Acoustic design of SEN (Special Educational Needs) classrooms

 

 

If you design a school, you will probably need to have at least one SEN (Special Educational Needs) classroom.

Given the very specific acoustic design requirements of such spaces, this post sheds light on:

• why the acoustic environment is important for SEN classrooms.
• what type of acoustic environment is suitable for SEN classrooms.
• what you need to think about for the acoustic design of SEN classrooms.

This post mostly draws on the following British documentation:

• BB93: acoustic design of schools – performance standards.
• Acoustic of Schools: a design guide.

 

Why is the acoustic environment important for SEN classrooms?

Some pupils in SEN classrooms will have special hearing requirements. 

As listed in the Acoustic of Schools: a design guide, these pupils could be those:

 

Of course, the ability of the teachers to organise and manage the SEN classrooms is paramount. But the acoustic environment also has to be suitable for these pupils to be able to hear, concentrate, learn and communicate. 

 

What type of acoustic environment is suitable SEN Classrooms?

If you design a (several) room(s) for pupils with special hearing requirements, you need to be aware that the acoustic quality of the environments will have to be a step further compared to most standard school spaces. 

Generally, you will need:

• Quieter environments.
• Less reverberant (or less echoey) environments, especially at low frequencies. Too much low frequency reverberation can mask the sound of the teacher’s voice and make it harder to clearly hear what (s)he says (If you need to better understand what sound reverberation is, you can read Sound reverberation – Part 1: Basics).

 

Note: For open-plan spaces, BB93 states the following:

“Open plan teaching and learning spaces should not be regarded as a simple alternative to traditional classrooms and may be unsuitable for some children, particularly those with special hearing or communication needs.

In order to fulfill their duties under the Equality Act 37 2010, school client bodies should anticipate the needs of deaf and other disabled children as current and potential future users of the space when open plan accommodation is being considered.”

 

What do you need to think about for the acoustic design of SEN Classrooms?

Quiet environments

To design quieter environments, you could need to control the noise from various areas. 

First, you need to control the noise from outside. This could include:

• Designing the building façade to control the external noise breaking into the classroom.
• Selecting the right ventilation strategy. The external noise environment could be too loud to ventilate the rooms by simply opening the windows. Even during the hottest days of the year. In this case, you need to set a suitable mechanical ventilation strategy.
  

  Note: under BB93, the internal noise level requirements in schools can be relaxed to improve thermal comfort in summer (during the hottest 200 hours in peak summertime exactly) at the expense of high indoor ambient noise levels. But this relaxation doesn’t apply to SEN classrooms!

 

Secondly, you need to control the noise from the services installed for the school, like the ventilation systems and/or heating & cooling systems. They are likely to require:

• Attenuators to control the fan noise transmitted through the ductwork.
• If some units (for example fan coil units, heat recovery units, VAV boxes, etc) are hung from the soffit, you could need a dense and sealed suspended ceiling to control the noise from the units themselves. Alternatively, sealed cupboards or encasement systems made with dense materials could be required if the units are in other places of the room.

 

 

• Controlling the noise from inside the school by improving the sound insulation of the floors, walls and doors that separate the SEN spaces from other spaces. Zoning the SEN rooms and/or creating buffer spaces around them is also possible.

Less reverberant environments

To create environments that are less reverberant, especially at low frequencies, an efficient acoustic design could involve the following:

• Installing more acoustic absorption on the walls or fixed on/hung from the soffit.
• Installing some materials or elements that absorb more acoustic energy at low frequencies. You can efficiently do this by installing a dense porous suspended ceiling with a (large) cavity above.
• Planning smaller spaces, as the smaller the volume the less reverberant (echoey). SEN rooms are generally smaller anyway, hosting between 4 to 8 pupils most of the time.
• Laying a carpet. On its own, a carpet will not make you achieve the reverberation time you target for a classroom. However, it can help you reduce the need for (expensive) acoustic materials.

 

 

Design Note: Sometimes it might not be practically possible to achieve the very stringent sound reverberation requirements for SEN classrooms. 

