PRINCIPLES OF NOISE CONTROL ENGINEERING

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.

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.

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)

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.

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

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

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 &

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.

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)

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)

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

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)

Modes of a plate forced into vibration

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

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!

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

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.

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 running @ 2000rpm = 200 Hz 2f0 = 400 Hz, 3f0 = 600 Hz, etc dB f0 2f0 3f0 etc Axial Fan dB f0 2f0 3f0 etc Centrifugal Fan

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

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

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 0.1 1 10 Frequency ratio (f/f0)

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

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

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

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.

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 10 - 15 dBA reduction. Absorptive lining

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

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

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)

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