NOISE IN VENTILATION DUCTS

Sound in ductsThe primary source of noise is the fan. It will radiate tonal sound at frequencies related to the motor, rotational, frequency and the blade frequency. It will also radiate broadband sound due to any turbulent airflow off the blades. The sound power is constrained within the ducting. The sound power propagating along the duct will reduce only where energy has been:

absorbed by the duct walls (and possibly then radiated outside the duct) reflected back up the duct by a sharp bend divided between ducts where branches diverge reflected back up the duct due to changes in the cross-sectional area of the duct

In the simplest possible case of a long straight duct with very rigid walls, the power will be the same at all points along the duct.

Fan NoiseNoise energy generated by the fan rotation is dependent on the diameter and the rotational speed of the fan, typically proportional to the fifth or sixth power of the linear velocity at the tip of the fan blades. Generally a large diameter fan at low rotational speed produces low frequency and comparatively low noise level. The same volume flow rate can be achieved by increasing the rotational speed and reducing the diameter. In this case the frequency and the level of the noise are higher but the higher frequency noise can be more effectively attenuated by lining the duct with sound absorbing materials. In general the SWL of fans varies greatly with types, sizes, and speeds. It should be chosen carefully to ensure low noise levels. Manufacturer’s data on the source SWL of a fan should always be sought.

Flow generated noise: In empty straight ducts noise from turbulence is generally very small compared with other

factors, especially with velocty, u<10 m/s. Bends, Branches, Junctions and Turns cause abrupt changes in flow conditions. Noise

level increases approximately 15 (power u5 ) to 20 (power u7 ) dB per doubling of flow speed depending heavily on the duct size and configurations. Noise can be reduced by rounding the bends or introducing streamlined turning vanes.

Interaction with Obstacles, e.g. silencers etc causes turbulence wakes. Noise level again depends approximately on u5, and also on the size and geometry of the obstacle. Shapes that create abrupt changes in the flow are best to be avoided.

Inlet/Outlet Grilles, (air) Diffusers and dampers. Similar noise generating mechanisms as other obstacles. Noise level again u5 to u6.

For good, low flow noise design: Avoid abrupt changes. Graduate transitions are preferred to minimise the generation of

turbulence. Avoid grouping duct/flow changes near each other. Spread out changes to “straighten” the

flow before hitting another transition. Porous absorptive duct linings should generally be used to attenuate flow noise in high

specification cases. Significant flow noise generated by a duct fitting/change should be attenuated by a secondary attenuator placed immediately downstream of the fitting.

Most importantly, since flow generated noise are generally ~u5 or higher, the flow velocity must be kept low. For example, for a NC20 design, the flow speed in the main ducts, branch ducts, and final run out sections should respectively be lower than 4.5 m/s, 3.5 m/s, and 2 m/s. For a fixed volume flow rate, a lower flow velocity can be achieved by using larger duct sections.

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Predicting the sound power level of the ventilation grilleIn order to determine the sound pressure level in a room due to the noise emitted from an air inlet or air extraction grille of a ventilating system, the sound power level, LW of the grille as a noise source in the room has to be determined. The sound pressure level in the room can then be calculated from:

where

Q Example of grille locationGrille radiates into

1 In the centre of the volume of the room (e.g. a long duct down from a high ceiling) Whole space

2 In the middle of a plane surface (e.g. the ceiling or a wall) Half space

4 At the junction of 2 plane surfaces (e.g. where a wall meets the ceiling) Quarter space

8 At the junction of 3 plane surfaces (e.g. a corner where 2 walls and the ceiling meet) Eighth space

The sound power level of the fan is usually acquired from the manufacturer’s data. Attenuation in the duct will be due to a combination of the following:

i. straight duct runs;ii. bends in the duct;iii. branches in the duct; andiv. the duct termination at the grille.

For each case, i. to iv. above, reference is made to an appropriate graph or reference table, as explained below. NB: the following does not include or predict any addition to the sound power within the duct. Self, or regenerative, noise is due to turbulent air flow which occurs when the air is being moved at high speed and/or encounters irregularities or obstacles (e.g. flow control flaps/dampers, sudden bends, duct wall irregularities, grille louvres).

i. Straight duct runsDuct walls are not rigid and, consequently, absorb sound power whilst vibrating in response to sound pressures in the air in the duct. Some of the absorbed power is then radiated as airborne noise outside the duct and some is converted into heat due to the internal friction of the duct wall material. Both lead to a reduction in the sound power inside the duct. The table below shows typical attenuation in dB/metre for unlined sheet metal ducts.

