TMD to Improve Isolation at Mount?

[Rob45]

Greetings:
I have a situation where an isolated vehicle cab structure has poor dynamic stiffness in one direction, at approximately 300 Hz, and therefore has poor isolation. This results in noise at the operator's ear at 300 Hz. Since in this case redesigning the structure is not an option, would inserting a tuned-mass damper at this location be likely to give an improvement in isolation and thus a reduction in noise?
This is an idea I've been toying with for a while, but I thought I'd seek some outside opinions before I started making a prototype and running a test.
Any thoughts would be appreciated.

Best regards,
Robert

 

[GregLocock]

Yes, that should work. At 300 Hz you may find that merely bolting a large mass (say 5-10 kg) near the mounting point may be enough. More than one car has gone into production with that inelegant solution. (BMW and Volvo)

The way i'd approach it is to use an accelerometer and a mic to find the actual noise path (ie check that the noise is coherent with the vibration), find where the vibration is strongest, measure the mounting point's inertance at that frequency with a bonk test, get a mass at least 10% of that, and then tune the spring.

What's the noise source?



Cheers

Greg Locock


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[Rob45]

We did in fact run a Transfer Path Analysis showing the cab mounts were major paths for powertrain noise to the driver's ear: mount FRFs were taken, sound and vibration levels were taken in operation. The mount FRFs are available, so I have or can re-take the inertance.
This is a diesel-powered commercial truck, and the noise levels are "higher than desired."

Thanks!
- Robert

 

[Strong]

I suppose a TMD could work. Have you tried adding a small "point mass" to change vibration mode? What about a 2-stage spring mount with a softer spring to handle the 300-Hz issue and a stiffer spring to limit large deflections?

Walt

 

[Rob45]

I'm primarily interested in the TMD because of the lack of space to make a significant improvement in the isolator. It already has durabilitiy issues, and softening it would make them worse.

 

[GregLocock]

Typical size for a 150 Hz damper on the powertrain (for driveline bending) is around 1 kg of mass.

A bodyside damper would be larger, but at higher frequency it could be smaller. So I'd expect you to need a couple of kg, at least at the development stage.

You should be able to pull 6 dB out of it, if there is only one path.

Have you thought about curing the problem at the source?



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[FeX32]

First, I would go for changing the static stiffness if possible.
If that 300Hz changes in the future, the TMD may not be such an elegant mechanism anymore.
You could design an 'optimal' TMD. That is, design the absorber mass, stiffness, and damping for optimal performance over a larger frequency band. That way if the 300Hz changes in the future, it won't be as detrimental.


Fe

 

[GregLocock]

Yes, but then it doesn't work as well at the actual peak.

I know NVH is usually a horrible bodge, but it seems to me the correct engineering solutions are twofold

(a) redesign the isolators so they isolate and are durable

(b) cure noise problems at source.

Back in the real world...




Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[FeX32]

True. The normalized amplitudes at the actual peak would then be around 3 to 4, as opposed to 1 or less then 1.
Although, I like your (b).



Fe

 

[Rob45]

Yes, reducing the source would be ideal, and I am pursuing that, having already redesigned the engine mounts, which gave a worthwhile improvement.
The source in this case btw is a 15L 6-cyl turbocharged Diesel engine making 450 HP and 1550 lb-ft of torque.
But the reason for wanting a "band-aid" approach is to avoid the glacial pace of a redesign program, and management's unwillingness to devote any resources to such a project: the 300 Hz problem is due to a weakness in the cab structure, and not due to any particularly bad vibration from the engine.
I'm expected to just wave my magic NVH wand over the truck and "make it better."
Greg, thanks for the helpful hints as to how to actually go about this.
Best regards,
Rob