In this case, you can raise an Alternative Performance Standard (APS). You will need to justify why you can’t practically (and not financially!) achieve the initial requirement and suggest an alternative performance with the help of the acoustic consultant. BB93 documents a thorough procedure to follow for Alternative Performance Standards.

 

Procurement tip: During the early stages of a school project, it is normal to have minimum specifications for the acoustic absorption materials proposed in each space. They will generally be selected in terms of acoustic absorption Classes (i.e. A, B, C, D, …).

As the detailed design approaches, you will have a better idea of the products you want to use. 

Once the products are selected, it might be useful to ask the acoustic consultant to review the quantities of materials. This will involve gathering the absorption test data for the specific products and re-running the calculations. By doing this, you are likely to reduce the quantities of materials and therefore reduce cost.

 

 

 

 

 

Laboratory or on-site sound insulation rating. Which one to use?

 

 

 

 

You are probably trying to select dry lining products for your project and can see two different ratings on the acoustic specification (or the mark-ups). 

One is R_w, which is a laboratory sound insulation rating. The other is D_nT,w , the on-site sound insulation rating. 

 

    Note: for ease of reading we will stick to R_w=lab rating and D_nT,w= on-site rating. But be aware that the terms can vary depending on the situation including the types of spaces separated by the partition, the type of building and the country. 

 

They both represent the reduction in sound transmission through a building element. In more technical terms, we say they correspond to the airborne sound insulation performance of the element. 

This post explains:

• which sound insulation rating to use for your selection
• what both ratings actually mean and what you use them for
• why, on the acoustic specification, the ratings have different values for a single partition type
• why some partitions only have a laboratory rating
• how to remember which one is which without reading this post again

Which sound insulation rating to use for your selection?

The quick answer is: use the laboratory rating (R_w).

 

What are the laboratory and on-site sound insulation ratings? and what are they used for?

In practical terms, both ratings represent the difference between the noise levels measured on either side of an element. 

The general principles for the measurements are as follows:

• generate sound in one room, the source room.
• measure the sound level in this source room.
• measure the sound in the room that shares the partition studied, the receiver room. 

 

 

Both airborne sound insulation ratings correspond to different types of testing conditions.

The paragraphs below explain this a little bit more.

Laboratory sound insulation rating

The laboratory rating, R_w, is useful for suppliers or manufacturers. This way, they give us an idea of the performances their products can achieve.

In a lab, you test the products in very specific and standardised conditions:

• The samples tested have a set size and they are connected to specific elements.
• The testing rooms have particular sizes, shapes and acoustic characteristics. 
• The performances are measured and calculated in a certain way.

These allow the acoustic consultants to use the laboratory rating as a benchmark for their design. Although they know there is more work to do to achieve what they need.

On-site sound insulation rating

The on-site rating, D_nT,w , is given as a requirement in most standards, guidelines and building regulations. 

It represents the sound insulation performance you are contracted to achieve once a building is completed. 

Therefore, this is the sound insulation performance the acoustic engineer will test during the commissioning of a building.

Ok, but why are the ratings different on the acoustic specification?

As mentioned above, the testing conditions in a lab are very particular.

They are designed in a way to minimise the sound transmissions around the samples.

In technical terms, you call it sound flanking. It happens via:

• the connections
• the services
• the flooring systems/slabs
• the ceiling systems/slabs
• other paths that connect the building elements

 

Acoustic consultants know that the sound flanking is more important in actual buildings than in laboratories. So they underestimate the lab rating by applying a negative correction to take account of the sound flanking. 

This correction is generally around 5-7 dB for drylining partitions. It is less for masonry partitions. 

 

    Note: if you are working with “lightweight” partitions that need to achieve ratings with an extension at low frequencies, you might need to use a higher correction. In other words, you need to underestimate even more the laboratory rating.

This is particularly the case for timber constructions.

 

Be aware that, even if you apply this correction, you are not exempt from spending time designing the junction details between the partitions and the other building elements. 

 

And why do some partitions only have one rating?

