06.04 Acoustics

Categories: Building Comfort

Introduction
Aural comfort is dependent on the level of noise in the environment.  Noise, defined as unwanted sound, can intrude, distract, annoy and in some cases cause injury.  Noise criteria can be grouped in two classes.  The first specifies the conditions required at the listener.  The second specifies the reduction in noise (insulation or attenuation) required to achieve these conditions.  When considering the facade one is primarily concerned with sound insulation from the external environment, however when automating the facade one must also consider the noise generated by motors and its affect on the desired noise levels within the room.

No matter how well sound insulated a facade is, if that facade has ventilation openings that are slightly open, noise will be able to penetrate into the space almost as if the facade wasn't there at all.  Therefore it is often vital when designing naturally ventilated buildings in noisy environments (not an ideal situation) to appreciate that the occupants may want the windows to be closed at certain times.  Unfortunately, sound level meters are not commonly used in building control systems due to their expense and therefore it is often wise to provide a user over-ride for an automated system so that the occupants can make their own assessment of noise.

For a diagram outlining man's physiological, behavioural and artificial mechanisms for aural discomfort click here.
 

Noise is unwanted sound, whether it originates from road traffic, railways, aircraft, factories, discos or merely from neighbours. It is a nuisance and may cause annoyance, impair work performance and, in the extreme, contribute to illnesses, both physical (tinnitus) and psychological.

The building envelope plays an important role in reducing the noise levels inside buildings and may sometimes be required to reduce the level of noise emanating from a building.  This Section deals predominantly with controlling noise transmission from outside to inside the building.

Noise transmission from one room of a building to another may be affected by the joint between a curtain wall and the relevant floor or wall.

Noise may arise as a result of the cladding for instance the drumming of rain on sheeting.

This Section firstly explains the physical characteristics of noise and sound insulation.  It then contiues to describe the performance of different components and constructions.  Finally it describes some of the noise environments that may be encountered.
 

Decibels and Frequency
The threshold of the level of hearing is 0 decibels (dB), and 130 dB represents a level at which physical pain can be felt. Typical examples of intermediate sounds are shown below.
 

As with the other human senses, hearing sensitivity cannot be represented by a linear scale but by one which is related to relative changes (logarithmic).  This logarithmic scale is measured in decibels (dB).  The basic energy is measured as a sound pressure level (SPL) and the relationship between the two is shown below:
 

When multiple sounds combine to give a louder noise the sound levels are not simply additive because of the logarithmic scale. If, say, there are two noises of 78 dB and 84 dB, their difference is 6 dB.  Reference to this chart indicates that a corresponding correction of 1 dB is appropriate, and should be applied to the higher level, (84 dB), so that the resultant of these two sounds is 85 dB. Multiple sounds can be compounded by replacing successive pairs of sounds by a single equivalent and, by repeating the process to derive a final overall equivalent.

Our ears respond to the range of sound frequencies, or pitch, from around 20 hertz (bass) to 20 kilohertz (treble) though our response is not linear, being generally more sensitive to the high frequencies than to the low.

Loudness is a subjective composite judgement of the frequency mix of the noise and its Sound Pressure Level.

In order that measuring equipment can give realistic indications of how people are likely to react to noise, it is usual for it to incorporate electronic circuitry whose response is weighted to simulate that of our ears.  The internationally agreed standard for the appropriate corrections is called the ‘A’ Weighting Curve.  Measurements made with this facility are, therefore, termed ‘A’ Weighted Decibels or, more concisely, dBA, to discriminate from those made in plain dB, which do not account for human reaction.
 

Frequently, full analyses of noise problems are not required, and use is made of shorthand methods in describing the offending noise, the acoustic performance of the building envelope components and target values for interior noise quality. The most common of these are defined below.
 

Acoustic Indices
R_m – Mean Reduction
The complete way of specifying the acoustic performance of any other building element is to establish its sound insulation over a wide range of frequencies. The British and European preferred frequency range is 100 - 3150 Hz, in which case the corresponding sound insulation value, (or Sound Reduction
Index, SRI), should be determined at all the 16 third octave bands between 100 - 3150 Hz.

The arithmetic mean, or average, of these insulation values is a simple indicator of performance, designated R_m , or Mean Sound Reduction Index, and is measured in dB.
 

