Most gearheads would agree that a vehicle’s exhaust sound gives it its soul. People can identify engines sight unseen, just by the distinctive song emanating from their tailpipes. This love for the sound of internal combustion is the reason why aftermarket exhausts are the most popular modifications for a car or truck. Whether it’s the sonorous wail of a V12, the intimidating rumble of a V8, or the blatty warble of a flat four or six, a properly tuned exhaust is literally music to an enthusiast’s ears. But for better or worse, we live in a “society” with so-called “normal” folks who don’t like the songs of our people.
While most of us can fall asleep to racing videos; a straight piped muscle car roaring down the street at two in the morning makes regular people complain and write letters to their alderman or whoever. Because of this, there are laws and regulations and international ruling bodies who throw their weight around and force automakers to quiet it down or they lose their ability to sell cars.
Joking aside, this all makes sense. Life would be much more annoying if garbage truck makers decided it was cheaper to build their vehicles with open headers, for example; or if you lived anywhere near civilization and every vehicle on the road was as loud as a fart-canned Civic. That’s why countries all over the world have had vehicle noise limits on the books for the past of 50+ years.
Automakers have to pass what is called a “Pass-by test” with every engine/transmission combination for each vehicle model they sell. Thankfully, inside the 100+ tedious pages of these international test standards written by the most lawyerly engineers, or engineery lawyers, is a tiny clause that lets our beloved vehicles belt out their spine tingling melodies whenever we hit the loud pedal. It gives powerful vehicles a significant advantage compared to your average car or truck. Without this three word stipulation, the world would be a much sadder place for anyone with gasoline in their veins.
To be clear, this test has nothing to do with how awesome a car sounds. There is a whole art and science that goes into designing an exhaust that does its job of quieting the combustion events happening in your engine, while invoking the subconscious, subjective feelings the company wants you to associate with their vehicle. This isn’t a group of sommeliers debating the subtle notes of a new batch of bourbon. It’s the scientist confirming the alcohol level is within the legal limits.
I’m Steve Balistreri, the engi-nerd from Detroit who specializes in NVH (noise, vibration, and harshness). Pass-by testing is something I’m very familiar with, as I’ve run hundreds of tests for a half dozen automakers at tracks across the country. I was in charge of all the pass-by testing at a Michigan automotive proving ground for five years and tested anything from electric cars to 700 horsepower trucks to boring crossovers to super cars that cost twice as much as my house. Some tests were quick and dirty, done on barely running prototypes held together with duct tape and hope; others were official certification tests where every detail was poured over by an independent third party witness. Pass-by testing involves repeated wide open throttle events which is fun, but the targets are very tightly controlled and take a bit of finesse and skill to hit.
How A Pass-By Test Works
Before I spill the beans on this secret loophole I have to explain how a pass-by test works. The whole purpose of the test is to quantify, in a controlled and repeatable manner, how loud a vehicle is during normal driving in urban traffic. The first pass-by test specification came out in 1967 with J986. Times were simpler back then, cars had 2 or 3 speed transmissions, and the spec filled a whole 8 pages. It was revised in 1984 with J1470 or ECE324 which ballooned to 17+ pages depending on the revision. The current spec, UN/ECE R51.03 introduced in 2016 is a whopping 98 pages long. It’s kind of confusing as there are international ISO specs, their equivalent SAE specs, specs for different countries, plus all their amendments. Overall, the current specification is run the same way everywhere.
Let’s start with the track you do the test on. Since tire noise is a major contributor to a car’s overall noise level, the track has to be built to an ISO specification and certified every two years. It has to be perfectly flat and meet targets for texture, surface irregularity, and noise absorption. An area 50 meters around the track needs to be free of any objects that may reflect sound back to the test microphones.
It helps if the track is in a quiet place as there are ambient (background) noise requirements to run the test. Ideally in NVH testing you want the background noise to be at least 10-15 dB (decibels) quieter than the thing you are measuring. I’ve spent many hours sitting on the test track waiting for hundreds of migrating geese to fly over us to the nearby pond because their stupid honking was too loud. The worst case was a proving grounds I tested at where the pass-by track emptied onto an active skid pad area where a driver training/autocross event was happening. If that wasn’t bad enough, on our left side was the giant oval track, where the curve hits the straight, a popular place for the dozens of test cars out that day to accelerate hard out of the corner. If THAT wasn’t bad enough, the right side of the track had water cannons spraying a tile surface where they were doing emergency brake testing on school buses. Needless to say, it was way too loud to get any testing done. We later found out most people rent this particular pass-by track at night.
