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 at the sound-absorption coefficient usually range from about 0.01 for marble slate to almost 1.0 for long absorbing wedges often used in anechoic rooms.
The sound absorption coefficient can be measured in 1/3 octave bands according to ISO 354 and weighted as αw acc. to ISO 11654.

Acoustic impedance
The acoustic impedance Z (or sound impedance) is the ratio of sound pressure p to particle velocity v in a medium or acoustic component and is usually represented in complex notation as Z=R + iX. The real and imaginary components are called, respectively, acoustic resistance and acoustic reactance.
See also characteristic impedance.

Airflow resistance
See specific airflow resistance.

Airflow resistivity
See specific airflow resistance.

Air absorption
Air absorption is usually neglected in calculations of reverberation times for auditoriums, but for large enclosures it may become signifi cant. Air absorption is greater for high frequencies and is dependent upon air temperature and relative humidity.
In calculations of reverberation times the air absorption can be included by adding an equivalent absorption area for the sound attenuation by air as (acc. to EN 12354-6):

A_air=4*m *V
V - room volume

Airborne sound
Airborne sound is the sound propagation in air as pressure waves. The sound propagation in solid materials is called structure borne sound propagation.

Airborne sound insulation
The reduction of the sound intensity for a sound wave passing through a building element. (level difference for the incoming airborne sound wave and transmitted airborne sound wave).
The sound insulation is normally expressed as the sound reduction index R.
R depends on the frequency of the sound passing through the element and is measured in 1/3 octave bands.

Ambient noise
The composite of airborne sound from many sources near and far associated with a given environment. No particular sound is singled out for interest.

Amplitude
The instantaneous magnitude of an oscillating quantity such as sound pressure.
The peak amplitude is the maximum value.

Anechoic Chamber
An anechoic chamber is a room where there is no reverberation and no echo.
An acoustic space without echo or reverberation. Often used for the acoustic testing (e.g. microphones and loudspeakers) or sound recordings without any refl ections for use in auralization examples.

ASTM E90

Standard Test Method for Laboratory Measurement of Airborne

Sound Transmission Loss of Building Partitions

This test method covers the laboratory measurement of airborne sound transmission loss of building partitions such as walls of all kinds, operable partitions, floor-ceiling assemblies, doors, windows, roofs, panels, and other space-dividing elements.

ASTM E413
Classification for Rating Sound Insulation
The STC value (Sound Transmission Class) is calculated from 125Hz to 4000 Hz. The Weighted Sound Reduction Index (Rw) is the ISO equivalent of STC.
It is determined in accordance with ISO 717, Rating of Sound Insulation in Buildings and of Building Elements. The reference contour extends from 100 to 3150 Hz and there is no 8 dB rule. Usually STC and Rw are approximately equal.

ASTM C423
Standard test method for sound absorption and sound absorption coefficients by the reverberation room method Measurement of sound absorption coefficient a in a diffuse field (room methode) and calculation of the NRC value (Noise Reduction Coefficient)

Attenuation
When sound travels through a medium, its intensity diminishes with distance. This weakening in the energy of the wave results from two basic causes, scattering and absorption. The combined effect of scattering and absorption is called attenuation 

Audible frequency range
A human ear can normally perceive frequencies from 20 Hz to 20.000 Hz. This is called the audible frequency range.

Background Noise
Background noise is a definition of all kinds of sound beyond the particular sound wanted.

Baffle
A flat board or plate, deflector, guide or similar device. In acoustics free installed baffles of absorbing material (e.g. mineral wool) can be used to control the room acoustics or can be mounted in a ventilation duct to attenuate airborne sound.

Bandwidth
The range of frequencies, expressed in Hertz (Hz), for the filters used for the frequency spectrum analysis. Often used bandwidth are 1/1 octave band or 1/3 octave band, but also 1/12 octave band.

