CPD 21 2018: Introduction to acoustics

INTRODUCTION

Sound is all around us. How the passage of sound is managed – whether to enhance or to limit – depends on the ultimate use of the building concerned.

This CPD will look at the fundamentals of acoustics and how we understand sound, how it is measured and how sound is dealt with in order to comply with relevant aspects of Building Regulations. It will also examine specific applications to deal with sound generated within the internal environment and sound generated in adjoining dwellings.

SOUND AND ITS CHARACTERISTICS

Sound is an auditory sensation produced by an air pressure variation (pressure wave) propagated in all environments (water, air, concrete, wood, glass, etc.) except in a vacuum.

As this pressure wave propagates through the air it is perceived by the ear and the information gathered is sent to the brain where it is analysed.

The sound we hear in a building can come from a variety of sources – from outside (such as traffic), from equipment within the building (such as ventilation systems), from our neighbours (their voices or TV noise) or even from ourselves.

NOISE AND THE HUMAN EAR

Physically, a noise is a set of sounds of differing frequencies and power levels. The average human ear can detect sounds of frequencies between 20 and 20,000 Hz with varying degrees of sensitivity depending on the frequency, as well as areas outside of the normal range of hearing.

We are less sensitive to deeper sounds and to a lesser extent higher pitched sounds. At these frequencies sound needs to be at a higher amplitude (i.e. louder) for humans to be able to detect it.

To try to simplify matters a little, the dB (A) scale was developed to reflect typical human hearing – the effect of frequencies we are less sensitive to are minimised.

NOISE

When talking of the desire to minimise sound we generally talk about noise – noise is the annoying or unpleasant sounds we don’t wish to be exposed to. That said, noise is very subjective, and there are a number of factors that contribute to us finding a particular noise annoying or unpleasant. If the sound is unwanted then it is considered noise – think of a neighbour practising on his guitar. If we’re exposed to a particular sound for an extended period of time then it would be considered noise – a fire alarm test or an emergency vehicle’s siren would be an example of this. 

A sound that is masked by other louder sounds during the day may become noise if it continues unabated when all the other masking sound has disappeared – think of a dripping tap. And we also have the cocktail effect, where simple conversation becomes impossible in a room full of people due to the droning noise of dozens of simultaneous conversations.

NOISE LEVEL SCALE

The amplitude of the air pressure fluctuations gives rise to the volume of a sound, measured in decibels, and this illustration shows how these amplitude differences are perceived by the human ear; it also gives examples of these decibel levels that you will encounter.

Very quiet sounds, such as whispering, would be between 10-40 dB, while a quiet office environment would be in the 40-60 dB area. A noisy classroom would be in the region of 60-80 dB, while dangerous noise is above 80 dB, the sound of a passing train for instance.

Deafening noise is 100 dB-plus. Something like an aircraft on take-off would easily exceed 100 dB. At this amplitude permanent and irreversible damage to your hearing can occur in as little as a couple of seconds.

After understanding how we measure sound and the noise level scale now we’re considering noise level addition rules. Since the sound level is a logarithmic scale the normal rules of addition do not apply. If you have two sounds both at 83 dB you will perceive the combined sound (which technically is doubling the sound intensity) to be at 86 dB and not at 166 dB.

For the addition of 2 sounds with a large difference in volume, the louder sound simply hides the quieter sound – so 95 dB plus 80 dB would simply be perceived as 95 dB.

NOISE SOURCES

Moving onto more building-specific sound there are four main sources of noise when it comes to building acoustics:

• airborne noise from external sources – think of aircraft, traffic, shouting in the street etc.
• airborne noise from internal sources – conversations in other rooms, children playing upstairs, your neighbour’s TV etc.
• impact noise – if you’ve ever lived below someone with a laminate floor and a love of walking round in high heels at all hours of the day you’ll be all too aware of impact noise
• equipment noise – noise produced by lifts, generators, ventilation fans, etc.

All of these noise sources should be considered during building design and some are the subject of performance requirements set within building regulations.

THE EFFECT OF UNWANTED NOISE

The impact of unwanted noise should not be underestimated, nor the importance of acoustics.