Due to the massive expense required to build and certify these tracks, there are only a handful in the US. The lack of grading on the pavement also means these tracks take forever to dry if it rains since there’s nowhere for the water to go. There is a track on the east coast where it would rain every time we made a trip out there. It got to the point where we’d pack giant squeegees in the test vehicles so we could push standing water off the track and not lose too much test time.
Marked out on the track is a line down the center of the lane you drive down, called CC’ in the spec. The PP’ line is in the middle of the track perpendicular to CC’ and is where the two measurement microphones are located, 7.5m from CC’ and 1.2m above the ground. The portion of the track where the measurement actually happens is marked off by lines AA’ and BB’ which are 10m on either side of the PP’ line. The peak noise level from the time your front bumper touches the AA’ line to when your rear crosses the BB’ line is used to calculate the score for that test run. You can see this layout in the diagram below.
The pass by test requires four steady speed cruise runs where you drive 50 kph (31.1 mph) +/- 1 kph through the track; and four wide open throttle runs where you drive down the track and WOT the throttle before AA’ and hold it until your rear bumper passes BB’. For the WOT test, your vehicle speed must be 50 kph +/- 1 kph when your front bumper hits the PP’ line.
The spec requires linear acceleration between AA’ and BB’ so pre-acceleration is encouraged, ie. mashing the accelerator before your front bumper hits AA’. This way any delay between your WOT and the vehicle fully accelerating from turbo lag or whatever happens before AA’ and your velocity line is nice and straight. You can see some test data below, the WOT point is around -14 meters (where the RPM line gets wavy), and the curve in the vehicle speed line where the car starts accelerating happens before AA’ (-10m).
Hitting the PP’ speed target takes a bit of skill and throttle control. You have to pick a spot a couple meters before AA’ and WOT it at that point consistently. You also have to control your entry speed precisely because the speed target 10+ meters from your WOT point has a window about 1 mph wide. At the same time, you have to keep the car centered on the CC’ line and operate a laptop to control the test software, and occasionally a second laptop to control the automatic transmission. The peak noise level from these four runs at each microphone have to be within 2 decibels of each other, which adds more chances for error. These WOT runs mess up newbies the most. It’s definitely a great feeling when you can crack off four perfect WOTs in a row.
The cruise runs are nothing to sneeze at either. Your speed can’t exceed 51 kph or dip below 49 kph at any point during those 20+ meters you’re driving through AA’ and BB’. With some electric vehicles, regen can be touchy so you really need to feather the throttle in that Goldilocks zone to keep your speed steady.
Here is what the WOT and cruise runs look like in our software. For all the graphs, the X axis is the position of the vehicle’s front bumper on the track in meters. -10m is AA’, +10m is BB’ and 0 is PP’ where the microphones are located. The top-left graph is vehicle speed, bottom left is engine speed. The two right graphs are the noise level from the microphones. Each graph is displaying the four required runs, and you can see the consistency required to do the test correctly. For the WOT runs, the entry speed is about 46 kph and the WOT starts around -19m so the vehicle speed line is nice and linear between AA’ and BB’. All the runs are within 1 kph at PP’. The peak noise level between AA’ and BB’ + the vehicle length is what is used to calculate the final result.
At the top of this section is a little video of a pass-by test being done with our equipment.
The software measures the peak noise level as you WOT or cruise through the test zone and records it. It does a bunch of math with those levels and you get your score. The current limit will be 68 dB in the US in 2024, which is 6 dB lower than it was in 2016. For reference, that is about the noise level of a vacuum cleaner.
The way the software works is pretty cool as well. There are two separate data acquisition systems, one on the track and one in the vehicle. The track, or “ground station”, measures data from the microphones, meteorological data from a weather station, and two IR triggers that send a voltage spike when the vehicle crosses a beam at the entrance and exit of the track. This data acquisition system is connected to a network switch that sends data to the ground station laptop and a large antenna.