Because you either won’t have to or won’t be able to test them.

Examples are:

• partitions with openings, doors, serving hatches or windows/vision panels 
• service risers
• lining or boxing systems for services and structural elements
• lift shafts

 

How to remember which one is which?

Any easy way to remember is to look at the subscripts. 

As described above, both ratings are based on the sound level difference measured between a source room and a receiving room separated by the partition studied.

R_w is the rating measured in a laboratory under standardised conditions, i.e. the same for all the measurements. It can be used as a benchmark and the measurements are weighted. Hence the “w” in the subscript (it corresponds to the way it has been calculated but we won’t go into details here). 

D_nT,w is the rating measured on-site under building conditions, i.e. different for most measurements. So as well as weighting the measurements (“w” in the subscript), you also need to standardise them. Hence the “nT” in the subscript.

 

    Note: Standardising the measurements takes account of the specific sound reverberation conditions in the receiving room. It gives a more subjective performance for a partition depending on how and where it is installed in a building.

 

So the subscript for the on-site rating is longer than for the laboratory rating, because you need to consider the particular conditions on-site. 

 

 

Room Geometry

Prior to setting any acoustic criteria, it is important to know the use of each room.

A room could be for:

• teaching theory with no instruments played.
• single musicians or singers to practice.
• small ensemble or group practice.
• loud instruments. 
• large ensemble or group rehearsal.
• performances. 

With this in mind, the acoustician can help you set the right dimensions, volume and shape for each room.

Room dimensions and volume

As mentioned inAcoustic design of schools for performing arts – Part 1: Design aspects, finding a balance between the sound reverberation and the loudness of a room is key to obtain ideal acoustic conditions.

This is done by setting the dimensions and the volume depending on:

• the number of musicians practicing, rehearsing or performing
• the type of music played (amplified or acoustic)
• the instruments likely to play inside (ex: pure singing, quiet instruments or loud instruments) 
• the size of the audience or the orchestra (for performance spaces or rehearsal rooms)

 

For early guidance on ideal room dimensions and volume, check Acoustic design planning of music spaces. 

Room shape

The shape of a room is a basic feature that allows you to direct the reflected sound where and when you want.

Acoustic experts are unanimous about the shoebox shape for most music spaces. For performance spaces, that shape is useful to reflect sound, coming from the stage, back to the side of the audience. This is proved particularly beneficial for the acoustic comfort of the audience and the perception of the musical performance. 

Other shapes are also well known to be a “no-no” for acoustics. Any domes, curved shapes or hipped roofs can focus sound and should be avoided.  

It is also common practice to angle the opposite walls of small rooms and recording studios. This is avoid any flutter echos and standing waves to be created between parallel walls. 

Building layout

The sound insulation design of a school for performing arts generally requires more consideration and higher performances, leading to higher construction costs. 

You can minimise this by optimising the layout of the school. 

Here is what you could do:

• Position the sensitive spaces away from the loud areas.
• Create acoustic buffers around or adjacent to the sensitive spaces (like the rehearsal and performance spaces). They can be quiet corridors, quiet spaces or lobbies.  
• Position the sensitive spaces away from the external sources of noise. That way, you minimise the need for high performance building façade to control the external noise getting in the building. Buffer areas, like corridors, can also be used in this case.
• Position the loud areas away from the external noise sensitive receptors. This can avoid the need for a high performance façade.

Internal finishes and furniture

For music rooms, especially for the larger spaces dedicated to performances and formal rehearsals, choosing the right finishes and furniture will be one of the most important design tasks. It is also a very specialist, scientific and (even) artistic task.

Although a lot could be said about room acoustic design, below are some general trends that are worth thinking about early.

Sound reverberation at low frequencies

As mentioned in Acoustic design of schools for performing arts – Part 1: Design aspects, if you want large spaces to accommodate acoustic music rehearsals or performances, you need to consider their reverberation characteristics at low frequencies (below 500Hz).

