Repack silencer, what noise reduction?

[Dave Bilyk]

I just checked my noise level in preparation for the Sussex GT at Goodwood which will be a 105dB at 3/4 max revs event. I got 106-107 at 4850rpm , so I think that repacking the silencers would do it. These are simple straight through silencers as supplied by DAX all those years ago,so I wouldn't really expect to get a big margin, just enough.
So what is the experience out there, can anyone let me know what reduction would generally be expected?

regards
Dave

 

[Dave Bilyk]

bump
I just checked my noise level in preparation for the Sussex GT at Goodwood which will be a 105dB at 3/4 max revs event. I got 106-107 at 4850rpm , so I think that repacking the silencers would do it. These are simple straight through silencers as supplied by DAX all those years ago,so I wouldn't really expect to get a big margin, just enough.
So what is the experience out there, can anyone let me know what reduction would generally be expected?

regards
Dave

 

[Jason Lindsey]

Have you thought about using some Supertrapp slip on silencers? I used these on my GTD and they made a big difference.

Jas

 

[Dave Bilyk]

Thanks Jas, will look into it.

Dave

 

[Jason Lindsey]

These are the ones I used

Supertrapp Disc Only Clamp On Silencers - Demon Tweeks

I made up a pipe that slipped over the exhaust pipe and used exhaust clamps to hold them on. I used 6 discs and it dropped the noise level by 5 db at 3/4 revs.

I liked my car to be noisy, so this temporary measure worked well for me.

Hope this helps

Jas

 

[Dave P]

Dave:

Years ago on my Oval track car I had to repack the muffler. It had a Coleman 5" straight through muffler. It was much quieter after repacking. The packing before was almost all burned out. But I have no idea on decibel drop but seems your only looking for a little. Also turndowns pointed at the ground may help some.

 

[Dave Bilyk]

Jas,
5dB is a decent reduction, did it change power at all?
I looked on the Supertrapp site and saw some others too some of which are internal to the pipe, I don't really want to change the look from the back, so I have a couple of ideas I might try first. It should be easy to test simple inserts that I can make myself. I'll let you know how I get on.

Dave, that's interesting, my car has done about 6000 largely gentle miles, so I have no idea whether the packing has deteriorated, but then again, I have no idea what was put in there in the first place. The mufflers don't have cover plates, so I have to cut-out holes and make covers. I may do the insert idea first, and if it works delay the repacking until later. So many things to do!

Dave

 

[Dave Collins]

Dave,

I have just used acousta-fil to repack my exhaust boxes and this appears to have good write ups as far as audio performance goes with people claiming several db drop over standard material. It was very easy to handle and fit.

Also I know that several people use Merlin Motorsport additional silencers and I may investigate a pair myself if car is too close to 105. I have another track session at Castle Combe before the Sussex GT so should be able to get car sorted with sufficient silencing.

https://www.merlinmotorsport.co.uk/p/merlin-mild-steel-racing-muffler-m-ex-muff-4

Look forward to meeting up in May.

Dave

 

[Dave Garner]

I repacked my GTD silencers with acoustafil 2 years ago. From memory I got a 4 or 5 dba reduction, which put me just below the crucial 105 dba in the standard noise test.

When I took the back off the silencers to put the acoustafil in ALL the original packing had gone (see photos).

I checked the packing last month and the acoustafil is still there and looking fine after a reasonable amount of track driving.

I have not tried the 'insert discs' type add on silencers but some of the tracks do not permit the use of removable silencers, so it might be worth checking.

 

[Keith]

By the way, and sorry for the intrusion but I have a TESA Professional Sound Meter (with on-board calibration) that anyone is welcome to borrow along with a tripod that will hold the meter at any angle from .5 metre height.

Borrow FOC.

Would have to be picked up and returned to me @ Lymington but I could arrange to meet at Rownhams Services near M3 Junction on M27.

If it's any good to anyone.

 

[Dave Bilyk]

Intrusion? from you Colonel Mustard, never! A very kind offer. (I know you didn't do it btw, you're so kind you couldn't kill anyone in the kitchen or anywhere else for that matter)

I messed about with bits of metal up the exhaust for a few minutes, and thought, well this won't work without strangling the power, so spurred on by Dave P, and Dave G's comments, I marked off the silencer to install a hatch. As soon as I drilled the corner holes, I could see there was no packing left, so I guess that's what I need to do. From Dave C, Acoustafil seems good, very clever that the threads melt and allow the matting to expand, so I guess the Daves have it all sewn up between them! Thanks Guys! :thumbsup:
Dave C, look forward to seeing you and your car in May!

