Humans are sensitive to vibrations. Before any instruments became avail- able all vibration analysis was done by listening and feeling. This method
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is still used by those who do not have access to instruments. We have some built-in sensors in our skin and our ears. These biological transducers served a survival function and still do. The human ear is amazingly sensitive to the smallest pressure change. The pressure sensors in our skin can detect constant pressure, and also oscillating pressure or vibration.
Humans can directly detect vehicle noise, loudness and vibration smooth- ness by listening and feeling before any electronic instruments are con- nected. This is a valid sensing method for vibration analysis if certain precautions are taken, namely calibration and frequency analysis. It is dif- fi cult to compare one vehicle with vibration ‘readings’ separated by several days, especially if many similar vehicles are seen during the interval. The
‘measurements’ are also highly subjective. One person’s judgement of
‘rough’ could be another person’s ‘acceptable’. This system of human per- ception gives an overall vibration reading. The best that can be obtained with hand sensing of vibration is a crude, overall, subjective vibration mea- surement that sounds an alarm when mechanical failure is imminent. The method of hand sensing works satisfactorily in vehicle noise, vibration and harshness (NVH) departments where some human calibration techniques are employed by NVH engineers and technicians. First, one NVH engineer/
technician is given responsibility for a specifi c vehicle line program, which is his or hers and no-one else’s. So the human variable is removed. Second, this one person checks the vehicle on a daily basis, so there is not a long interval between ‘measurements’; and third, there are usually identical vehicles nearby to compare against. Using these methods of calibration the NVH engineer/technician can be successful in identifying noise and vibra- tion problems within the vehicle by human hand feeling measurements.
In order to identify the sources of the noise and vibration problems, frequency analysis is necessary. Humans are equipped with a frequency analyser. The combination of the human ear and brain is actually a pretty good spectrum analyser and is extremely sensitive. Human hearing has a sensitive bandwidth of about 40Hz or less for people with a good musical ear. The human ear is sensitive to air vibrations from about 20Hz to 20,000Hz. This is also the frequency range of most annoying vibrations of mechanical equipment. 30Hz is barely audible for most people but it can certainly be felt with the pressure sensors in the fi ngertips. A metal object, such as a coin, can be held between the fi ngertips while probing for vibra- tions. Evidently there is some amplifi cation from the metal object pressed against a vibrating surface. Hand sensing works for low-frequency vibra- tions (less than 100Hz). For higher frequencies, you should listen to the tones, as the majority of mechanical vibrations are within the frequency range of the human ear to detect.
Using the human ear for frequency analysis of vehicle noise is more effective when the ear can be coupled directly to the vehicle. This means
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using a stethoscope, a wooden stick or a screwdriver. Any vibration analysis should always be done fi rst by listening and feeling to give the analyst a subjective frame of reference.
If no objectionable vibration or noise can be felt or heard, you can be confi dent that there is probably no vibration problem in terms of mechanical design and wear. Humans are well calibrated in this respect to mechanical damage criteria, but this judgement depends on no interfering background noise.
Human judgement of vibration severity can be quantifi ed by the level of acceleration:
• 0.001–0.01g is the threshold of perception.
• 0.1g is considered unpleasant.
• 0.5g is considered intolerable by most subjects.
Vibration problems and human sensitivity to them are well correlated in the absence of background vibration. If vibration is uncomfortable to people, then it is probably causing serious damage to the vehicle or machine.
As vibration analysts we are looking for vibration. We want to amplify and take a closer look at it. This is the whole purpose of vibration instruments – to amplify vibration and display it in a way that we can better understand.
Instruments increase the signal-to-noise ratio. Suppose you wanted to examine the vibration from one of three running pumps. You could approach the pump of interest and place your hand on it, or couple your ear to its housing with a wooden stick. This will increase the signal of inter- est above the background noise. Alternatively, you could turn everything else off and run only the pump of interest – this will reduce the background noise and again increase the signal-to-noise ratio. By placing a transducer directly on a vehicle structure of interest, we couple directly to that source of vibration and separate out the background vibrations. The signal- to-noise ratio is increased.
To perform any signifi cant vibration analysis the important parameters to measure are frequency and amplitude. Hands and ears can make both of these measurements subjectively. The rotational speed is the most fun- damental number required to do any vibration analysis. A stroboscope with accurate speed readout, or a tachometer, should be the fi rst instrument to be purchased. The stroboscope is, however, not reliable for ‘stopping’
motion below 300rpm.
The methods described above using the hands, ears and a stroboscope/
tachometer (and perhaps a stethoscope, wooden stick or screwdriver) are not yet outdated. Keep these tools available in your toolbox and use them frequently to correlate with your instruments as a check.
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