VENTILATION NOISE

Components Air from outside Conditioned air to rooms via ducts Distribution system Air-handling Unit Fan with filters, heaters, coolers, to condition the air. Rectangular or circular

Noise Sources and Transmission Paths TRANSMISSION VIA STRUCTURE Case Radiation & Vibration Vibration Via Supports Duct Breakout (SECONDARY PATHS) FAN (PRIMARY SOURCE) TRANSMISSION THROUGH DUCTWORK (PRIMARY PATH) SOUND ENERGY REACHING THE OCCUPANTS OF THE BUILDING TRANSMISSION TO OUTSIDE REGENERATED NOISE DUE TO TURBULENCE (SECONDARY SOURCES)

Dealing with Secondary Sources & Paths These can be minimised by:- Minimising turbulence and reducing velocities Enclosing the fan Cladding ductwork Flexible couplers between fan and ductwork Anti-vibration mounts for the fan Anti-vibration hangers for the ductwork This leaves primary source and path as our main concern – and can be split into the atmospheric side and the room side (on which we will concentrate)

Fan Noise Calculation (Room Side) Methodology start with the fan sound power level, Lw and correct this for attenuations along the ductwork and into the room. Attenuation of straight duct Reflections at bends Attenuation at branches Reflections from the end of the duct Lw at the diffuser = Fan Lw – attenuations Lp at a position in the room depends on Lw at diffuser and the effect of distance (diffuser to position) and room absorption. Note:- these calculations are for each octave band

Regenerated Noise Note that in high velocity systems there will be secondary noise sources at bends and dampers within the ductwork. These will need to be considered separately and the appropriate attenuations applied from where they occur within the ductwork to the room as before. The levels in the room from the secondary sources must be added to that from the fan to obtain the overall level in the room. Secondary sources are ignored in this simple method.

Fan Power The best data is that supplied by the manufacturer based on tests of octave band sound power levels in a special reverberation chamber. Alternatively, a generic formula based on the power of the fan (kW), the flow rate (Q), and the pressure produced (P), although it is is less accurate. Lw = kW + 10logQ + 10logP + C C is a constant depending on the fan efficiency Note that this is the total sound power level and must be split between room and atmospheric sides (-3 dB?) And, corrections applied to get the octave band levels

Fan Performance Curve Pressure Developed, P (Pa) Fan Curve (flow rate achieved when the fan is developing a given pressure) Operating Point (pressure and flow when this fan is used in this ductwork system Pressure Developed, P (Pa) System Curve (pressure required for a given flow rate in the ductwork) Volume Flow Rate, Q (litres/s)

The test to determine the sound power level is similar to the one you performed in the reverberant chamber at UWE

Typical Fan Data Sheet Overall sound power level (A weighted) ,LwA at various fan operating points Fan Curve Pressure developed,P (Pa) Volume flow rate, Q (m3/s)

Corrections for the Octave Band Levels

Attenuation of Sound Power Level straight duct (energy removed to support duct wall vibration) bends (certain frequencies are reflected back at bends) plenum chambers / changes in x-section (sound is reflected if the cross section changes abruptly) branches (sound power is shared if the duct splits into branches) End reflection (due to impedance mismatch)

Attenuation -- Straight Duct Mainly due to duct wall flexing which takes energy out of the passing sound wave Note:- less attenuation per metre for large ducts less attenuation per metre for circular ducts less attenuation per metre for high frequencies

Attenuation of bends Long wavelengths negotiate bends with little reflection Short wavelengths travels by cross reflections and go round bends easily When the duct width is ½ a wavelength sound is strongly reflected back at bends

Plenum Chamber Used to distribute the air and can be used in noise control Attenuation depends on location of inlet and outlet, size and absorption of internal surfaces.

Attenuation --- Branches Sound power is split between the branches Use graph or equation Atten. = 10lg(A1/(A1+A2)) A1 is the cross sectional area of the branch in question, A1 + A2 is the total cross sectional area of all the branches

End Reflection Low frequency sound is reflected back due to the change from an enclosed duct to the larger room (impedance mismatch)

Lp in the Room Sound Power Level, Lw at the diffuser is the fan Lw minus the attenuations plus any regen noise. Q = geometric directivity 2, 4, or 8 Rc = room constant (m2) r = distance (diffuser to position) usually 1m for the worst position (closest to the diffuser)

Is it a problem? Plot the octave band room Lp’s on NR curves to find the attenuation required. The difference between the Lp’s and the target NR gives the additional attenuation required Say target is NR35 NR35

Acoustic Attenuators Simplest is to line the duct with sound absorbing material More effective if we split the airway to make a number of narrower ducts Typical performance

Typical attenuators (silencers) Louvre Circular Pod Rectangular Splitter

Calculator @ http://www.engineeringtoolbox.com/accoustic-calculation-ventilation-d_70.html