Noise & Vibration

Noise – any unwanted disturbance, such as undesirable sound waves in a car passenger cabin. The word “noise” is derived from the same Latin root as the word “nausea”.

In the Roman times of Julius Caesar, chariots were prohibited from the streets of Rome after nightfall. The Roman laws at that time were basically enacted to restrict vehicle noise and explicitly controlled the night time chariot traffic in residential areas.

In the 13th century, Nuisance Laws were part of English Common Law which recognised the problems with noise. It pronounced noise as a public nuisance which was regarded as a crime or semi-crime.

Vibration – An oscillation about an equilibrium position or shape.

Vibration is generally interpreted as the cyclical (symmetrical or nonsymmetrical) fluctuations in the rate at which an object accelerates. In longitudinal vibration the direction of motion of the particles is the same as the direction of advance of the vibratory motion; in transverse vibration it is perpendicular to the direction of advance.

Below is a list of terms that I will gradually expand to give useful examples, calculations and hints:

Acoustic Filter Elements – there are a number of types of acoustic filter and they have varied applications.

Aerodynamic Noise – noise generated by the flow over an object or through a duct.

Attenuation of Sound in Air – The attenuation of sound in air at 20°C due to viscous, thermal and rotational loss mechanisms is 1.6×10-10f2dB/m.

A-Weighting – This is the most generally used filter when making overall noise measurements. The attenuation of the sound signal with an A-weighted filter corresponds to the fact that the human ear is not as sensitive to sound of the lower frequencies as it is at the higher frequencies.

Chatter – a vibration issue when using a machine tool where the workpiece or the cutting tool are too flexible and so vibrate – the result is a vibration or squealing sound and a repetitive pattern cut into the workpiece.

dB(A) – A sound-level meter reading with an A-weighting network simulating the human-ear response at a loudness level of 40 phons.

Decibel – A difference of 20 dB between two sounds means that the more intense one has 10 times the amplitude (100 times the power) of the softer. A change of 3dB is commonly thought to be the smallest change in sound pressure level that can be remembered.

Exhaust Silencer Design – The exhaust system on an internal combustion engine may be used to reduce noise and/or increase power. A pressure tapping is also used on many exhaust systems to pressurise the fuel tank and hence allow the engine to operate at any attitude.

Fan Noise – Some simple ways to reduce noise levels from a fan are to increase the diameter and reduce the speed, ensure there is no structure in front of or in the close wake of the blades.

Flow Noise – A term generally used to describe aerodynamic noise produced when a gas flows within a duct or when the gas exits the duct. This is a typical problem observed in the exhaust systems of internal combustion engines and high engine rotational speeds and hence high gas flow rates.

Propagation of Sound – If the noise source is outdoors and its dimensions are small compared with the distance to the monitoring position (ideally a point source), then as the sound energy is radiated it will spread over an area which is proportional to the square of the distance. This is an ‘inverse square law’ where the sound level will decline by 6dB for each doubling of distance.

Sound Pressure Level – when we measure noise level we normally measure the sound pressure level and express the result as something like 85dB(A)