R_w – Weighted Reduction
R_m is now less used since the publication of BS 5821: 1984, in which the index, R_w (Weighted Reduction) is defined, which incorporates a correction for the ear’s response. It is derived from comparing the building component sound insulation/frequency curve with a family of reference curves and selecting one to obtain the
‘best fit’ so that its average adverse deviation over the 16 third octave bands between 100 - 3150 Hz is only 2 dB. The corresponding sound insulation of this reference at 500 Hz then defines the R_w Index of the building component.

Numerically, it may be up to 5 dB higher than the corresponding R_m value for the same window data. Hence, it is most important to differentiate between these indices.
 

R_TRA – Traffic Noise Reduction
Neither R_m or R_w can be used directly to estimate interior noise levels because of their independence of the spectrum of the actual noise climate. By adopting an idealised, but typical, spectrum of road traffic noise in town and city centres, the index R_TRA (Reduction of road traffic noise) can be derived, by processing this with the basic sound insulation of the building component, frequency-by-frequency. This represents the attenuation, in dBA, which the building component can achieve in mitigating road traffic noise and gives a very useful guide to in service performance.
 

STC – Sound Transmission Class
Occasionally, requirements may be stated, in terms of Sound Transmission Class (STC) values, which appears in the American Standard ASTM E413. Its derivation is similar to the R_w index, except that the relevant frequency range is 125 - 4000 Hz (i.e. shifted upwards by 1/3 octave from the British Standard). For this reason STC is, typically, around 1 dB higher than its R_w equivalent, owing to panel materials being generally better performers at high frequencies.
 

Average peak level
Research has shown that disturbance or annoyance does not correlate well with the prevailing average noise level, but is better ranked by the corresponding average peak level which, to some extent, takes account of the ‘startle’ effect. This is most often represented by the noise level which is exceeded for only 10 percent of the measuring period, and is designated L₁₀ . If measured in dBA, it is often referred to as L_A10.

L₁₀ values are those normally used to describe the ambient road traffic noise exposures for design purposes. The L₁₀ (18-hour) value is the value derived from measurements over the period from 6 a.m. to midnight on a normal working day. It is this which currently is used to determine eligibility under the Noise Insulation Regulations 1975, by which a sound insulation package may be offered to certain householders, satisfying specified conditions, when their homes are subjected to traffic
noise levels of 68 dBA or more.
 

Equivalent Noise Levels - L_eq Values
Particularly with intermittent sounds or noises, (train noise, discos), it is helpful to determine the notional steady noise level which contains the same total energy as the real, varying noise. This is called the Equivalent Noise Level (L_eq) and is increasing in use, especially where different or multiple exposures are involved (e.g. a building situated close to both a railway and an airport). BS 8233: 1987 ‘Sound Insulation and Noise Reduction for Buildings’ is the most recent authoritative document to give recommendations of noise climates for a variety of building types. Throughout, it adopts L_eq (dBA) values, which are referred to as L_Aeq.

To a close approximation

L_Aeq = L_A10 – 3
 

Noise reduction
The noise reduction achieved through any wall will depend on the component parts of the wall and the joints between them.  In general the noise reduction achieved is controlled by the weakest component in the envelope.  If the sound insulation of the solid or opaque wall of a facade is at least 10 dB higher than that of the glazing, noise transfer through the wall can be ignored and transmission through the windows, and other openings, alone may be considered. This is most common as typical single brick walls have an R_m of 45 dB and cavity brick walls of better than 50 dB.

In most cases, the effective performance of a facade is determined by the glazing. Clearly, the bigger the window, the more noise energy can be admitted (or escape) but, owing to the way in which sound levels are additive, this is a relatively small effect. It can be assumed, without serious error, that doubling or halving the window area produces an aggregate corresponding noise level change of 3 dB or – 3 dB respectively, which can only just be noticed.

Glazing and windows clearly have an important role in attenuating outside noise so that building occupants are not unduly disturbed. Achieving this may involve employing thick glass, double or multiple glazing, laminated glass, and their combinations.

The appropriate design solution depends on a number of factors, including the nature of the noise, when and for how long it occurs, and the activities carried out inside the building.

