Anyone ever tested the sound level of your Cobra?

[750hp]

I'm interested to see if anyone has checked the volume of their car with an accurate decibel meter.

I went to an exhaust shop today to see for certain how loud my car was. Our current laws require testing to be performed from a point as though you were basically taking a close "rear 3/4" photo. The decibel reading is taken at an engine speed equal to 3/4 of peak torque rpm (ie: peak torque at 4000rpm, decibels tested at 3000rpm). Maximum allowable is 90 decibels.

At idle, my car was 103.

At 3,500rpm, it went off the gauge. The gauge read to 130.....

A loud car never hurt anyone, did it? I'm going to have to do something to take the edge off the noise, but I really don't know for sure what to do....

[CobraDan]

Craig,

There called mufflers
Dan

[Hotfingrs]

When I was doing some testing for inserts, mine read 104 at idle and at a distance of 30'...got it down to 89-90 with some inserts. Every 10db is = to double the noise.

[Ant]

Craig:

There trying to bring in a decibel reading here across the ditch, mutings of 96 db for a start! What you might be able to do is restrict the throttle fit a stop, to be used for any WOF or if the Police have meters.

Your car must be incredibly noisy, as my 650bhp injected Cobra had Supertrapps which ran all the plates, I ran them without plates and it didn't really sound any louder, but that was at about 100db, and it was noisier when I backed off, all the wanking race officials came running over - bleet bleet!

I am considering running Flowmaster race mufflers fitted underneath my car for road and track, hopefully it wont be to noisy.

[750hp]

Dan, huh.... what??? Did you say something.... Oh, mufflers? Yeah, I've got 'em. I'm using perforated 3.5" tube inside the sidepipes, with 5" outside stainless pipe. The packing inside is the really fine white fiberglass packing that the big rig truck muffler shops use. I have a horrible feeling that my first day at the roadrace track destroyed most of the muffler packing. It was quieter before that day. The foot-long flames shooting from the sidepipes on deceleration might have had something to do with that...

Jack - did you fit a spiral style insert into your pipes? What brand? Did this drop the idle noise, or WOT noise, or both? Any effect on power? What compression ratio are you running in your motor? More compression seems to really increase the sound. Sorry to hear about the Fling. Your car was one car that I slapped myself for not seeing at last year's event. I'd read about it previously, and was disappointed to see that we were both there, and didn't meet up. Next year?

Ant, noisier when you back off? When I see a Police car, I can't help but back off..... Did you still have perforated muffler running the length of the sidepipes, and then fit the Supertrapps to the ends, or only use the Supertrapps?

Thanks,
Craig

[Wayne Maybury]

Craig

I tried last year but it didn't work out too well. I borrowed a decibel meter from a friend and decided to check my side pipes as follows: one with nothing in it and one with the insert in.

I fired up the car and immediately put the meter off the scale, which went up to 128 or 130, on the side with no insert. This was just above idle and back 5 or 10 feet. Unfortunately, I got exactly the same result for the pipe that had the insert. So much for science. I can tell you that the car is much louder without the inserts. I also have shorty pipes that are even louder than the empty long pipes.

Wayne

Wayne

[Hotfingrs]

http://www.spiralturbobaffles.com/ this is the link to the website that I was using. comp ratio on the engine I was using for testing was 11.62 and db's were dropped across from idle to wot. We didn't get a chance for any performance runs because the engine was going sour and it wouldn't have been a fair test. Was going to test them again this year after the fling....but you know that story....car should be back on the road in 2-3 weeks...nothing major....broken motor mount, bent rim, bent header, and brake problem....biggest decision now, is what side exhaust I want to go with....

[DanElam]

Our 5.0L Spec Racer is typically 92 or 93db depending on ambiant air temp. You are supposed to wear hearing protection if exposed to sounds greater than 80-85db. The scale is logrithmic, so an increase to 103 is quite significant. If you got to 130db, I suspect that the guage was broken: rock drills typically max out at 130db. In any case, even short term exposure causes hearing loss, especially of high end frequencies that are useful to determine impending engine failure. (I said: "YOU ARE GOING TO NEED A NEW MOTOR!") Ear drum perforation is about 160db and standing next to a jet is about 140db. You would have to be gulping enormous amounts of air in order to do that.

As a practical matter, most tracks have a limit of 102db. If you exceed that, you go home. Even most localities have a limit on how much noise the car can produce. I should point out that most race cars don't have a problem staying below the noise limit.

[casaleenie]

I'll let you know after the next condo meeting.. I think my neighbors are measuring it on a daily basis... Apparently they don't share my enthusiasm for the car... When I moved here I didn't realize that most of the other neighbors moved here to die... Rich widows with butter faces...

[mikiec]

At the fling Turk measured his car and got a reading of 118 at idle. Of course he was using shorty pippes with no mufflers.

Mike

[Turk]

128.

TURK

[Rob of Hamburg]

The only measure I have is the setting off car alarms parked three cars away. I Luv this car in the summer!!!!

[Ant]

Craig,

I was using the large Supertrapp Muffler and diffuser plate packs, I think there was at least 20 plates, or whatever they came with! there was some sort of perforation inside the mufflers and some of the glass pack left, but it might have been mostly gone, having said that I removed four of the diffuser plates and that solved the problem. Unfortunately I haven't got accurate readings of the car.

