Exhaust Noise

[mreguest]

Hello,

I have measured exhaust noise using a couple of different silencers in my 2.5" mongoose system. Tests were done by me using a professional type 1 sound level meter, so spectral frequency data is available and I'll make a nice graph one day when I get some spare time.

As per SVA, IVA and 70/157/EEC the measurements were undertaken at a distance of 0.5m from the exhaust pipe at 3/4 engine power etc...

As you will see in this document from 2009 imported car exhausts are not supposed to exceed 99dBA, and prior to 2009 I think the limit was 102dBA. I have not looked into whether this only applies at the time of import or if we are expected to comply with this as it gets stricter. I assume it would be completely unreasonable for us to comply with the new regulations...

http://www.minimag.co.uk/files/2011/08/M1-Inspection-Manual-May-09-1.pdf

I did the tests at 5625rpm and my car is standard boost & ecu fueling etc...

5-6 year old mongoose with decat pipe.

= 105dBA (f'ing loud)

mongoose with unknown age second hand (off bob) hks/mongoose decat silencer. - one of these:
http://www.ebay.co.uk/itm/Nissan-Sunny-Pulsar-GTiR-Mongoose-Decat-Pipe-/221210600200

= 98dBA (still loud but should comply with many track day noise tests)

mongoose with hks/mongoose decat silencer and my own 'custom bodge job' with additional 500mm Jetex silencer - one of these:
http://www.ebay.co.uk/itm/Jetex-Uni...-5-Mild-Steel-Case-Length-500mm-/290616387048

= 91dBA (much quieter, comparable to friends 15 year old knackered but standard Toyota MR2 exhaust)

At idle (~1000rpm) I got...

mongoose with decat pipe.

= 82dBA

mongoose with hks/mongoose decat silencer.

= 81dBA

mongoose with hks/mongoose decat silencer + my 500mm bodge.

= 74dBA

Note at idle the small silencer makes chuff all difference because of the frequency response. At idle there is not a great deal of higher frequency noise, so the silencer has nothing to work on and as such the dBA value is dominated by low frequency noise. As you rev up you generate more higher frequency noise which the silencer can attenuate. Due to its length the big silencer can attenuate noise quite well at most frequencies.

Obviously different peoples car will produce different noise levels/frequencies depending on the type of mods, tuning etc... but this gives you a good idea of what is possible.

Naturally as your exhaust gets older it also gets louder because the mineral wool packing material degrades over time.

I'll probably do a 'how-to' sometime documenting my bodge job fitting a 500mm silencer across the back of the car - i'm quite proud of it and it did not take any welding, just a grinder, bit of additional pipe, exhaust putty, hangers and clamps... these volvo clamps are really good for clamping and hanging:
http://www.ebay.co.uk/itm/Volvo-V70...st-Tailpipe-Bracket-Hanger-64mm-/360418504473

When I get some spare time I'm also going to document an experiment with some material that is very similar to Dynamat. See these three experiments on their website:

http://www.dynamat.com/technical_sound_dbates_intro.html

I hope to recreate these experiments at my work using my car as a demo and demonstrate the properties of some advanced (and should be cheaper and lighter) material that I hope to start selling... I can't wait to kill off lots of road noise and panel vibrations

Vroom Vroom!

Adam

 

[keastygtir]

Cool info mate. this is why I went to a scorpion exhaust as its very quiet. One the downside it sticks out down low too much to fit one of those nice diffusers

 

[watoga]

Good write up. Informative and very helpful.

Have you had any experience with re-packable silencers? Just wondering if some decats can be re-packed with new wadding to give even more sound-deadening ability. And if so, what type(s) of wadding are best for optimum sound reduction?

Many thanks!
Dave

 

[webadmin]

Ive just read this.

Great post.

Thanks

 

[stevepudney]

Really interesting, I wasn't aware of the rules change. Brilliant right up :-D

 

[PobodY]

Am I right in reading that rule change only applies to the SVA test; so only when it's first registered (or "compliance tested") in the UK?

 

[red reading]

could do with you noise testing my car, how much for you to do it?

