steve smith wrote:Hello Mark and thank you for your post. Sorry it has took so long to reply, I have seen your post here and been gagging to reply but found myself side tracked.
NVH using the microphone moves us into the realms of audio frequency analysis (above 200 Hz) and of course all the challenges that come with it.
I have posted an NVH file below from a Toyota Yaris with an whining alternator. If you playback the NVH file you will hear the whine via your PC speakers.
The first step to capturing audio frequencies is to ensure we have a large enough frequency spectrum to view the amplitude of the sound ( in this case a high frequency whine)
When looking at vibration our spectrum is around 0-200 Hz ( low frequency) which we experience as vibrations we can feel and so detected using the accelerometer.
Because we have a whining sound ( which we experience as a sound we hear) we switch to the microphone for detection and increase our spectrum to cover up to 20,000 Hz. Above this frequency humans cannot hear as we move into Dolphin territory and ultrasound.
Coming back to our whining alternator, looking at the frequency graph we can see the fundamental frequency identified by the letter "A". The is derived from the pulley dimensions entered into the NVH software during the set-up process.
Crank pulley diameter 164 mm driving an alternator pulley with a diameter of 55 mm at an engine speed of 3514 rpm.
164 / 55 = 2.98 (Crank pulley to alternator pulley drive ratio)
1 x crank pulley rotation equals 2.98 alternator rotations.
Engine speed at 3514 rpm / 60 = 58.56 Hz
Crankshaft frequency is therefore 58.56 Hz at 3514 rpm when the alternator whine is present.
Alternator fundamental frequency with a crankshaft frequency/speed of 58.56 Hz is therefore:
58.56 Hz x 2.98 = 174.5 Hz Identified in the frequency graph by our letter "A"
However our whine is a lot higher than 174.5 Hz otherwise we would feel a vibration rather than hear a whine. Looking at the frequency graph we can see a high peak detected by the microphone around 1250 Hz, how could this be attributed to the alternator?
1250 Hz is certainly audible (94dB) and happens to be visible when the alternator is whining at high speed. Let's look at the frequency of the alternator AC output (before rectification)
Alternator output frequency = Number of alternator poles x RPM of alternator / 120
Our alternator has 14 poles: 14 poles x (174.5 x 60)= 14 x 10,470 = 146,580 / 120 = 1221.5 Hz
Therefore our whine can be linked to our alternator as a result of generator noise from an internal electrical error during generation, not a mechanical failure.
The microphone has enabled the capture of the relevant frequency (whine) whilst the operating principle has enabled the application of the theory to confirm the component failure.
When looking for intermittent or sporadic noises, the typical "Chassis Ear" is still a viable option as we rely upon the noise level being present for a period of time before the NVH software can identify and display the frequency/amplitude.
Body creaks/ speaks etc. are evident for minuscule amounts of time, and whilst they can be detected by the sensitive NVH microphone, they would be lost to background noise and delayed transfer to the frequency graph.
In these scenarios we require multiple microphones placed about the suspect area of creak/ squeak with a rapid live update speed to in order to display the amplitudes on screen.
Here the NVH kit is not applicable but watch this space!
A big thank you to Alex (NVH Guru) for his input here at Pico as I was lost looking at this one
I hope this is of some help, take care......Steve
What do you mean, Steve? We only have one microphone within NVH kit. Sorry, bad EnglishIn these scenarios we require multiple microphones placed about the suspect area of creak/ squeak with a rapid live update speed to in order to display the amplitudes on screen.
Users browsing this forum: No registered users and 0 guests
Pico Technology — automotive lab scopes and diagnostic equipment