need to loose some DB's... help
 

ok folks

borrowed a db meter from work today to check noise for IVA

as i gather it needs to be 99DB or less at 75% of full engine power so for me that 7500 RPM sounds dificult alredy dosnt it

ok my problem is im getting about 117 DBs at the required distance and angle.

i think some of it is engine noise not exhaust noise so any ideas of a cheep is way to sound deaden my engine bay dont fancy they expencive matting at 60 quid a meter

also any ideas on how to drop the alcual exhaust noise i was thinking maybe stuffing it with wire wool?????

all sujestions would be great...

chears tilly

ps went for a quick drive round the block tonight only at about 30mph but felt good

well done on reaching the point of driving your car :cool:

may i ask if your exhaust can is a bike can that came originally with your engine?

if so here,s what we did when racing bikes ( some tracks are noise sensitive ), open up the can & put more exhaust padding ( not wire wool ) around the existing padding, if you remove the old stuff you will be even louder as the wool takes a while to "soot up", rebuild the can & hopefully it,ll help. ( the wool is approx £15 per roll ).

cheers
andy

If your using a bike can, try one of these,

http://cgi.ebay.co.uk/Motorcycle-EXH...item5884430802

or

http://www.demon-tweeks.co.uk/produc...ode=BIKEXCKILL

I have just ordered a DB killer in case I need it for this Sunday - you are more than welcome to borrow it to try.

"Position the vehicle within the authorised test area. With the sound meter horizontal, set the microphone height so that it is at the height of the exhaust outlet or 200mm from the ground, whichever is higher.
a.
At an angle of 45 degrees to the exhaust outlet in the direction which gives the greatest distance between it and the vehicle contour
b.
At a distance of 500mm from the exhaust outlet.
With the vehicle at normal operating temperature, run the engine at ¾ of its ‘maximum power’ speed, and note the sound level reading obtained."

Abstract from the IVA manual, check your distances are correct. But it states that it's 3/4 of the rpm at engine max power so you can get the test revs reduced by showing that the max power is at, say, 6000rpm so testing the noise at 4500rpm. Maybe take some documentary info stating this.

Should reduce the dB a bit, 'tis a lot to loose though 18dB

Regards....AndrewH

thanks guys iv taken a few of your ideas and modified them slightly really like the DB buster idea but i think i can make it more efective by custom making one with a blanked off end so it goes down the inside of the back box and is perforated so that the gasses are fored out threw the sound reducing materil surounding the pipe most of the off the shelf ones seem to still have a through pipe though it is a smaller diamiter, i am awere this is going to kill power but im not bothered for IVA i can loose it afterwards.

also i was talking to me old man about it last night and between us we came up with the idea of double skinning the inside of the engine bay then putting fibreglass insulation like loft insulation in the gap so kindo like outer pannel (aluminium) 25mm of fibeglass wool then the very thin stainless since it has supirior heat disapation qualitys and would not add to much weight i think we have a rool of 0.2mm or 0.1mm ss somewere anyway

also i was thinking carpet underlay on the underside of the bonnet to add some mass and dencity to prevent resonence of the bonnet

your oppiniuns would be great

chears tilly

You often get quite a lot of inudction noise. How is your air intake arranged? Generally, sound will eminate from openings like a big spherical ball - is there a direct line of site between the intake/sound meter? If so can you shroud the air intake on the one side to prevent the sound travelling in that direction. You should bear in mind that at the edges of the shroud, the sound will 'wrap around' the sharp corners but at a much reduce amplitude. Another little thing is that sound levels are not straight additions - best to look here http://www.engineeringtoolbox.com/ad...ibel-d_63.html
but basically if you have two noise sources, that are the same sound pressure level in dB, you will get a 6dB increase in the measured level.

Hope that makes some sense

on previous cars ive owned i always found wraping the manifold in heat proof tape also helps reduce the engine noise quite a bit, wont do much at the can tho, hope it helps ;)

It would not be the first ime I have used fiberglass insulation to stuff an exhaust can to quieten it down,another little trick we use to use on racing single seaters was a coke can over the outlet pipe just before the end and painted black. Worked a treat.

retard the ign timing if you can that can help if it doesn't mess up your emissions