Acoustic Ratings Explained: Rw, DnT,w, L’nT,w and αw

Acoustic specifications can contain a confusing mix of letters, numbers and symbols.

You may see a wall described as achieving Rw 55 dB, a completed building tested to DnT,w + Ctr, or an acoustic panel given an αw rating and absorption class.

These figures all describe acoustic performance, but they do not measure the same thing.

Some ratings relate to how well a building element blocks airborne sound. Others measure the performance achieved between completed rooms. Some relate to impact noise through floors, while others describe how much sound a material absorbs within a room.

Understanding the difference helps architects, developers and contractors specify the right performance and avoid comparing figures that are not directly equivalent.

What is an acoustic rating?

An acoustic rating turns a set of measurements across different frequencies into a simpler, single-number value.

Sound behaves differently at different frequencies. A wall may perform well against speech but less effectively against low-frequency music or mechanical plant. Testing therefore produces a range of results rather than one figure.

A rating method converts those frequency-by-frequency results into a single number that is easier to:

• Compare between products or constructions
• Include in an acoustic specification
• Use during design calculations
• Check through testing
• Compare with a project target
• Refer to in planning or building control documents

The rating only makes sense when you know what has been measured and how the test was carried out.

An Rw rating, for example, cannot be treated as directly equivalent to an on-site DnT,w result.

A quick guide to decibels

Acoustic ratings are generally expressed in decibels, written as dB.

The decibel scale is logarithmic rather than linear. This means that a change of a few decibels can represent a meaningful difference in sound energy.

A 3 dB increase represents approximately twice the sound energy, although it will normally be only just noticeable to a listener. A change of around 10 dB is often perceived as approximately twice or half as loud.

This distinction matters because sound energy and perceived loudness are not the same thing.

Ratings also need to account for frequency. The human ear is more sensitive to some frequencies than others, while different walls, floors and acoustic materials can perform very differently across the frequency range.

What is the Rw rating?

Rw stands for weighted sound reduction index.

It is a laboratory rating used to describe how well a building element reduces airborne sound passing through it.

Products and constructions that may be given an Rw rating include:

• Walls
• Floors
• Doors
• Windows
• Glazing systems
• Façade elements
• Roof constructions
• Acoustic partitions
• Ventilation components

The element is tested between two specially designed laboratory rooms. Sound is generated on one side, and the amount transmitted to the receiving room is measured across a range of frequencies.

Those results are then converted into the single-number Rw rating.

Is a higher Rw rating better?

Yes. A higher Rw value generally indicates better airborne sound insulation under laboratory conditions.

For example, a construction rated Rw 55 dB provides greater laboratory-tested sound reduction than one rated Rw 45 dB.

However, the difference should not be interpreted as an exact prediction of what someone will hear in a finished building.

The result applies to the element and test arrangement used in the laboratory. Its performance on site will also depend on the surrounding construction, installation quality and alternative sound paths.

What are C and Ctr?

An Rw rating may be shown with a spectrum adaptation term, such as:

• Rw + C
• Rw + Ctr

These terms help account for different types of sound.

The C term

The C term is generally associated with sources containing a broader spread of mid- and higher-frequency sound.

Depending on the project, this may include everyday living activity and some forms of transport or building noise.

The Ctr term

The Ctr term gives greater weight to lower-frequency sound.

This can be relevant to sources such as:

• Road traffic
• Aircraft
• Music
• Some mechanical plant
• Other sound with strong low-frequency content

Ctr is usually a negative number. Adding it to the Rw value therefore produces a lower result.

A wall described as Rw 50 dB may, for example, have a lower Rw + Ctr value. This shows why the headline Rw figure alone may not tell you how the construction will perform against the source that matters to the project.

What is DnT,w?

DnT,w is the weighted standardised level difference.

Unlike Rw, it is normally measured between completed rooms in a real building.

During the test, sound is generated in one room and measured in both the source room and the receiving room. The result is adjusted to account for the reverberation conditions within the receiving room.

