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  1. #11
    Join Date
    Mar 2014
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    Marham
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    42
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    236
    Really this is nothing to be concerned about, as Dave A alluded to sound pressure attenuates very quickly in free air - 99db at 0.5m falls to 89.5db at 1.5m, so unless the noise 'cameras' are mounted lower than 5' we should never trigger them.
    AK Gen II - Delivery May '15

  2. #12
    Join Date
    Nov 2016
    Location
    Stevenage
    Age
    47
    Posts
    251
    Bought my wadding from cbs
    Also ss wire along the od of the baffle and inserted them down the pipe x2 each side, much nicer and for the iva its hopefully quiet enough, only a 2 inch internal baffle

    Might need something else down the tube aswell

  3. #13
    Join Date
    Sep 2010
    Location
    Cambridge
    Age
    35
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    1,188
    Quote Originally Posted by Richie View Post
    Really this is nothing to be concerned about, as Dave A alluded to sound pressure attenuates very quickly in free air - 99db at 0.5m falls to 89.5db at 1.5m, so unless the noise 'cameras' are mounted lower than 5' we should never trigger them.
    Some of the guys I work with are boffins in the field of acoustics and so I had a chat with them concerning how much vehicle noise will reduce from a distance. This is relevant to me at the moment I want to track my car and the drive-by limit is 95dB so our conversation went as follows:

    You've got your perfect world "free field" so the sound is radiated in a sphere from the source which is floating in mid air with no reflections. If you double the distance, the sound pressure level will be half. On the dB log scale, thats a reduction of -6dB for twice the distance.

    Unfortunately we don't live in this magical fantasy land, we are stuck in the real world. We can crank up reality one notch and assume the sound eminates in a hemisphere from a point on the floor. Some maths occurs in the background and that cuts our attenuation in half so now we have a reduction of -3dB for a doubling of distance.

    This doesn't take into account the reflections off the ground surface. Surface type plays a big part. If its hard and flat, very bad. If its lumpy bumpy i.e. vegetation, low bushes, or even gravel then that helps dissipate the sound. Add in extra solid things typically found at the road side like roads and houses for extra reflections and you can see how this chips away at our estimate. Conservatively we're probably now looking at around -2dB reduction for a doubling of distance.

    So far the car has been stationary. When its moving you've got tyre roar from the road surface as well as probably increased induction noise from the engine under load when compared to just revving freely when stationary. Also the static IVA test is at 3/4 max power revs so clearly there's a few more revs/more noise there as well. Lets do some more arm-waving, considering all the other complicated factors introduced above and we are probably realistically worst case looking at a reduction as small as -1.5dB for a doubling of distance.

    Working in our favour for cobras though if they are measuring dB(A) its weighted to certain frequencies that the ear is sensitive to so our low frequency rumble from our engines probably wont register as high on this scale. We're into pure guess work here though so I wouldn't want to put a number on it without reading some books first so we'll ignore that.

    For my specific case, assuming the car is IVA compliant and measured 99dB in the static test at 0.5m, the worst case conservative prediction of -1.5dB for each doubling of distance puts us somewhere around 91dB at 20 metres.

    If your still awake at this point, one of the exhaust baffle configurations I experimented with before the IVA measured 105dB at 0.5m. Our theorising above gave us a range of between -2dB to -1.5dB for a reasonable realistic guess at noise attenuation as a function of doubling distance. For a distance of 20m that gives a range between 94dB at best, to about 96.5dB worst, so right on the very limit for your average 95dB drive by track day and probably in the region where these acoustic cameras would likely be set.

    regards

    Dave A
    Last edited by hifihedgehog; 18-06-19 at 12:39 PM.

  4. #14
    Join Date
    Feb 2007
    Location
    West Sussex
    Posts
    8,535
    I am wondering whether the noise is to be measured at a set distance where the camera microphone is placed, or is the camera and its computer making an inference based on knowledge of local conditions/gemoetry and inferring noise emitted at, say 0.5m from the vehicle?
    Crendon Chassis No.49
    Huddart FE428 + toploader

    Not listed in the Shelby Register.

  5. #15
    Join Date
    Sep 2010
    Location
    Cambridge
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    35
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    Quote Originally Posted by KevinW View Post
    I am wondering whether the noise is to be measured at a set distance where the camera microphone is placed, or is the camera and its computer making an inference based on knowledge of local conditions/gemoetry and inferring noise emitted at, say 0.5m from the vehicle?
    I can't see it working any other way than having a microphone a set distance away and then recording the peak noise at that distance. It starts getting way too complicated otherwise I can't decide if sidepipes are a disadvantage or not - the sound is directed straight at the side of the road, but then the microphone can only "listen" to one side at a time, the other side of the car will be almost completely shielded, unless there's an unfortunate wall or building to give a reflection. We could all probably help ourselves (Assuming sidepipes) by turning the exits down to face the ground, but then I personally don't like the way that looks, I mean I'll try and stick to the rules if they're reasonable but if it makes my car look silly....

    If you spot one, take note of what side of the road its on and drive as far to the other side of the road away from it as possible, every single metre will help!
    Last edited by hifihedgehog; 18-06-19 at 02:00 PM.

  6. #16
    Join Date
    Jan 2006
    Location
    Chelmsford Essex
    Posts
    821
    I was at Goodwood track day and they had a noise limit on the day. ( Not one of there unrestricted days in the calendar for major events) .

    If the noise limit was hit then you were ask to restrict your revs to 3,000 rpm and have to suspend the session for a certain amount of time. Now surprisingly a Jag V12 5.3 cobra with under slung hit the noise limit. It was more to do with the low noise and rumble as the car sounded the quietest on the track yet exceeded the noise limit. It is misleading on how noise travels and what frequencies are measured to go over the limit.

  7. #17
    Join Date
    Apr 2012
    Location
    Wrexham
    Age
    47
    Posts
    529
    Bought acoust fil today.
    If anyone wants to know with these pipes, from the calculator you need 700mm long, 0.6m per pipe. Tweeks sells 350mm so i ordered 2x350mm 1m per pipe, i.e 4 lots of 350mm wide 1m lengths and cut down to 0.6m. Probably get away with 3x 350x1m but i didnt want mess...
    Sounds better than my Fulton boiler flue lagging....
    Regards
    Gav
    AK Ls1/t56
    Body on April 2014
    Passed IVA March 2017
    Registered April 2017.

  8. #18
    Join Date
    Dec 2009
    Location
    Barton Mills,Bury st Edmunds,Suffolk
    Age
    49
    Posts
    260
    Hi all I’m writing on the back of original thread, I’ve got noise issues with pipes from AB Joner from an auction site! The pipes look great and fit well and they come with an internal baffle that’s not removable and 2 additional baffles that fit in the end of the side pipe, I failed IVA on 101 and as mentioned 99 or less required for a pass, I’ve tried wrapping with exhaust wadding the fibreglass type a strangely got mixed readings, 1 side lower nearly there and the other louder. Tried wrapping with more and got louder and some information says shorten the length of the baffle? So before chopping the baffles I’ve just ordered some acoustafil any advice greatly received.
    Thanks Mark
    mark chambers
    Dax de Dion 350 Chevy

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