This proposal could open a proverbial can of worms.  Noise measurement (or I should say ACCURATE noise measurement) is very difficult.  Vehicle noise testing by manufacturers is carried out in proper acoustic laboratories. It cannot be done in an closed space like a garage MOT bay, simple as that.  Trust me, many years back I did a lot of industrial noise tests and this idea isn't going to fly.  One thing I get involved in these days is motorcycle race meeting noise tests and we do those to the best of our ability, but it isn't easy.  It's always done in the open, away from buildings.  I reckon for the MOT this is going to be down to the opinion of the individual tester to decide if a vehicle is acceptable. 

As for your degraded silencer, it's certainly the case that over time fibreglass (usually) packed straight through silencers do degrade.  The packing can either escape or consolidate.  It's fairly common for motorcycle racers to have to repack silencers to pass the noise test.  That's a much easier job on a motorcycle silencer where the end cap can be relatively easily removed than an MX5 back box. 

JS

Edited by user 10 October 2018 18:38:54(UTC)  | Reason: Not specified

My twin outlet cheapo off e-bay got through mot noproblems. No surprise there really it still sounds like a vacuum cleaner, not noisy at all.

Ah memories.  My ancient Ford with the tuned Kent engine would have easily failed that increased revs noise test; when the big second choke kicked in it went from family balloon mumble to hooligan snarl, building to a banshee howl.  Lovely. 

The original middle box on that had an in-pipe capped off at about 70% of the way into it by a baffle, an out-pipe similarly capped by another baffle, and an inner resonator pipe open the full length through the baffles.  The wadding around these three perforated pipes in the mid compartment was clogged up by the soot from the knacked previous engine, so I just punched through the caps to bend them back and made it a straight through exhaust with an S-bend in the box.

From the Driver & Vehicle Standards Agency ( Individual Vehicle Approval )

As I make, amongst other things, exhausts for race cars I do have a sound meter, but what might be of more interest to yourselves is that

the apps that are free to download for your iPhone are accurate to within 1dB of the meter I have.

Obviously if a customer is paying £xxxxx for his Lotus 20 system, or whatever, he doesn't want to see it tested with a phone, hence I have the meter.

I have my own can on my 2.5 Sport so if I get a mo I'll test it later.

Paul G

Originally Posted by: Gerryn

 

Afraid I disagree with this. Sound checks in a soundproof booth (acoustic laboratory) achieve nothing IMHO, the soundproofing in that environment also negates much of the sound anyway. I used to work as a Public address engineer, and sound checks should be read at 1M (max) from the outlet - in my case, a loudspeaker. or horn. I also helped install and test a language lab in a University sound proofed room, which deadens external noise and also internal, so I speak from experience.

Check any race circuit or sprint event. Under MSA rules sound checks must be made in the vicinity of the track, and read with a  (specified) sound meter held at 1M from the exhaust outlet. That F1 has it's own rule book is beside the point.

Donny some years ago, had it's sound level meter at the worst possible point, at the lowest point of the track, part of boarding and facing more boarding, and at the point where noise is at it's nosiest point, after the S just before the pits, and where everyone has their foot hard down to recover speed lost going through the off camber S. Exceed that, and you were red flagged. Meanwhile MSA scrutineers carried out their own tests elsewhere. Hopefully things have changed since then. Not all MSA scrutineers check about 1M from the exhaust, as I have observed. MOT test is as I stated above, whereas MSA checks are made at 2/3rds revs.

 

Absolutely, the whole point of an acoustic booth is that it isolates the sound source being measured. I was merely commenting that attempting to replicate such data in a normal/garage environment isn't possible.  Luckily, it's irrelevant anyway, as exhaust noise is not shown in the MOT manual as being a measured parameter; it's just down to the tester's opinion.  Yes, I agree most industrial noise measurement  is at 1 metre.  For reasons best known to the ACU they use 0.5 metre at 45 degrees to the outlet.  It's a static test at a specific mean piston speed.  We don't carry out 'drive-by' tests.  Circuit rules for community noise vary; for example Castle Combe have metering set up at the site boundaries as required by the local authority.  Luckily they are flexible as a dispensation a couple of years back allowed some demonstration laps by a Honda 6.  All I can say is it failed the normal ACU noise test.  It did pull in the crowds though.  

JS

This discussion about how the sound from an exhaust might be measured at an MoT testing station is all very interesting fellas, but I'm going to attack this subject from a slightly different perspective - how can sporty-sounding exhaust manufacturers sell their products knowing there is a possibility of cars to which they are fitted might fail the MoT under the new regulations ? 

