ACOUSTIC CONSIDERATIONS IN NATURALLY VENTILATED BUILDINGS Professor Steve Sharples School of Architecture, University of Sheffield Professor David Oldham

ACOUSTIC CONSIDERATIONS IN NATURALLY VENTILATED BUILDINGS

Professor Steve SharplesSchool of Architecture, University of Sheffield

Professor David Oldham Acoustics Research Unit, University of Liverpool

Dr Max de SalisPDA Acoustic Consultants, Warrington

Concerns regarding sustainability, health and energy use have stimulated interest in the use of passive means to achieve ventilation for new and existing buildings.

Background

To enable the designer to select an approach that will satisfy requirements for air flow and noise attenuation the acoustical performance of a ventilator needs to be presented in conjunction with airflow performance data

Only small pressure differentials available to drive natural ventilation systems in buildings.

Requires a system to have inherently low airflow resistance

Low airflow resistance can be achieved by opening large areas of the building façade

Thereby significantly decreasing the noise insulation of the building fabric

Thus natural ventilation systems may offer little resistance to the ingress of externally generated noise

It is necessary to look at measures which will render NV a viable option in areas with higher background noise levels.

Problems with natural ventilation

The aim is to devise noise control techniques which optimise both the acoustical and airflow performance of ventilation inlets and outlets

There is a need to be able to quantify the acoustical and airflow performance of different ventilation strategies so that their benefits can be compared

Quantifying the airflow and acoustical performance of ventilation systems

Air flows in naturally ventilated buildings result when a pressure differential, P, is created across the façade of a building by wind and/or buoyancy forces

The equation of flow for a thin orifice plate is valid for a large opening of simple geometry, such as would be employed as a ventilation aperture in a building façade:

Quantification of airflow performance

Qv = 0.827 A (P ) 0.5

Qv is the volumetric flow

A is the open area of the aperture

ΔP is the pressure difference across the aperture

Volumetric flow rate is directly proportional to the area of the aperture

Similar expressions exist for ducted flows

Design of natural ventilation systems involves specification of an appropriate value of the equivalent open area of an inlet to achieve a desired airflow for estimated pressure differentials

Natural pressure differentials are typically small – of the order of 10 Pa

Quantification of airflow performance

The standard equation for the Sound Reduction Index (SRIW+A) of a composite wall consisting of

A main structure of area AW and Sound Reduction Index SRIW

An element of area AA and Sound Reduction Index SRIA is:

Quantification of acoustic performance

AW

SRI

A

SRI

WAW AA

AAdBSRI

AW

1010 1010log10)(

The effective sound insulation of a composite façade is a function of the sound reduction indices and relative areas of each component.

The net sound reduction index is not in simple proportion to the area of the aperture as is the case for airflow.

The sound reduction index of a simple aperture is approximately 0 dB

For a naturally ventilated building, the composite façade Sound Reduction Index SRIA+W will thus tend to be dominated by the poor performance of the ventilation aperture.

For a high performance wall the effect of even a very small aperture is to dramatically reduce its effective performance.


The sound reduction index of a wall varies with frequency and thus the effect of an aperture in a given wall will be a function of the frequency of the incident sound

For practical applications need to devise a simplified measure of the acoustical performance of a façade

Concerned with the effect of different acoustic treatments on noise in urban areas where road traffic is the major noise source

The technique adopted in this work was to use a single figure SRI to express façade performance

Calculated by logarithmically summing the ‘A’ weighted spectrum of traffic noise as attenuated by the façade and subtracting it from the sum of the un-attenuated spectrum


The acoustical performance is a function of the ratio of aperture area to total facade area

•Express airflow performance in a similar manner

•Expressed as a function of the percentage of façade area occupied by the ventilation opening per square metre

•Conversion of this data to façades of different area involves simple mathematical manipulation

Based upon the assumption that the size of the apertures and surrounding un-perforated wall are such as not to perturb significantly the pressure field at the façade

Relating airflow and acoustic performance

Curves are based upon the acoustical performance of a wall with a Sound Reduction Index relative to a traffic noise spectrum (SRIroad traffic) of 40 dBA.

Combined presentation of airflow and acoustical performance

SRI and airflow characteristics for a wall (SRI 40 dBA) containing a simple aperture

Curves enable the designer to:-Assess the acoustical consequences arising from ventilation requirements- Identify the need for acoustical treatment.

ExampleA large open plan room measuring 20m by 20m and 1 storey height would require airflow:

20 m3 per hour per m2 of façade area to achieve 1 air change per hour to control air quality

100 m3 per hour per m2 of façade area to achieve 5 air changes per hour for cooling.


