1. PRINCIPLES OF NOISE CONTROL ENGINEERING

2. INTRODUCTION
Noise control engineering is often seen as a remedial measure for noise problems.
However, it is more effective if a noise problem can be predicted before it occurs since solutions are easier and cheaper if applied at the planning and design stages.
In either case good housekeeping is essential to ensure that the performance of any measures is maintained.

3. Noise generation is often complex
E.g. car engine noise involves many sources
explosions
gears meshing
bearing noise
fluid flow in the cooling system
air intake and exhaust.
There will also be associated resonances of parts of the engine
These sources will be transmitted
via vibration, and/or
through the air to appear inside the car interior
or away from the car as environmental noise.

4. SOURCE-PATH-RECEIVER MODEL
A simple approach is needed to break the problem down into individual elements
Modifying any or a combination of the three elements can provide noise control.
SOURCE
OF ACOUSTIC
ENERGY
TRANSMISSION
PATH(S)
RECEIVER(S)

5. CONTROL AT SOURCE
The first step is to analyse the source and decide if the level can be reduced by,
Changing the process (Alternative method)
Modifying the existing process so that it is less noisy.
Modifying an existing process generally involves,
Reducing the forces in the process
Isolating the vibration
and/or
Introducing damping.

6. CONTROL IN THE PATHWAY Modifying the path could involve
moving the noisy machines
grouping noisy and quiet processes in separate rooms
enclosing or screening the source
isolating the source from the building structure
increasing the absorption in the room

7. NOISE CONTROL AT THE RECEIVER
The person receiving the noise could be moved to a quieter area
(it’s surprising how many workers are subject to high levels of noise when they can be relocated to quieter surroundings without affecting productivity).
Noise havens could be provided; these are the reverse of noise enclosures
And, as a last resort hearing protection could be issued

8. NOISE CONTROL APPROACHES
In practice noise control is generally concerned with reducing noise at source and/or attenuating the energy in the transmission path
REDUCE SOURCE
STRENGTH
REDUCE
ENERGY IN
TRANSMISSION
&

9. REDUCE SOURCE STRENGTH
The basic rules apply, based on actions that:
Reduce forces
(lower speeds, shorter drop heights, reduced operating pressures, longer application times)
Isolate forces so that they do not vibrate panels
(anti-vibration mounts and resilient connections)
Reduce the amplitude of vibration
(increase the mass, increase stiffness, apply damping).
Inevitably, there is a limit to the reduction that can be achieved without a fundamental redesign or rethink of the process.

10. REDUCE ENERGY IN TRANSMISSION
Note that there are many transmission paths, all of which must be considered.
We can see that, in this case, the flanking path limits the overall attenuation. Increasing the sound insulation of the separating wall will not provide greater attenuation.
100 dB
40 dB (direct)
Overall = 50 dB
50 dB (flanking)

11. MECHANISMS OF NOISE GENERATION
In order to decide on a suitable method of reducing noise at source we must first understand how noise is generated.
Here are two basic mechanisms:
Vibration of surfaces
Aerodynamic, which may be split into:
Fluctuating force on a fluid (fan blades, etc)
Shearing of a fluid (air jets)

12. Free Vibration of a Surface
Plate subject to hammer blow will have two parts to the noise
Hammer blow is an impact so will be impulsive and contain all frequencies
Plate will then ‘ring’ at its natural frequencies which will die away depending on how it is damped. The frequencies excited will depend on the plate dimensions, thickness, material and how it was struck.
Chladni’s figures -
created by using
a violin bow to excite
a metal plate

13. Forced Vibration of a Surface
If the surface (plate, panel, frame) is attached to a source of vibration it will itself be ‘forced’ to vibrate at the same frequency.
The plate will vibrate most at its natural frequencies (resonances or modes) as these frequencies are easy to excited.
Natural frequencies and amplitudes of vibration will depend on, the material that the surface is made of,
elasticity, internal damping and inertia
and its dimensions and how it is fixed (edge constraints)
The noise that is radiated from the surface will also depend upon:
The size of the surface (sound power proportional to area)
Wavelength to size ratio (radiation efficiency)

14. Modes of a plate forced into vibration

15. Methods of Noise Control
Isolate forces
Use anti-vibration mounts
Increase the mass and stiffen panels
Reduces vibration amplitude and increases resonant frequency
Use Damping
visco-elastic
unconstrained
constrained layer

16. FLUCTUATING FORCE ON A FLUID
AIRFLOW AROUND AN OBJECT
TONES FROM REGULAR
VORTEX SHEADING
BROAD BAND NOISE FROM IRREGULAR SHAPES
CHIMNEY WITH SPIRAL TO PREVENT REGULAR VORTEX SHEADING – Chimney stacks may suffer structural failure if the frequency of excitation = stack resonance frequency!

