Reverberation

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Typical architectural solutions are designed to ensure a building or space conforms to requirements laid down under Building Regulations (Approved Documents) and acoustic guidance documents for specific sectors (such as Education) with BB93.

One of the main causes for poor acoustic performance within a space is reverberation.  If left untreated this can have numerous negative effects. 

The following information on reverberation can be found below:

Diffusion and room modes may have an effect on reverberation but are two independent acoustic properties that are worth knowing about.  

What is Reverberation?

Reverberation is the persistence of sound within a space after it has been produced. Any  sound that is produced will continue to bounce around a room until enough energy has been absorbed. The levels are measured in decibels and calculated based on how long (in seconds) in takes for the noise to decay to the required level (eg. 60dB) once the original noise source has stopped.
If left untreated reverberation can intensify in levels that can exceed that of direct sound.

Wall Panels
Effects of poor reverberation

As most rooms are built with hard, heavy materials to meet sound insulation requirements,  reverberation can, therefore, be a major issue. Given these issues there are mandatory requirements in some sectors (such as Education) to ensure reverberation times meet minimum target levels.Negative effects of poor reverberation levels include:

1. Speech intelligibility

2. Cognitive fatigue and reduced attention span

3. Reduced productivity

4. Loss of profit

Treating Reverberation

Treating reverberation is generally very easy. Simply by adding a suitable quantity of an absorptive  material within the space will reduce reverberation to acceptable limits. An example of such requirements can be found BB93 (examples of tables can be found here).

Treatment may also be undertaken purely to improve ambiance, i.e. as in a restaurant to create a more pleasant atmosphere.  

We identify requirements,  either through reverberation testing, predictive assessment or acoustic modelling. Once we understand a room’s base line reverberation time we can then calculate the amount of material required to meet target performance levels.

Once we know the amount of treatment required we will explore available products according to their sound absorption characteristics, deciding the best place to fit them within the room, depending on available free wall or ceiling space. 

Sound Absorption Information

All acoustic absorbers should have at least the following information detailing how they perform. Normally these are calculated through laboratory testing a suitable amount of material within a reverberation chamber although site testing can be undertaken to determine the performance criteria.

        • Sound Absorption Coefficients: shown as a figure between 0 and 1 (may exceed 1 in some cases) that represent how much of the sound energy is absorbed at each frequency (0= 0% and 1 = 100%)

BS EN ISO 354 is the laboratory based standard used for calculating coefficients

        • Sound Classification: represents how well a product works within a predefined range of absorber classes. These bands go from Class A (best) to Class E (worst). As a general rule you need around 50% less material at Class A as opposed to Class C products.  

Classification bands are assessed to BS EN ISO 11654: 1997

        • α w: is a single figure A weighted sound coefficient rating derived from the individual sound coefficients (250hz – 4000Hz). The rating is calculated through the use of a reference curve and rules.
Thickness
(mm)
Sound Absorption Coefficient (BS EN ISO 354)α wAbsorber Class
125Hz250Hz500Hz1000Hz2000Hz4000Hz
250.100.450.881.001.001.000.75C
500.400.950.900.951.001.000.95A
Absorptive Materials

Unlike sound reduction that uses hard, heavy materials to reduce noise from travelling between two spaces, with reverberation we need to add porous materials (usually soft) that are capable of allowing sound energy to pass through them. As sound energy passes through these materials friction occurs between the material and sound energy that creates heat. This in turn means that the sound energy is diminished and  the reflected sound energy is considerably reduced. 

The majority of acoustic absorbers are manufactured from one of three main core types:

      • Mineral Fibre (Glass or Stone based products)
      • Foams
      • Polyester

The thicker the core material the better the product will perform in controlling reverberation. For example most Class A products tend to be between 40mm and 50mm thick and at Class C a panel would be 25mm thick.

Panels at 25mm thick can be used as a Class A absorber if they are installed with a 25mm air gap between the rear of the panel and the substrate or soffit.