The “vehicle station” measures the vehicle speed via GPS, the engine RPM, and any other extra microphones the engineer is interested in getting data from, such as tire, exhaust, or intake mics. This station is also connected to a test laptop and a smaller antenna perched on the roof.
Both stations have their own GPS antenna, not to measure position, but to record the super accurate time signal from the GPS satellites. The ground station is sending live microphone and weather data to the vehicle station through the antennas and knows when the car passed through the track via the IR triggers. After a pass, the ground station sends all this data, along with the GPS time data, to the vehicle station. The vehicle station lines up the time signals to overlay the data, and back calculate the vehicle’s position on the track, since it knows its speed and when it crossed the known position of the IR sensors. This all happens in a few seconds so you get almost instant feedback whether a test run was good or not.
The Three-Word Loophole
So where does this loophole come in? If you are measuring wide open throttle events, how could a Corvette or Mustang ever be on the same level as your average crossover? The key is the gear you test in. Every vehicle gets an acceleration target, or Awot ref, which is calculated using the vehicle’s power and weight via the equation below. You do pre-test WOT runs in multiple gears to calculate their acceleration and see which one is closest to this target. This is why you use pre-acceleration, so the vehicle speed line between AA’ and BB’ is as linear as possible to get an accurate number. If no gear is within +/- 5% of the target you test in the gears above and below the target.
A wot ref = 1.59 * log10(PMR) – 1.41
PMR = Power to Mass Ratio
For example: if your A wot ref target is 1.68 m/s2 while your 4th gear acceleration is 1.9 and 5th gear acceleration is 1.52, you’d do 4 WOTs and 4 cruise runs in both 4th and 5th gear, since neither is within 5% of the target. If your 5th gear acceleration is 1.6 m/s2, you’d do the 4 WOT and cruise runs in 5th gear because it’s within 5% of the target.
What about electric cars, or other cars without gears like CVTs? We use a hand operated accelerator pedal that plugs into the vehicle’s wiring harness. There is a set screw that lets you limit how much you can push down the pedal, i.e. limiting the throttle percentage. You do practice runs where you dial in this pedal until you are within 5% of the target.
This acceleration target is where the loophole comes in, see the bold portion of paragraph (a) from section 3.1.2.1.4.1 of the specification.
If one specific gear ratio gives an acceleration in a tolerance band of ±5 per cent of the reference acceleration awot ref, not exceeding 2.0 m/s2, test with that gear ratio.
There is a 2.0 m/s2 cap to the acceleration rate! [Ed Note: Re the headline, we’re calling the three words “not exceeding two.” -DT] If you don’t understand how this sentence is a gift from those lawyers holed up in the mountain lair where these standards come from, read on. Your typical car with over 300 hp will have an acceleration target right around 2.0. For example a VW GTI with 220 hp has an acceleration target of 1.86, while the 310 hp Golf R’s target is 2.14. When you get into the 400+ horsepower range it’s well over 2. A new Mustang GT’s target will be around 2.17, a BMW M5 is around 2.33.
According to the specifications, in that situation you choose the gear with an acceleration as close to 2.0 m/s2 as possible without exceeding it. So instead of doing your WOT and cruise runs in third gear where you are really winding out your engine, you do it in 5th or 6th because the acceleration rate will be under 2.0 m/s2.
If you’ve driven a manual and mashed the pedal in too high a gear you know what this is like. Your entry speed for your WOTs is in the low 40 kph zone (25 mph), in 6th gear you are just barely above idle. You floor the accelerator, the engine kind of groans and you exit the track at BB’ a few hundred RPM higher than you were when you entered.
In a more typical vehicle with an acceleration rate under 2, the runs are more like the ones in the plots above, where your RPMs reach 3k or sometimes higher. Engines typically get louder the higher they go in the rev range so this acceleration cap is basically limiting how loud these vehicles can get during a test.
To be clear, the engineers still have a lot of work to do to keep these cars below the limit even in this situation where they’re in a high gear. But doing the test in 6th gear where the engine never cracks 2000 RPM is significantly quieter than if it was racing towards redline.
