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MG MGB Technical - Exhaust noise

I have a single box big bore exhaust, it sounds great, I want to try a track day. But there is a noise limit of 100db at half a metre from the tailpipe. I think it may fail, is there anything I can do to temporarly to reduce it.
c cummins

It's worth doing a noise level check if you can get your hands on a meter. Failing that the only thing that you can do is to fit the second box to be sure.
Iain MacKintosh

It is common practice to insert a socket or tube of some kind to restrict the noise for the test, then remove it for the track. The test is usually conducted at 45 degrees to the pipe and at around 3/4 rpm, it is also not unusual to discover rev counters over reading for this very purpose :)

The tail pipe bore being narrower than the rest of the system is the way ex system manufacturers get rid of a lot of the noise.


Remember that every 4 dB is double the noise( something to do with an exponential function).

Peter
P Burgess

bit of trivia

a bel means simply twice as loud to the human ear

bel from Bell Laboratories (Alexander Graham Bell)

a deci (tenth), so decibel is tenth of a bel

as a youngster I thought about replying to a BBC advert for a trainee sound engineer but it meant working nights and I was too thick anyway - so I can't say that 4 db isn't double the noise and don't want upset Peter anyway (keep yourself safe, my car is falling apart just at the wrong time, as usual)

sound meters are cheap enough to buy if you're thinking of doing many track days
N Atkins

Doubling the power, or in this case doubling the sound pressure level, is actually a 3db gain, not 4db.
Steven 67GT

Hi Steven

Thanks for putting me right with 3 not 4 doubling. I was trying to point out how a 'small' number difference with dB testing is a massive difference in sound level. Our main involvement with metering has been keeping the rolling road noise down to acceptable levels for our neighbours. It was interesting working with Environmental Health trying (and finding it impossible)to isolate the noise specifically from our unit. Testing approx 60 yards away (other side of the road which services the industrial estate we are on) all passing traffic was the same noise-type and 'louder' than us. So we do not annoy too much the housing estate the other side of the service road we do not run after 5 O'clock and we do not dyno at weekends. Previous people complaining have been told by the council we actually have planning permission for 24/7/365 working (we are in an old pit building) so we are trying to be reasonable whilst earning a living. Life is a matter of compromise.

Nigel

All trivia is most interesting to read and hopefully tucked away in the recesses of the mind till required. I hope your Midget is ok for the visit to us on Wednesday. The water will be hot in the urn so you can wash away the road dust with some coffees or teas.

Peter
P Burgess

Merlin Motor sport (Castle Coombe) do a extra silencer that fixes over the tail pipe, looks like a German stick grenade this reduced the noise by 8db on my Sprite.
K Harris

K Harris, thanks for that, Castle Coombe is where I intend to go. How much over was your Sprite, and did the potato masher strangle much power?
c cummins

I was 104 db at 4500 after fitting 96 db, yes it did drain at bit of power but not enough to ruin your day.

Just see what it reads on the day my mate's RS went through with no problem on a single box andI thought he was louder then the Sprite but the metre said he was OK.

Since fitted a new backbox (the wadding has'nt burt out yet :-) )
K Harris

O/T - Peter the car will make it to you unless I attempt to repair it and that goes wrong - as if !

tea is never refused thank you although a toilet may be handy as a pub visit will be included sometime that day subject to whether the wife's car goes in for bodywork en-route to you
N Atkins

This thread was discussed between 04/06/2011 and 05/06/2011

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