Broadband noise
In the frequency domain, a broadband noise has a continuous spectrum - that is, energy is present at all frequencies in a given range. This type of sound is often referred to as noise because it usually lacks a discernible pitch. White noise and pink noise are typical broadband noises used for  acoustic measurements.

Characteristic impedance
The characteristic impedance of a medium (usually air, but also for instance exhaust gases in a silencer) is the ratio of sound pressure p to particle velocity v in open field (i.e. in a condition of no reflecting waves). This impedance is a material constant and equals the product of the density of air ρ (rho) and the speed of sound c:
ρ (rho)=density of air in kg/m3
c=speed of sound (acoustic wave velocity) in m/s
Note: Z0 is expressed in Pa·s/m. Before the Pascal was introduced by the SI, impedance was expressed in N·s/m3.
The characteristic acoustic impedance of air Z0 equals:
410 Pa·s/m at a temperature of 25°C (77°F)
413 Pa·s/m at 20°C (68°F).
°C=degree Celsius and °F=degree Fahrenheit.

Clarity C80, C50 (dB)
The measurement of Clarity is the ratio of the energy in the early sound compared to that in the reverberant sound, expressed in dB. Early sound is what is heard in the first 80 msec (C50 - 50 msec) after the arrival of the direct sound. It is a measure of the degree to which the individual sounds stand apart from

one another. If the clarity is too low, the fast parts of the music are not “readable” anymore. C80 is a function of both the architectural and the stage set design.
If there is no reverberation in a dead room, the music will be very clear and C80 will have a large positive value. If the reverberation is large, the music will be unclear and C80 will have a relatively high negative value. C80 becomes 0 dB, if the early and the reverberant sound is equal. Often the values for 500Hz, 1000Hz and 2000Hz are averaged.
This will be expressed by the symbol C80(3). For orchestral music a C80 of 0dB to -4dB is often preferred, but for rehearsals often conductors express satisfaction about a C80 of 1dB to 5dB, because every detail can be heard. For singers, all values of clarity between +1 and +5 seem acceptable. C80 should be generally in

the range of -4dB and +4dB. For speech, in comparison to music, the clarity will be measured as the ratio of the first 50 msec (C50) instead of 80 msec (C80) for music.

Critical frequency
In building acoustics (sound transmission) the frequency, where the sound of speed in air equals the propagation speed of bending waves in the partition.
The main mechanism of sound transmission through the partition changes at the critical frequency. At this frequency the sound reduction index drops down drastically.
The critical frequency depends on the material type (bending stiffness) and thickness of the partition.

dB(A)
See under A-weighting.

dB, Decibel
The dB is a logarithmic unit (logarithm with base 10) used to describe a ratio. The ratio may be power, sound pressure, voltage or intensity or several other things. In acoustics the sound power P [in W] and the sound pressure p [in Pa] are expressed in dB values as ratio to a defined reference value:

Sound power level in dB

Decay rate
At a stated frequency, time rate at which sound pressure level decreases in a room after the excitation has stopped. For a room with total diffuse sound field (reverberant room) the decay rate d is constant and is related to reverberation time T by T=60 dB.

Directivity index, directivity factor
The directivity factor, Q, is defined as the ratio of the intensity (W/m2) at some distance and angle from the source to the intensity at the same distance, if the total power from the source were radiated uniformly in all directions.

where Iq=Sound intensity at distance r and angle q from the source I=Average sound intensity over a spherical surface at the distance r And the directivity index (DI) is defined as:

Double construction
In building acoustics a partition consisting of two outer membranes and a cavity between, where the interactions between the outer membranes and the cavity determine the acoustic behaviour for the sound transmission. Normally the sound insulation increases by using absorbing material as Rockwool

inside the cavity. The positive acoustic behaviour of a double construction with absorption is partly reduced by sound bridges (like steel frame in gypsum walls), which mechanically connects the two membranes. Typically used double constructions are lightweight partitions Marine Panels. Mathematically double constructions can be described as mass-spring-mass systems with the two membranes as mass and the cavity (compression of air) as the spring.