It makes sense why they would do this. This standard is meant to measure a vehicle’s sound during its “typical” operation in an urban area. To get around town doing your daily activities you don’t “need” to accelerate more than 2 m/s2. If you are driving a low horsepower car you have to wring the engine out a lot more to keep up with traffic compared to a high horsepower car that can do it in its sleep. The standard is pretty much assuming you are going to drive your new sports car like a grandma. Even though your sports car can accelerate much faster and be a lot louder than the test measures, they are trusting that you’ll be a smart and courteous member of society. This type of trust seems to be severely lacking these days, but is definitely welcome.
They could have easily left this clause out, and it would’ve made the test a lot simpler. However, if this acceleration cap didn’t exist, our automotive landscape would be a lot different. It would take a ton of work to get a V8 performance car in the 70 dB range doing a full WOT in 2nd or 3rd gear. The exhaust would probably be one long muffler, and you’d lose all the character and joy that sound brings. Identifying vehicles by their exhaust sound would get significantly harder, driving them would be much less exciting. It’s not a world I want to live in. So the next time the wail from a car’s exhaust puts a smile on your face or a tingle down your spine, thank the engineer/lawyers up in their mountain lair who added those three critical words, making it all possible.
Want to hear a sad sound? Perhaps not, but here is our very own Jason Torchinsky hiccuping his way through a message to us.
If you feel as bad for Jason as I do, you’ll donate to the Torch Medical Fund right now.
Do these tests have a correction factor for temperature/humidity? If not, that’s another way to game the results.
There is a temperature range which you are allowed to run the tests. In Michigan we have about 5 months where the temp is in range. I’d have to double check what the exact temps are.
BMW seemingly did this with the stock E39 540i exhaust. It makes no exhaust note under any circumstances. It has catted headers, secondary cats, a large Y-resonator that dead-ends on one side, and double mufflers, along with the one tip being a turn-down. The result is a V8 that’s quieter than most 4-cylinders. I learned to drive stick on one of those and I purely had to rely on the tach for my shifting, because I couldn’t rely on sound at all.
The specs were a bit different when the E39s came out. There wasn’t an acceleration target from what I recall, and the limits were a bit higher. I had an E38 with the V8 and they were pretty quiet. It’s not as much an issue with luxury cars but sports cars are another story.
I very much appreciate this loophole, although I find it interesting that these tests seem to require WOT in a higher gear than is good for the motor for high powered cars to stay within range.
Does it matter if the car is damaged during testing? It just has to be operating in that 20 meter long strip. If the engine dies 1 meter past BB, who cares? Not the enthusiast, since they’ll never drive their car that way.
That is very true. I learned the hard way as a novice manual driver that lugging is something to avoid.
This article is a perfect example of how the enforcement of government regulations is at odds with the reality of driving a car in the real world. Same can be said for the EPA, I suppose.
Except, of course, smog controls do work, unless someone tampers with or removes them. I was born and raised in L.A. and I distinctly remember the horror of “smog-alert” days. Sorry, but when your parents have to ship you out of the state in the summer so you can breathe, the good work of the CARB and the EPA becomes a little personal.
Flooring a car in too high a gear isn’t necessarily bad for the car, and is done in other types of tests to measure an event they call lugging. For some automatics though the vehicles computer won’t let us test in the gear we need so we make due with what we can test.
I mean it’s not bad if you do it a handful of times for the purpose of the test. If you are doing it constantly it would be bad.
Yep, this does nothing regarding enforcing local noise abatement laws. I wonder if there are classes that teach law enforcement how to do noise measurements that stand up in court, just as there have for radar gun use?
Are manufacturers still resorting to outright skulduggery in addition to rules layering?
In days of yore Suzuki notoriously put a flat spot in the GSXR ignition map at the rpm range used for the European drive by test and it common knowledge that clipping a jumper wire on the ECU removed the flat spot.
There is a separate, super complicated test called ASEP where you do runs in most of the gears at several different speeds. They look for an expected linear increase in the noise level as your speed and acceleration increases. If a carmaker sandbagged a particular gear it would be caught by this test.
Well, it’s just a matter of time before some busy body reads this 2nd or 3rd hand and decides it is a civic duty to fix this “problem”. And they will likely succeed. Sometimes a little knowledge just for fun is best left unlearned.
Hunh? What? I can’t hear you!
One simple trick to passing a noise test they don’t want you to know about
Interesting stuff!
Is this kind of testing required for *all* vehicles? Semis? Just passenger cars, etc.?