 

[mreguest]

could do with you noise testing my car, how much for you to do it?

just realised i didn't write back to this, sorry dan! if i'd bloody noticed i'd of brought my stuff to the northweald track day... :doh:

(i'm sure when people write back to a thread i used to get an email update? any ideas?)

if it's just for track day comparison then to be honest i would use a maplin SLM rather than paying me, as a company we charge £80 per hour. so including travel time etc could be expensive...

http://www.maplin.co.uk/digital-sound-level-meter-46362

stick it on a tripod, do a few tests and set it to: high range, 'A' weighted, fast response, max hold. then park in a big car park as far away from stuff as possible to avoid reflections and copy the test requirements from the track you want to visit, i.e. 1m away from exhaust tip, 45 degrees, 0.5m above ground, rev to 5625rpm etc - Maplin SLM is +/-1.5dBA accuracy which is not bad for £50, the sound level meters we use in work are >£2,000, accuracy +/- 0.1dBA.

been meaning to give you a shout about a gearbox i have that needs inspecting because it won't stay in first gear, i'm physically not strong enough to hold it in and it shudders like crazy in reverse, (it is no longer in my car) the gears 2nd/3rd/4th/5th seem fine!?!? maybe we could meet up at yours one day and have a noise testing gearbox inspection party? regardless of noise testing i would probably need you to fix gearbox and we can work out £££... what you think?

WATOGA DAVE : re-packable silencers you can take apart fairly easily because of the design. most normal silencers are seam welded/sealed and would need cutting open repacking and then sealing up again.

have a chat to these guys - http://www.thermalvelocity.co.uk/ - i have no experience with them i just googled - my line of work really involves huge industrial silencers but the science is still the same with a car.

check this silencer for a boat - it's massive!
http://www.expeditionyacht.org/exhaust/new-main-engine-exhaust

 

[red reading]

Hi ya, well we can have a deal, if you can do the noise test...dan Fennell has a maplin noise meter and needs his car done too....while your doing them I will look the gearbox over and tell you what is needed....I think I know what it is and I have second hand bits there to do it so all in all it will cost nowt.....what do you say?

 

[mreguest]

Hi ya, well we can have a deal, if you can do the noise test...dan Fennell has a maplin noise meter and needs his car done too....while your doing them I will look the gearbox over and tell you what is needed....I think I know what it is and I have second hand bits there to do it so all in all it will cost nowt.....what do you say?

sounds brilliant mate! i would bring a full frequency analysis sound level meter from work and do a few tests, different distances, rpms etc, then at least you know where you stand as different tracks can have slightly different test rules... we can write down basic statistics on paper and in my spare time if i download the data on a computer in work i'll make some proper frequency response graphs.

this is all probably doable in an evening afterwork if you need noise testing asap? i finish work in Woking at 5.30pm. my weekends are all fully booked up in August with holidays etc. if there is no rush then maybe a weekend in september? (i live in guildford so not that far from reading)

cheers

Adam

 

[red reading]

Ok I need to do just a basic test to make sure I am below 100db at 4000rpm for a track day up coming so hopefully dan f's maplin meter will be fine for that, but a proper test will be cool in the future just to learn and understand more for myself, and you will get a working gearbox in return...happy days !

 

[mreguest]

Ok I need to do just a basic test to make sure I am below 100db at 4000rpm for a track day up coming so hopefully dan f's maplin meter will be fine for that, but a proper test will be cool in the future just to learn and understand more for myself, and you will get a working gearbox in return...happy days !

Dan - what track are you going to? do you have a website link to info about their noise testing rules? (be interesting to read) when are you going? it's probably pretty straight forward but you must be sure to follow the procedures exactly, usually they want a test at 0.5m from the tip of the exhaust and 0.5m above ground and 45 degrees horizontally away from the centre line of the tip, at 3/4rpm. Some tracks say at 1m at 3/4rpm. You obviously know it's <100dBA at 4000rpm but at what distance etc?

it's worth a little practice revving to 4000rpm at home and get dan-f to watch how the sound meter reacts and then you do it to his car. I'm guessing with your cars both being highly tuned there could be a fair bit of snap crackle and pop to avoid which could throw the readings... most noise testers at tracks understand this and might do a few attempts to see how/if it averages out or settles down. Some testers will just say 100 is the limit so 100.1dBA go away! To be on the safe side remember to add 1.5dBA to what ever you measure because Maplins meters claim +/-1.5dBA accuracy. Your settings should be 'A' weighted, fast & high range, and you want the Lmax value. You may use a 'max hold' button to help.