The starting point is to work on the right density and stiffness of the surfaces (walls, floors, ceilings, joinery, sound reflectors/diffusers, etc). You need to find the right balance between:

• The surfaces being too light and flexible, so they absorb too many low frequencies and will cause the music to sound too dry.
• The surfaces being too heavy and stiff, so they don’t absorb enough low frequencies and will cause the music to sound too “boomy”.

 

Variable acoustics

In some spaces, you may want to be multi-purpose by organising various types of activities and events. These could be acoustic music, quiet music, loud music, amplified music, pure teaching or drama performances. 

Each of these types have different optimal reverberation conditions. You therefore need to be able to change reverberation conditions.

You can do this with variable acoustic systems. Their general principle lies in revealing or hiding an acoustic absorber. So that, a room is less reverberant when the absorber is revealed and more reverberant when the absorber is hidden.

Such systems are:

• Thick acoustic curtains extended along some walls or stored in dedicated cupboards.

• Acoustic banners hung along the walls.

• Rotating acoustic systems that usually have one absorptive surface and another (flat) one reflective. Some systems have a third surface that diffuses the sound.

• Retractable seating that make a room less reverberant when pulled out and more reverberant when stored.

• other systems exist like openable timber slats, panels with variable perforation or even inflatable absorbers (aQflex and aQtube from FlexAcoustics). 

Seating

If you choose to have a performance space for acoustic music (i.e. non amplified), you may want to invest in a high-end seating system. 

In this type of space, most of the acoustic absorption is provided by the audience. Therefore, you run the risk to have very different reverberation conditions depending on the size of the audience.

To avoid this, you can design the seating system to have similar absorption characteristics whether the seats are full and empty.

Also, a good seating system should not generate noise when operated. You don’t want to hear the noise from people walking to or off their seats in the middle of a performance. Neither do you want to hear the noise from the seats tipped up or down.

 

    Procurement tip: You should purchase the seating system early in a project. And ideally, you should do it outside of the main building contract. This is because the potential laboratory tests of the system selected by the contractor could be delayed and data not available prior to the completion of the design.

 

Acoustic diffusing surfaces

Diffusing surfaces can be of many forms and shapes. They allow to scatter the reflected sound energy in many directions. 

For some spaces for performing arts, you could benefit from using diffusing surfaces to: 

• avoid flutter echoes and room mode effects from sound waves bouncing back and forth between two hard parallel surfaces. You could also use absorbing materials, but some rooms might sound too dead with the additional absorption. 
• spread the sound energy evenly on an audience or a large ensemble

 

Building services noise and vibration

The acoustic design implications of exposing CLT floor slabs

 

 

 

 

Are you designing a building with slabs made of Cross Laminated Timber? And would you like the underside of the slabs to remain exposed? Great idea! It will look good and feel more relaxing. 

But if you expose the slabs, you have no ceilings (of course!). This is likely to make the acoustic standard of the building harder and dearer to achieve. 

Ceilings might not always be visually appealing, especially when you can expose a timber structure instead, but they have several acoustic design functions that can lead to important cost savings. 

This post firstly presents the acoustic functions of suspended ceilings. It then summarises some design alternatives you could think about if your budget allows you to expose the CLT slabs. 

Acoustic design functions of suspended ceilings

Acoustic designers can advise for the installation of suspended ceilings to control:

• the airborne sound insulation performance of floor constructions.
• the impact sound insulation performance of some floor constructions.
• the sound flanking:
  • above partitions (through the slab and via the gap at the head).
  • via building services and structural penetrations through partitions.
  • via structural elements (like the beams) running across partitions.
• the sound reverberation within the spaces.
• the noise of building services hung from the soffits.

The sketch above pictures the above functions. 

Alternative design solutions, to using suspended ceilings, exist but very few of them combine all the above functions. Each alternative has its own cost, which overall is likely to require a higher budget than when suspended ceilings are installed.

 

Acoustic design alternatives

You could justify a higher budget to benefit from the aesthetics of the exposed timber and other qualities like smell, healthy feeling or also absorption of humidity.  