All building envelopes will provide a degree of noise reduction.  The CWCT Standard for Curtain walling states that 'Double glazed curtain walls provide an adequate level of sound reduction in most applications.  In areas of high traffic noise specialist advice should be sought'
 

Sound Insulation of Glazing
Monolithic Glass
The theoretical Mass Law indicates that a 6 dB increase in sound insulation should result from doubling the glass thickness. However, resonances interfere with this trend, and in practice the incremental increase is reduced to about 4 dB.

Measured values are shown below:
 

These values show a significant resonance frequency - where there is a ‘dip’ in the sound insulation.  This Coincidence Resonance frequency is inversely proportional to the glass thickness. This Critical Frequency (f_c ) is determined from the formula:

f_c = 12,000 / d  Hz

where d is the glass thickness, in millimetres.
 

Laminated Glass
Laminated glass is commonly used in safety and security applications, but lamination can also have acoustic benefits, particularly in the suppression of the Coincidence Resonance.

Two main types of interlayer material are used; polyvinylbutyral (PVB) in sheet form, and cast-in-place (CIP) resins, each with their own particular merits. The resins used for CIP laminates can be specially formulated to provide enhanced acoustic performance, and differentiate them from ‘standard’ PVB laminates. Because CIP resins are ‘softer’ than PVB sheet, the resonances occur at frequencies which correspond closely with the individual glass components of the laminate. With PVB this resonance occurs at the frequency which corresponds to the total thickness of the laminate (i.e. lower in frequency). Thus, a CIP laminate shifts these resonances to higher frequencies where, generally, they play a less important role in overall acoustic performance.

Sound insulation values for laminated glasses are shown in the tables above and below for comparison with other glasses.
 

Double Windows (Secondary Sashes)
Where high sound insulation of windows is required, airspace widths of greater than 100mm may be required. The same principles apply here as for insulating units, the use of dissimilar glass thicknesses, one of them being thick (at least 6mm, and preferably 10mm thick).

Lining the reveals with acoustic absorbent material (fibreboard) is beneficial because it can reduce reverberation in the cavity, giving an overall improvement of 2-6 dB, according to its area and absorption characteristics.

Increasing the width of cavity produces an increase of sound insulation, but not pro rata. Beyond a spacing of about 200mm it is normally uneconomical to install such windows because the incremental acoustic improvement is small. This trend is illustrated here.

Sound insulation levels for some typical arrangements are shown below:
 

Insulating glazing units
Insulating glazing units were originally introduced to reduce heat loss through windows. However, it is possible to use them to achieve moderately high acoustic insulation. The main principles to employ are in order of significance:

• The use of thick glass,
• Ensuring that the component glasses differ in thickness by at least 30 percent (e.g.10 + 6mm or 6 + 4mm) in order to offset individual resonances (suppress sympathetic resonances).
• The use of laminated glass

The lamination of one pane produces a further small improvement.  This generally ensures that the components are of different thickness and this is achieved regardless of which glass is laminated.  It is worth noting that identical sound insulation is obtained irrespective of which way round insulating units are installed. A 10/12/6 unit gives the same performance as a 6/12/10 unit.

Over the usual cavity width range of 6 to 20mm for insulating units, there is little variation in acoustic performance although there is a more significant change in thermal insulation. This acoustic performance plateau is due to the relatively strong coupling of the component glasses.

Sound insulation values for various insulating units with standard 12mm air-filled cavities are shown below:
 

Negligible error is introduced if these values are adopted for all cavities within the range 6 to 20mm.
 

Effect of Gas Filling
Insulating units may be thermally improved by filling the cavity with argon gas, Section 09.04. Such units exhibit exactly the same acoustic performances as standard air-filled units of the same glass combination.

For applications where middle frequency acoustic performance is the most critical (e.g. speech), units may be filled with sulphur hexafluoride (SF₆) gas mixtures. This elevates the corresponding R_w index. Simultaneously, SF₆ introduces a significant resonance at 200-250 Hz and, for noises dominated by low frequency components (road traffic, railways, aircraft on take-off, etc.), this is detrimental. Such units generally offer lower effective sound insulation in these situations than standard air-filled units.  They may also provide lower thermal performance than argon filled units, Section 09.04.
 