While I’ve been learning a lot of car trivia and such over the past years, the only engine I confidently know by sound is the 7.3l PowerStroke.
“No wonder he’s shouting all the time”–a friend, after their first time driving the dump truck with that engine someone else drove a lot
Thanks! Yes it applies to all vehicles although over a certain weight and passenger capacity the test is run differently. So busses, dump trucks, heavy duty vehicles like an F750 are covered under this spec but have a different procedure. Motorcycles have a different specification that I’m not familiar with.
Many racetracks have sound limits. Every racer has a toolkit with items like angled tips, turndowns, and in a pinch, a sound reducer insert than can be clamped in the pipe.
The first thing you learn is the side of the track where the sensor is placed and then point your nozzle away. Sometimes you even have to resort to lifting at the sensor which is a real pain while racing.
That’s super interesting. I’ll look into that, could be a good article.
Good old Laguna pipes.
Several years ago, several of us East Coast Spec Miata racers trucked our cars across the country to race at Laguna. Before we departed, we cleaned out every auto parts in the area buying various exhaust adapters. One guy still got nabbed by the sound police.
Wow! Great stuff. You obviously know WOT you’re writing AABBout.
Thanks!
Having done sound testing and designed test boxes in the past, this was a fascinating read. I always imagined getting a baseline low ambient would be tough for outdoor testing. Even in double-box sound chambers in a lab, ambient could vary a fair amount even when it seemed by ear that there wasn’t much change. (Of course, it was probably overkill to worry so much about the variance at such low relative ambient sound levels, especially where the out-of-spec electronic ballasts were very obviously loud—up to 90db @ 6″—but we wanted to be as consistent as possible when presenting the results to prove a major problem in the field was legitimate and not “oversensitive” end users and a minuscule number of failed units as our relabeled product’s OEM engineering kept trying to claim. It all worked out though when we found that the loud units eventually became flamethrowers that would shoot flame and heavy, oily smoke several inches after burning a hole through the steel case and they stopped arguing against a recall.)
:0 holy cow! Glad my experience with test objects starting on fire is pretty limited. Background noise can occasionally be a problem, but since we are measuring stuff in the 60-80 dB range we are good if the background is in the low 40s. There are indoor passby tests on NVH dynos that allow for testing in any weather and get rid of any background interference. I recently helped outfit a passby track in the middle of the Arizona desert which is definitely the quietest outdoor spot I’ve been in.
There are minimum noise tests for electric vehicles and there was a bunch of controversy because they added a ambient requirement that the level couldn’t change more than 2 dB during a 30 minute measurement, so one bird tweeting in the distance would scrap the test or make you re-record the ambient. From my understanding pretty much everyone ignored that requirement.
2db at ambient ~40?! Yeah, that would have to be ignored. Sounds like that was specced by someone who has never had to set up a noise test. What about aircraft flying overhead, too? I have a damn train that’s over 4 miles away past town and woods that I hear hitting the horn for a crossing late at night. Hell, even just one of the endless squirrels crunching over fallen leaves would be a problem. I can’t imagine trying to find a place outdoors so consistently quiet for so long.
Thanks for the lesson, Teach!
I would hazard a guess that this measurement standard is part of the landscape behind that flap in California over the Hyundai Elantra N that got ticketed for noise.
The official standardized test was probably fine, and therefore the car was legal. The measurement taken of the car in CA was ad-hoc by the roadside under completely different operating circumstances.
Yes the test has to be done in the loudest mode that the car is sold under typically. Some jurisdictions use the results of a standardized stationary noise test for ticketing, while many others just seem to make it up as they go along. Either way I don’t believe most of these roadside measurements are being done properly.
Chevy must have taken advantage of that loophole, because I’ve read of more than a few Camaro SS / ZL1 with the stock 2 mode exhaust get tagged at Laguna Seca for noise, although not sure if it was a 90 or 92dB day.
Also at least one ZL1 that tried to run in quiet mode that ended up melting the rear bumper..
Live next door to 2 fart cans that rev up every morning. Problem is, they don’t have a snooze button
Mustang behind me had some weird muffler and the guy started it up at 515 every day and revved the snot out of it, went out one morning and the other neighbor was screaming at they guy. It was loud enough to set off a car alarm two doors down. He moved shortly after that and before I could fill the thing with spray foam
Oh, I dreamed of spray foaming them on more than one occasion. Funny, I’m not the only one out there that thought of this.