Dynamic range
The range between the maximum and minimum sound levels that a sound system can handle. It is usually expressed in decibels.

Early Decay Time EDT(s)
The Early Decay Time is measured in the same way as the reverberation time, except that it relates to the early part of the reverberation curve (0 to -10 dB). EDT is also called “Early reverberation time”. The EDT can vary locally in a room much more than the reverberation time T20 or T30 and is therefore more used as a local parameter for a part of the room (e.g. the stage in a concert hall) and not as a global parameter for the whole room.

Echo
An echo is the delayed reflection of a sound, which is detected from the human ear independent of the direct sound. An echo can be very disturbing for the human ear and can make speech unintelligible.
Equivalent absorption area The area of a surface multiplied with its absorption coefficient. The equivalent

absorption area of an element differs with the frequency and is normally stated per octave band or 1/3 octave band. The total absorption area of a room is used to calculate the reverberation time acc. to Sabine’s formula.

Filter
An acoustic filter passes or blocks components of sound of different frequencies. The passing components may be amplified or attenuated. Most used filters are:
Octave band filter:
Only the desired frequency range inside the octave is transmitted, outside this range all frequencies are blocked.
dB(A)-filter: the different frequency components are attenuated (or amplified) according to the A-filter values (see dB(A)).
High-pass filter: All the low frequencies below the cut off frequencies are blocked.
Low-pass filter: All the high frequencies above the cut off frequencies are blocked.
Band pass filter: A combination of a high-pass and low-pass filter with different cut-off frequencies so that only a defined range of frequencies passes through the filter. Octave band filters and 1/3 octave band filters are typical band pass filters.

Flanking transmission
Flanking transmission occurs when sound is transmitted from one space to another indirectly, through adjoining parts of the structure. For example, impact sound may be transmitted from one room to another through a timber floor, but also through the supporting wall.

Flutter echo
In room acoustics, a series of specific reflective returns caused by large surfaces being parallel to each other.

Free field
A sound field whose boundaries exert a negligible influence on the sound waves. In a free-field environment, sound spreads spherically from a source and decreases in level at a rate of 6 dB per doubling of distance from a point source, and at a rate of 3 dB per doubling of distance from a line source. Free field conditions can be used as a reference situation for defining room acoustics parameters, e.g. Strength G

Frequency
Frequency is measured in Hertz [Hz] and is defined as the number of times an event is repeated per time unit. For example with sound it is the number of repeated waveforms per second.
Human hearing 20-20.000 Hz

Geometrical acoustics
Simulations based on geometrical acoustics rely on ray theory, i.e. the approximation that sound propagates between two points along straight, ray-like paths. Thus, an un-occluded linear path between a source and receiver is sufficient to model the direct sound, and specular reflections can be simulated by piecewise linear paths that obey Snell’s Law when they encounter flat, reflecting surfaces.

Hard room
A hard room is defined as a room with surfaces which absorb a limited amount of sound waves. The surfaces will reflect a large part of the sound waves and the room is the opposite of an anechoic room. The average absorption coefficient may be about 0.1 or less.

Hearing loss
The difference between the level of sound that can just be heard by an individual with impaired hearing and a standard level that has been determined by averaging measurements from a group of young hearing people. It is usually expressed in decibels. The hearing loss is normally different at different frequencies.
People exposed to sound levels can get noise-induced temporary or permanent hearing loss. Sounds of less than 80 decibels, even after long exposure, are unlikely to cause hearing loss.

Helmholtz resonator
A Helmholtz resonator is a device which is designed like a closed container with a small hole. It can be shaped like a cylinder or a kind of sphere. A normal bottle can be looked at as a Helmholtz resonator. The device is used in acoustics to trap sounds with low frequencies. It can be used as a passive noise control device.

Hertz
Hertz is the unit of frequency and is abbreviated Hz. Se also frequency.

