NATURALLY VENTILATED BUILDINGS

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ACOUSTIC CONSIDERATIONS IN NATURALLY VENTILATED
BUILDINGS
Professor Steve Sharples School of Architecture,
University of Sheffield Professor David Oldham
Acoustics Research Unit, University of
Liverpool Dr Max de Salis PDA Acoustic
Consultants, Warrington
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Background
Concerns regarding sustainability, health and
energy use have stimulated interest in the use
of passive means to achieve ventilation for new
and existing buildings.
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
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Problems with natural ventilation
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.
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Quantifying the airflow and acoustical
performance of ventilation systems
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
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Quantification of airflow performance
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
Qv 0.827 A (?P ) 0.5
Qv is the volumetric flow A is the open area of
the aperture ?P is the pressure difference across
the aperture
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Quantification of airflow performance
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
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Quantification of acoustic performance
The standard equation for the Sound Reduction
Index (SRIWA) 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
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Quantification of acoustic performance
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 SRIAW 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.
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Quantification of acoustic 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
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Relating airflow and acoustic performance

• 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

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Combined presentation of airflow and acoustical
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.
SRI and airflow characteristics for a wall (SRI
40 dBA) containing a simple aperture
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Combined presentation of airflow and acoustical
performance

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

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Combined presentation of airflow and acoustical
performance
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.
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Combined presentation of airflow and acoustical
performance
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
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NOISE CONTROL STRATEGIES
Attenuation mechanisms will have optimum
performance over a restricted region of the
frequency spectrum.
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NOISE CONTROL STRATEGIES
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
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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.
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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

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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

--- Without ANC
With ANC
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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.
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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.

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Acoustic and airflow design chart.

• Example
• A 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.

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Acoustic and airflow design chart.
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.
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Acoustic and airflow design chart.

• 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.

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Acoustic and airflow design chart.
Alternative approach to the use of the design
chart The designer specifies the airflow and
acoustic requirements and locates appropriate
treatments from the chart.
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Acoustic and airflow design 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.
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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.