ACOUSTIC GLOSSARY

Superscript, subscript.

Diacritic symbols, letters or numbers positioned in at the upper right (superscript) or lower right (subscript) accompanying symbols of acoustic quantities and altering their meaning, used to identify the means of measurement and expression to which the quantities refer, written in a smaller font than the symbol they accompany.

Example: the following symbols of phonoinsulating power (through the air) R, Rw, and R’w identify, respectively:
•R= the phonoinsulating power of a partition, indicating the difference in the level of sound that the partition can maintain between a disturbing space and a receiving space under controlled conditions in the laboratory (with no lateral transmission)
•Rw= the addition of the subscript “w” means that this is an indicator of the partition’s phonoinsulating power measured in the laboratory or obtained by calculation (without lateral transmission), expressing the decibel value of the reference curve at 500 Hz, after shifting the curve according to the method specified in standard ISO717,
•R’w= addition of an apostrophe as superscript with the symbol means that the evaluation index refers to the measurement on site of the same quantity and therefore includes lateral transmission and, in this case, is defined as the apparent phonoinsulating power evaluation index.

Acoustic absorption.

The ratio between the acoustic energy absorbed by a surface and the acoustic energy incident upon it; a smooth, hard surface reflects all the sound that strikes it, and in large rooms it forms an echo, while porous, absorbent surfaces reduce reflection of noise in a room, reducing reverberation time.

Forecast calculation.

Evaluation obtained through standardised UNI ratios, software, laboratory experiments and testing on site, with the goal of accurately estimating the insulating power of partitions in construction, which must then be measured on site.

Compressability.

Ability of a resilient material to be elastically deformed while maintaining its original thickness and mechanical properties.

This value may be estimated in laboratory tests on the basis of the instructions contained in Standard UNI 12431.

Acoustic correction.

Assessment, analysis and solution of problems concerned uneven perception of sound (a problem typical of conference halls or cinemas) caused by incorrect geometry or incorrect choice of covering materials. Correction of this problem permits highly faithful perception of sound emissions in the room.

Decibel.

Unit of measurement of sound, the logarithm of the ratio between measured pressure and the minimum pressure of 0.00002 Pa, corresponding to the minimum threshold of audibility, used as a reference. The decibel is a mathematical artifice which is also used to express other quantities in physics which have a very wide range of variability. This measurement system does not, however, permit linear appreciation of the sum of two sounds, or the difference between them; while we are tangibly capable of perceiving that the length of a 10 m rope is twice the length of a 5 m rope, this is not possible in the case of sounds measured in decibels because it is not a linear measurement such as a metre but a logarithmic measurement. As a result, two sounds emitted at the same time by two adjacent and equal sources, such as two washing machines each producing a noise level of 60 dB, will not together produce a noise level of 120 dB, but “only” 63 dB. Using the logarithmic scale of decibels as a unit of measurement, doubling the sound pressure generated by the two washing machines corresponds to an increase of “only” 3 dB.

On the other hand, insulation decreasing the noise level by 3 dB is a major accomplishment, as it decreases the intensity of the sound as much as turning off one of the two washing machines! Another “strange thing” about decibels is that if the same two washing machines make noise with difference of more than 10 dB between the two machines, the resulting noise is practically the same as the noise produced by the noisier washing machine, so that if one of them produces a noise level of 50 dB and the other 60 dB, the total noise will be 60 dB.

Dodecahedron.

Omnidirectional sound source for measurement of acoustic insulation of airborne noise.

DPCM.

Decree of the Presidency of the Italian Council of Ministers.

Phonometer.

An instrument for measuring the level of sound pressure, consisting of a microphone which translates the energy of acoustic vibration into an electrical signal, which, appropriately amplified, expresses the noise level in linear decibels, dB or dB L. The signal is filtered and corrected with appropriate filters reproducing the “sensitivity curves” of the human ear; three types are used for different intensities of sound:

• A, imitating the ear’s sensitivity to sound levels of 0 to 55 dB
• B, for levels of 55 to 85 dB
• C, for levels above 85 dB

There is also a fourth type, D, which is used to assess the disturbance caused by airplane noise. The level measured by the phonometer supplied by filters is in this case expressed not in dB L but in dB(A), dB(B) and dB(C), respectively: ponderal decibels, that is, taking into account the sensitivity of the human ear, as if the ear had been used to measure sound in place of the phonometer. DPCM 5/12/1997 states that linear dB, that is, the phonometer used without ponderal filters, must be used to measure the insulating performance of a dividing wall, a façade or a floor slab, while dB(A) is used to determine the disturbance caused by technical installations.

