How to pass the "noise test"???

[Plums]

G'day all,

I am slowly getting the Cobra ready for engineering and had a friend with a sound meter test the noise level.

I have quietened down (read bunged up) my sidepipes so much that it sounds just like a standard Commodore at the sidepipe exit. Even at 4000 rpm the noise coming from the pipe outlet is negligible. My problem is the noise coming from the engine bay...... The mechanical noise from the engine is far louder than anything else!!! This noise doesn't really seem to come from the extractors either, it's mostly just the mechanical noise of the engine. (And for you beer can knockers, it doesn't have any piston slap issues). I realise that alloy engines are noisier than boat anchors too...

With the engine at 4000 rpm it reads 93 dB, 50cm from the sidepipe exit (either side, doesn't matter)... If you hold the sound meter out in front of the radiator at 4000 rpm it reads nearly 96 dB. At the rear of the car at 4000 rpm the sound level is down to 88 dB. As a bit of a quick test, we put the front of the car in the shed and then closed the sliding garage door right up to the car, the sound level dropped down to 91 dB (not exactly perfect science but it did prove something)... If you could completely isolate the sidepipe exit, I reckon it would probably only read 86 or 87 dB....
For info, the sound meter is used for jap import certification and just came back from calibration in Sydney.

How on earth is this thing going to pass the driveby noise test?

How did you chaps pass the noise test??? Am I missing something or doing something wrong???

Thanks in advance,

Bobby Brown

[Merv and Sharon]

Have to try and see Bobby. dB is a tricky measurement. A measured 5 dB increase is equivalent to a perceived loudness doubling as it is a logarithmic scale not a linear one. So they may be picky. Also, I am not sure whether they use dBA, or dBC, etc . Can make a difference. As I understand the engineer does a drive-by test and a standing (RPM) test. Others could be more accurate on this?

Seems like you have the exhaust system output covered but the motor noises (induction and that LS1!) all add to the accumulation. I also suspect that they will do an average of the noise on test and not just peak values. Again others may have a better idea on this point.

Merv

[albanycobra]

This is how I passed with the Coupe. Single 1 3/4 inch under body exhaust using a custom baffle box centre muffler. There is no exhaust note at all just a choking whooshing noise.

[Rebel1]

Bobby, I have been advised to really address INTAKE noise (ie Pod filters etc.) as well as exhaust noise.

[sideshow]

yer intake noise on those rattly holdens is noisier than the exhaust

put a throttle stopper on the butterfly or if its fly by wire put a block under pedal

put a pipe around 1m or so on the intake to the air cleaner box then put 1 or 2 car wash sponges in there

helps reduce power and air into enigne

will go **** but this might help passing

[Plums]

Thanks for the prompt replies guys.

The noise isn't from induction roar. I should have mentioned that I tried it without any filter at all, then with the pod, then with the standard MAF and airbox, it made no difference to the sound level at all. This is because the motor is under virtually no load when just free reving to 4000rpm so it has very little airflow. At WOT is a different story though and I will use Sideshow's idea of stuffing foam into the stock airbox, (thanks Sideshow)

It's basically just the mechanical noise of the engine itself that's causing the problem. I seem to remember reading about somebody covering the engine in a blanket to pass the driveby noise test, then almost having a fire!!!

Help...

[sideshow]

ben just went for engineering last week

he borrowd about 15kgs of lead padding

covered engine and sides and underneath

everything gets hot but thats what u have to do sometimes to pass

he also had to do brake test at 160 kph

that is the new rule for brake tests

[Rebel1]

Originally Posted by sideshow

he also had to do brake test at 160 kph

that is the new rule for brake tests

Are you joking! ..... they ask you to exceed the speed limit.?.

Can I see some objections to that.

[OZCOBRA]

Originally Posted by sideshow

he also had to do brake test at 160 kph

And just where does or is this test performed.... common sense says it's BS....SOUNDS MORE LIKE A TESTER WANTING A QUICK JOYRIDE...AND "YES SIR YOU'VE PASSED THE BRAKING TEST"

[sideshow]

this is the reason y some engineers are giving up on cars

the rta bring in these dumd ass rules

i recond some new factory cars might have problems stopping at 160

well to do this properly u have to hire a track or go on a track day and do it

this is ahrd to plan and u need afew cars to do at once just to make it cheap


i just finished wirign an austin healy and it cost the guy 1g for engineering

didnt do brake test car was factory sort of except for 20v engine

engineers are goin up and up in price and the rta aint helping

[Rebel1]

Sideshow....I'd just ask the engineer to supply me with documentation that the STATE insist on this test. Remember it is the STATE which interprets, or accepts, and enforces, the EPA recommendations. That's why the states have different requirements.

