Good vibrations

Frank explains the science behind the way-out sounds made by today’s cars that can be the cause of much head-scratching in the workshop

By Frank Massey |

Published:  31 December, 2018

In a previous topic I expanded on the availability of focused test tools for independents. It’s not often that we see a technical breakthrough which has real application potential, but there was a breakthrough recently when Pico introduced a new NVH kit. It has come just in time, as noise, vibration and harshness is a challenge that’s not getting any easier, so what is it?

You must first start by accepting that the motor vehicle is a series of mechanical systems in permanent conflict. There are components travelling in different directions, subject to acceleration, deceleration, changing direction, and of considerable mass differential.

What I have just described there is the internal combustion engine. adding chassis and body systems to the mix. I think you will agree the problem we have is in identifying noise and vibration.

 The difference between noise and vibration is based on frequency and amplitude. Noise is a single event with a diminishing synodal pattern. It looks like a trumpet. Vibration however has a repetitive frequency and amplitude. Both of which will change with speed and a whole host of influences, resonance, beating, and mass differential are just some.  

So why has it become more difficult for us techs to bend our ear and diagnose an issue with confidence? The answer is due to the technical innovations of today’s vehicles. These include the dual mass flywheel, active engine mounts, cylinder cancellation, Audi anc system, infinite computer control of chassis dynamics, and the most obvious of all-  lack of accessibility.

Let’s begin with the basics. As we have seen, Vibration is classified by frequency and amplitude. A large mass will by nature have a lower frequency and a greater mass, while  a small mass will present the exact opposite. Two or more mass that converge with the same frequency combine their mass value increasing the amplitude. This is called resonance. Mass that have a similar but close frequency differential, within say 10hz, cause beating; “wo, wo, wo, wo.” An example of this would be a worn wheel bearing.
Vibration has three elements: Cause, transfer path, and respondent. In almost all cases we experience the respondent. Let’s think about the vibrating ash tray, wedged with paper to stop the noise! Vibration also falls into three other simple categories, vibration we feel, vibration we see, and vibration we hear. We humans can only hear noise between 25hz/22,000hz.

The next consideration is how many events per rotation frequency is experienced these are called, first, second, third, orders etc.
Now let’s do some simple maths. It’s getting interesting now isn’t it? We must convert everything into frequency, the unit is hertz, or cycles per second. For simplicity, a four-cylinder engine revolving at 3000rpm, in top gear 1:1, differential ratio 4:1.
3000/60=50hz divided by final drive ratio 4:1=12.5hz.

Therefore crankshaft vibration will be @50hz and tyres, rim, brake disc, and drive shafts will be at 12.5hz. So, you will now appreciate is a simple matter of separating the various operating frequencies.

Well not quite, but by now I’m hoping you view vibration in a more clinical way and not just based on experience or opinion. Vibration can have different direction or vectors, something tyre fitters more often or not get wrong.

Bring on the technology. The kit which can have an infinite flexibility of accessory options, uses a three-dimensional accelerometer, for vector differential, measuring mass, and a microphone recording sound, together with 1+3 channel interfaces, and bnc connection leads. The engine speed data is collected via the serial port with a drew tech mongoose serial interface. This can also be achieved optically if preferred. The accelerometer has a magnetic base and is directional sensitive, fore/aft, vertical, and lateral. Its initial position should be on the driver’s seat frame. After all that’s where the complaints start! The microphone could be positioned close to a known noise source or in the cabin.

Navigating through the software wizard is straightforward, you will need to select number of cylinders and configuration, in line, opposed, v config, and direction of mounting. You will then need to establish the various gear and final drive ratios, with tyre size data.

The software will then gather data over an infinite timeframe and scaling which is of course adjustable. The most challenging aspect in my opinion is control of the style of driving technique, speed, gearing, direction, braking and the influence of the road surface. The vehicle may have selectable drivetrain and suspension options, which will affect the potential effects of noise and vibration.

Did you remember not to omit the obvious or obscure effects? Has the vehicle been modified in any way whatsoever? Wheel size, spring rates, power output, etc, etc. Record your driving technique and environmental influences into the microphone. After all it is recording sound, all sound!

There are several options in the display menu, from bar chart, frequency, and 3D. you will quickly establish exactly which one of three vehicle systems the problem originates based on visual evidence. Engine, transmission or tyres.
You can then reposition the sensors to further locate the position of the source. Vibration will increase in amplitude, as will noise the closer you are to the source. This is due to the reduction in the length of the transfer path, and any devices that may absorb it.  
I can confidently monitor discrete combustion anomalies based on the transfer of mass energy from the pistons to crankshaft orders, simplifying connectivity issues with coil on plug multi-cylinder engines. I could show you images from a test I conducted recently, but a simple static image does not fully demonstrate the effects of vibration.

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