What are the input data useful for reducing the reverberation of a room?

The input data useful for reducing the reverberation of a room are mainly:

- a physical indicator of the existing acoustic situation i.e. the exiting reverberation time (*)
- the use of the premise (*)
- the level of acoustic performance wich is required (desired reverberation time, desired sound pressure level, desired spatial sound decay) (*)
- the surfaces available for the implementation of absorbent materials (area of available surfaces, acceptability of overweight for wall supports or for roofs).

In very numerous cases, the reaction to fire of considered materials is also taken into account as well as considerations of appearance (architectural style, colors, materials ...).

In the case of realization in the food industry, in pharmaceuticals, or in hospitals, specific constraints related to hygiene are involved and may necessitate the use of particular absorbing coatings (with waterproof washable cover) or the use of stainless steels.

Other parameters may be involved such as the authorized mass, the required longevity or the available budget.

*limit values ​​of maximum reverberation duration (reverberation time) or of minimum equivalent absorption area must be taken into account for certain premises in tertiary buildings, minimum values ​​of spatial sound decay are to be considered for work spaces

Which metallic wall panels for the reverberation control of a room?

As far as metallic wall panels for the reverberation control of a room are concerned (also known as acoustic insulation panels) such as those used for current applications in the field of industrial soundproofing or in premises of tertiary buildings, like reducting the reverberation of a room or increasing the spatial sound decay in a room, they generally consist of an absorbent filling (quite often: mineral wool) covered by a surfacing (mostly: a fiber glass) and also by a perforated protection (mostly: galvanized or painted steel). A powder coating usually provides the best possible protection against corrosion: up to classification C5 according to ISO 12944 whereas providing a visual aspect appropriate for the context. As required, the perforations of the protection can be generally circular (perforated plate) or in the form of slots (absorbent panels with metal blades).

In some cases, the frame of metallic wall panels (metallic absorbing panels) can be made of stainless steel (SS 304, SS 316) or of aluminum.

As required, the absorbing lining may consist of acoustic foam or of polyester wool and for some applications, protected by a waterproof surfacing (polyurethane film and for extreme cases: painting).

In order to increase the absorption coefficient in a given frequency range, an intermediate plate (usually made of steel) and also a additional absorbent spacing layer may be inserted into the metallic wall panels, the performance of such absorbent panels then being (partially) based on the effect of resonant membrane (also known as resonator).

The prediction of acoustic performance of metallic wall panels can be performed with the simulation software SILDIS.

The verification of acoustic performance of metallic wall panels can be made in accordance with ISO 354 Acoustics - Measurement of sound absorption in a reverberation room

Why choose baffles made of melamine foam, rather than any other suspended baffle?

Such a choice is appropriate in all cases where it is looked for a baffle with both a very good absorption factor, a minimum mass, a minimum cost and minimum installation costs according to different possible techniques allowing all architectural aspects.

Verifying the performance of acoustic baffles made from melamine foam can be conducted in accordance with ISO 354 Acoustics - Measurement of sound absorption in a reverberation room

Wich efficiency for the reverberation control of a room ?

The efficiency for the reverberation control of a room (in workplaces, in a building for tertiary use, in a building for residential use ...) is very frequency dependent and is mainly related on the one hand to the performance of absorbent materials which constitute the partitions or of which are covered the partitions of the room (characterized by their absorption coefficient or by their Sabine factor) and also to their area, and on the other hand to the geometry of the room, to its volume, to its fitting density and to the level of its acoustic quality before soundproofing. In some particular cases, humidity and room occupancy rates are also taken into account.

This performance can be expressed in terms of difference (with and without the implementation of materials for reverberation control) of reverberation times or (for rooms with a particular use) of averages of reverberation times at some frequencies.

For orders of magnitude, a difference of reverberation time (or of averages of reverberation times at some frequencies) up to 40% can generally be obtained without special requirements after implementation of a suspended acoustical ceiling (or of a net of suspended acoustic baffles) in the case of premises where the ceiling height is low enough and where the partitions were highly reflective before soundproofing (e.g. concrete wall, metal deck roof) whereas a difference of reverberation time (or of averages of reverberation times at some frequencies) above that shall involve a special achievement that may necessitate the implementation of absorbing wall panels.

This performance can also be expressed (namely in workplaces) in terms of difference (with and without the implementation of the materials for reverberation control) of the sound level decay per doubling of distance from the source.

