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:

anechoic chmaber

Acoustic Chambers and Rooms – some are defined very specifically and others more loosely. These rooms are mainly used by noise and vibration measurement specialists.

Acoustic chambers and rooms are quite specialist rooms to design, build, operate and maintain. Often the design of these rooms will be done by specialist design acoustic design companies.

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 curves

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.

Community Noise Criteria – there are a number of different community noise criteria measurements: CNEL, LAeqT, Ldn, LN, LEPN, LNP, LPK, NEF, SEL, SENEL, TWA

Damping – the reduction of vibration amplitude by dissipation of the energy.

Using sand bags to add damping down the vibrations of the Sieg SX2.7 milling machine.

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.

Degrees of Freedom – a fundamental definition when thinking about the motion or vibration of a system. The number of degrees-of-freedom of a mechanical system is equal to the minimum number of independent co-ordinates required to define completely the positions of all parts of the system at any instant of time.

6 degrees of freedom
For a solid body there are 6 degrees freedom.

Electric Motor Noise – There are numerous applications of electric motors and the type of noise produced by each may be very specific to the installation and type of motor. There are a number of reasons why the motor is perceived as being noisy

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.

frequency wave speed wavelength equation

Frequency, Wavelength and Speed – Understanding frequency, wavelength and speed is fundamental to everything around us. From noise and vibration, chatter on a lathe tool to the light we experience from objects.

felt in place to stop any rattling

Milling Machine Stiffness = Low Vibration = Top finish – adding a bracket to close off the column U-section and connect it to the base has dramatically reduced the vibration on the milling machine and hence improved the surface finish.

cast iron dust

Particle Damping

  • Energy dissipated through shear friction
  • Broadband frequency damper
  • Position at the maximum amplitude response
  • Can be used in harsh environments
  • Solid, liquid or gas based design
    • Simple as a bag of dried sand
  • Possible to increase the damping of a structure with only a small weight gain

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.

Room Acoustics – a huge subject in it’s own right, but I wanted to share some basics. A basic level of knowledge will help if you are trying to reduce the noise levels in a workshop or position your speakers in the optimal location.

oblique mode

Room Acoustic Modes occur at frequencies at which sound waves in a room resonate (in the form of standing waves), based on the room dimensions. The acoustic modes will “colour” the sound, ie enhance certain frequencies and dull others.

Therefore, the standard modal approach for designing a room with good acoustics is to create as many different resonances as possible, and to spread them as evenly as possible across the frequency spectrum.

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

Speed of Sound – The speed of sound depends on the elastic and inertial properties of the particular medium. Gases In air, the speed of sound depends on the temperature of the air, at 20°C the speed of sound is 343ms-1.