17. FLUCTUATING FORCE ON A FLUID
AIRFLOW OVER AN OPENING
If you blow across the top of a bottle the air in the neck bounces up and down on the ‘springy’ mass of air in the bottle
WOOD PLANER
Here a cutting blade is held in a slot in a drum. As the drum rotates the air in the slot ‘bounces’ and produces a tone - just like the bottle.
Noise Control
Fill any voids to prevent resonances

18. FLUCTUATING FORCE ON A FLUID
FAN NOISE (Centrifugal Fan)
Broadband noise is due to:
Vortex noise as air detaches from the blades and creates eddies.
Intake turbulence as the air entering the fan becomes turbulent.
Tonal noise is due to:
Propeller noise as the blades chop the air and create +ve and -ve pressures in front and behind the blades.
Interaction noise as the blades pass fixed edges of the casing.

19. FLUCTUATING FORCE ON A FLUID
Frequency content of fan noise
Broadband noise plus blade-pass tones
f0 = No of blades x rpm/60
E.g. a six blade fan 2000rpm = 200 Hz
2f0 = 400 Hz, 3f0 = 600 Hz, etc dB
f0 2f0 3f0 etc
Axial Fan dB
f0 2f0 3f0 etc
Centrifugal Fan

20. FLUCTUATING FORCE ON A FLUID
Noise is increased if the are air entering the fan is turbulent
POOR DESIGN GOOD DESIGN

21. Methods of Noise Control
Select the quietest fan
Make the ducts runs as ‘smooth’ as possible
Avoid abrupt changes to ductwork
Avoid closely spaced components
Use lined ducts and or attenuators

22. SHEARING OF A FLUID
An air jet produces a cone of turbulence where the fast air contacts the stationary surrounding air
Most noise is at an angle of 45o to the direction of the jet
The FREQUENCY SPECTRUM has a fundamental (peak) at:-
f0 = 200V/d
V is velocity in ms-1,
d is diameter of the jet in mm
[e.g. 50 ms-1 & 5mm nozzle
f0 = 2000 Hz ]
Sharp velocity profile
Area of turbulence dB
Spectrum Shape
Frequency ratio (f/f0)

23. SHEARING OF A FLUID
The SOUND POWER LEVEL of a pure jet is proportional to the sixth or eighth power of the velocity (depending on the exact flow regime) and also proportional to the square of the diameter (area) of the jet nozzle.
Sound Power varies with V8 and also with d2
Lw = 80log(V) + 20log(d) + constant
Small changes in velocity give big changes in noise output, and vice versa

24. Methods of Noise Control
Making the velocity profile flatter reduces the turbulence (induced flow nozzle)
Flatter velocity profile
Typical Nozzle -- reductions of up to 30 dB

25. Methods of Noise Control
Similar effects with air knives – used for drying / cleaning in industry -- Typical reduction 15 dB

26. Methods of Noise Control exhaust air / gas or steam
Directivity of the a jet can be used as part of the noise control
45o
Alternatively,
(a) reduce the velocity of the jet or
(b) use some type of muffling device.

27. Shroud Control with a ‘shroud’ (a) hid the jet (about 5 dB)v1 v2 d D
(b) Muffle by lining the shroud with absorptive material
this reduces the jet noise that can emerge from the open end.
A straight through lined shroud typically give about
dBA reduction.
Absorptive lining

28. Shroud and Pepper-pot
Convert low frequencies to high frequencies (more easily absorbed)
This is a shroud and pepper-pot silencer
Absorptive lining
Perforated pipe

29. Safety Valves Typical silencer for a high pressure steam safety valve
Absorbent filling
Perforated
Many Small outlets
Single large outlet
Diffuser
Exhaust stream

30. Summary For successful noise control we need:-
Knowledge of the mechanisms of noise generation
To identify the main sources & pathways
Be able to apply the correct treatment's
At source
In pathway
At receiver
Think about other considerations (holistic approach)

31. THE END