The final DnT,w result reflects the combined performance of the completed construction, including:

• The separating wall or floor
• Junctions with surrounding elements
• Workmanship
• Doors and openings
• Service penetrations
• Voids and cavities
• Flanking sound transmission
• The geometry of the rooms

Is a higher DnT,w result better?

Yes. A higher DnT,w value indicates better airborne sound insulation between the two tested rooms.

However, it should only be compared with another value expressed using the same rating method.

What is DnT,w + Ctr?

DnT,w + Ctr combines the site-measured airborne sound insulation result with the low-frequency spectrum adaptation term.

It is used within Approved Document E in England when assessing airborne sound insulation between certain dwellings and rooms for residential purposes.

Because Ctr is normally negative, the DnT,w + Ctr result will usually be lower than DnT,w on its own.

This rating helps account for lower-frequency sound that may be more difficult for a construction to control.

A project specification should make clear whether the target applies to:

• Rw
• Rw + Ctr
• DnT,w
• DnT,w + Ctr

These values should not be used interchangeably.

Why is the site result often different from Rw?

An Rw rating is measured under controlled laboratory conditions, where sound passing around the test element is reduced as far as possible.

A real building is more complicated.

Sound can travel through the main separating construction, but it can also follow other routes. This is known as flanking transmission.

Potential flanking paths include:

• Floors and ceilings
• External walls
• Internal partitions
• Structural columns and beams
• Raised access floors
• Suspended ceilings
• Façade systems
• Continuous cavities
• Ventilation routes
• Poorly sealed service penetrations

Installation quality can also have a major effect.

Small gaps, unsealed edges, incorrectly installed insulation or poorly detailed junctions can reduce the completed performance.

For this reason, it is not safe to assume that an element’s laboratory Rw rating will be achieved between finished rooms.

The construction needs to be considered as part of the whole building.

What is L’nT,w?

L’nT,w is a weighted rating used to assess impact sound insulation between rooms in a completed building.

Impact sound is created when something strikes or moves across a building element.

Typical examples include:

• Footsteps
• Furniture being moved
• Objects being dropped
• Activity on stairs
• Impacts on a floor finish

During field testing, a standard tapping machine is normally placed on the floor in the room above. The resulting sound is measured in the room below.

The measured impact sound level is then adjusted for the receiving room and converted into the single-number L’nT,w rating.

Is a lower L’nT,w rating better?

Yes.

This is the opposite of airborne sound insulation ratings such as Rw and DnT,w.

A lower L’nT,w value indicates that less impact sound is reaching the receiving room.

A useful way to remember the difference is:

• Airborne sound insulation: higher is generally better
• Impact sound level: lower is generally better

What is a sound absorption coefficient?

Sound absorption is different from sound insulation.

Sound insulation concerns sound travelling from one space to another.

Sound absorption concerns what happens to sound after it enters a room and reaches a surface.

A hard surface may reflect much of the sound back into the space. An absorptive material converts a greater proportion of the sound energy into a small amount of heat within the material.

Absorption can help:

• Reduce reverberation
• Improve speech clarity
• Control echo
• Reduce noise build-up
• Make busy spaces feel more comfortable
• Improve conditions in teaching, meeting or performance spaces

It does not necessarily mean that less sound will pass through the wall or ceiling behind the material.

A lightweight acoustic panel may provide strong absorption within a room but very little sound insulation between rooms.

What is αw?

The symbol αw, pronounced “alpha w”, represents the weighted sound absorption coefficient.

It converts frequency-based sound absorption test results into one single-number rating.

The value generally ranges from:

• 0.00, indicating very little sound absorption
• 1.00, indicating very high sound absorption

A higher αw value indicates that the tested product absorbs a greater proportion of the incident sound under the test conditions.

Unlike the old article’s wording, αw is not an “A-weighted absorption coefficient”. A-weighting is used for sound level measurements that reflect the frequency response of human hearing. The αw rating is a separate measure of material absorption.

What do absorption classes A to E mean?

Sound absorbers may also be assigned an absorption class under BS EN ISO 11654.

The classes run from:

• Class A: highest absorption performance
• Class B
• Class C
• Class D
• Class E: lower classified absorption performance

Some products may remain unclassified if they do not reach the relevant class threshold.