Impact noise
In building acoustics noise generated by mechanical impact excitation like foot steps or hammer. According to ISO 140 impact noise transmitted in a building through the construction is measured using a standardized tapping machine. The measured sound pressure level Li in a room is called the impact

sound pressure level and expressed as normalized impact sound pressure level Ln=Li + 10 log(A/10 m2) (A is the equivalent absorption area of the room). See also structure borne noise.

Impact sound transmission
Sound transmission of impact noise through the building elements. Impact sound transmission happens if something is dropped on the floor or even just when someone is walking on the floor. Impact noise can be transmitted as vibrations in the building elements over a long distance and be radiated to rooms far away and not directly adjacent to the source room.

Impulse response
Measured or calculated sound pressure signal (versus time) in a receiving point, when an impulse is emitted in a source point. Knowing the impulse response, all acoustic local parameters can be calculated. (e.g. reverberation time, speech intelligibility,). The impulse response indicates all reflections between source point and receiving point.

Insertion loss
Sound attenuation of an element which is inserted in a construction. E.g. the attenuation of airborne duct sound inside a ventilation duct using a ducted silencer. The insertion loss is often measured as the difference of noise level in a receiving point with and without the considered element.

Intelligibility
See speech intelligibility

ISO 140

Acoustics -- Measurement of sound insulation in buildings

and of building elements
Part - 1 ... 18 - Definition of sound reduction index, impact noise and measurements (R, Dne, L)

Most relevant:
Part 3: Laboratory measurements of airborne sound insulation of building elements
Part 4: Field measurements of airborne sound insulation between room
Part 6: Laboratory measurements of impact sound insulation of floors
Part 7: Field measurements of impact sound insulation of floors

ISO 717

Acoustics -- Rating of sound insulation in buildings and

of building elements
Rw, (C, Ctr)
The Rw value (Weighted Sound Reduction Index) is calculated from 100 Hz to 3150 Hz based on results from e.g. ISO 140-3 measurements, different adaptation terms for extended frequency ranges and noise spectra available.
Ln,w
The Ln,w value (weighted normalized impact sound pressure level) is calculated from 100 Hz to 3150 Hz based on results from e.g. ISO 140-6 measurements

ISO 15186

Acoustics - Measurement of sound insulation in buildings and of building elements using sound intensity
ISO 15186-3:2002 specifies a sound intensity method to determine the sound reduction index and the element-normalized level difference of building elements at low frequencies. This method has significantly better reproducibility in a typical test facility than those of ISO 140-3, ISO 140-10 and ISO 15186-1. The results are more independent of the room dimensions of the laboratory and closer to values that would be measured between rooms of volume greater than 300 m3. ISO 15186-3 is applicable in the frequency range 50 Hz to 160 Hz but is mainly intended for the frequency range 50 Hz to 80 Hz.
The results found by the method of ISO 15186-3 can be combined with those of ISO 140-3 and ISO 15186-1 to produce data in the frequency range 50 Hz to 5 000 Hz.

ISO 10848

Acoustics - Laboratory measurement of the flanking transmission of airborne and impact sound between adjoining rooms

ISO 10848-2:2006 applies to light elements such as suspended ceilings, access floors, light uninterrupted facades or floating floors. The transmission from one room to another can be simultaneous through the test element and via the plenum, if any. With measurements according to ISO 10848-2:2006, the total sound transmission is measured, and it is not possible to separate the two kinds of transmission. ISO 10848-3:2006 applies to structurally connected light elements forming a T or X junction. The performance of the building components is expressed either as an overall quantity for the combination of elements and junction, or as the vibration reduction index of a junction.

ISO 354
Acoustics – Measurements of sound absorption in a reverberation room Measurement of sound bsorption coefficient in a diffuse field (room methode) 

ISO 11654
Acoustics – Sound absorbers for use in buildings – Rating of sound absorption. Calculation of ap (octave band values) and aw (weighted sound absorption) and definition of sound absorption classes A ... E.