I would then seek from the Minister an explanation why his department is insisting a member of the public put his/her life at risk in meeting the standard.

I don't care if it's tested on a track or not....it is still a risk at those speeds. Not every member of the public is comfortable driving at 160klm.

Who accepts the responsibility if some-one is injured, or worse, disabled or killed during this testing?

[Baz]

Originally Posted by Rebel1

Sideshow....I'd just ask the engineer to supply me with documentation that the STATE insist on this test. Remember it is the STATE which interprets, or accepts, and enforces, the EPA recommendations. That's why the states have different requirements.

I would then seek from the Minister an explanation why his department is insisting a member of the public put his/her life at risk in meeting the standard.

I don't care if it's tested on a track or not....it is still a risk at those speeds. Not every member of the public is comfortable driving at 160klm.

Who accepts the responsibility if some-one is injured, or worse, disabled or killed during this testing?

Phil reckons my LS2 won't pull 160? Does that mean it won't qualify for engineering/? :

Baz

[Rebel1]

Baz, If that was the Phil with an M, ie. the Phil who has inappropriate murals painted on his car, then I'd take little notice.

We have proof of his respect for the chev on video. His action in leaping backwards during Andrews first start, and in warning his missus to stand clear, is clear to see.

The man is just a closet Chev admirer.

[jams]

I knew a guy who put coffee can lids in the collector before the test. The lids had a hole big enough to let exhaust through but small enough to keep the sound down.

After the test, unbolt the collectors and remove the lid!

[stephen low]

Bobby Vic requirement for our test is that the noise meter be level in height with exhaust outlet, 600mm away and 45 degrees outwards. But this is for a standard rear facing outlet. Still reckon you'll have a similiar requirement particularly not measured directly in front to eliminate other created noise effects.

In fact my tips arc down towards the ground and although I was asked to only rev to 3500- 3600 when strictly it should have been 3950 rpm, I achieved 89dBA or 1 dB under the limit.

I tried making progressively smaller orfice plates mounted just after the cats but even with a 25mm hole was still getting low 90's readings at my correct revs.

I ended up going with the attitiude that if I needed to add hot dogs or similar just to get over the line I might argue my way out of that at the test else I'd get them installed and passed and they'd have dropped off afterwards.

Merv was nearly right at the start, for every 3 dBA increase it is a doubling of sound power. And A weighting replicates the human hearing and is what the test is done as.

What the WA limit?

Cheers

[CobraEd]

Just put three cans of STP in it for the test. Then drain it back out. That stuff is like molasses and will quiet all mechanical noise.

.

[Merv and Sharon]

I was struggling with my 20-years ago audiological training there! Steven is right and 3dB is the doubling-sound power difference, not 5dB.

However, it is interesting that they use dBA (filtered to somewhat replicate human hearing; and is less sensitive under 1000 Hz) rather than dBC which is a more flat response. I would be interested to know where in the sound spectrum the Cobra exhaust/s is/are most prominent. That could also influence how we try to attenuate the noise sources. I suspect that there is a good deal of low frequency noise there. Different strategies could be applied for attenuating higher and lower frequencies. It is also good to see that they use a standard set up for the measurement. I am still wondering if they use the average measurement of output over several trials or only record the 'peak' or most intense point? Obviously with cars being so close to the limit an average is an advantage.

Last, the exhaust guy I spoke to at the Gold Coast yesterday, who does lots of Cobras, said that he always fits plates to restrict sound output for registration. I am interested to see what he does.

[martrogers]

I believe the static test requires you to rev to 75% of peak HP, not a particular RPM, but anyway 4000rpm seems popular.

I failed the driveby test by 3db, 3db is a signficant amount. As far as I recall the driveby test consists of approaching the test area at 50km/h then flooring the throttle. In my case the auto then kicked down and I failed - unfair on autos but anyway.

So I did the following:

Stuffed the air intake

Put an adjustable bolt behind the accelerator (fly by wire) so that it accelerated but didnt kick down at 50km/h

Built a custom muffler and stuffed with fine wire wool.

Sealed the major holes in the engine compartment (including the whole of the underside with a sheet of ply.

Put in barium doped sound absorbant mat 20mm thk including behind the radiator to stop noise going out the front. Mat not cheap but performs well.

Passed second time.

I just missed the higher speed braking requirement but it means that only certain tracks have a long enough straight which will certainly add $$$$ to testing.

Mart

[427 S/O]

Man! you guys in Perth must have some stringent noise laws?, around here I have no fear of being ticketed as long as those Harley's cruse up and down the road. There really loud.

[ptrefry]

wear ear plugs !!!!