Have you had anything to do with Flowmaster race mufflers?

[nolimits]

750HP, 130 dB(A)!? Can you hear me yet!
Check to see if the regulation is 90dB or 90dB(A), some EU countries use dB(C) for pass by regulations. The A scale is closest to human hearing which weights each frequency accordingly, with 1000Hz being the most heavily weighted and tapers off above and below 1KHz. dB with no weighting has even weight across the freq. spectrum, and our engines are spiking frequencies from 10Hz to 250Hz giving the potentially deafening dB values being posted. In A scale, your 130dB is probably more like 108dB(A), and 112dB(C). Also, distance from the car and location in relation to the sidepipe is very important. A couple of feet could change a couple dB, and move forward of the exhaust exit as much as regulation allows. Microphone direction and equipment calibration is also a factor. Unless you are using a very good sound pressure level meter, error can easily be 5dB. There are decibel conversions for scales calculations available on the web. We were recently looking into building a car for a Swiss customer, which has an 80dB(A) regulation, and determined its virtually impossible to meet with cats and sidepipes
Hope this helps.
Jay
Classics by Elite LLC.

[Ant]

80DB would be getting to the stage of ridiculous over regulated laws, why would anyone want their classic car, especially a road racer from the 60's to be that quiet, I will never ever muffle my car down to that, I would find a way around it, some wanker in an office who hates our pastime can bring a new law in like that, to justify their position or salary, what a corrupt society we live in.

Having said all that I am not into noise and pissing neighbours off, I go away to the race track or out in the country for some fun, with the road racers causing problems with wheelies etc!

[750hp]

Thanks for all the info!

Jay - I honestly don't know for sure what scale is used over here, but suffice to say the shop that did the testing for me is one of only 3 shops in the whole state whose sound testing is recognised by our Department of Transport. They are required to have the meter calibrated frequently, and all testing is performed in a controlled environment. They did the test to the letter of the law. The owner is a friend of mine, so he wasn't just scouting for more work either It was interesting that they mentioned that our local GM product with a V6 Buick derivative shows an 87dB reading, while the LS1 Chev motor in the same cars is right on 90dB (at around 3000rpm). They noted that one totally standard LS1 powered car that was 6 months and 10,000 miles old, gave a reading of 93dB..... If an untouched, stocko LS1 is over the limit, what chance in the world would any Cobra over here have?!?

Ant, the only Flowmaster cars with any power that I've heard are the Transam style racers (Sports Sedans). I must say that they "sound" louder than other mufflers to me. It seems like they have a much higher note, rather than a deep, booming note. What that translates to on a decibel meter is anyone's guess...

Dan, these guys only run their testing equipment up to 130dB, stating that anything over that is beyond the pain threshold. Once anything hits that level, they're out of there. Blowing all that sound out through the sidepipes at 6500rpm in top gear on the racetrack actually sounded quite nice with the helmet on, but if the revs go above 4000rpm on the street, it does give your ear a workout. More incentive not to do silly things on the street..... In any case, what with the exhaust, three noisy fuel pumps, solid roller camshaft, cog driven supercharger with it's own internal straight-cut gear drive, and great volumes of air rushing past the bypass valve whilst in vacuum, I think the engine would have to seize solid before I'd hear anything untoward.... With the rear of my hoodsoop open (RS Camaro cowl-induction style), any mechanical noise just hits you straight in the face as if the windscreen wasn't even there. http://www.clubcobra.com/photopost/d...122rollbar.jpg

Regarding the 102dB track limit. I wonder what 130dB at very close distance roughly equates to from 100ft away?

Jack, thanks for the link. A friend of mine has similar spiral mufflers in his 65 SS Impala. His 13:1 comp 509 cube streeter is insanely noisy at idle to the point that I cannot even hear the modified 460 running in my Falcon while sitting beside him at the lights. At the track however, his car at full throttle sounded only half as loud as my traditional mufflers. It's interesting that your car is quieter at idle with these. Maybe it's the combination of spirals and perforated tube that works best

[NeedAntiVenom]

Craig,

Sound intensity decreases with the square of the distance. For example, going from 3 feet away to 6 feet away causes the sound power to be reduced by 6 dB. dB is a log scale, each change of 3dB represents twice the power (+3 dB) or half the power (-3dB). The chart linked to below show the relative change in dB relative to distance.

SOUND PRESSURE LEVELS vs DISTANCE
http://www.warehousesound.co.uk/Hire...versusdist.htm

Humans hear with their own log scale. To a person, something sounds twice/half as loud when there is a change of 10 dB. In addition, the perceive intensity of sound in humans changes both with frequency and volume. This was empirically measured by Bell labs.

Fletcher-Munson Curves
http://www.webervst.com/fm.htm

Hope this was what you were looking for.
Regards,

Jim

[750hp]

Thanks for the links, Jim. It's amazing how the measured noise drops off with such relatively short distances. You learn something new everyday!

Craig

[Shelby American Inc.]

Originally posted by Turk


128.

TURK

This unit was rated at a peice of paper towel crammed in each ear brought the sound level back to "I can hear myself think"


BEFORE he traded it out.

[ac andy]

Now i think i might have mentioned something about this on that day in your garage Craig. You just looked at me with a puzzled look on your face! If you get pulled over, just tell the cops you are deaf and you didnt know it was that loud!

Andy.