ISO 10534

Acoustics - Determination of sound absorption coefficient and impedance in impedance tubes
Specifies a method for the determination of the sound absorption coefficient, reflection factor, surface impedance or admittance of materials and objects. The values are determined by evaluation of the standing wave pattern of a plane wave in a tube, which is generated by the superposition of an incident

sinusiodal plane wave with the plane wave reflected from the test object.

ISO 9052

Acoustics - Determination of dynamic stiffness
Relates to the unit area of resilient materials with smooth surfaces, including mineral wool. Does not apply to loadings lower than 0,4 kPa or greater than 4 kPa, that is not for materials in wall linings and not for machinery foundations. 

ISO 9053
Acoustics - Materials for acoustical applications -- Determination of airflow  resistance

ISO 3382
Acoustics – Measurement of room acoustic parameters
ISO_2923-1997 + ISO_2923_Cor Measurements of noise onboard vessels

kHz (kilo Hertz)
Unit for the frequency : 1 kHz=1000 Hz

Lateral Energy Factor
LEF is a parameter that, somewhat simplified, describes how much early sound arrives from the sidewalls to a listener’s position in relation to the sound that arrives late. This parameter is correlated to the subjective perception of spaciousness in a concert hall.

Lightweight partition walls
Internal walls usually formed from timber or metal studs with outer panels (gypsum, plywood) and a cavity filled with absorbing material (e.g. mineral wool) forming a acoustic double construction with high sound insulation properties. This can be compared to standard double marine panels.

Ln
Normalized sound pressure level for the impact noise in a room generated from a standardized tapping machine in another room.

Mass law
An approximation that describes the sound transmission loss TL (sound reduction index R) of a limp, flexible barrier in terms of mass density and frequency. For each doubling of the weight or frequency of a partition, mass law predicts a 6 dB increase in TL (or R).
For most building elements the mass law is a valid approximation only in a limited frequency range and other effects as the decrease of the sound reduction at the critical frequency has major influence on weighted values as Rw .

Noise
Noise is sound which is undesirable. It is any kind of sound and depends only on which sounds are wanted and which sounds are not. Noise interferes with the sound you wish to hear. The degree of noise depends on volume and frequency.

See also broad band noise.

Noise rating
To define demands for the maximum sound pressure level for background noise like noise from HVAC components, different type of noise rating curves are defi ned:
NR curves – value of the curve is the SPL value at 1000 Hz
NC curves – value of the curve is the SPL value at 2000 Hz
RC curves (room criteria) – value of the curve is the SPL at 1000 Hz

A given background noise has to fulfil the requirements in all frequencies of the chosen noise rating curve.

Noise Reduction Coefficient

The Noise Reduction Coeffi cient NRC is the average sound absorption coefficient measured at four frequencies: 250, 500, 1,000 and 2,000 Hz expressed to the nearest integral multiple of 0.05.

Octave
In music, an octave (sometimes abbreviated 8ve or 8va) is the interval between one musical note and another with half or double the frequency. For example, if one note is pitched at 400 Hz, the note an octave above it is at 800 Hz, and the note an octave below is at 200 Hz.

The ratio of frequencies of two notes an octave apart is therefore 2:1. Further octaves of a note occur at 2n times the frequency of that note (where n is an integer), such as 2, 4, 8, 16, etc. and the reciprocal of that series. For example, 50 Hz and 400 Hz are one and two octaves away from 100 Hz because they are 1 / 2 (1 / 21) and 4 (22) times the frequency, respectively, however 300 Hz is not a whole number octave above 100 Hz, despite being a harmonic of 100 Hz.

Octave band
A frequency range with an upper limit that is twice the frequency of its lower limit. The audible frequency range is often divided into bands of frequencies because sound transmission through solid barriers can vary dramatically with the frequency of the sound. The broadest bandwidth commonly used is an octave. An octave is any band where the highest included frequency is exactly two times the lowest included frequency.