Given that the core materials tend not be visually attractive an outer layer is normally added to give these panels an aesthetic finish.

Finishes

The different acoustic cores each have their own strengths and weaknesses with mineral fibres (for example) tending to be lightweight and cheap but the fibrous nature of the material means that fibre migration in to the atmosphere is an issue and can cause irritation if you come in to contact with it.

Foam acoustic cores may be left untreated depending on the use and design required but as with mineral fibre based panels they can usually be treated using the same materials and methods.

A pre-requisite of any product or finish an architect or designer is looking to achieve is one of an aesthetic and pleasing style that suits the design and feel of a space. Commonly used products or finishes include:

            • Acoustically transparent fabrics from suppliers such as Camira Fabrics
            • Timber panels or slats (cannot be solid as sound will not be able to pass through in to the core)
            • Perforated steel liner systems or trays that can be powder coated to pretty much any RAL colour
            • Flock or other adhered coating

Some materials such as polyester or foam may already be coloured and not need an additional facing but normally the range of colours and finishes are limited.

 
Types of absorbers

A variety of Acoustic absorbers are available, depending on where and how they are going to be fitted. Fixing methods will vary by product and manufacturer and should be checked at the time of specification for suitability.

Absorbers are generally placed on the wall or ceiling and for best results should be placed on both where possible.

Wall panels tend to be stand-alone feature panels or a liner panel system that is designed to blanket cover an entire wall. The panels can be either bonded direct to the wall or fixed using a clip system to make them removable.

Panels can be shaped or finished in fabrics that have bespoke printed images, according to a customer’s requirements.  

Timber and steel finishes are also widely available  and popular given the finishes available and how well they fit in with architects’ design requirements.

Ceiling panels are usually supplied as  suspended rafts and comprise an acoustic panel hung in the horizontal plane. These panels come in a variety of shapes and sizes allowing a wide range of design options. One of the most common suspended ceiling systems is to use a suspended grid with proprietary ceiling tiles that have an acoustic rating.

A series of acoustic baffles can be used on larger spaces or areas with high ceilings and consists of long narrow panels hung vertically in rows, usually at centres of around 600mm between rows.

Acoustic panels may also be bonded to the ceiling however these tend to be manufactured from lightweight foams to reduce the loads imposed on the chemical bond.

As with wall panels, timber and steel systems are popular due to the number of available finishes. 

Specialist systems such as stretched fabric solutions usually consist of a profile that is fixed to the wall.  Fabric is gripped by inserting it in to the profile. Such systems are popular as the panel size is only restricted by the fabric width and length (installation issues may also limit panel size) and can also use printed imagery on the fabric. The choice of acoustic core can also be varied to allow for more control in treating the various frequencies. 

Diffusion

When dealing with some fields in acoustics, (music for example) we do not want a completely dead room (totally free from reverberation). We incorporate diffusers into the design in order to direct sound in a particular direction or directions. 

Diffusion is the practice of evenly spreading sound energy around a room or environment. A room with perfect diffusion would have a reverberation time that is the same at every point within the room. 

Diffusing sound can be achieved by reflecting sound energy back in to the room in a specific direction. Typical diffusers can be plain angles or curved panels, pigeon hole or stepped panels. The choice of diffuser will depend on a number of factors including shape of room, sound energy within the room (including reverberation) and specific frequencies that need to be treated. 

Recording studios require the right acoustic atmosphere
Diffuser
Room Modes

There are three different types of room modes and each one describes how sound energy is reflected off various surfaces.

Axial room modes tend to move parallel with the floor and ceiling, bouncing off opposite walls. This mode can be across the length, width or height of the room. 

Tangential room modes bounce off each wall in a diagonal path, in effect diagonally across the room either from wall to wall or floor to wall and ceiling.

Oblique room modes allow sound to bounce off  the corners of walls at an angle, alternating between a high position in one corner to a low position in the opposite corner.

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