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Dunham-Bush
1. Introduction to Fan Coil Unit Acoustics and Fan Coil Room Noise
Levels
The selection of fan coil units is governed not only by thermal
or air volume flow rate requirements, but also by any constraints on
the level of noise permitted in the room. Frequently, the
manufacturer is required to select fan coil units based on the
predicted noise levels in the room as a result of the fan coil unit
application, however these are influenced not only by the fan coil
unit but also by the room itself. During the preliminary design
stages, not all the room conditions may be known but certain
assumptions allow the room noise levels to be approximated.
2. Sound Pressure Level
In generating sound, the source will expend energy.
The rate of transfer of acoustic energy from the source to the medium
though which the sound is transmitted is known as the sound power level
(SWL). In a similar manner to sound pressure levels, sound power levels
are measured on a logarithmic scale, in dB above the power corresponding
to the threshold of hearing for a typical human ear.
Sound pressure level (SPL) is given by.. SPL = 20 Log10 P / Pr
where P = sound pressure being measured (Pa)
Pr = sound pressure reference i.e. limit of hearing (2 x 10-5 Pa)
3. Sound Power Level
In generating sound, the source will expend energy.
The rate of transfer of acoustic energy from the source to the medium
though which the sound is transmitted is known as the sound power level
(SWL). In a similar manner to sound pressure levels, sound power levels
are measured on a logarithmic scale, in dB above the power corresponding
to the threshold of hearing for a typical human ear.
Sound power level (SWL) is given by.. SWL = 10 Log10 W / Wr
where W = sound power of source (W)
Wr = sound power reference (1 x 10-12 W)
4. Frequency & Octave Band Spectra
Sound is oscillatory in nature and the speed of
oscillation is measured as a frequency in Hertz (Hz). In practice,
virtually all sound consists of components at different frequencies;
each component has a sound power level at its relevant frequency. The
human ear is sensitive to sound with a frequency between 30Hz and 20kHz.
Since frequency is continuous variable,
sound levels (both SPL and SWL) are grouped in frequency bands for
convenience. These are referred to as octave bands (since their width is
typically one octave) and the mid band frequency
is used to identify them.
Octave band - mid-frequencies (Hz)
32 63 125 250 500 1000 2000 4000 8000 16000
For fan coil unit applications, octave bands 125Hz, 250Hz, 500Hz,
1kHz, 2kHz and 4kHz are critical.
Each octave band, for a piece of equipment such as a fan coil unit, has
a corresponding sound power level, from which an equivalent sound
pressure level at the same frequency may be derived.
5. The Relationship Between SWL and SPL
In simple terms, the SWL is the rate at which
acoustic energy is transferred to a room and the SPL is a measure of the
effect of the SWL at a given point within the room. A useful analogy is
to consider a heater; the output of a heater is measured as power in
Watts (W) and the effect of the output of the heater is measured as
temperature in degrees Celsius (°C).
6. Sound Path
It is important to consider that sound can be emitted
from different sources to the listener.
1) Inlet and case radiated sound is transmitted
through the ceiling into the room space.
2) In duct sound is carried along the duct and
through the supply air diffusers to the room space.
It is also important to consider that sound is
transmitted via more than one route.
1) The direct sound path is a straight
uninterrupted path between the source and the listener.
2) Reverberant sound paths are any path,
beginning at the source, along which sound is reflected off one or
more surfaces prior to reaching the listener.
7. Factors Affecting Room Sound Pressure Levels
Figure 1: A typical fan coil unit application, with factors affecting
room sound levels
A In-duct discharge sound power levels
B Inlet and case radiated sound power levels
Figure 1 represents a typical ceiling void fan coil unit installation.
Certain criteria must be known In order to determine room sound pressure
levels.
1) Sound power levels of the fan coil unit -
Sound power levels are specified as two separate sets of acoustic
data, namely: discharge in duct sound power levels, and; inlet and
case radiated sound power levels.
2) Outlet ductwork - The size, length and type of
ductwork, together with the quantity of bends incorporated into it,
all affect the level of discharge in duct sound that reaches the
supply air plenum.
3) Supply air plenum - The acoustic
characteristics of the supply air plenum are affected by its size,
the number of duct connections and whether or not it is acoustically
lined.
4) Supply air grille - The type and size of the
supply air grille affect the level of the discharge in duct sound
radiated into the room. High air velocities through the grille may
result in noise regeneration.
5) Position of supply air grille - The position
of the supply air grille, i.e. its proximity to one or more major
room surfaces, affects the level of discharge in duct sound radiated
into the room that is transmitted via the direct sound path to the
listener.
6) Ceiling construction - The level of inlet and
case radiated sound transmitted into the room space is dependent
upon the acoustic characteristics of the ceiling.
7) Return air grille - The position of the return
air grille affects the inlet and case radiated sound emitted into
the room. Return air grilles should be installed as far as possible
from the inlet to the fan coil unit.
8) Room conditions - The sound pressure levels
resulting from the component of sound transmitted from the ceiling
and the grilles via the reverberant sound path is affected by both
the surface area of the room space and the quality of the room
surfaces.
9) Position of the listener - As sound travels
further from the source, both acoustic energy and hence sound
pressure levels reduce.
8. Noise Assessment
Noise rating (NR) curves are the most commonly
used method for assessing noise level. Each curve connects
predefined sound pressure levels, which consider the response of the
human ear at each of the octave bands.
The noise level at a particular point in space is
represented by the value of the NR curve whose constituent sound
pressure levels are either greater than or equal to those calculated
or measured.
Figure 2 shows a set of sound pressure levels
with a noise level of NR35.
Figure 2: Noise Rating (NR) curves with typical SPL data plotted. The
noise level in this case is said to be NR35.
9. Common Problems with Fan Coil Unit
Installations
1) Undersized outlet ductwork - Undersized outlet
ductwork leads to high air velocities and external static pressures,
and noise regeneration in the duct. As a guide, the maximum air
velocities should be limited to 3m/s in typical office environments
and to a lower figure in noise critical areas. Recommended air
volume flow rates for duct sizes of Ø250mm, Ø200mm and Ø150mm are
145l/s, 95l/s and 55l/s respectively.
2) Excessive quantities of flexible ducting - For
a given air volume flow rate, flexible ducting produces a higher
pressure drop than an equivalent length of rigid ducting. Higher
pressure drops may lead to noise regeneration.
3) Position of return air grilles - The
installation of return air grilles too close to the inlet of the fan
coil unit results in more of the inlet and case radiated sound
reaching the room via the grille. The length of the air path between
the inlet of the fan coil unit and the return air grille should be
maximised to minimise the level of inlet and case radiated sound
that is transmitted through the return air grille into the room
space.
4) Hard acoustic conditions - Predominantly hard
surfaces in the room space results in higher sound pressure levels.
The addition of decorations and furnishings yields lower noise
levels.
5) Poor ceiling attenuation - Some of the inlet
and case radiated sound will reach the room space via the ceiling.
The use of acoustic ceiling tiles with low sound transmission and
good absorption properties reduces the level of inlet and case
radiated sound transmitted in this manner.
6) Listener too close to supply air outlet -
Sound pressure levels reduce with increasing distance between the
listener and the supply air outlet.
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