Attenuation due to a straight duct run (in dB/m duct)

Duct dimension (mm)O.B.C.F.(Hz)

63 125 250 500 1k ≥ 2k

Round 100 - 300 0.1 0.1 0.1 0 0 0

Round > 300 0 0 0 0 0 0

Rectangular 150 x 150 0.65 0.65 0.5 0.35 0.35 0.35

Rectangular 600 x 600 0.65 0.65 0.35 0.15 0.15 0.15

Rectangular 1800 x 1800 0.35 0.35 0.15 0.03 0.03 0.03

Rectangular ducting gives higher attenuation than circular ducting because the flat walls of the former are less rigid and vibrate more readily. Notice how the flat duct walls attenuate more at low frequencies, reflecting the absorbent behaviour of panel (or membrane) absorbers. It follows from the last sentence that duct break-out is greater at low frequencies.

ii. Bends in the ductAt bends sound power is reflected back towards the fan. The effect is greater for 90˚ bends. In addition to the reflection there is also resonant behaviour for frequencies where the wavelength is twice the duct width. The tables below show typical attenuation/bend for unlined and lined sheet metal 90˚ bends.

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Long radius, curved, bends give much less attenuation and are usually assigned 0dB attenuation. 90˚ bends may well create turbulent flow and generate additional sound power inside the duct; one solution is to place turning vanes in the bend to encourage the air to flow through the corner in a streamlined way.

Attenuation due to a bend in the duct (in dB per bend)

Duct typeWidth B

(mm)

O.B.C.F (Hz)

125 250 500 1k 2k 4k 8k

Without absorber

125 0 0 0 6 8 4 3

250 0 0 6 8 4 3 3

500 0 6 8 4 3 3 3

1000 6 8 4 3 3 3 3

Absorber in front of elbow

125 0 0 0 6 8 6 8

250 0 0 6 8 6 8 11

500 0 6 8 6 8 11 11

1000 6 8 6 8 11 11 11

Absorber after the elbow

125 0 0 0 7 11 10 10

250 0 0 7 11 10 10 10

500 0 7 11 10 10 10 10

1000 7 11 10 10 10 10 10

Absorber in front and after the elbow

125 0 0 0 7 12 14 16

250 0 0 7 12 14 16 18

500 0 7 12 14 16 18 18

1000 7 12 14 16 18 18 18

iii. Branches in the ductThere is no net loss of sound power from the ventilation system at a branch. Rather, the sound energy divides between the main and take-off ducts in the same way that the airflow does. Consequently, the power entering a branch is less than that in the run of duct immediately before the branch. The attenuation, i.e. the reduction in sound power level in going from the feeding main duct into the branch, can be calculated from:

The sound power level in the main duct after the branch will be less than that prior to the branch and may be determined by decibel subtraction.

iv. Duct termination at the grilleWhen the sound waves finally reach the grille not all of the power passes out into the room. Some of it is reflected back into the duct. The reason is the change in acoustic impedance between the air in the duct and the air in the room. This behaviour is frequency dependent. Low frequency sounds have long wavelength which are greater than the cross-sectional dimensions of the duct and are more prone to attenuation due to impedance change. High frequency sounds tend to propagate along the axis of the duct and radiate freely into the room. The table below shows typical data for an open duct termination attenuation.

Attenuation due to end reflection (in dB per termination)

Duct dimension (mm)O.B.C.F. (Hz)

63 125 250 500 1k ≥ 2k

125 17 12 8 4 1 0

250 12 8 4 1 0 0

500 8 4 1 0 0 0

1000 4 1 0 0 0 0

2000 1 0 0 0 0 0

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Duct silencersThe propagation of acoustic power along a duct may be reduced by the use of duct silencers. This may involve simply lining the inner surfaces of the duct with acoustic absorber. The following graphs show the attenuation achieved, per metre of rectangular cross-section sheet metal duct, by lining it with porous absorber 25 mm and 50 mm thick respectively.

Back on page 2, the data relating to duct bends also includes information relating to lined bends.Duct silencers may be classified as:

Silencer Classification

Comment

Active Microphones and electronics to determine and attenuate (tonal) noise. Not often used.

Diffuser Perforated pepper pots that slow down velocity and are mainly used for nozzles, jet engines etc.

Dissipative Based on porous absorbers, the dissipative part, and widely used for HVAC duct systems.

Packless Used for clean room activities and in zones where acoustic materials can cause fire hazards.

Reactive Tuned cavities or membranes which are designed to attenuate low frequency noise from machines.

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Splitter silencers

The commonest form of silencer is the splitter silencer, which is inserted into a straight duct run.

Above is a photo of a splitter silencer with a cross-sectional diagram of its construction. The following is taken from the manufacturer’s description of the diagram.

