, a door slam, or a shout, usually characterized by a rapid rise time in the initial pressure pulse of less than a few milliseconds, and by a decay time of less than a few seconds.

Insertion Loss, IL, dB

of a silencer or other sound-reducing element, in a specified frequency band, the decrease in sound power level, measured at the location of the receiver, when a sound insulator or a sound attenuator is inserted in the transmission path between the source and the receiver.

Mass Law

Relationship used to predict the approximate sound reduction index (SRI) of a uniform single-skin dividing barrier based on its mass.

Maximum Noise or Sound Level, dB LAmax,T

The highest value of the A-weighted sound pressure level that occurs during a given event or time period and with which the time-weighting should be specified. Noise Unwanted sound resulting in distraction and disturbance, interference with speech, and stress or damage to hearing. However, noise can be subjective – a noise that upsets one person may go unnoticed by another.

Noise Isolation Class, NIC

A single-number rating calculated in accordance wit h ASTM E 413 using measured values of noise reduction. It provides an estimate of the sound isolation between two enclosed spaces that are acoustically connected by one or more paths.

Noise Rating (NR) & Noise Criterion (NC ) Curves

The NR curves are a set of internationally agreed octave band sound pressure level curves based on the concept of equal loudness as reference in ISO/R 199 6 Acoustics – Assessment of noise with respect to community response . They represent the human’s perception of noise where less annoyance is caused by low frequency noise than higher frequency noise at the same sound pressure level. Hence higher levels may be tolerated for low frequency noise. Noise spectra, measured or specified, are often quoted as a NR value for ease of reference. The US uses similar rating curves called Noise Criterion (NC) curves. Used to describe sound from mechanical ventilation systems in buildings 

Noise Reduction, NR 

The difference between the average sound pressure levels either at two well defined locations based on existing conditions, or at a single location before and after some mitigation measure is implemented. 

Noise Reduction Coefficient, NRC

A  US  single-number  rating  for  random-incidence  sound  absorption  coefficients.  The  term  is  defined in ASTM 423 as the arithmetical average of the measured sound absorption coefficients for the four one-third octave band centre frequencies of 250Hz, 500Hz, 1,000Hz and 2,000Hz, which is then rounded to the nearest 0.05. 

Normalized Noise Isolation Class, NNIC

A  US  single-number  rating  calculated  in  accordance with  ASTM  E  413  using  measured  values  of normalized noise reduction.  

Normalized Noise Reduction, NNR 

Between two rooms, in a specified frequency band, the value that the noise reduction in a given field test  would  have  if  the  reverberation  time  in  the  receiving  room  were  0.5  s.  The  normalized  noise reduction  is  intended  to  approximate  the  noise  reduction  that  would  exist  between  two  ordinarily furnished rooms. 

Normalised Impact Sound Pressure Level, Ln 

The corrected measurement of impact sound pressure level, to take into account the sound absorption of the receiving room. 

Normalised Level Difference, Dn 

The  corrected  value  of  field-measured  level  difference  between  two  rooms,  to  take  into  account  the sound absorption of the receiving room. 

Octave Band 

A  group  of  adjoining  frequencies  where  the  value  of  the  upper  limiting  frequency  is  twice  that  of  the lower limiting value. The nominal mid-band frequencies of “preferred” octave bands as defined in ANSI S1.6/  ANSI  S1.11  fall  in  the  series  16,  31.5,  63,  125,  250,  500,  1000  Hz,  2000Hz,  4000Hz,  8000Hx, 16000Hz. The octave bands between 63 Hz to 8 kHz are most often used. 

One-third octave band  

A group of adjoining frequencies (formed by dividing each octave band into three) that are used when more detailed information is required than is available when octave bands are used. 

Outdoor-Indoor Transmission Class, OITC

Of a building facade, in a specified frequency band , ten times the common logarithm of the ratio of the airborne sound power incident on the exterior of the facade to the sound power transmitted by the facade and radiated to the interior. The quantity so obtained is expressed in decibels.

Percentile Level Ln, dB

The level exceeded n percent of the time during the stated measurement period.

Pink Noise

Electronically generated noise with a continuous frequency spectrum and with equal power per constant percentage bandwidth. For example, equal power in any one-third octave band. Which is often used as a sound source for acoustic measurements.

Random Incidence Sound Absorption Coefficient, αs

A measure of sound absorption derived from tests undertaken in a reverberation chamber of an acoustics laboratory, over the one third octave fre quency bands 100-5000Hz, in accordance with ISO 354 or ASTM C 423.