Sound-dampening.

An element which reduces the vibration caused by transmission of sound.

Assessment indicator (of the insulating performance of partitions).

The acoustic requirements imposed by DPCM 5/12/1997 on partitions in buildings are assessment indicators. They are determined by calculation in accordance with standard UNI EN ISO 717 parts 1 and 2:1997 for insulation from airborne noise (717-1) and the sound of footsteps (717-2) in buildings and built elements, in order to convert the results of measurement of acoustic insulation taken on the basis of frequency in bands of a third of an octave or an octave (shown in a graph referred to as an experimental curve) in an assessment indicator which sums up the insulating performance of a partition in a building in a single value in dB, permitting rapid, practical comparison of the performance of different partitions.
In order to determine the indicator, the legislation established the reference values in dB for each range of frequencies, which, appearing in the decibel/frequency graph, form a reference curve (limit curve) shaped like a line broken into three sections, which is then translated parallel to itself onto the experimental curve graph until the deviation between measured values and reference values falls within the limits set by the legislation. At this point we have one and only one reference curve representing the acoustic insulating performance of the partition, and its dB value at 500 hz is its index.

Noise pollution.

Exposure to noise levels which can damage the hearing (very high levels over a short period of time or high levels for a long period of time), produced by noise sources of different types (vehicle traffic 70 dB, machinery 100-105 dB or other sources).

Standard acoustic insulation for façades (index)

D2m,nTw. acoustic insulation on façades to eliminate airborne noise generated outside buildings, expressed in linear dB as an indicator thereof, identified by the symbol D2m,nTw representing the difference in sound level, measured on site, which the façade can make between the outdoor environment where a noise source is located and a space inside the building behind the façade. If the sound that can be produced by the loudspeaker used in the test is exceeded by the noise present in the outdoor area, the noise source used for measurement on site will consist of the prevalent traffic noise. DPCM 5/12/1997 establishes a minimum value of D2m,nTw for insulation of façades.

Acoustic insulation of horizontal and vertical partitions in buildings.

Reduces the transmission of airborne and footsteps noise between spaces separated by a partition (wall or floor) in a building.

Footstep noise Ln.

• L’n,w standardised (index). Insulation from the sound of footsteps between spaces, measured on site on a completed floor slab including flooring and insulation, expressed as an index in linear dB, representing noise transmitted directly and laterally, measured in the space receiving it (including neighbouring spaces on the same level) when the tapping machine shaking the floor above is turned on. DPCM 5/12/1997 sets the maximum value of L’n,w for floor slabs separating separate property units.
  •Ln,w,eq: equivalent standardised (indicator): level of footstep noise, transmitted strictly directly, from a bare floor slab with no floor over it or floating screed over resilient material, expressed as an index in linear dB, measured in the laboratory or obtained through calculation under the test conditions described above.
  • ∆Lw: attenuation of the level of footstep noise (indicator): expressed as an index in linear dB, characteristic of a floating floor slab of a certain weight made of the type of resilient material taken into consideration, measured in the laboratory or obtained by calculation if the dynamic rigidity of the resilient material is known. Represents the contribution to insulation made by the floating screed to the bare floor slab.

Absorbent materials.

Porous or fibrous materials used to cover the surfaces of walls and ceilings in reception halls or theatres capable of absorbing incident sound used for acoustic correction of the halls. Some absorbent materials are also used to fill in the gaps in double walls and false ceilings to improve their acoustic insulation. These are materials of a fibrous or porous consistency which, by resisting the passage of air, (r=resistivity to air flow dependent on the density of the material) impose a loss of energy dissipated through friction (heat) on the sound emission.

Laboratory measurement.

A procedure of measurement of acoustic insulation of horizontal and vertical partitions and noisiness of equipment performed using instruments in a conforming laboratory with standardised test methods, the results of which are used to plan insulation for buildings in conformity with standardised calculation methods.

Measurement on site.

