Unit 7 - Building Services Noise

Question 1

The noise levels inside and outside have to meet different design criteria.

Answer 1

• For ‘room-side’, this may be octave band levels as decreed by NR or NC curves.
• For the exterior, the criterion may be similar octave band level criterion, a simple L**Aeq,T , or compliance with a BS4142 assessment.

Question 2

Meeting design criteria requires the following;

Answer 2

1. Fan Noise Data – this may come from the fan manufacturer or predicted from generic formula.
2. System Attenuation - ductwork acts a wave guide leading to efficient sound transmission but each duct length, bend, branch and termination adds some attenuation. There can also be some flow generated noise to add.
3. Receiver Conditions – as the sound exits the ductwork it spreads out into space. The resulting level at some defined point will depend upon directivity, distance, room volume and room reverberation time.
4. Noise Control Measures - if additional attenuation is required to meet the criterion the system layout could be modified, a quieter fan used, silencers or plenum chambers added.

Question 3

Centrifugal Fans

Answer 3

air enters the centre of a rotating impellor that spins the air radially outwards forcing it to pass through a constricted throat (from which it is discharged tangentially at the outlet.

The blades of the impellor can be forward curved, backward curved or straight radial blades.

Question 4

Centrifugal fan

forward blade

Answer 4

lower speed but higher exit air velocity = higher output sound power.

Their lower = lower mechanical and bearing noise.

Question 5

Centrifugal Fans

backward fans

Answer 5

require a higher speed to give the same output = higher frequency noise but lower sound power due to lower air exit velocity.

Question 6

Centrifugal Fans

Straight radial blade fans

Answer 6

give greater pressures and as such are employed in dust and particle extraction systems but find little use in air handling due the higher sound power generated.

Question 7

Axial Fans

Answer 7

impellor is mounted on a shaft surrounded by a cylindrical housing.

As the impellor rotates it moves air axially from the inlet side through to the outlet side of the fans.

noise outputs but are biased towards higher frequencies.

Axial fans provide higher volume flow rates but at lower pressures compared with centrifugal fans.

Using aerofoil cross sections on the fan blades gives greater fan efficiency and lower noise output, but at a greater fabrication cost compared to simple flat profile blades

Question 8

Mixed Flow Fans

Answer 8

These fans are a combination of the centrifugal fan and the axial fan and can give higher pressures and higher volumes at lower noise emissions.

Question 9

Vortex noise from turbulence

Answer 9

Question 10

Real duct attenuation

Answer 10

In real ducts, there is a limited amount of absorption by the air in the duct and some loss through the duct walls.

The more rigid the duct walls then the smaller the internal attenuation via the duct loss

Circular metal ductwork has less attenuation that oval or rectangular ducting but less breakout

Question 11

External lagging of the ductwork with an absorber

Answer 11

• increases the low to medium frequency attenuation within the duct
• reducing breakout from the duct.
  
  • increases the damping of wall vibrations thus soaking up some duct noise.

Question 12

Internal lining of the duct with absorber,

Answer 12

effective in increasing duct attenuation

need to avoid mineral wool due to fibres getting into air supply

Question 13

Sound is partially reflected back to the source whenever there is a change in direction.

Answer 13

The greater the change in direction = the greater the reflection = greater the decrease in the sound power in the duct.

sharp right-angle bends can provide useful medium to high frequency attenuation.

Question 14

what happens when the duct branches

Answer 14

the total sound energy is divided between the branches in direct proportion to the relative cross-sectional areas of the branches.

If a new side branch is markedly smaller than the main branch then there will be extra loss due to a back reflection at the marked change in cross-sectional area.

Question 15

what frequencies does end reflection affect

Answer 15

The end reflection loss is significant at low frequencies and for small termination areas.

Use multiple small room terminations (diffusers) rather than a single large diffuser to reduce the low frequency noise into a room

Question 16

Receiver Conditions

direct level

Answer 16

Question 17

receiver conditions

Rev level

Answer 17

Question 18

Silencers normally have 3 parameters of interest;

Answer 18

• the attenuation they provide (this varies with frequency)
• the self-generated noise which is added to that passing through (also frequency dependant)
• the pressure drop across the silencer, to overcome which the fan power may need to be increased. As a result the fan noise will increase.

Question 19

To calculate the effect of the silencer;

Answer 19

take the octave band sound power level of the fan

subtract the octave band attenuation of the silencer

if the silencer’s self-generated noise is within 10 dB of that passed through, do a decibel addition of the attenuated and self-generated levels.

Question 20

Splitter attenuators

Answer 20

divided the airflow into two or more channels which pass between porous absorbers contained within perforated metal sheets.

The porous absorber will attenuate high frequencies but has less effect on low frequencies.

Can reduce turbulence with careful design and therefore regen noise

Question 21

Reactive silencers

Answer 21

exploit the fact that a certain proportion of the sound wave energy is reflected whenever the area of the duct changes. In its simplest form it consists of a plenum; a large box into which the air flows and then leaves

. Sound energy is reflected at both the inlet and the outlet.

Question 22

Active Silencers

Answer 22

Have the advantages of having good low frequency attenuation and can be used with passive silencers to cover the full frequency range.

They are smaller than passive silencers of the same attenuation and offer very low pressure drop

high initial cost has limited their general use.

Question 23

active noise cancellation process in duct

Answer 23

1. The incoming noise is sampled by the signal microphone and the signal is processed in the controller to produce an anti-noise signal that is fed to the loudspeaker.
2. The noise from the loudspeaker aims to be opposite in phase and amplitude to the in-duct noise.
3. The result is that the two signals cancel each other by superposition.
4. The resulting noise is read by the control microphone that feeds back to the control allowing further refinement of the anti-signal.

The noise in a ventilation system is fairly constant and predictable which makes the generation of the optimum anti-signal easier.

Question 24

assessing against NR/NC curve

Answer 24

• Measure sound pressure levels in octave bands under specified conditions (eg room unoccupied, ventilation on)
• Plot the results on a graph or compare with a table of figures
• Select the curve for which all points just fall below as the rating number

Question 25

NR/NC compared to dBA

Answer 25

If a background noise specification in terms of overall A weighted Lp is set it will be 5 dB above the equivalent NR value.

In many cases meeting a particular NR is a more stringent requirement than a specification of overall A-weighted level because due to compliance at each octave band.