The diagram below. to the by the. outlet into. calculation. Since TRANSMISSION VIA STRUCTURE. Vibration Via Supports Duct Breakout

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1 NOISE CONTROL IN VENTILATING SYSTEMS INTRODUCTION The diagram below shows the various noise sources and transmission paths of plant rooms and ventilation systems. For the ventilation system this can be summarised; the schematic diagram shows fan noise and regenerated noise due to turbulence inside the ductwork where there is a change of direction (sharp bend) or obstruction (flow control damper). The noise (and vibration) is transmitted along the ductwork, which is the primary path or via other secondary paths which may then add to the noise entering the occupied rooms. Case Radiation & Vibration Vibration Via Supports Duct Breakout TRANSMISSION VIA STRUCTURE (SECONDARY PATH) TRANSMISSION TO OUTSIDE FAN (PRIMARY SOURCE) TRANSMISSION THROUGH DUCTWORK (PRIMARY PATH) SOUND ENERGY REACHING THE OCCUPANTS OF THE BUILDING REGENERATED NOISE DUE TO TURBULENCE (SECONDARY SOURCES) Secondary paths can be minimised by enclosing the fan and, where necessary, cladding the ductwork and by the use of a spring isolator system for the fan and anti-vibration supports for the ductwork. This leaves the transmission of fan noise through the ductwork as the main problem. Duct borne noise can be reduced by the use of an acoustic attenuator located near to the fan outlet. Additionally, in high velocity systems regenerated noise may be attenuated by the use of sound absorbing lined plenums and ducts located close to the diffuser outlet into the room. In order to determine the amount of attenuation required to meet a given Noise Rating specified for the room in question it is necessary to perform a fan noise calculation. Since noise will also be transmitted via the fan inlet it will also be necessary to calculate the noise that will be radiated to other buildings outside.

2 FAN NOISE CALCULATION (ROOM SIDE) The method starts with the fan outlet sound power level usually given in octave bands. Corrections are applied to account for attenuation through the length of ductwork system (sound energy transferred to the duct walls (which flex and convert sound energy into heat), reflections at bends and changes in duct section, reduction in sound power when the duct splits (sound energy is shared to each branch), and reflections at the opening where the duct connects into the room (end reflection). When all these corrections are applied the resulting sound power level is that which enters the room through the diffuser. The sound pressure level at any point in the room can then be determined in octave bands and plotted on a Noise Rating chart to obtain the noise rating. If any of the predicted octave band sound pressure levels are above the recommended NR for the room the differences reveal the attenuation required and hence enables a suitable acoustic attenuator (silencer) to be selected. In high velocity systems it may also be required to predict the regenerated noise caused by turbulent air as it passes round bends, through branches or other obstructions in the ductwork system. The regenerated sound power levels are then subject to the same attenuations through the ductwork and each regenerated noise source one will need to treated separately as they will be at different positions within the system. The total sound power level at the diffuser is calculated by summing (logarithmically) all the sound power levels METHOD 1. DETERMINE THE SOUND POWER LEVEL ENTERING THE DUCT Manufacturers test data is best and most accurate. If this is not available the total fan power level, L w for each octave band can be estimated from a generic formula and the fan operating conditions. An assumption is made that the sound power is split 50/50 between the inlet and outlet (-3dB in each direction (inlet and outlet)) where Lw = K w + 10log10Q + 10log10P + C K w is a correction depending on the type of fan and the octave band frequency Q is the volume discharged (m 3 s -1 ) P is the fan static pressure (Pa) C is a constant depending on the fan operation condition (% efficiency) Corrections are then applied to obtain the octave band sound power levels 2. CALCULATE THE TOTAL ATTENUATION OF SOUND POWER LEVEL DUE TO; a. straight duct (energy removed to support duct wall vibration) b. bends (sound of certain frequencies is reflected back at sharp bends) c. plenum chambers (sound is reflected if the cross section changes abruptly) d. branches (sound power is shared if the duct splits into branches) e. End reflection 2

3 a. Attenuation of sound in unlined sheet-metal ducts. As sound travels down a duct the duct walls flex as the compressions and rarefactions pass by. This flexing takes energy from the sound wave and is dissipated due to damping in the duct or lost as sound radiation from the duct. Large rectangular ducts are less stiff than small ducts or circular ducts and therefore have higher attenuation per metre. The following table is an extract from CIBSE B5 Noise and Vibration Control Duct section Rectangular Mean dimension or diameter I mm Attenuation in db/m for stated octave bands 63 Hz 125 Hz 250 Hz 500 Hz and above < > Circular < > b. Attenuation of bends Sound is reflected back where there are sharp bends. The reflected sound energy is then lost due to flexing of the duct wall etc. Low frequencies (long wavelengths) tend to negotiate bends without reflecting back and high frequencies (short wavelengths) travel by cross reflections so they also are not reflected as much as mid frequencies. Reflection is greatest when the wavelength is twice the duct width. Attenuation of 90 o bends (db) Octave band centre frequency Hz Duct dimension * Rectangular 75 mm Rectangular 150 mm Rectangular 300 mm Rectangular 600 mm c. Attenuation due to a plenum chamber Plenum chambers are just large boxes or extra large sections of the ductwork --- they are used to equalise the air pressure and distribute air into the branches of the duct system. Usefully, sound energy is reflected at each change of cross section and they can give significant sound attenuation particularly if this is combined with lining the plenum with sound absorbing material. The insertion loss of a plenum can 3