The required open area of facade to achieve these airflow rates with a 5 Pascal pressure differential across the façade would reduce the SRIroad traffic to

~25 dB(A) to achieve air quality

~18 dB(A) for sensible cooling.


Example illustrates the need for noise control measures at ventilation openings in order to achieve both adequate acoustical insulation and airflow rates

Designers need to be aware that the noise control measures will tend to increase the resistance of the ventilation openings

This may necessitate an increase in the size of the openings to maintain the desired flow rate


NOISE CONTROL STRATEGIES

Attenuation mechanisms will have optimum performance over a restricted region of the frequency spectrum.

In reducing a broad band noise, such as road traffic noise, reductions must be made in all the prominent frequency bands

Most conventional noise control treatments (e.g. louvres, screening) are not effective at low frequencies

When these devices are used to combat road traffic noise their effectiveness at reducing the ‘A’ weighted sound pressure level is limited

NOISE CONTROL STRATEGIES

Noise control strategies

• Most conventional treatments are effective at high frequencies

• Limit to benefits that can can be obtained from their application

• Need to complement with low frequency treatments such as Active Noise Control

Aperture 0dBA

Single louvre 11.3dBA

Double louvre 17.7 dBA

Brick cavity wall 40dBA

Normalised internal noise spectra for different configurations.

Active Noise Control

• Used in industrial and building services systems

• Little evidence of use for traffic noise control in NV

• Theoretically, ANC should add no loss to the natural ventilation flow

• Problem of varying traffic noise signal

Active Noise Control

• Figure shows traffic noise attenuation in duct

• At the difficult low frequencies attenuation 5dB to 13.5 dB

• Combining passive and active noise control has the potential to give high SRI with high natural ventilation rates

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--- Without ANC With ANC

DESIGN GUIDANCE

Design methodology has been developed involving the presentation of acoustic and airflow data on a single chart.

The airflow in cubic metres per hour per metre squared of façade area per Pascal0.5 , denoted by Q, may be calculated as a function of the proportion of ventilator inlet area to total façade area.

For a given façade construction e.g. brick cavity wall, a chart of sound insulation to road traffic noise against flow rate may then be compiled.

This can be applied where a specified flow rate and sound insulation are required to give either:

The necessary open area for an assumed pressure differential.The required pressure differential for a given open area to achieve the design conditions.

Acoustic and airflow design chart.

Predicted performance of apertures in a brick cavity wall to normally incident sound when treated with

•Acoustic louvres, •A lined duct•A lined duct plus active noise control.

ExampleA large room measuring 20m x 20m in which the airflow rates are:

-to control air quality ~ 20 m3 per hour per m2 -for cooling ~100 m3 per hour per m2 of facade area

For a pressure difference of 5 Pa the corresponding values of Q are approximately 9 and 45.


For the simple aperture, (bottom curve)

Air quality can be achieved with an open area of approximately 0.3% and an effective sound reduction index of approximately 25 dB.

Cooling can be achieved with an open area of approximately 1.4% and effective sound reduction index of 18dB.

If these values are not adequate then the effect of different treatments can be investigated.


With regard to cooling, the required airflow can be achieved using:

•Simple louvres occupying approximately 7% of the façade area with an effective sound reduction index of approximately 24 dB. •Double louvres occupying approximately 9% of the façade area with an effective sound reduction index of approximately 28 dB. •Lined duct occupying approximately 1.4% of the façade area with an effective sound reduction index of approximately 32 dB.

•Lined duct supplemented by active noise control the area is again approximately 1.4% but the effective sound reduction index is approximately 38 dB.


Alternative approach to the use of the design chart

The designer specifies the airflow and acoustic requirements and locates appropriate treatments from the chart.


For example, if considerations of required airflow and design pressure differential result in a calculated value of Q of 30 m3hr-1Pa-0.5m-2 and the Sound Reduction Index required to achieve acceptable indoor conditions is 25 dB

From the chart that it can be seen that acoustic louvres occupying approximately 4.5% of facade area would be suitable.


CONCLUSIONS

If a natural ventilation approach is to become more common in noisy urban areas then more information needs to be provided to designers about different approaches to noise control.

The acoustical performance of a ventilator needs to be presented in conjunction with airflow performance data so that the designer can select an approach that will satisfy both attenuation needs and other design requirements.

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ACOUSTIC CONSIDERATIONS IN NATURALLY VENTILATED BUILDINGS Professor Steve Sharples School of Architecture, University of Sheffield Professor David Oldham