You will probably find revving smoothly up to 4k, hold it briefly and then smoothly throttle back down you'll probably dump less fuel and as such less snap crackle pop... i'd be surprised if you don't meet <100dBA at 4,000rpm. Do you have one or more silencers on the car? If you do have at least one I think you'll be fine... If need be you can clean the insides with a bit of white spirit and a bog brush, the inner perforations can get clogged up... but if the packing is totally burnt out you'll need an alternative - http://www.jetex.co.uk/website/custom_parts.php?mat=Stainless&dia=3.0+inch - jetex have some good stuff...

here's some food for thought:
http://www.ebay.com/itm/Exhaust-Control-Valve-with-Vacuum-Actuator-for-2-1-2-63mm-Pipe-/380606319348

and this is why AMG mercedes, lambos etc are loud as hell when you poodle around town in first gear with sports button pressed:
http://ipe-japan.com/wp-content/uploads/2013/02/AMG-C63-iPE-valve-muffler.jpg

obviously you don't get as much boost/vacuum when revving stationary so you could block off a pipe somewhere and pass air through a big silencer. (you would nail static noise testing.) then when you're moving, the engine is loaded and with the boost/vacuum generated you could pull a valve open and deafen people

some tracks have state of the art noise monitors like rockingham so they can basically follow your noise on the way round the track and then remove you. but because you passed the static test they would probably give you your money back and tell you to get lost.

the whole point of enforcing a static test is to meet average moving noise levels around the track (or a building site) to protect neighboring houses.

FYI - i do stuff like this in work for building sites so we look at diggers etc rather than race cars
http://www.rockingham.co.uk/community/pdf/noise_management_plan.pdf

what ya doing sunday 15th September? fancy a noise test/gearbox fix?

 

[red reading]

Adam, the tracks will be brands,castle coombe,silver stone .....I have no chance of Bedford on there 85 db days, the car has 1 18" x8" silencer then a further 4.5" round at about 16" long and finally a 9" x3.5" round all 3" bore it's side exit with the exit pointing towards the ground, the track days I am doing are private hire /invite only so are a bit more laid back but I don't want to upset them too much.

September 15th on a Saturday is actually good for me

i will look up the rest of the info you have posted later on, my other concern is with he noise limits is what my car makes on drive by as I have a very vocal turbo and straight cut gearbox to add to it all as well!

cheers

Dan

 

[watoga]

WATOGA DAVE : re-packable silencers you can take apart fairly easily because of the design. most normal silencers are seam welded/sealed and would need cutting open repacking and then sealing up again.

have a chat to these guys - http://www.thermalvelocity.co.uk/ - i have no experience with them i just googled - my line of work really involves huge industrial silencers but the science is still the same with a car.

Great, thanks for the reply. I'll take a look at these guys (and similar) to see what's best. I'll also be interested to see your frequency analysis for your own 3 tests once you get it completed.

Many thanks,
Dave

PS - thanks for employing punctuation in your posts................ makes the world of difference!

 

[mreguest]

Adam, the tracks will be brands,castle coombe,silver stone .....I have no chance of Bedford on there 85 db days, the car has 1 18" x8" silencer then a further 4.5" round at about 16" long and finally a 9" x3.5" round all 3" bore it's side exit with the exit pointing towards the ground, the track days I am doing are private hire /invite only so are a bit more laid back but I don't want to upset them too much.

that should be plenty of silencing, (i'm gonna guess 90dBA at 4000rpm.) i remember thinking at northweald your car wasn't scary loud like typical mongoose setup.

September 15th on a Saturday is actually good for me

15th september is a 'Sunday' which i suggested, i'm free all day... is this what you meant rather than 'Saturday'? (saturday 14th is possible but would have to be really early, i need to be in london for about 4pm)

i will look up the rest of the info you have posted later on, my other concern is with he noise limits is what my car makes on drive by as I have a very vocal turbo and straight cut gearbox to add to it all as well!

We can measure drive by noise if we can find a suitable venue. Ideally we need a road with fields either side to mimic a track. Probably straight road because officially you need pedal to the metal acceleration in third gear from about 50mph onwards over a straight line distance of roughly 100m between points A & B. The sound level meter would be on a tripod in the field 20m away from the side of the road halfway between points A & B, if that makes sense? You approach point A at a steady 50mph and then put your foot down, you need to be doing more than 60mph as you pass the microphone and when you hit point B chuck it in neutral, slow down and stop. We do the test a few times reducing the distance between points A & B to see how the levels change if at all.

 

[mreguest]

dan - why <100dBA at 4000rpm? i can't find a reference to that - it seems like most places suggest a dBA value at 3/4 of max rpm?

see here:
http://www.clubmsv.com/car-trackday-noise-limits.aspx
and Bedford
https://secure.motorsportvision.co.uk/MSV-info/trackday_terms/Bedford Autodrome Trackdays Terms.pdf
Brands
https://secure.motorsportvision.co.uk/MSV-info/trackday_terms/Brands Hatch Trackdays Terms.pdf
Oulton
https://secure.motorsportvision.co.uk/MSV-info/trackday_terms/Oulton Park Trackdays Terms.pdf
Cadwell
https://secure.motorsportvision.co.uk/MSV-info/trackday_terms/Cadwell Park Trackdays Terms.pdf
Snetterton
https://secure.motorsportvision.co.uk/MSV-info/trackday_terms/Snetterton Trackdays Terms.pdf

