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

The reduction of the reverberation of a room (due to the reflections of sound waves on the walls which delimit it and also on objects of different nature) is required in many contexts:

• spaces with young children
• classrooms and training facilities of educational establishments
• music rooms (with or without amplified music broadcast)
• sports halls and gymnasiums
• catering areas (e.g. canteens)
• work premises (e.g. workshops, open space offices, meeting rooms)

Depending on the case, the reduction of the reverberation of a room can be envisaged in the perspective of:

• improving speaking and listening comfort
• increasing confidentiality and discretion
• limiting of ambient noise levels e.g. with regard to the prevention of hearing risks

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 ^[1]
• the use of the premise ^[1]
• the level of acoustic performance wich is required (desired reverberation time, desired sound pressure level, desired spatial sound decay) ^[1]
• 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.

In the particular case of industrial premises, the location of noisy equipment (machines, production lines), their sound emission spectrum and the location of workstations (which one wishes to protect) constitute additional useful data in the framework of a project for which the objective is a reduction of the reverberated noise (resulting from the reflection of acoustic waves during their mean free path between the point of emission and the point of reception).

All of the useful input data for reducing the reverberation of a room can be used (as at ITS: by an acoustician qualified in building physics):

• for predictive acoustic calculations (in some cases: with numerical simulations based on sound ray tracing involving a 3D modeling of the considered space)
• for the selection or development of construction systemshaving appropriate acoustic characteristics (i.e. an acoustic absorption coefficient often as close as possible of 100 % in the frequency range of interest)
• for the prediction of values possible to be obtained, after works, for the reverberation time and when it makes sense, for the spatial sound decay, for the reduction of noise levels; of course the evaluation by ITS of these acoustic indicators before works is also possible

Soundproofing materials and corresponding installation works can also be marketed: ITS masters all aspects of the question of useful input data for reducing the reverberation of a room.

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^[1] 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 efficiency for acoustic screens (noise barriers) ?

The effectiveness of acoustic screens (noise barriers) is very frequency dependent and is primarily related on the one hand to the performance of acoustic insulation panels (characterized by their sound reduction index) as well as when appropriate (for applications other than road noise barriers or railway noise barriers) - to the performance of silencers but also to the number and sizes of cutouts and openings, to sealing imperfections, and to the transmission of bridging noise. Moreover, this performance depends on the screen size and on the relative positioning of the noise source and receiver with respect to the acoustic screen (sound barrier) - and also on the size of the sound source -.

The prediction of the effectiveness of an acoustic screen (noise barrier) can be performed with the simulation software SILDIS^®®.

Verifying the performance of a baffle (noise barrier) can be performed (as appropriate) in accordance with ISO 11821 Acoustics - Measurement of the in situ sound attenuation of a removable screen or NF EN 1793-1 Road traffic noise reducing devices. Test method for determining the acoustic performance. Part 1: intrinsic characteristics of sound absorption or NF EN 1793-2 reducing devices road traffic noise - Test method for determining the acoustic performance - Part 2: intrinsic characteristics related to the airborne sound insulation.

>> Which acoustic office screens?

Which acoustic insulation panels for an acoustic screen (noise barrier) in the workplace in the industry ?

With regard to metallic sound insulation panels (also called soundproofing panels) used for current applications in industrial soundproofing such as building acoustic screens (noise barriers) at workplaces in industry with respect to noisy machineries or equipments, they generally consist of an outer casing (usually made of non-stainless steel, most of the time: with paint) and of an absorbing filling (quite often: mineral wool) covered by a surfacing (mostly: a fiber glass) and also a perforated protection (mostly: galvanized steel). For the outer casing, a powder coating usually provides the best possible protection against corrosion: classification up to C5 according to ISO 12944.

In some cases, the envelope of acoustic insulation panels of cabins can be made of stainless steel (SS 304, SS 316) or aluminum.

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

In order to increase the sound reduction index in a given frequency range, an intermediate plate (usually made of steel) and an additional spacing absorbent layer may be inserted into the acoustic insulation panels, the performance of such insulation panels being based on the double shell effect.

For specific cases, a damping material (viscoelastic) can be incorporated.