It sounds to me (sorry, no pun intended), that as the amount of noise coming from an exhaust is purely down the the individual tester as to whether or not it is excessive, this opens up a potentially huge can of worms.  It seems to be a big grey area, and testers, only being human after all, and subject to the vagaries of human nature, might not be as consistent in their appraisals as one would hope, so catch him on the wrong day and you've had it ! 

Either testers should have strict guidelines as to how and where the noise from an exhaust is tested, and have proper equipment to assess this, or it should not come into the MoT at all.

And as for sporty-sounding exhaust manufacturers, should they have their product tested and perhaps certified that when fitted to a vehicle, it is in fact road legal ?

Originally Posted by: Chris Phillips

 

And as for sporty-sounding exhaust manufacturers, should they have their product tested and perhaps certified that when fitted to a vehicle, it is in fact road legal ?

This is what happens in most of Europe, ie. "TUV certified", which creates a black market for non-TUV approved parts. the most miserable experience seems to be in Norway.

Originally Posted by: Chris Phillips

This discussion about how the sound from an exhaust might be measured at an MoT testing station is all very interesting fellas, but I'm going to attack this subject from a slightly different perspective - how can sporty-sounding exhaust manufacturers sell their products knowing there is a possibility of cars to which they are fitted might fail the MoT under the new regulations ? 

It sounds to me (sorry, no pun intended), that as the amount of noise coming from an exhaust is purely down the the individual tester as to whether or not it is excessive, this opens up a potentially huge can of worms.  It seems to be a big grey area, and testers, only being human after all, and subject to the vagaries of human nature, might not be as consistent in their appraisals as one would hope, so catch him on the wrong day and you've had it ! 

Either testers should have strict guidelines as to how and where the noise from an exhaust is tested, and have proper equipment to assess this, or it should not come into the MoT at all.

And as for sporty-sounding exhaust manufacturers, should they have their product tested and perhaps certified that when fitted to a vehicle, it is in fact road legal ?

Couldn't agree more.  That's why I pointed out a definitive measurement wouldn't help because it's effectively impossible to make.  The problem is, most things in the MOT depend on the opinion of the tester and could be considered grey areas.  Phrases like 'excessive play', 'excessive movement', 'excessively deteriorated' etc are standard phrases occurring throughout the testers manual.  These things are all entirely subjective, so adding a subjective noise check is just another one to add to the list.

JS

Edited by user 11 October 2018 15:40:23(UTC)  | Reason: Not specified

Originally Posted by: Gerryn

Lot of contradictory Information being posted on this thread, one says No actual sound level test, and other gives specific details of that test, including microphone set up. I don't really see how this helps, other than the comment that "It's up to the individual tester to come to his own conclusion". Such being the actual reality, then we all need info as to which test station gives the best verdict (from choice of silencer POV.) If one is a bit deaf, or wears a faulty hearing aid, then that's got to be a good one! Within 50 miles would help a lot. My exhaust is now legal - unfortunately - - - -.

All I'd say is that I quoted the Govt MOT Testers manual, which confirms there's no definite test or measurement - it's the opinion of the tester, like many other things in the MOT.

JS

Was worried this topic would appear at some point! I read about two years ago that the government was trying to push a bill through to make it illegal to fit after market components to any car. This included exquisite, and the rules were going to be that any exhaust other than OEM would deem the car illegal, regardless of noise levels. The bill proposed that the fitting of any non standard parts would the mean the car fails the type approval it gained from the manufacturer when built.

The whole bill was flawed, as even getting a "standard" exhaust from the likes of quik fit or national etc would technically mean it fails to comply, as they get their parts from various manufacturers so tolerances would vary. The bill when read through was scary to say the least, and the documents the police had for determining what was allowed/illegal when carrying out a vehicle check were brutal. After market exhaust......car doesn't meet type approval. A/m wheels? Doesn't meet type approval. The list of things they were allowed to check was lengthy, and any fails could result in the car having a notice put on it and removed from the road.

Wish I could remember where I read it all, cos at the time I was like "they can job on if they think they can stop us modding our cars"! Seems this could be the start of things to come? And imo, this all boils down to one thing. They want all petrol/diesel cars off the road and want us all in stupid electric cars.

I wouldn't worry too much about the noise situation or any of the other plans the government is trying to impose. There are far too many implications for any of these plans to actually be enforced. If it fails, take it to another MOT station. If it still fails, dispute it. 

Oh, and for qualify exhaust, I'd recommend Powerflow. We got the cat back system custom made for our civic Aerodeck over 7 years ago and the sound is exactly the same as the day it was fitted. Exhaust still looks like new!

Dave