 

[red reading]

Yep Sunday I ment sorry, castle Combe is the first one and that is a private hire, they stipulated 100db at 4000rpm to us that are going as originally it was mk1/2 escorts which redline at 6.5krpm as stock cars (which none of us are stock), since the sprint day I have changed the exhaust as it had an issue with the flexible coupling collapsed (which did suppress a lot of noise) and it is louder now comes the but....it's a lot louder at tick over but as the revs go up with a warm exhaust it may well be quieter.....also bere in mind I am running a vent to atmosphere wastegate which means when the car is on boost it is louder too.

and as for the drive by tests, I think I know a few places we can go

 

[mreguest]

Nice one - Sunday 15th is in my diary - i'll PM you to get address/time etc...

Castle Combe - looks like you you might need 100dBA at 4500rpm unless you're sure 4000rpm is on your private hire day, easy enough to test both rpms, they don't imply a drive by test and my gut feeling is you would be fine at both rpms:
http://www.castlecombecircuit.co.uk/trackdays/carinformation.asp

Interesting you suggest a warm exhaust may well be quieter. There's some truth in that temperature can have an affect. Cold is typically a little quieter than hot because of a heat related frequency shift in moving air vs the performance of the silencers. ( maths - http://jcaa.caa-aca.ca/index.php/jcaa/article/download/784/497 ). Always test hot/normal running temperature.

It will be interesting to see how much noise you get on the drive by tests, am i right in saying an atmospheric vent wastegate has what some people call a screamer pipe? sounds like a jet plane! if it turned out to be a problem i'm sure you could weld up some pipe to turn the noise away from the exhaust side of the car and use a little motorbike silencer along the way...

 

[mreguest]

I'll also be interested to see your frequency analysis for your own 3 tests once you get it completed.

Note the three different tests where: 1) straight through 2.5" mongoose with decat, 2) 180mm long secondhand decat silencer & 3) 180mm long decat + 500mm brand new back box. For each test the car engine had no more or less modifications and was running standard boost/fueling.

Each were tested at idle ~1000rpm and again at 3/4 rev range 5625rpm.

Unfortunately each test happened about a year apart so there are other factors to consider such as: engine fueling, possible vacuum leaks, spark plugs quality, degradation of decat silencer, idle fluctuation, panel rattle on the car body, ambient air temperature, engine/exhaust temperature, etc...

You can see in the results that at some frequencies the silencers inserted would appear to make it a little louder at some frequencies. (WRONG) If the tests were all done on the same day at the same temperature on an engine test rig with no panel rattle, in a perfect world every addition of a silencer should be quieter than before at all frequencies.

Also note that at 5625rpm it looks like the 180mm decat silencer has more of an effect than a 500mm silencer. Not true. If we tested only a 180mm and then only a 500mm, the 500mm would win easily. Length wins, width helps... (That's what she said ) To understand this further, having two silencers of equal performance put inline does not give double performance out. In layman's terms 5+5 is not equal to 10, you'll find 5+5=8 :wtf:

IDLE:


5625rpm:


cheers!

Adam

 

[red reading]

Nice one - Sunday 15th is in my diary - i'll PM you to get address/time etc...

Castle Combe - looks like you you might need 100dBA at 4500rpm unless you're sure 4000rpm is on your private hire day, easy enough to test both rpms, they don't imply a drive by test and my gut feeling is you would be fine at both rpms:
http://www.castlecombecircuit.co.uk/trackdays/carinformation.asp

Interesting you suggest a warm exhaust may well be quieter. There's some truth in that temperature can have an affect. Cold is typically a little quieter than hot because of a heat related frequency shift in moving air vs the performance of the silencers. ( maths - http://jcaa.caa-aca.ca/index.php/jcaa/article/download/784/497 ). Always test hot/normal running temperature.

It will be interesting to see how much noise you get on the drive by tests, am i right in saying an atmospheric vent wastegate has what some people call a screamer pipe? sounds like a jet plane! if it turned out to be a problem i'm sure you could weld up some pipe to turn the noise away from the exhaust side of the car and use a little motorbike silencer along the way...

Yep it's also known as a screamer, funnily enough I have just bought an extra motorcycle silencer for exactly as you have said, the tempreture thing is just something I noticed that when the car has been standing being made of steel it gets very moist inside from condensation when it has previously cooled.....it sounds to me to be a fair bit louder till the car has been run up to temp properly.....could that be the case?