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Dunham-Bush
1. Introduction to Fan Coil Unit Acoustics and Fan Coil Room Noise
Levels
The selection and application of fan coil units is based on not only
heating/cooling duties or airflow rates, but also on the level of sound
in a room generated by the fan coil unit. Often, it is the room sound
levels which are the principal criteria. However, it is not only the fan
coil which affects room sound levels, but the room itself. In the early
stages of design, room conditions will not be known. Although this
prohibits an accurate assessment of sound levels, estimates can be made
if certain assumptions are made.
2. Sound Pressure Level
The human ear reacts to the fluctuations in air pressure caused by
sound. The level of sound heard by the human ear (and measured by a
noise level meter) is quantified as the Sound Pressure Level (SPL); the
ear reacts logarithmically over a very wide range of sound levels, so
sound pressure levels are quantified on a logarithmic scale, with a
reference point taken as the pressure corresponding to the threshold
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 is quantified
as the Sound Power Level (SWL). As for sound pressure levels, sound
power levels are measured on a logarithmic scale, with a reference point
taken as corresponding to the threshold of human hearing.
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 oscillations is
measured as frequency in Hertz (Hz). In practice, virtually all sound
consists of components at different frequencies; each component will
have a sound power level at the respective frequency. The human ear is
sensitive to sounds with frequency in the range from 30Hz up to 20kHz.
Since frequency is a continuous variable, sound levels (both SWL and SPL)
are quantified in frequency bands for convenience.
These are referred to as octave bands (since their width is normally one
octave) and the mid-band frequency is used to specify each.
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,
1000Hz, 2000Hz and 4000Hz are critical.
Thus, for a piece of equipment such as a fan coil, each octave band
frequency has a corresponding sound power level, which in turn leads to
corresponding sound pressure level at the same frequency.
5. The Relationship Between SWL and SPL
In simplistic terms, SWL is the rate at which acoustic energy is
transferred to a room. SPL is a measure of the effects of SWL at a
certain point within the room. A useful analogy is to consider a heater;
the heater’s output is measured as power in Watts, but the effect of the
heater’s output is measured as temperature in Kelvin or degrees Celsius.
It is important to remember that sound can be emitted from a number of
sources to the listener:-
i) Inlet and case radiated sound will be carried through the ceiling
into the room space.
ii) In-duct sound will be carried down the duct and through the supply
air diffusers to the room space.
Inside the room space, sound will follow a direct path from the source
to the listener as well as being reflected off room surfaces before
reaching the listener.
iii) Sound that travels directly between source and listener is direct
sound path.
iv) Sound reflected off room surfaces before reaching the listener is
reverberant sound path
6. Factors Affecting Room Sound Pressure Levels
The diagram in figure 1 represents a typical fan coil installation.
In order to determine room sound pressure levels, the following criteria
must be known:-
Sound power levels of the equipment i.e. fan coil unit
Sound power levels are specified as two sets of acoustic data
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
1 Outlet ductwork The size, length, number of bends and type of ductwork
will influence how much in-duct outlet sound reaches the supply diffuser
plenum.
2 Supply air plenum The supply air plenum acoustic characteristics will
be affected by size, number of duct connections, acoustic lining etc.
3 Supply air grille The type and size of the supply air grille will
affect the in-duct sound radiated into the room. High air velocities
through the diffuser may create regenerative noise.
4 Position of supply air diffuser The position of the supply air
diffuser will affect the sound that travels directly to the listener
(direct sound levels) as well any sound reflected off room surfaces
(reverberant sound levels).
5 Ceiling construction Some inlet and case-radiated noise will be
transmitted through the ceiling, depending upon the acoustic
characteristics of the ceiling.
6 Return air grille The position of the return air grille will affect
the inlet and case-radiated noise emitted into the room. The return air
grille should be as far as possible from the fan coil unit inlet.
7 The room conditions The sound transmitted from the ceiling and air
grilles will travel directly to the listener as well as be reflected off
all the room surfaces before reaching the listener. Hence the room
surface area and quality of room surface will affect sound pressure
levels.
8 The position of the listener As sound travels away from the source,
acoustic energy reduces leading to lower sound pressure levels.
7. Noise Assessment
To assess the level of noise, the sound pressure levels are compared
with a set of curves which reflect the response of the human ear at
given octave bands. The most common curves employed are Noise Rating
(NR), Noise Criterion (NC) and Room Criterion (RC) curves.
Measured or calculated sound pressure levels are plotted on the curves,
and the highest value curve that sound pressure levels reach is taken as
the criterion for the noise at that point in space. The example in
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.
8. Common Problems With Fan Coil Installations
a) Outlet ducts are small - Supply air ductwork with small diameters
will lead to high air velocities
and external static pressures. As a guide, air velocities should be
limited to 3m/s in typical office spaces and lower for critical areas.
Greater air velocities lead to noise-regeneration within the duct.
Recommended air volumes for various duct size are:-
Ø250mm - 145 l/s : Ø200mm - 95 l/s : Ø150mm - 55 l/s
b) Excessive quantities of flexible ducting - Flexible ducting has a
higher pressure drop than rigid ducting. Higher pressure drops can lead
to noise regeneration.
c) Return air grilles too close to fan coil units - More of the inlet
and case radiated noise sound will reach the room via the return air
grille, so the longer the air path to fan coil inlet the better.
d) Hard acoustic conditions - If the room has predominantly hard
surfaces, then sound pressure levels will be higher. The addition of
furnishings will cause noise levels to reduce.
e) Poor ceiling attenuation - Some of the inlet and case radiated noise
levels will be transmitted to the room via the ceiling. The use of
acoustic ceiling tiles with low sound transmission and good absorption
properties is recommended.
f) Listener too close to supply air outlet - Sound pressure levels will
reduce the further the listener is from the supply air grille.
Request information above
Dunham Bush Air & Water Cooled Packaged Chillers
Request information above
Dunham Bush AH-Us Radiant Heating Fan Coil Units
Technical information associated with:
Dunham Bush - Fan Coil Unit Acoustics & Room Noise Levels
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Dunham-Bush Ltd
Downley Road
Havant
Hampshire
PO9 2JD
Email this company
Tel : 023 9247 7700
Fax : 023 9245 3601
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