For example, the frequency band that covers all frequencies between 707 Hz and 1,414 Hz is an octave band. The band is identified by its centre frequency, which is defined as the square root of the product of the highest and lowest frequency. In the above case the centre frequency is 1000 Hz. The entire frequency range of human hearing can be covered in the following 10 standard octave bands: 31 Hz, 63 Hz, 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, 8000 Hz, and 16000 Hz

One-third octave band
A frequency band that has a width (in Hz) that is only 1/3 of the width of an octave. It takes 31 one-third octave bands to cover the entire frequency range of human hearing. One-third octaves are used when octave band analysis does not provide adequate resolution in the frequency domain.

Overtone
A whole-number multiple of the fundamental frequency of a tone. The overtones (also called harmonics; 1st overtone is 2nd harmonic) define the harmonic spectrum of a sound. E.g. a fan with a rotating frequency of 45 Hz (=60*45=2700 rpm) can generate vibrations at 45 Hz and overtones in 90, 135, 180 Hz. 

Peak level
The maximum instantaneous amplitude of the wave (e.g. sound pressure).

Phon
The phon is a unit of perceived loudness, which is a subjective measure of the strength (not intensity) of a sound. The phon value of a given sound is the sound pressure level in dB of a 1000 Hz sound, which is perceived as loud as the given sound. E.g. if a given sound is perceived to be as loud as a 60 dB sound at 1000 Hz, then it is said to have a loudness of 60 phons.

Pink noise
Pink noise is a noise containing equal power per octave band.

Plenum
In suspended ceiling construction, the space between the suspended ceiling and the main structure above or analogue for a raised floor the space between raised floor and main construction. The acoustic properties of the plenum influence the sound propagation in the building and the room acoustics in the room.

Porous absorption
Sound absorption in a porous material like Rockwool wool caused by
friction between air movement and the material (e.g. fibres). Other forms of absorption can be based on wave interference in Helmholz absorbers or membrane absorbers.

Pure tone
A pure tone is a sound which only contains one single frequency, the wave form is a simple sine wave (sine tone).

Rapid Speech Transmission Index RASTI
Rapid Speech Transmission Index RASTI is a measure for the speech intelligibility like STI. The measurement method for RASTI uses a limited frequency range compared with the STI method.

RC (Room Criteria) curves
Noise rating curves used mainly in the USA to evaluate background noise from e.g. HVAC components.

Reflection
Reflection occurs when a sound wave hits a surface which has a low absorption coefficient. The sound wave is reflected by the surface and returns into the room.

Resonance
The condition that results when a system is acted upon by a periodic driving force the frequency of which coincides with one of the natural frequencies of the system. The steady-state amplitude of the system, for fixed amplitude of the driving force, is a local maximum at a resonance frequency.

Resonance frequency
The frequency at which any system vibrates naturally when excited by a stimulus. A tuning fork, for example, resonates at a specific frequency when struck. In building acoustics the resonance of the mass-spring-mass system for a double construction reduces the sound insulation of this construction at this frequency.

See also Resonance.

Reverberant sound field (diffuse sound field)
A sound field made of reflected sounds in which the time average of the mean square sound pressure is everywhere the same and the flow of energy in all directions is equally probable. This requires a room with hard surfaces with very low absorption coefficients like a reverberation room for acoustic measurements.

Reverberation
Reverberation is an expression of the fact that a sound continues after the sound source has stopped. See reverberation time.

Reverberation time
Reverberation time is a measurement used in acoustic design. It is defined as the length of time taken for a sound to decay 60 dB after the source has stopped.

Some times, the reverberation time is called T20 – indicating that the 60 dB decay time is extrapolated from the decay -5 dB to -25 dB or T30, extrapolated from the decay -5 dB to -35 dB.

