Info on NVH testing in your own backyard 2

[leroi]

I'm (hopefully) putting the finishing touches on my next drive for the next decade, which, being a decade or so behind the times as I generally am, is going to be a '94 Geo Metro 2-door hatch. I've been following what the main website for these cars say, and have followed up their suggestions for suspension/handling/brakes upgrades--I always thought the first gen of Geo's suffered badly from cost-engineering in their suspension and brakes and have corrected that problem as best as I can with the superior later Geo Mk II parts. Still keeping the 3-banger, not doing anything much to it, although putting MPFI on it might be in the works. Fuel economy is one of the main reasons for this car--personal sociopolitical statement on reduction in use of irreplaceable fossil fuels and all that--and the fact that the car has enough headroom for me were the main factors in my picking the Geo. (My hypothesis as to why the Geo has so much headroom--more than any other GM sedan, near as I can tell--is because it is, in its Suzuki iteration, a vehicle co-produced with Maruti in India, and the requirement for headroom there comes from all those Sikhs who wear turbans. Globalization does have its benefits, after all.)

Big question I have is seeing what the results are of the NVH work I've done on this car. I've filled all the unibody sections with 3X expanding foam (can't find the two-part stuff in post-industrial Austin) and put down a gallon of pickup truck bedliner (urethane/rubber particle mastic) on the interior sheetmetal. Looking at the SAE materials on foam in body sections, I haven't found much where people have actually foamed in vehicles and tested the results. There are also questions concerning the difference in structural properties between 3X foam and 2-part foam that I can't answer from the SAE literature. I'm thinking about doing some backyard NVH tests on this car, which should be interesting as I've got access to equivalent vehicles of both marks of Geo's, to drive and gather data from.

It seems to me that first I need to read some more out of some general automotive engineering text on NVH. Could someone out there steer me to one? Second, financial constraints on test equipment are steering me towards one of the laptop PC sound o-scope softwares. Could someone out there recommend me one? Third, is anyone out there in automotive engineeringland interested in seeing my results?

Thanks all--Dan White

 

[GregLocock]

The efficacy of most sheet metal treatments comes down to lb/sq ft.

For acquisition/analysis a laptop is fine, but bear in mind that most laptops have only one line input, any serious work needs two. I haven't used any of the cheap packages to generate calibrated spectra. Goldwave might do it. I use a little program called winscope to check that the data is ok, then import the wav into matlab or whatever.

I don't know of any good texts on general NVH testing, mostly we learn by experience and reading the technical papers and textbooks. Se what you can find secondhand and ask here before buying it - someone is bound to have read it. Anything by Beranek or called "automotive sound and vibration " is a good start.

If you look on the b and k site they have some introductory leaflets that are very useful.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Rob45]

leroi:
If you're still around watching this old thread and still interested in the daunting task of making a Geo quiet:

I'd echo what Greg says: the next book you find on automotive NVH will be the first one!

You can do some work inexpensively with a laptop, with the built-in "Sound recorder" application and using an inexpensive Radio Shack microphone, if you can work with comparative results only, since you won't be able to calibrate the input level on a .wav file.

You can find a freeware version of CoolEdit software, something like "CoolEdit 97" (or is it "96") that is available on the web; it has a good analysis function that will permit doing FFT analyses. This won't tell you how loud it is inside your Geo (I suspect that answer to be "very") but you will be able to determine the relatively highest level frequencies, and comparison testing should give you an indication of whether or not you have reduced those peaks.

A bit of detective work will tell you the source of the various peak frequencies, more easily if you construct an "alignment diagram," which is a plot of frequency vs engine RPM on which you have plotted the lines showing engine firing frequency, alternator rotation and firing frequencies, and any other roational inputs you may have.

For example, your 3-cylinder engine fires 3 times for every two rotations of the crankshaft, and so is called a 1.5-order phenomenon: at for example 1500 rpm, engine firing is (1500/60)*(1.5) = 37.5 Hz
So your engine firing is a line on the alignment diagram from the origin and passing through the point (37.5 Hz, 1500 rpm). Alternator would be a line at the alternator drive ratio, e.g., 2.5 x engine speed perhaps, and another line at 3 x that, owing to the nature of alternators. That is, at the same 1500 rpm, alternator rotation might be 1500/60 * 3 = 75 Hz, and alternator "firing" would be at 75 * 3 = 225 Hz.

You'll have to decide on a repeatable test procedure, someplace where you can drive a steady speed at a constant load, without distractions, excessive noise from passing cars, with a uniform level road surface - in fact, if you find such a place let me know where it is... You'll also have to make sure your test conditions are well-documented: wind speed, ambient temperature, etc., and make sure the car is fully-warmed-up. Small variations can sometimes make large differences in noise.
Now, when you detect a noise peak in the frequency spectrum, put its frequency/rpm point on the alignment diagram, and with luck, it'll land on an existing line, and point you in the direction of the problem.
In order for this to work, of course, you'll need a fairly accurate tachometer; you should be able to check the accuracy of your tach by finding the firing-frequency spike in the noise spectrum (rpm = freq * 40, as above) and comparing it to what the

Simply knowing what the source is doesn't always suggest the solution: for example, say you know that engine firing is the problem at highway speed: is the noise coming from the muffler, from the aircleaner, or in a car this old, via bad engine mounts, or an exhaust system that's contacting the chassis? More detective work is required, and space and my time prohibits me from describing all the common techniques for doing this.

One more point: spray on stuff looks great, but may or may not do any good. For your interior, try to maximize the amount of carpet and fabric upholstery, preferably backed with thick open-cell foam.
Heavy vinyl barrier sheet or (better yet) a thin sheet of lead under the carpet will make an effective sound barrier.

Or maybe after reading this you'll decide that a better sociopolitical statement would simply be to drive a modern, non-polluting car, and carry a sign somewhere on the weekends: I'm sure Austin abounds with opportunities to "demonstrate" in favor of or against SOMEthing vitally important...