For orders of magnitude (and with respect to a noise spectrum like "pink noise"), a difference of decay of the sound level per doubling of distance from the source up to 25% can usually be obtained without special requirements after implementation of a suspended acoustical ceiling (or of a net of suspended acoustic baffles) in the case of premises where the ceiling height is low enough and where the partitions were highly reflective before soundproofing (e.g. concrete wall, metal deck roof) whereas a difference of noise levels decay per doubling of distance to the source above must involve special achievement that may necessitate the implementation of absorbing wall panels.

This performance can also be expressed (namely at workplaces) in terms of difference (with and without the implementation of the materials for reverberation control) of overall A weighted sound pressure levels or of sound pressure levels in octave bands at specified locations well away from noise sources (in general: no value to a work station too close to a noisy machine).

For orders of magnitude (and with respect to a noise spectrum like "pink noise"), a level difference of up to 2 to 4 dBA can usually be obtained without special requirements at the end of implementation of a suspended acoustical ceiling (or of a net of suspended acoustic baffles) in the case of premises where the ceiling height is low enough and where the partitions were highly reflective before soundproofing (e.g. concrete wall, metal deck roof) whereas a higher level difference must involve a special achievement that may necessitate the implementation of absorbing wall panels.

Predicting the effectiveness of a the reverberation control of a room can be performed with the simulation software SILDIS.

Wich acoustic performance levels and criteria for offices and associated areas or for other tertiary buildings excluding school premises?

In France, the standard NF S31-080 (January 2006) Acoustics - Offices and associated areas - Acoustic performance levels and criteria by type of area addresses the following areas: individual office, collective office, open space, meeting room, relaxation area, restaurant, circulation, tray to accomodate.

Depending on the case, the reverberation time or spatial decay are the basic technical requirement.

For each type of room, the standard defines and classifies the acoustic environment in three performance levels:

- "Current" level is what is required by regulation, and in the absence of legal texts, the minimum functional level which does not ensure acoustic comfort.
- "Powerful" level is acoustic performance beyond the level of "current". This level provides an acoustic comfort conducive to good working conditions.
- "Very Powerful" level corresponds to maximum acoustic performance made ​​possible by action on all different elements of the buiding construction (design, architecture, materials, ....). This level aims at the perception of useful noise and at non-perception of unnecessary noise: so there is a qualitative notion suitable for the purpose and activity to be conducted in the premise.

In addition, NF HQE Tertiary Buildings certification distinguishes buildings of which acoustic performances reflect best current practices. This certification covers the phases of planning, design and construction for new buildings and renovations.

The acoustic criterion of a space and its interactions with neighboring spaces is quantified through two concepts: the sensitivity of the space and the aggressiveness of the space.

The sensitivity of the space relies to the acoustic environment expected by the occupants. The more sensitive the space is, the more hearing emergences (from adjacent spaces or from space itself) are troublesome. One thus distinguish:
o very sensitive areas, such as:
- individual offices, rest areas, infirmary,
- customers private spaces (hotels)
- etc.
Sensitive areas, such as:
- open spaces, empty space awaiting conversion , meeting rooms, dining rooms
- lounges, entrance hall, sports area (hotel)
- etc.
o not much sensitive areas such as:
- hall, waste premise, restrooms, corridors,
- some sales areas, common areas dedicated to customers traffic
(trade)
- bathing areas (hotel)
- warehouse (Logistics)
- etc.

The aggressiveness of the space quantifies the impact of the space on the neighboring space. The more aggressive the space is, the greater the average sound level of the space is and the more the space impacts on neighboring areas. We thus distinguish:
o Highly aggressive areas such as:
- dining areas, auditorium,
- delivery areas, common areas dedicated to customer traffic,
sales areas,
- bar, disco, games rooms, bathing areas (hotels)
- warehouse (logistics)
- etc.
o Aggressive areas such as:
- open spaces, meeting rooms,
- sales areas,
- rooms (hotel)
- etc.
o The non-aggressive areas such as:
- individual office, rest areas, infirmary,
- customers private spaces, reading rooms (or quiet lounges)
library (hotel)
- etc.

Thus, by combining the notion of sensitivity with the notion of aggression, we can classify the different spaces found in buildings in categories according to the sector (office, education, retail, hospitality, logistics).

In terms of generic requirements, the concern referred to as "base level" aims at accounting the location of sensitive and very sensitive spaces with respect to aggressive and very aggressive spaces for inner and outer disturbances while the concern referred to as "performance level" and the concern referred to as "high performance level" aim at optimizing the shape and volume of spaces in which the internal acoustics is an issue.

Depending on the case, the reverberation time or the spatial decay are the basic technical requirement, when this is not the equivalent absorption area.

Wich acoustic performance levels and criteria for school premises?