(1) Die-formed single-piece splitter construction throughout.(2) Acoustic baffles designed for maximum attenuation at low frequencies, the toughest job of all.(3) Straight-through air passages designed for maximum air handling at minimum pressure drop.(4) Solid, rounded noses increase noise reduction.(5) Bell-mouth entrance and exit minimize turbulence.(6) No protruding fastener heads to cause turbulence or self-noise.(7) Solid air-impingement surfaces and self-cleaning air passages minimize dirt entrapment.(8) Acoustic fill protected against erosion by perforated-metal containments.

Insertion LossInstalling a silencer in a duct will lead to a reduction in the sound power level in the duct, when measured at a point beyond the silencer away from the fan. The reduction in the sound power level at that point is called the insertion loss. It is quoted in dB and is frequency dependent.

Self noiseClose examination of the manufacturer’s text for the eight points listed above, shows that attention is paid to maintaining laminar air flow through the silencer. Any turbulence in the flow will generate noise. Self noise is the sound power level generated by the turbulent air flow through the silencer. The sound power level of the self-noise will add (by decibel addition) to the sound power level of the fan noise.

Self noise may be generated elsewhere in the ducting, wherever the air flow is not laminar. Turbulent flow may be encouraged by: irregularities in the inner surfaces of the duct; abrupt changes in direction of flow; abrupt changes in the duct cross-section; intrusive features or devices in the duct (e.g. air flow control dampers), or high flow velocity. Air handling systems for concert halls and recording studios make use of very large cross-sectional area ducts along which the air is delivered or removed slowly.

Active silencersActive silencers make use of noise-cancelling technology, whereby an acoustic signal is injected into the duct. The signal has identical amplitude and frequency to the duct noise, but is in anti-phase (180 degrees out of phase) with the duct noise. Destructive interference occurs when the duct noise and acoustic signal combine, thereby reducing the level of the duct noise. This technology is most appropriate for persistent tonal noise. The diagrams below show one manufacturer’s design. It is a hybrid, as it includes a dissipative (“passive”) silencer.

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The reference microphone detects the duct noise. The control unit determines and generates the anti-phase acoustic signal to be injected into the duct by the loudspeaker. The error microphone detects the resulting, combined, acoustic signal and provides feedback to the control unit, which then refines the noise cancellation signal to the loudspeaker.The manufacturer of the above hybrid silencer produced the following graph for three noise spectra in the duct. In descending order they are: before inserting the silencer; relying on the dissipative absorbers alone; and activating the noise cancellation equipment. The reduction in levels at mid to high frequencies due to the porous, dissipative, absorber can be seen; as can the noise cancellation technology’s ability to reduce low frequency sound.

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Positioning silencersBreak-out is the transmission of duct noise through the duct walls into the surrounding spaces. Break-in is the reverse process. Plant rooms can be very noisy spaces and if the silencer is positioned close to the fan, the noise break-in into the duct before it passes out of the plant room may lead to noticeable plant room noise radiating from the room grilles. The break-in problem may be addressed either by lagging the exterior of the duct to reduce break-in, or by positioning the silencer as close as possible to the point where the duct passes out of the plant room.

Structure-borne noiseVibration from the fan and motor may be transmitted into the structure of the building, either directly from the fan and motor, via their supports, or via transmission along the steel ducts and their hangers and supports. The vibrating building structure (the building elements) would then radiate sound into the rooms.

The way to reduce structure-borne noise is to provide vibration isolation between the ventilation system and the building structure. This implies having no rigid connection between the two.

Commonly, the fan and motor assembly (see left) will be supported on vibration isolation mounts (see below). This is often a heavy platform resting on resilient mounts (e.g. helical steel springs).

The ratio of the driving frequency of the fan motor to the natural frequency of the assembly should be in the region of 3 to 4, depending upon the percentage isolation required..

Having assembled the fan and motor on the vibration isolation mounts, the connections (e.g. electricity supply, inlet duct and outlet duct) should not be rigid; otherwise they provide bridging routes for vibration.

The flexible duct coupler, as shown to the right here, helps to isolate the metalwork of the duct from the structural vibration of the fan and motor; but could cause turbulence in the air flow due to irregularities in the inner wall of the coupler..

Where vibration is transmitted along the metal of the duct, rigid supporting hangers would convey the vibration into the building structure. In this case, use is made of resilient hangers which may include rubber blocks or helical springs as the resilient element. The latter are shown to the left.

