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3SOUND AND VIBRATION/APRIL 2001

Welcome to this special SAE issue ofSound and Vibration. Although a lot ofemphasis has been put into automotive

acoustics in the last fifteen years, thisfield is at least sixty years old. We are allmaking measurements and/or working inautomotive acoustics, but do we reallyknow what we are doing? In other words,if we are the users, do we know whetherthe information we are receiving and us-ing is correct, valid and usable?

Here are two graphs (Figure 1) takenfrom a report to make a point. The titleon both graphs indicate Noise ReductionEvaluation. However, the y-axis on onegraph says Noise Reduction and the y-axis on the other graph says InsertionLoss. If you are the user of these data and

had not been involved in acquiring thedata, would you accept the data providedas Noise Reduction or Insertion Loss orwhat?

Some More Examples. Now let us lookat the three basic acoustical materials

absorber, barrier and damper. We all testacoustical materials and we do it right.Then, why do we have so much of a dis-crepancy?

First, look at some data (from: O.Nwankwo, et. al., SAE paper) taken on anabsorber where measurements weremade in accordance with ASTM Stan-dard C423 (Figure 2). In all cases, thesame sample was tested at different fa-cilities. In a few cases, measurementswere made by the same person using thesame equipment but at different facilities

of course. Results show that the perfor-mance of this sample is all over the place,especially at frequencies above 1000 Hz.

If I am the user, which one should I trust,and on what basis? This is just the begin-ning. There is so much variation in dataof the same material that I wonder whatwill happen when different suppliersprovide me with data on different mate-rials, each tested at a different facility?Should I select a material solely based onits numerical values knowing that resultsobtained at different facilities are differ-ent?

Now look at a barrier assembly that wastested at different facilities under a roundrobin study program of SAE in 1996. Thesample tested in this case was a system

consisting of a steel panel, a decouplerand a barrier material. All the facilitiesreceived the sample (but not the same testpiece). Each test piece was carefully mea-sured and weighed to obtain the vital sta-tistics in terms of thickness, weight andsurface weight, etc., and to ensure thatthese are all the same sample. The studyshowed that there was a wide discrep-ancy among results obtained from differ-ent test facilities (Figure 3). Certainly ifthe results obtained from some laborato-ries are closer to the correct performance,

Do We Know What We Are Doing?

Pranab Saha, Contributing Editor

EDITORIAL

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Figure 3. Sound transmission loss of a double-wall system per SAE J1400, 1996.

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then the results obtained from many oth-ers are far from the truth. The question iswhat do we do with these results? Themost important purpose of the roundrobin test is to compare test results ob-tained at different facilities in a system-atic way. Comparing results is fine, butwhat do we do when we find large varia-

tions as they show in the figure? Thechallenge is to try to understand why thetest results are different. To do this suc-cessfully, you have to sit down with yourcompetitors and share information withothers to understand why we have varia-tions and how to correct these variations.Often one may need to look at the stan-dard used for the measurements moreclosely to understand whether the stan-dard has any limitations.

Figure 4 shows the results of a similarround robin study of a different double-wall sample that was reported in 1982.The results are more consistent in thiscase. One wonders what we have done toourselves. With modern tools and gadgetswe cannot reproduce results as consis-tently as we could earlier. Or, is there a

basic f law in the measurement.Figure 5 shows the variation in damp-

ing performance of a given test bar mea-sured at two different laboratories usingthe same test protocol. The sample was aviscoelastic material tested under exten-

Figure 4. Sound transmission loss of a double-wall system per SAE J1400, 1982.

Figure 5. Vibration damping of the same testsample measured by two different laboratories.

sional layer configuration. It is consid-ered that by doubling the thickness of adamping material, the performance mayincrease three times and that is a measur-able, noticeable and meaningful differ-ence. Then when I test the same test barat a different facility and find at sometemperature and mode of vibration, the

performance of the sample obtained fromone laboratory is 2 to 3 times better thanthe other one, what am I supposed to do?You wonder, is the variation due to testlaboratory operating procedures or due toother reasons. There were more varia-tions in test results where tests were con-ducted using automated procedures thanmanual procedures. This is really strangeas automated procedures are expected to

be more repeatable and idiot proof.Standards. A close look at the data pre-

sented here brings up a question, are ourstandards okay? Again, we all can helpwrite standards but are we doing it right?In order to do things right, we not onlyneed to have the proper background andright participation but we also need toknow what had been done until now, andwhy.

Almost any work that is done has a fewgroups who get affected by the outcomeof the work. These groups include:a Users (they are often the final custom-

ers)

a Manufacturersa Test laboratories and consultants.

Therefore, in developing these stan-dards we need to have representationfrom the above groups of people so wecan share and understand the concerns ofthe test methodology and results and caneventually recommend proper solutionsto come up with a good standard. Onceround robin tests are conducted, all testfacility participants need to discuss thetest results together to understand whatmay be the cause of the variation in testresults and how these variations could beminimized. Everybody has to be ready toaddress these issues together and readyto solve the problem. Otherwise, in spiteof laboratory accreditation, certificationand/or registration, we will not do justiceto our vehicle and/or the noise problemwhere we are going to use certain acous-tical materials based on test results ob-tained from these studies.

The author may be contacted at: prsaha@kandse.com.