The way homes are used is changing; children are staying in the parental home for longer, the prevalence of home offices is increasing, and there are more noise sources in a typical home now from multiple TVs, games consoles, mobile devices and stereos, etc. All of this can give rise to increased discomfort due to noise. Similarly in schools, noisy environments can have a detrimental effect on learning and subsequent pupil performance, while in an office, productivity can suffer in a noisy environment. And in healthcare, patient recovery rates can be hit if noise levels are too high.

So it’s important that we consider how to design our buildings properly in order to control the noise within them.

SOUND INSULATION 

The aim of acoustic insulation is to reduce noise transmission from one room to another, and as a result the noise is reduced and comfort improved.

Three types of noise can be reduced by means of sound insulation:

1. Airborne noise
2. Impact noise
3. Equipment noise – note that this type of noise is not covered in this CPD.

The acoustic performance sought for a room relative to the adjoining rooms is achieved through insulation, it depends on three parameters: 

1. The acoustic properties of the products and systems used
2. The techniques implemented and the installation quality
3. The architectural context: junctions between walls and the materials used

AIRBORNE NOISE

Two values are used to measure sound insulation performance against airborne noise (in dB):

Sound Reduction Index (Laboratory measurement):

R measures the quantity of sound stopped by the wall (or floor), taking into account solely direct transmission, at each frequency f (in Hz). The overall value of the sound reduction index is given by the index R_w. The higher R_w is, the better the walls’ sound insulation. 

Standardised Level Difference (Field measurement):

D_nT measures the quantity of sound stopped between two rooms, taking into account all transmission (direct and flanking). The insulation D_nT varies depending on the frequency f (in Hz). The overall value of the sound insulation is given by the index D_nT,w.

The higher the D_nT,w value, the more effective the insulation between the two rooms. D_nT,w is often cited with the low frequency adaption term C_tr, as a correction factor to account for low frequency noise. 

IMPACT NOISE

As well as airborne noise it is important to consider impact noise under Building Regulations.

Direct transmission through the floor is often the main vector. However, sound can also be transmitted through all of the building’s walls, depending on the type and number of junctions and how they are detailed. The regulatory value relating to impact noise is the Weighted Standardised Impact Sound Pressure Level L’_nT,w (in dB).

With L’_nT,w the lower the figure means the better the performance of the structure in question, so for example a floor of 56 dB (L_’nT,w) performs better than a floor of 62 dB (L’_nT,w).

This measurement is taken in the field with a standard tapping machine and the lower the value is, the less the noise is noticed in the next room.

MASS AND MASS-SPRING-MASS LAWS

Solid walls are constructed of a single material. The Mass Law states that the heavier and thicker a construction, the better the level of sound insulation it can achieve; sound insulation increases as the wall get heavier and thicker.

In an ideal world we don’t want to be building numerous thick and heavy constructions within our homes. The mass-spring-mass type of partition can help here; it consists of two faces (the ‘mass’ – usually plasterboard) separated by a cavity (the ‘spring’ – usually filled with mineral wool). Two mass faces and the spring between them act in a way to offer a high level of sound insulation without the need for thick, heavy walls described by the Mass Law. This allows us to build slim lightweight partitions to reduce the sound transference from room to room, rather than having to build thick and heavy walls.

APPLICATIONS

INTERNAL PARTITION WALLS

Non-loadbearing timber or metal stud partitions

This usually consists of a 63 x 38mm timber stud or a 48mm metal ‘C’ stud finished with plasterboard either side and with mineral wool insulation placed within the stud zone.

Building Regulations mandates a sound reduction index of 40 dB (R_w) to be achieved by these internal partition walls (Approved Document E – England & Wales & Section 5 – Scotland). A minimum of 25mm mineral wool is typically specified within the stud zone to achieve this figure, although fully-filling the stud zone with insulation will always give the best performance.

A typical Building Regulations compliant partition wall at 40dB (R_w) means that a normal level of speech in the next room wouldn’t be intelligible but would still be audible to a listener. Increasing the partition’s performance to 48 dB would mean that you would barely hear loud speech from the next room. And further increasing this performance to 50 dB would mean that you wouldn’t be able to hear loud speech at all, ensuring a higher level of acoustic comfort than a lower-performing partition wall would achieve.