Receiving Room

In architectural acoustical measurements, the room in which the sound transmitted from the source room is measured.

Reverberation and Reverberation Time (RT)

The time, in seconds, taken for a sound within a space to decay by 60 dB after the sound source has stopped. The symbol Tmf represents the mid-frequency arithmetic average of the reverberation time in the 500 Hz, 1 kHz and 2 kHz octave bands. Reverberation time can be measured using the procedures set out in ISO 3382-2. The symbols T20 and T30 are the reverberation times extrapolated from a 20 and 30 dB dynamic range, starting at the -5 dB point, in order not to introduce errors due to irregularities in the early reflections. Reverberation is significant in determining the quality and level of sound within internal spaces

Signal-to-Noise Ratio (S/N)

This is the difference between the source noise level and the background (or ambient) noise level. The higher the difference the better the speech intelligibility of a public address (PA) or voice alarm (V A) system. For PA/VA system announcements, it is preferable to have an S/N ratio of at least 15 dB but preferably higher.

Sound Absorption

The process of dissipating sound energy. The reduction of energy is generally due to dissipation into heat by friction but it may also be lost when sound passes into adjacent areas, or to the outside through an opening, but does not return. Most materials abs orb sound to a greater or lesser extent and the more common forms of sound absorbers can be conveniently classified into three main types – dissipative (porous), membrane (panel) and resonant (cavity). Sound absorption is measured in Sabins, which is equal to the sound absorption coefficient multiplied by the surface area in square meter.

Sound Absorption Average, SAA

A single number rating, the average, rounded to the nearest 0.01, of the sound absorption coefficients of a material for the twelve one-third octave bands from 200 through 2500 Hz, inclusive, measured according to the test method described in Test Method C423.

Sound Absorption Coefficient, α

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. It is expressed as a value between 1.0 (perfect absorption or no reflection) and 0 (total reflection or no absorption). The value varies with frequency and the angle of incidence. Single figure descriptors include the practical sound absorption coefficient (αp ) and weighted sound absorption coefficient (αw ) as defined in ISO 11654. The single-figure rating can have a modifier added to indicate if the spectral shape is dominated by a particular frequency range:

L – Absorption is predominantly in the low frequency region.

M – Absorption is predominantly in the mid frequency region.

H – Absorption is predominantly in the high frequency region.

The absence of a letter following the rating indicates that the absorber has no distinct area of sound absorption and has an essentially flat spectral shape.

Sound Attenuation

The reduction of sound pressure 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 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 Isolation

The degree of acoustical separation between two locations, especially adjacent rooms. Sound isolation is achieved by using sound-insulating or sound-attenuating elements. Structure-Borne Sound Sound for which a significant portion of the transmission path from source to receiver takes place in a solid structure rather than through a liquid or gas .

Tapping machine

A portable electro-mechanical device used as a standardised impact sound source to rate the normalized/standardised sound pressure levels or impact noise insulation of floors.

Vibration Isolation

The use of devices such as steel springs, rubber mounts or similar resilient materials used to support a vibrating source or structure and thus minimize the transmission of its vibration energy into the surrounding structure. Vibration Level Vibration is generally measured in terms of velocity, in mm/s, m/s and acceleration, in mm/s2, m/s2, but can also be measured in terms of amplitude, in mm, m. These values are often converted into dB values on a logarithmic scale with peak and root mean square (rms) values often used.

Weighted Sound Level Difference, D w , dB

This term is not defined in ISO 717-1. The Dw is us ed widely in the UK and Europe to characterize the sound insulation performance between two spaces and is basically the sound level difference between them accounting for flanking paths around building elements and at wall, ceiling and floor interfaces.

Weighted Standardized Level Difference, DnT,w, dB

When laboratory airborne sound insulation tests are carried in accordance with ISO 10140-2 and field tests are carried out in unfurnished buildings in accordance with ISO 16283-1, the tests determine the standardised level difference, DnT, at each frequency, which is then rated in accordance with ISO 717- 1 to calculate the single-figure weighted standardised level difference, DnT,w.

Weighted Sound Reduction Rw, dB

The weighted sound reduction index, Rw, is a single figure description of sound reduction index which is defined in ISO 717-1.The Rw is calculated from measurements in an acoustic laboratory to ISO 10140-2 and rated to ISO 717-1. Sound insulation ratings de rived from site (which are invariably lower than the laboratory figures) are referred to as the apparent weighted sound reduction index (R’w) and measured to ISO 16283-1.