A procedure for measurement of the acoustic insulation provided by horizontal and vertical partitions and the noisiness of equipment, performed using instruments in the building itself and testing for acoustic requirements in conformity with the standardised calculation methods required under DPCM 5/12/1997.

Subscript: refer to “Superscript, subscript”.

Auditory perception.

The human ear transforms changes in air pressure into auditory perceptions which do not depend solely on pressure but also on the frequency with which changes in atmospheric pressure occur. While the microphone on the sound level measurement instrument faithfully measures sound pressure at any frequency, the ear is an imperfect instrument which perceives sounds with a frequency of 20 to 15,000 Hz, with greater sensitivity in the frequency range of 500 to 5,000 Hz. Within this “sensitivity range”, the human ear is more sensitive to high frequency sounds than to low frequency ones, so that, for example, it perceives as the same a 35 dB sound emitted at a frequency of 4,000 Hz and a 90 dB sound emitted at 20 Hz. Differences in the sensitivity of the ear at various different frequencies may be represented on an intensity/frequency graph, defined as a normal audiogram, by a curve of equal sensation (isosensitivity) referred to as a “sensitivity curve”, with more marked differences in sounds of low intensity, which are gradually cancelled out for sounds with an intensity over 85 dB. At 1,000 Hz, a frequency to which the ear is particularly sensitive, the physiological level of sound perceived by the ear on the sensitivity curve coincides precisely with the physical level that may be measured using the instrument.

For the same reason, the ear has an “audibility threshold” for sound, which varies according to the frequency of the sound; for example, the human ear can perceive a sound of 8 dB emitted at 250 Hz but cannot perceive a sound at a level of 50 dB emitted at 31 Hz. The way the human ear perceives sound is taken into account in construction, in designing or judging acoustic insulation for a wall or a floor. The ear cannot bear sounds more intense than 120 dB, known as the “pain threshold”, which produce a sensation of pain.

Acoustic insulating power (index):

Rw and R’w. Performance of acoustic insulation of a partition (floor or wall) of airborne noise generated in a building, expressed in linear dB as an indicator thereof, identified by the symbol Rw, representing the difference the partition makes to the noise level in the laboratory, between the chamber where the noise is generated and the chamber receiving it, completely disconnected from it, when the noise passes solely by direct transmission through it. If, on the other hand, it is identified by the symbol R’w, it represents acoustic insulation of airborne sounds between the rooms separated by the partition, that is, taking into consideration the difference in the level of noise that the partition makes in the context of the building, between the chamber where the noise is generated and the chamber receiving it when the noise crosses it, both directly and laterally through bordering partitions. The ratio Rw≥R’w still applies, because lateral transmission of sound decreases the partition’s insulating performance when assembled on site. DPCM 5/12/1997 establishes minimum values of R’w for elements separating separate units of property.

Passive acoustic requirements for buildings.

Heading in DPCM 5/12/97; Decree implementing Framework Law on noise pollution no. 447 of 1995, establishing the maximum noise levels of continuously and discontinuously operating systems and footsteps on floor slabs and establishing the minimum insulating properties of masonry in façades and dividing walls between two separate property units, measured on site, forming a part of all the buildings shown in Table A to Annex A of the decree.

Resonance.

A phenomenon in which, under particular conditions, the amplitude of vibration of a material or system under periodic forces takes on particularly high values, resulting in a drop in its insulating capacity.

Airborne noise.

Noise generated in the air and transmitted through variations in air pressure (such as speech and the sounds produced by televisions or radios). In a building, we distinguish between noise from outside, such as traffic noise, etc., which can be cut by insulating the outer walls of the building, and noise generated by activity inside the building, such as televisions, conversations among inhabitants, etc. which can be cut by insulating the vertical and horizontal walls dividing different property units within the building.

Sounds produced by footsteps, shocks and blows.

In a general sense, this term refers to all impactive sounds, that is, noise generated inside a building by mechanically impacting an element of the building, which is transmitted directly by vibration of structural elements of the building (such as blows of a hammer driving a nail into the wall, dragging a chair or other item of furniture over a floor). The most common source of this kind of noise is footsteps on the floor. In a narrow sense, the term also refers to the sound of blows generated by the “tapping machine”.

Background noise.