INTERNAL FLOORS

Intermediate internal floor between rooms

These usually consist of a timber deck atop 150 x 50mm timber joists finished with plasterboard beneath and with insulation placed within the joist zone. Building Regulations in England & Wales mandates a sound reduction index of 40 dB (R_w) to be achieved by these internal floors (Approved Document E) or 43 dB (R_w) if in Scotland (Section 5). 100mm of mineral wool within the joist zone will typically achieve this figure.

The construction will usually be based upon a previously-tested floor make-up that achieved or exceeded the mandated minimum performance when tested in a laboratory. 

PARTY (SEPARATING) WALLS

Building Regulation requirements

Approved Document E mandates a performance standard of 45 dB (D_nT,w + C_tr) for new separating walls for dwellings in England & Wales.

Section 5 mandates a performance standard of 56 dB (D_nT,w) for new separating walls for dwellings in Scotland.

The construction of the separating wall will typically be masonry cavity or timber stud – this will match the construction type of the main external walls.

Whereas the main external wall will only need to achieve a thermal performance, the separating wall will have both a thermal and an acoustic performance requirement.

This usually means the residual cavity between the two leaves of blockwork or timber studwork must be fully filled with appropriate acoustic mineral wool insulation.

ROBUST DETAIL SCHEME

Robust Detail Limited offers a third-party certified construction scheme which applies to both party (separating) walls and separating floors. Using constructions certified by RDL avoids the need for pre-completion testing (which can cost around £500 per plot) to prove compliance with Building Regulations. The scheme applies to England, Wales, Scotland and Northern Ireland.

RDL publishes a handbook that contains all the separating wall & floor constructions that they certify and is usually updated three or four times a year. These constructions are certified by RDL after an extensive program of testing to prove a certain minimum level of performance can consistently be achieved.

A house builder consults the handbook and uses the data and comparison tables within to select their preferred RD for their separating wall and/or floor. The house builder registers its scheme with RDL, and RDL supplies the compliance certificates and associated checklists that Building Control will be asking for upon completion. The house builder then builds to the specified RD.

It is worth noting that RDL sound tests approximately 2% of all completions and visually check a further 1% during the build stage to ensure compliance with the registered RD.

MASONRY PARTY (SEPARATING) WALL

An example of a masonry separating wall Robust Detail is E-WM-17:

E-WM-17 consists of the following:

• 8 kg/m² (min) plasterboard facings
• No parge coat
• 1350-1600 kg/m³ block density (or Plasmor Aglite Ultima at 1050 kg/m³ density)
• 75mm (min) cavity filled with Isover RD Party Wall Roll

Note that Besblock Star Performer blocks can be used with this detail, but be aware that the minimum cavity width increases to 100mm with this option. 

The RD specifies the type of blockwork that must be used, the minimum cavity width that separates the two leaves of blockwork, the type of insulation that must be used within the cavity, whether a parge coat is required, and also the type of plasterboard that needs to be used to finish the party wall.

The image detailing the basic construction is taken from the front page of the RD; there are also subsequent pages for each RD that cover specific construction and junction details and also the checklist that needs to be adhered to, thereby ensuring compliance with the RD.

PARTY (SEPARATING) FLOORS

Building Regulation requirements:

Approved Document E mandates a performance standard of 45 dB (D_nT,w + C_tr) and 62 dB (L’_nT,w) for new separating floors for dwellings in England & Wales.

Section 5 mandates a performance standard of 56 dB (D_nT,w) and 56 dB (L’_nT,w) for new separating floors for dwellings in Scotland.

The construction of the separating floor can be timber, concrete or steel/concrete composite. The structure generally consists of a secondary floor deck isolated from the main structure of the floor by a resilient layer. Note that not all floor types can be used with all wall types – RDL publishes compatibility tables that show acceptable combinations of wall & floor.

TIMBER SEPARATING FLOOR

E-FT-1 is a timber separating floor for use in England & Wales constructed using timber I-joists. 100mm of mineral wool can be used within the I-joist zone as an absorbent material, while 25mm of mineral wool can be used within the floating floor treatment as the mineral wool quilt.