4 be calculated the following equation incorporates all these factors to give an insertion loss (attenuation) in db. The insertion loss of a plenum, Where, A OUT is the cross sectional area of the outlet (m 2 ) Q is the directivity factor for the inlet duct (8 in this case) θ is the angle between outlet and inlet r is the distance between inlet and outlet (m) α is the average absorption coefficient of the lining A is the internal surface area of the plenum (m 2 ) d. Attenuation due to branches When the duct branches to divert air into different rooms the sound energy is split between the branches. This is calculated either from the cross sectional areas or the volume flows in each branch. Use the graph or the equation below: Attenuation = 10 log(a 1 /(A 1 +A 2 )) A 1 is the cross sectional area of the branch in question, A 1 + A 2 is the total cross sectional area of all the branches e. Attenuation due to end reflection This is due to an impedance mismatch between the air in the duct and the air in the room. The sound energy travelling down the duct is reflected back and is lost via flexing of the walls on the duct system. The maximum reflection occurs when the wavelength is large compared to the duct dimension, i.e. low frequencies. Attenuation db Octave band centre frequency (Hz) 2000 Duct dimension * and higher

5 3. FIND THE FAN SOUND POWER LEVEL ENTERING THE ROOM The sound power that enters the room is calculated by subtracting the total attenuation in each octave band from the fan sound power entering the duct at the fan. Lw (entering room) = Lw (at fan inlet) total attenuation through the ductwork 4. DETERMINE THE SOUND PRESSURE LEVEL AT A POSITION IN THE ROOM For a noise source in a room, the sound level is the sum of direct and reverberant sound and is given by: Lp = Lw + 10lg Q πr 2 R c where, Q = geometric directivity 2 if the diffuser is in the middle of the room (hemispherical spreading), 4 if it is located at one edge of the ceiling 8 if located in a corner R c = room constant (m 2 ) r = distance from the diffuser to position of interest in the room (usually 1m for the worst position just below the diffuser) Room constant R c = S α / (1- α) wheree S = total surface of the room (m 2 ) α = area averaged absorption coefficient of the room surfaces 5. PLOT THE OCTAVE BAND SOUND PRESSURE LEVELS ON A NOISE RATING CURVE A plot of the octave band levels will reveal the attenuation required to reduce the noise to the specified NR value and hence enable a silencer to be selected. Example 63 NR35 64 Calculated 65 Atten. R qd k k 4k

6 The secondary noise sources including the diffuser are treated similarly. Predicted Lw from the air velocity and type of obstruction (from tables or empirical equations) and then attenuated from their position in the duct system to the room as above. All the individual sound power levels from all the secondary sources are added at the diffuser and the room Lp is then calculated as above. Note that is best to keep these as separate Lw s so that the most problematic source can be identified, thus allowing decisions on which to treat. Additional attenuation (lined duct or plenum) may then be specified between the secondary source and the room. DESIGNING DUCTWORK FOR LOW NOISE The fan sound power level is based on measurements by the manufacturers using British Standard :2004. The method requires the fan to be installed in ductwork in the most favourable configuration to minimise noise generation. If the fan is installed where the air flow is disturbed turbulence and therefore noise will increase. 6

7 Regenerated noise is the result of turbulent air. Smooth airflow through the duct and adequate spacing to allow turbulent air to become laminar before encountering another obstruction will minimise noise. 7

8 NOISE CONTROL TECHNIQUES Select the quietest fan for the duty required this is usually achieved by selecting a fan which will be operating at maximum efficiency at the flow rate and pressure equired by the ventilation system. Design the ductwork system to give laminar flow (see diagrams above) and prevent regenerated noise Make sure that the fan is enclosed and vibration isolated from the building structure. Disconnect the ductwork from the fan using a flexible connector (made from canvass) 5. Use appropriate acousticc attenuator (or louvre for inlet) to reduce fan noise Louvre an attenuator designed to stop rain penetration 6. Select a diffuser to minimise regenerated noise and also to reduce low frequency noise by back reflection --- several small diffusers will be more effective than one large one. 7. Use lined bends, plenums and ducts to reduce noise within the ductwork system