An acoustic screen (noise barrier) with two absorbent faces can be used in case of separation of two adjacent noisy workstations (boxes for hammering, grinding, deburring ...)

Acoustic performance of sound insulation panels for cabins (in terms of sound reduction index) is very frequency dependent, being related mainly (besides the quality of the joint between panels basing their airtightness ):

- for panels with simple shell effect: mainly to the total mass density

- for panels with double shell effect: mainly to the mass density of each of the facings and to their spacing (the mass density of the inner absorbing lining may also be accounted if significant)

The prediction of acoustic performance of the partitions of an enclosure can be performed with the simulation software SILDIS^®®.

The verification of acoustic performance of the partitions of an enclosure can be conducted in accordance with ISO 140-3 Acoustics - Measurement of sound insulation in buildings and of building elements - Part 3: Laboratory measurements of airborne sound insulation of building elements

Which are the input data useful for sizing the walls of an acoustic screen (noise barrier) ?

The input data useful for sizing the walls of an acoustic screen (noise barrier) are mainly the conditions prevailing in the environment of the envisaged acoustic screen (noise barrier) (and the degree of aggressiveness with respect to the walls that may result from the existence of projections of liquids such as water in the case of an outer shield along a road, for example), the existence of projections of solids (wood chips in the case of grinding boxes for example, gravel in the case of an outer shield along a road, for example, ballast in the case of a screen outside along a railway track for example), the level of acoustic performance required depending on the frequency (insertion loss, residual noise level) and also - where appropriate - the existence of additional constraints such as possibility of disassembly, reaction to fire, protection against radiation or heat, or the need for the walls to be translucent to allow the vision etc ...

In the case of constructions located outside, the resistance to weather and to corrosion must also be taken into account, what may affect the definition of appropriate construction systems.

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

Whether divisions of large rooms, or noise reduction for individual workstations, or protection of individual workstations from other workstations being more noisy, or road noise barriers, or railway noise barriers are concerned, the dimensioning of the walls must account the needs in terms of sound reduction (i.e. the ability to oppose the transmission of noise), the effects of amplification of sound levels related to the existence of the screen itself and also (especially for applications in the workplace) accessibility requirements in terms of gates whose effectiveness is related to the airtightness (rotating doors, sliding doors, folding doors, rolling gates), the need for cuts for the passage of material flows (raw material inputs, outputs of finished products), and also (when for example the treatment of thermal generation equipment or of cooling equipment is concerned) the needs for air circulation (ventilation. ..), the need for cuts for passages of ancilliaries (cables, ducts, pipes ...).

These different input data are necessary for the selection of products and of construction systems appropriate to the acoustic performance required for the walls of an acoustic sreen (noise barrier) dependent on the acoustic performance and on the respective area of its various sub sets: acoustic insulation panels, access, glass frames, silencers, cuts and openings.

Which ventilation for a cabin (for soundproofing or soundproofed) ?

The design and implementation of a cabin - construction specially designed to protect people (e.g. machine operators) from ambient noise, consisting of a fully enveloping structure - being efficient in terms of sound insulation makes it necessary to involve constructions often very airtight and often having moderate heat loss.

The ventilation of a cabin is related to the need for health air renewal ventilation for the occupants and may contribute to their thermal comfort (in summer: if the air introduced into the cabin is fresh enough, in winter: if the air introduced into the cabin is warm enough).

The required ventilation flow rate is related to the number of occupants, to the potential thermal disspation of equipment installed inside the cabin (lighting, computer equipment ...), to the performance in terms of thermal insulation of the partitions of the cabin and to the acceptable inlet temperature gradient (difference between the temperature within the cabin and the temperature of the air of the ventilation system). The existence of a ventilation system in a cabin usually leads to the need to implement silencers (one for air inlet and one for the air outlet to limit the propagation of noise through these openings) and sometimes to have to silence the fan (especially for the acoustic comfort of the occupants of the cabin). The installation of booths in some particularly hot environments (in relation to climate, or due to the presence - close - of sources of heat like an oven or due to the existence of certain processes in the paper industry, the cardboard industry or glass manufacturing for example) may necessitate the implementation of air conditioning equipment.