Insulation of Spandrel Panels
Many modern buildings are designed to have a fully glazed external appearance, though some areas are opaque. This is achieved by using coated or opacified glazing, backed with insulating material in a metal tray in order to satisfy Part L of the 1991 Building Regulations, relating to the transmission of heat. As a generality, the addition of the insulation and tray may, because of resonances, impair the performance of the basic glass at very low frequency but enhance it at frequencies above 125 Hz.
 

Influence of Frame Material
In spite of hollow box sections being an integral feature of aluminium, PVC-u and composite framing systems, laboratory and field measurements have shown that, up to a glazing R_m of about 35 dB, the window frame is not often a serious leak path and the sound insulation of the glazing can be adopted as being representative of the window as a whole. Beyond R_m = 35 dB, it is prudent to evaluate the acoustic performance of the proposed framing; more substantial sections may be necessary to be compatible with very high performance glazing.
 

Airtightness
Very small airgaps can have a profound detrimental effect on the aggregate window acoustic performance. Airgaps of only 1 percent of the total window area can reduce the overall potential sound insulation by as much as 10 dB, which means that transmitted noises are heard twice as loud as they would be if fully sealed. If a window incorporates opening lights of any kind, it is essential that efficient seals are fitted.

It is important to point out that double windows will only achieve their high performance potential if all airgaps are sealed. Effectively, this means that the frames carrying the glasses must either be fixed or be casements, incorporating pressure seals all round and also featuring multipoint locking to avoid twisting.

Sliding sashes are not able to secure the required airtightness and their corresponding acoustic performance is impared (Ref. BRE Information Paper IP12/89).
 

Noise transmission within a building
High levels of sound insulation between floors are often necessary in multi-occupancy offices and are required in apartments.  High levels of sound insulation between individual rooms may also be necessary.

Noise will be transmitted through any gap left between the building envelope and a compartment floor or internal partition wall.  Where high levels of sound insulation are required these junctions may require special detailing.
 

Major Noise Sources
Most noises are not of a single pitch or frequency, but consist of a wide range of frequencies, or spectrum.

The identification of these frequencies and their relative strengths, (tonal mix), is important, as well as the corresponding overall noise level.

The most common noise problems involve:

• Road Traffic Noise

This is dominantly of low frequencies, being influenced by vehicle speed, engine type, road surface, local topography, etc.
• Railway Noise

This has a broadly similar spectrum to road traffic, except that more middle frequency tones are present and, generally, there is a more rapid fall off in high frequency content. Dominant influences here include speed, type of rail, type of sleeper, mix of rolling stock, embankments or cuttings, etc. Though noise levels adjacent to railways can be very high indeed, people’s tolerance is also greater, because the rise and decay of each pass by is predictable and the peaks are of short duration. It is widely accepted that railway noise can exceed road traffic noise by more than 10 dBA, whilst generating only the same degree of annoyance or disturbance.
• Aircraft Noise

This changes significantly with altitude, climatic conditions, type and load utilisation of aircraft, and on whether it occurs at landing or take-off. Take-off noise is dominated by low frequencies whereas landing noise contains strong high frequency components, characteristic of the engines being in reverse thrust.
• Speech

The most important frequencies of speech lie between 500-2000Hz (middle frequency dominated). Female speech is approximately an octave (or twice the frequency) above male speech. It is the suppression of the higher frequencies which is most important in providing privacy of conversation because these contain the essential aural clues of intelligibility, called the sibilants (s,sh).

Noise generation
The building envelope should be constructed so as to minimise the generation of noise.  The principal causes of noise generation are:

• Loads, movements, and changes in the environmental conditions may be accommpanied by levels of movement generated noise likely to be intrusive in and around the completed building.
• Rainscreens can be subject to creaking, clicking, and grinding noises as thermal movements occur unless connections are designed to permit noiseless movements.
• Wind and air movements can cause rattling and whistling noises, often emanating from external cover strips and vents
•  The possibility of noise generation by rain or hail striking against the surface of the rainscreen panel should be allowed for. The significance of drumming depends upon the design of the rainscreen panel.  Noise generation by drumming can be reduced by changing the properties of the panel, for example by the application of anti-drumming materials to the back of thin aluminium rainscreen panels.