In France, the Decree of 25 April 2003 on the limitation of noise in school addresses the following spaces:

- nursery lounge, nursery exercise room, nursery playroom
- teachning, music, education, hands-on activities premise ; dining room and polyvalent hall with a volume at most equal to 250 m3
- medical or social premise, infirmary ; toilets ; administration room ; hall ; meeting room ; library ; documentation and information center
- Local education, music, education or business practices of a volume greater than 250 m3, except noisy workshop
- Dining room with a volume greater than 250 m3
- Multipurpose room with a volume greater than 250 m3
- Other local circulations and accessible to students with a volume greater than 250 m3
- Gyms

In all cases, reverberation time is the basic technical requirement.

Which acoustic performance levels and criteria for workrooms?

The Decree of August 1990 the 30th used for the application of Article R. 235-11 of the Labor Code and relating to the reverberation limitation of working premises is applicable to the construction or layout of working premises, where envisaged machinery and equipment may expose workers to a daily sound exposure level greater than 85 dB (A).

This document establishes the minimum characteristics that these rooms must have in order to reduce the reverberation of the noise on the wall when this must significantly increase the level of noise exposure of workers (the increase in exposure is appreciated in relation to what would be the exposure of each of the workers in the same premises ideally treated, that is to say without any reverberation).
The technical requirements of this decree are applicable where it is established that reverberation, as assessed by a predictive acoustic method, would result in an increase of the daily sound exposure level of a worker equal to or greater than 3 dB (A ).

The wall of such spaces shall be so acoustically treated that the decay of the sound level by doubling the distance to the source measured in the empty space without any production machine or installation reaches at least the value given by a rule related to the workroom floor area.
When the decrease in the noise level by doubling the distance at the source is measured in the room after installation of the production machinery and apparatus, the DL value which must be at least reached is given by another rule related to the workroom floor area.

DL is expressed in dB (A).

What measurements in relation to the reverberation of a room?

In France, the measurement of the reverberation time is carried out in accordance with the standard NF S 31-057 - Acoustics - Verification of the acoustic quality of buildings (October 1982) or with the standard (which is often preferred) NF EN ISO 3382-2 - Acoustics - Measurement of acoustic parameters of rooms - Part 2: Reverberation time of ordinary rooms (September 2010).

Production of the acoustic field

The acoustic field is usually produced by an omnidirectional pink noise source (interrupted noise method)

Measuring equipment

The acoustic measuring equipment generally consists of a sound level meter (with a microphone) and of a calibrator.

If used, the equipment for measuring the temperature and humidity of the room is often a thermo-hygrometer.

ITS omnidirectionnal
noise source
ITS
sonometer
 

Transfer and analysis software

A transfer and analysis software is generally used, allowing a deferred exploitation of recordings carried out on site.

Reverberation time (reverberation time) evaluation

The reverberation time is the time needed for the sound pressure level to decrease by 60 dB after the noise source is turned off in a room.

Example of recording of the temporal sound decay allowing the determination of the reverberation time TR for the frequency band 4 kHz (in this case: TR = 0.87 s) with an evaluation range greater than that envisaged in the standard NF EN ISO 3382-2).

(measured)
reverberation time

Note

A value of the reverberation time too high (illustrating excessive reverberation phenomena) can impair the acoustic comfort in some spaces (given their intended purpose) and (in some cases) conformity.

What measurements in relation to the spatial sound decay in a room?

In France, the measurement of the spatial sound decay in a room is carried out in accordance with the appendix of the Decree of august 1990 the 30th used for the implementation of the article R. 235-11 of the Labor code and related to the reverberation limitation of working premises or with the standard (which is often preferred) ISO 14257 Acoustics – Measurement and parametric description of spatial sound distribution curves in workrooms for evaluation of their acoustical performance (October 2001).

Production of the acoustic field

The acoustic field is usually produced by an omnidirectional pink noise source.

Measuring equipment

The acoustic measuring equipment generally consists of a sound level meter (with a microphone) and of a calibrator.

ITS omnidirectionnal
noise source
ITS
sonometer
 

Transfer and analysis software

A transfer and analysis software is generally used, allowing a deferred exploitation of recordings carried out on site.

Evaluation of sound spatial decay

This is the difference in decibels between the sound pressure level in a given octave band and for a microphone position located at a given distance from the reference sound source and the sound power level of the reference sound source in the same octave band.

Determination of the spatial decay of the sound pressure level by doubling the distance

This is the slope, in decibels, of the spatial sound decay curve in a given range, when the source distance doubles.

Note

A value of the spatial decay of the sound pressure level by doubling the distance too small (illustrating excessive reverberation phenomena) can impair the acoustic comfort in some spaces (given their intended purpose) and (in some cases) conformity.

end Faq