The class can be useful for initial product comparison, but it should not replace the full frequency-based test data where detailed room acoustic design is needed.

Two products with similar αw values may behave differently at particular frequencies.

Sound insulation and sound absorption are not interchangeable

This is one of the most common misunderstandings in building acoustics.

Sound insulation

Sound insulation reduces transmission between spaces.

It is influenced by factors such as:

• Mass
• Layers
• Cavity depth
• Structural isolation
• Sealing
• Junction details
• Flanking transmission

Common ratings include Rw, DnT,w and DnT,w + Ctr.

Sound absorption

Sound absorption reduces reflections within a space.

It is influenced by factors such as:

• Material porosity
• Thickness
• Mounting depth
• Air gaps
• Surface area
• Frequency

Common measures include the frequency-dependent absorption coefficient and αw.

A product can be good at one and poor at the other.

Adding absorptive panels to a room may improve reverberation without making the wall significantly better at stopping sound reaching the next room.

What is STC?

STC, or Sound Transmission Class, is a rating commonly used in North America to describe airborne sound insulation.

You may see it on imported product data, international specifications or US-based websites.

STC and Rw are based on different rating methods and frequency ranges. Although their values can sometimes appear similar, they should not be assumed to be directly interchangeable.

For UK projects, the required performance should normally be specified using the rating and testing method relevant to the project, rather than relying on an STC figure alone.

Which acoustic rating does your project need?

The right rating depends on what you are trying to assess.

Comparing a wall or floor construction in a laboratory

Look for:

• Rw
• Rw + C
• Rw + Ctr

Testing airborne sound insulation between completed rooms

Look for:

• DnT,w
• DnT,w + Ctr

Testing impact sound through a completed floor

Look for:

• L’nT,w

Comparing acoustic absorption products

Look for:

• Frequency-dependent absorption coefficients
• αw
• Absorption class

Assessing reverberation within a room

Material ratings alone may not be enough. The assessment may need to consider:

• Room size
• Room shape
• Surface finishes
• Furnishings
• Occupancy
• Distribution of absorptive treatment
• The intended use of the space

How acoustic ratings are used in building design

Acoustic ratings may be used during:

• Planning and concept design
• Building regulation assessments
• Architectural acoustic design
• Product selection
• Partition and floor specifications
• Façade design
• Contractor design development
• Pre-completion acoustic testing
• BREEAM assessments
• Investigation of performance problems

The important point is to specify the right rating at the right stage.

A laboratory product rating may help the design team select a suitable construction. It does not prove that the completed building will achieve the required room-to-room performance.

Similarly, a high absorption class does not show that a product will provide adequate sound insulation.

Common mistakes when reading acoustic specifications

Watch for the following:

• Comparing Rw directly with DnT,w
• Assuming the highest number is always best
• Forgetting that lower L’nT,w values indicate better impact performance
• Treating sound absorption as sound insulation
• Ignoring the C or Ctr adaptation term
• Assuming laboratory performance will be achieved on site
• Relying on a single rating without checking frequency data
• Using an STC value as if it were an Rw result
• Specifying a product without considering junctions and flanking paths
• Treating an acoustic rating as a guarantee of regulatory compliance

Why involve an acoustic consultant?

Acoustic ratings are useful, but they need to be applied correctly.

An acoustic consultant can help the design team:

• Select the appropriate performance metric
• Set suitable project targets
• Review product test data
• Predict completed building performance
• Identify flanking risks
• Develop wall, floor and junction details
• Coordinate acoustic requirements with ventilation and building services
• Plan pre-completion testing
• Investigate why a completed construction is underperforming

Early advice is particularly valuable where several acoustic requirements interact.

A wall may need good sound insulation, for example, while also accommodating ventilation, fire stopping, structural connections and building services. Each opening and junction can affect the final result.

Talk to auricl about acoustic design and testing

Not sure which acoustic rating applies to your project?

Tell us what you are designing, specifying or testing. We’ll help you understand the relevant performance measure and suggest the right next step.