R_^w. R’_^w: weighted sound reduction index, dB
A single-number rating of the sound reduction through a wall or other building element. Since the sound reduction may be different at different frequencies, test measurements are subjected to a standard procedure which yields a single number that is about equal to the average sound reduction in the middle of the human hearing range.

The weighting procedure is standardized in ISO 717-1 for the sound reduction of airborne noise. R’w (dB) is the weighted apparent sound reduction index in a building, including flanking sound transmission. Rw is the laboratory value for the weighted sound reduction without flanking transmission.

Room acoustics
Room acoustics describes how sound behaves in an enclosed space. Deals with the sound propagation inside a room (sound absorption and reflection on surfaces, absorption in air). Important parameters are reverberation time, echoes and speech intelligibility.

Sabine
The originator of the Sabine reverberation equation. Wallace Clement Sabine, a founder of modern architectural acoustics (1868-1919). Developed a relationship between reverberation time and the amount of acoustic absorption in a room. Designed Boston Symphony Hall, considered being one of the best concert halls in the world.

Sine wave
The simplest form of periodic wave motion, expressed by the equation y=sin x, where x is degrees and y is voltage or sound pressure level. All other forms can be created by adding (mixing) a number of sine waves. The wave form of a “Pure tone” is a sine wave.

Sound
A pressure fluctuation, usually in the range of audible frequencies, resulting from a displacement of a gas, liquid, or solid, that can be detected by a mechanical or electromechanical transducer (e.g., a barometer, microphone, or the human ear). Sound propagation as sound waves in gas or liquids or vibrations in solid materials.

Sound absorption
Sound absorption is a material property which describes how well sound waves are absorbed in a material. When a sound wave is absorbed, it simply means that the sound wave is transferred into another kind of energy i.e. heat.

See also sound absorption coefficient.

Sound absorption coefficient
See absorption coefficient

Sound attenuation
The reduction in the intensity or the sound pressure level of sound, which is transmitted from one point to another.

Sound bridge
Analogue to thermal bridges in heat insulation constructions sound bridges transmit sound energy as structure borne sound between two part of a construction, which (besides the sound bridges) are not mechanically connected.

Typical example in building acoustics is the steel studs in double wall constructions (e.g. gypsum wall).

Sound bridges can reduce the sound insulation with many dB’s and be the main transmission path for sound transmission.

Sound insulation
The reduction of sound transmission through a building element.
Sound insulation material: Special material increasing the sound insulation of a construction as e.g. Rockwool.

Sound intensity
Average rate of sound energy transmitted in a specified direction at a point through a unit area normal to this direction at the point considered. Unit, watt per square meter (W/m{sup 2}); symbol, I.

Sound intensity level LI
Sound intensity I expressed as dB value:

LI=10*log(I/Io) dB

The reference value Io=10-12 W/m2 is defined so that the sound pressure level and the sound intensity level in propagation direction in a free sound field have the same dB values.

Sound power
The acoustic energy in W emitted from a sound source or transmitted through a defined transmission path (e.g. a duct).

Sound power level
The sound power expressed in dB
Sound power level in dB

Sound pressure
Sound pressure p (or acoustic pressure) is the measurement in Pascals of the root mean square (RMS) pressure deviation (from atmospheric pressure) caused by a sound wave passing through a fixed point. The symbol for pressure is the lower case p. The upper case P is the symbol for power. This is often misprinted.

The unit is Pa=Pascals.

Sound pressure level (SPL)
The sound pressure level is the sound pressure p expressed as dB value:
The reference value po corresponds to the average hearing threshold for the human ear.

Sound reflection
See reflection

Sound reduction index R
See also: Rw·R’w:weighted sound reduction index, dB. Acoustic property of a bulkhead / deck (partition) for reduction of air borne noise.

Sound Transmission Class (STC)
A single number rating according to ASTM E413 Classification for Rating sound insulation of the sound transmission loss (sound reduction) similar to the Rw weighting acc. to ISO 717. The STC includes frequencies from 125 Hz to 4000 Hz, the Rw from 100 Hz to 3150 Hz.