Duct Breakout NoiseAt frequencies above the transition between axial and multi-mode transmission of sound (f > c/(2*duct width/height)) in a duct, the sound field inside the duct may be considered as diffuse and the breakout of the noise from inside to the outside of the duct is protected by the sound reduction

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index or transmission loss (TLduct) of the duct wall. However since the duct runs over a significant length inside the receiving room, the total amount of the noise breakout increases as the length increases. For a simple calculation the following formula can be used:

(1)

where Sw is the total surface area of the duct in the room and SD is the cross-sectional area of the duct. It can be seen that no matter how high is the TL, all the sound power inside the duct will breakout into the room given that the duct length Lduct inside the room is sufficiently long. To limit the physical impossibility that SWLout > SWLin if Lduct is too long, the following condition applies,

(2)Another consideration is that flat duct walls will resonant and it is important to design the duct work such that the fundamental resonances of the duct walls should not be excited by any fan or flow generated noise, or by the speech or music signals inside the studio. Suitable damping materials should be applied to the duct to reduce resonance effects. In extreme cases the ducts may be “boxed in” with suitable partitioning, although this may result in undesirable bulkheads in the studio.

For high air flow velocity ventilation systems the use of flat rectangular ducts should be avoided as they can produce low frequency rumble. Circular ducts are stiffer and have less problems. However they are more expensive and there is also a “ring” frequency at which the duct circumference is equal to the wavelength of the sound travelling through the duct material as an acoustic (compressional) wave (f = cduct/D). Below this frequency the transmission loss is determined by the thickness to diameter ratio of the duct. Above this frequency the duct behaves very much like an equivalent rectangular duct.

CrosstalkCross-talk is the process whereby sounds generated in one room can travel along the duct and radiate into other rooms. Occupiers of one room hear the music, conversation, etc of people in other rooms. This can be addressed by paying attention to the closeness of outlet grilles (e.g. avoiding having outlet grilles, to separate rooms, facing each other across the duct) and/or by applying attenuation measures in the duct as described above for the fan noise.

The inside of a duct provides an excellent channel for noise transmission. Without proper precautions the noise from one room can be transmitted easily to another room via the ductwork, creating crosstalk that bypasses the noise insulation provided by the structural walls. The amount of noise generated by crosstalk may be estimated in 2 steps:

Noise break into the duct system from the noise source room and transmitted to the receiving room:

(3)where Lw,outside is the total sound power incident onto the duct section that exposes to the source room. The 3 dB reduction assumes that the sound power entering the duct will be divided equally in the two opposite axial directions, and only one of these directions will lead to the receiving room.

The formula for breakout noise from ducts can then be used to estimate the noise transmitted into the receiving room.

To avoid crosstalk, it is often necessary to use sound attenuators or silencers to block out any significant crosstalk paths. Apart from the air-borne transmission through the inside of the duct, the duct wall itself can sustain mechanical vibration and provide a path for structural-borne noise transmission. It is therefore important that the duct should be supported on resilient mounts which insulate it from the structural walls. This also applies to the opening through which the duct passes through a wall.

QuestionsProblem:The diffuse field sound pressure level in a noisy room is 80 dB at 1kHz. The room is serviced by a ventilation system that also services a studio. The cross-section of the ventilation duct has a width of 0.25m and a height of 0.25m. The length of the duct in the noisy room is 5m. Estimate the cross-talk noise level that would be created in the studio which has a volume of 200 m3, surface area 210

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m2 and a reverberation time of 0.3s. The length of the ventilation duct in the studio is also 5m. You may take the effective transmission loss of the duct wall, including the inlet and outlet grills, to be 20 dB. What is the sound attenuation required of a cross-talk silencer to be placed in the duct to reduce the cross-talk noise level to within 15dB at 1kHz?

Answer:Assuming a diffuse field, the relationship between sound pressure level in the room Lp and the sound power level on the duct Lw is:

(4)Where S is the surface area of the duct. So in this case Lw = 81dB.Using Equation (3), we can get the sound power level in the duct as: 58 dB.Sound power breakout using Equation (1) Lw,breakout = 58 – 20 +10log(5/0.25^2) = 57dBNote that this is within 1dB of the SWL in the duct, so adjust to 58-3=55dB.Level in Studio: =41.7dB (S is total area of room, and alpha bar the average absorption coefficient). The addition attenuation required = 27dB.

Answer:Since the sound power is equally distributed, the sound power level from the fan to the studio is 61-3=58dB.SWLfan-studio = 57dBWhen SWL is greater than the in-duct SWL-3, we takeSWLfan-studio = SWLfan –3 = 58 – 3 = 55dBThe SPL is thenSPLfan-stdio = 42dB (too high for a studio)

(b)Placing the silencer immediately after the fan only attenuates the fan noise. There are two other noise generating mechanisms in a ventilation system:1. Flow generated noise (see above)2. Cross-talk noise (see above)3. Structure-borne noise

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