Great article, but damn those noisy geese.
I’m all for enthusiasts being able to enjoy the sound of their machines when they are driving them. That’s why I advocate having speakers built into their headrests and the sound being piped to them, so they can crank it up as loud as they want and the rest of us don’t have to hear it.
And they could make them sound better or like a different engine altogether.
This. Is. Great!
Nice summary of something most of us never get a chance to see up close.
Thanks! Glad you enjoyed it!
Solid captioning right there.
Yup, I snorted.
Steve! I remember getting chewed out by Henry after shipping the wrong oil pan (due to…correctly following the wrong request) to NC for a pass-by noise test when I was working at Outer Drive lol.
Have been enjoying your writing here. Also, it’s pore over, not pour over. Somebody forgot to pore over your draft :p
-Graham
Hey Graham! Hope you are doing well! Really glad you are enjoying the articles, going to try to keep them coming.
Well, except for some coffee.
Fascinating read. Thank you.
Thanks glad you enjoyed it
You missed a number in there.
Great article though.
I’m glad I’m not the only one who noticed the absence of a reference to the cheerful popping of a three-cylinder two-stroke.
Thanks! I had to limit it a little bit or that paragraph could’ve gone on for a while 🙂
Inline 5? W16?
Inline-6 turbodiesel.
Just a friendly reminder. Like all pollution, noise pollution is harmful to human health.
For sure, one of the reasons why they have these regs in place
Yes, but who is actually doing anything about enforcing the regs? I walk my dog along a residential throughfare, and the noise from some vehicles along the 25mph road is painful!
This is cool, thank you – I always wondered about how they decided, esp as over the past 20 years or so, it definitely seems like volume from the factory has gone up in some (most?) cases.
Not that I’m complaining one bit – if there’s a true real-world problem, it’s not OEMs, but what some individuals choose to do their car’s exhaust systems on their own.
It’s interesting to me in PA how annual inspections are required statewide, but emissions testing is on a county-by-county basis.
Of course the coal region where I grew up was definitively not one of those places.
Same where I live. I’ve heard that some locales (I want to say Oregon but I may be misremembering?) are shifting to no more emissions testing at all, as I guess they figure there’s not enough older cars on the road anymore to make it worthwhile from a cost/benefit pov, and a safety inspection would catch any OBD2 MIL for most people.
WA stopped doing emissions testing in I think 2019. There isn’t a need for it anymore.
One of the reasons I felt good about moving here. I seem to struggle to own cars that do not throw emissions-related codes.
Knew I might be close but not quite right.
It seems a smart bang-for-the-buck measure. And good luck with the codes… 😉
Haha thanks. Currently have a car that throws an SAI code every few cold starts, one with a lean cylinder bank, and one with what I believe is a failed EVAP leak detection pump (or a leak i suppose-ha). All things that I can’t be bothered to fix, don’t have a significant impact on anything, and would fail me on a e-check.
The car I had before these had a light for a cat. The one before that did too. (probably just o2 sensors but the things are essentially impossible to remove on a rustbucket) Such is shitbox ownership.
IIRC, the EPA will require smog testing in metros or counties that fail to meet standards over a certain number of days. Hence, counties in Northern Virginia have smog testing, while counties in my own sprawling, industrialized and populated section of the state (which is also close to large bodies of water and lacks anything that anyone but a geologist would consider “terrain”) does not.
neat! Sadly, it seems like all this well meaning science is undone by local jurisdictions that take it upon themselves to define what loud means and how to measure it and how much to punish people for driving factory loud cars.
Thanks! I need to dig into it a bit more on how the results are used but some jurisdictions use the result of another standard noise test that is done when the vehicle isn’t moving for issuing tickets. Others seem to make it up as it goes. Would be a good future article.
A thing that I think is frustrating for all involved is that someone can absolutely create excessive noise by being a jackass, but someone who was not creating excessive noise could be cited just because they have the ability to.
I’m sorry to say in my neighborhood the ratio is something like 70% jackasses to 30% actual high-performance vehicles…
Pretty sure that’s better than most places. 90/10 here, at best. Actual high performance vehicles, when driven reasonably, should not be a problem in a residential neighborhood.