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

Sound waves
See sound

Specific airflow resistance Rs
According to ISO 9053 the quotient of the air pressure difference across the specimen divided by the linear velocity, measured outside the specimen, of airflow through the specimen.

For a homogeneous material the quotient of the specific airflow resistance divided by its thickness is called the airflow resistivity r.

The airflow resistance is a main product property of porous materials influencing the sound absorption in the material.

Spectrum
In mathematics, physics and signal processing, the frequency spectrum is a representation of a signal or other function in terms of frequency (in the “frequency domain”).

In audio, the frequency range is basically 20 Hz to 20,000 Hz. The frequency spectrum sometimes refers to the distribution of these frequencies. For example, bass-heavy sounds have a large frequency content in the low end (20 Hz- 200 Hz) of the spectrum.

E.g. the spectrum of background noise can be described in octave bands.

Speech intelligibility
A measure of the intelligibility of speech that indicates the ease of understanding speech. It is a complex function of psychoacoustics, signal-to-noise ratio of the sound source, and direct-to-reverberant energy within the listening environment. The intelligibility of speech (usually measured in the presence of noise or distortion) can be measured as Speech Transmission Index STI.

Speech Transmission Index - STI (%)
The measurement method for STI includes frequencies from 125-8000 Hz and results in a value between 0 and 1 which can be interpreted according to following subjective scale. The STI is a local parameter measured between a single speaker point and a single receiver point.

Speed of sound
(Also velocity of sound.) Usually taken as the mean value of the phase speed of an acoustic (or sound) wave. In an ideal, stationary gas the speed of sound c is a thermodynamic property depending only on the equilibrium state of the gas and is given by where is the ratio of specifi c heat capacities, at constant pressure and volume, respectively; R is the gas constant; and T is absolute temperature.

For normal air, for t=20°C ⇒ c ≈ 340 m/s In water the speed is about 1500 m/s. In solid materials the sound propagation depends on the wave form. For longitudinal waves the speed in m/s is about:

Glass 5,500-6,000
Aluminium, Steel 5,100
Wood 3,400-4,500
Concrete 4,000
Brick 3,600
Ice 3,100
Water 1,500
Mineral Wool 180

Standing waves
A resonance condition in an enclosed space in which sound waves travelling in one direction interact with the refl ections on hard walls travelling in the opposite direction, resulting in a stable condition as a standing wave. Standing waves in a room are characterized by sound pressure maxima in a distance of

half a wave length.

Structure borne sound / noise
As a result of airborne or impact excitation the building constructions are vibrating. The machines standing on or attached to the building constructions cause also vibration of the constructions. The components of vibration in the range of audible sound are called the structure borne sound. Generally they are the propagation of different forms of waves (bending, longitudinal, transversal, etc.), that is the vibrations are also propagating. They reach even rooms where there is no sound source operating. The normal component of the vibrations to the surface of the building constructions is responsible for sound radiation.

Threshold of hearing
The threshold of hearing is the sound pressure level SPL of 20 μPa (microPascals)=2x10-5 Pascal (Pa). This low threshold of amplitude (strength or sound pressure level) is frequency dependent. See the frequency curve in Fig. 2.

The absolute threshold of hearing (ATH) is the minimum amplitude (level or strength) of a pure tone that the average ear with normal hearing can hear in a noiseless environment.

The Threshold of hearing is frequency dependent, and typically shows a minimum (indicating the ear’s maximum sensitivity) at frequencies between 1 kHz and 5 kHz. A typical ATH curve is pictured in Fig. 1. The absolute threshold of hearing represents the lowest curve amongst the set of equal-loudness contours, with the highest curve representing the threshold of pain.

Wavelength
A wavelength λ is the distance between two peaks or two crests on a wave.

White noise
White noise is a noise signal which contains equal sound energy level at all frequencies.