The level and spectrum of sound normally present in an indoor or outdoor space acoustically characterises both the area taken into consideration and the different human activities, and may be defined as “background noise”. The noise level caused by traffic is high, and typical of urban centres, as is noise present in some industrial spaces which are considered noisy, while a rural zone may be defined as silent. Taking background noise into consideration is also important for acoustic insulation of partitions in buildings in high traffic areas or close to airports. Background noise is often variable at different times of day; for example, traffic noise is lower at night. The level of background noise masks sounds of lower intensity which are produced in the same place at the same time, which is why we don’t notice the sound of the neighbours’ television during the day but it keeps us awake at night: the level of background noise drops to below the level of the neighbours’ television. The level of background noise might be compared to the level of water in a river: when the water level is high, the surface of the water appears to be smooth and even, while when it is low, the rocks in the river stand out, like noise sources which have not actually decreased in intensity. A variation in the sound spectrum of background noise can also cause a sound that disturbs us to emerge, because the human ear has the ability to qualitatively identify a specific sound with a different frequency composition even when it has a quantitatively lower level. Measurement of background noise is an operation which is always performed in the receiving/disturbed space in order to assess the feasibility of measuring the acoustic insulation of buildings and determine the corrections to be made to the acoustic levels measured at various different frequencies. When measuring the insulation of a façade, background noise is also measured outside, in the emitting/disturbing space, in order to determine whether or not to consider it a noise source.

Noise from installations.

Noise caused by the level of sound of continuously functioning installations (such as a fan coil unit) ad discontinuously functioning installations (such as drains or elevators); the disturbance they create is measured in dB(A), and the limits on it, respectively Laeq and LASmax, were established by DPCM 5/12/1997 as values to be measured on site in the space subject to the greatest disturbance, provided it is not the same as the space in which the noise originates.

Mass-spring-mass system.

A model of a physical system in which two masses are kept separate by a spring in between them. In acoustic installation for construction, an example would be the behaviour of double walls (masses) separated by a sheet of air (the spring) which may or may not be filled with an absorbent material, normally of a fibrous constitution.

Resilient layer.

A resilient layer is an elastic layer separating rigid elements, the main feature of which is that it does not permit transmission of vibrations in the structure of the building caused by blows (such as footsteps) on the partitions in it.

Sound and noise.

A sensation in the human hearing organ caused by changes in air pressure generated by vibration of a body, a human larynx, a loudspeaker, a sheet of metal, etc. with properties (frequency and level) permitting it to be perceived by the human ear. It is characterised by the pressure level, measured in decibels (dB), and frequency, measured as a number per second, of variations in pressure around atmospheric pressure, expressed in hertz (Hz). All sounds with properties which make them unpleasant to the human ear are commonly referred to as “noise”, but in the construction industry in particularly, it is more appropriate to define noise as “undesired sounds”: not everyone wants to hear a Beethoven symphony in the middle of the night, though no-one would call a symphony noise.

Reverberation time.

Commonly referred to as the “echo effect”; a measurement of the amount of time required for the energy level of a sound signal to decrease by a significant percentage. Reverberation time is tested in large spaces, as a too long reverberation time would affect the intelligibility of words or music. Measurement is obligatory in the case of school buildings, and the limits are stated in Public Works Ministry memorandum no. 3150 of 22/05/1967.

Direct transmission.

The principal path of noise through a partition

Lateral transmission.

Indirect propagation of noise through rigid connections in partitions bordering on the partition in the building, which causes a drop in the potential insulation that might be expected or planned through direct transmission alone.

Ultrasound, infrasound.

Sounds which are not audible to the human ear because they have a higher or lower frequency than the range of sensitivity of the human ear. Animals can often distinguish both ultrasound and infrasounds that humans cannot detect, and in some cases emit them as a call or to orient themselves and capture prey. Ultrasounds are sounds with a frequency greater than 15,000 Hz, while infrasounds are sounds with a frequency below 20 Hz.

Speed of propagation of sound.

Sound is propagated through what is referred to as a “medium of propagation of sound”, which may be of many different types: air, water, metal, building materials, etc. Sound cannot be propagated in the absence of such a medium; no sounds are heard in vacuum. The speed at which noise “moves” depends on the medium in which it exists. The speed of propagation of sound in air is about 340 m/s, while in building materials it can travel as fast 5,000 m/s (5,000 m/s in steel, 3,000 m/s in brick, much less in insulating materials).

(source: index spa) http://www.isolantiindex.it