In that case, this section presents examples of solutions and materials that could help you achieve the acoustic standards you target for the building.

Note: Generally, the detailed solutions depend on the acoustic requirements, the structural design and the layout of the building. Anything shown below is just indicative.

 

CONTROLLING THE AIRBORNE AND IMPACT SOUND INSULATION 

CONTROLLING THE SOUND FLANKING ABOVE CRITICAL PARTITIONS

For any building, the acoustic designer pays important attention to the junctions between the construction elements. There is a need to control the vibrations transmitted from one element to another, up to the partitions (walls or ceilings) that re-radiate sound in the neighboring spaces. 

Timber constructions are very sensitive to such transmissions. Therefore an important part of the acoustic design is to find effective methods to decouple the elements. 

The most relevant methods depend on the structural design, the architectural design and the acoustic performances you target. Although, if you want to expose the underside of the CLT slabs, slab breaks and resilient strips are good solutions to think about from an early stage.

 

Slab breaks

Breaking the slabs above the partitions reduces the transmission of vibrations from one side of the partition to the other. Doing it might increase costs, but it can be very acoustically efficient in some cases.     

See the figure below showing where a slab break could be. 

 

Resilient strips (or resilient interlayers)

Resilient strips (also called resilient interlayers) are installed at the junctions of the CLT panels to control the transmission of vibrations in the rest of the structure. Most strips are made of elastomers but some use cardboard filled with sand. 

When exposing the CLT slabs, you install resilient strips above the critical partitions and possibly at the bottom of the partitions resting on the slab above.

See below an example to illustrate this.

To minimise acoustic bridging, resilient strips can also be used to decouple the fixings that hold the CLT elements together.

If you are looking for examples of resilient strips available on the market, visit Atelier Crescendo’s Acoustic Design Catalogue here.

Note: The anti-vibration characteristics of resilient strips depend on the load applied to them and how they are compressed. Therefore, the ideal specification for them will be the result of a thorough coordination between the structural designer and the acoustic designer.  

 

CONTROLLING THE SOUND FLANKING THROUGH THE BUILDING SERVICES PENETRATIONS

In the absence of a ceiling, the acoustic designer will generally advise you to not run the services (ductwork, cables, etc) through critical partitions (i.e. those acoustically rated and with an on-site requirement).

Instead, they should preferably run through the partitions that include a door. You will still need to carefully seal the penetrations to avoid any acoustic ‘leakage’. Dense mineral wool, plasterboard and non-setting mastic are examples of materials you can use for this. 

 

CONTROLLING THE SOUND FLANKING VIA THE STRUCTURAL ELEMENTS

In the absence of a ceiling, you are likely to require to box in the structural elements. Although it depends on the acoustic rating of the partition the structural element is going through.

Boxing in structural elements generally involves plasterboard and mineral wool inserted in the air cavities.

 

CONTROLLING THE SOUND ABSORPTION WITHIN CRITICAL ROOMS

If you expose the timber slabs, you can’t rely on the ceilings to control the sound reverberation in the spaces. Instead, you will need to use sound absorptive elements and finishes that are either suspended from the soffit or fixed to the walls and the floors. These could be:

• Horizontal acoustic rafts suspended from the soffit.
• Vertical acoustic rafts also suspended from the soffit.
• Acoustic wall panels fixed on the walls.
• Carpet.

Depending on the space, you need to find a combination of all the above to achieve suitable reverberation conditions. 

 

CONTROLLING THE NOISE GENERATED BY BUILDING SERVICES

If the building services (like FCUs, VAV boxes, etc) hung from the soffit are too loud and you don’t have a ceiling to attenuate the noise, the solutions would either be to insulate the casing of the units, encasing them (within bulkheads for example) or even sometimes fit new attenuators.

You could also think about reselecting the units for quieter ones. 

 

Final thoughts

If you expose the CLT slabs, you should expect more site inspections as any loss of quality in the workmanship will have a big influence on the acoustic performance of the walls and the floors.

 

Continue reading “Why we need to think beyond building regs for the sound insulation of CLT constructions” →