Dave

 

[Dave Garner]

In relation to noise meters.

Ian Cowan and I both bought low cost noise meters from Mapin and ebay respectively.

Ian 'calibrated' them using his GT40 at a Vehicle Inspection Center against their 'evidential' noise meter.

The unit from ebay was 5dba 'out' around the 105dba mark and thus mainly useful as a fishing weight. The Mapin unit was within 2dba which is 'close enough' to be useful - provided it is set to high range. (leave it set on 'low range' and it under-reads by approx 5dba - which is not helpful)

It is worth remembering at track days noise tests that our engines have a surprisingly LOW red line, often only 5250rpm and thus 75% full revs is 4000rpm...... it is probably worth spending some time practicing your 'honest face' ahead of the noise test............

Dave

 

[Graham T]

From a racing perspective only, I have yet to find the same db reading at any UK race meeting for years. With both the 917 and 40 having such different engines and noise characteristics I have had constant polite run ins with the noise scrutineers, a major problem as well with so many other "historic nature" cars.

I,ve yet to know if it is the meter itself or the totally different circumstances of operative, weather, or surroundings that give such varying results but, oh, can they can differ !!!

As Dave has just said ... put on an "honest face" and also cross your fingers if you decide to go racing, oh, and bribes don,t work !!! Another thing to realise is that circuits not only have static tests pre practice and pre race but the also have drive by meters and you can be black flagged !!

 

[Ron Earp]

Besides different meters reading differently, the location, distance, and enclosure of the meter can have profound effects on the recorded sound level. We too see this at a number of tracks.

And beyond that, the weighting of the measurement is paramount. There is no telling what weighting your meter operator might choose, or if s/he even understands anything about weighting. It's been reported that C, D, and A have been used at various tracks we race at over the years, making comparisons between levels at different tracks difficult or impossible. From Wiki on weighting, surely there are other more race oriented descriptions available on the web elsewhere.

Frequency weighting[edit]
The IEC 61672-1:2013 mandates the inclusion of an A-frequency-weighting filter in all sound level meters, and also describes C and Z (zero) frequency weightings. The older B and D frequency-weightings are now obsolete and are no longer described in the standard.
In almost all countries, the use of A-frequency-weighting is mandated to be used for the protection of workers against noise-induced deafness. The A-frequency curve was based on the historical equal-loudness contours and while arguably A-frequency-weighting is no longer the ideal frequency weighting on purely scientific grounds, it is nonetheless the legally required standard for almost all such measurements and has the huge practical advantage that old data can be compared with new measurements. It is for these reasons that A-frequency-weighting is the only weighting mandated by the international standard, the frequency weightings 'C' and 'Z' being optional fitments.
Originally, the A-frequency-weighting was only meant for quiet sounds in the region of 40 dB sound pressure level (SPL), but is now mandated for all levels. C-frequency-weighting however is still used in the measurement of the peak value of a noise in some legislation, but B-frequency-weighting - a half way house between 'A' and 'C' has almost no practical use. D-frequency-weighting was designed for use in measuring aircraft noise, when non-bypass jets were being measured and after the demise of Concord, these are all military types. For all civil aircraft noise measurements A-frequency-weighting is used as is mandated by the ISO and ICAO standards.​

As residences grow ever-closer to tracks sound will be important to all of us, even if we're only interested in a track day and not racing.

What's the sound level max at most UK tracks?

 

[Dave Bilyk]

All interesting stuff guys.
A couple of comments;
The A weighting is designed to mimic the sensitivity of the human ear so it is the weighting that should be used for this purpose and I would expect it to say that in whatever standard or procedure is applied to the noise test. The other weightings will give a higher reading so it is in our interest to ask what weighting is being used.

Regarding measurement differences other than weighting, I have done significant work investigating differences between vibration measuring instruments in the past. Perhaps there are similar issues with noise meters; What happens is that different vibration meter manufacturers have different measurement philosophies, the one that makes the biggest difference is True RMS measurement, or Peak Scaled RMS measurement, they will both give the same reading for a pure sinusoid, but for impulsive vibration the Peak Scaled measurement will give a higher reading (Peak/RMS crest factor comes in to this). Since exhaust noise is impulsive, so these sort of differences could also exist in noise meters.

Dave

 

[Dave Garner]

Typlically UK tracks run at static noise test levels of:
Unsilenced (rare and EXPENSIVE)
105dbA
102dbA
98dbA
95dbA (tough but achievable - see penultimate para for how)

The higher the noise limit the more the day costs!

A number of tracks also (or only in Donnington's case) have track side monitoring. Donnington quote 98dbA on thier track side monitor - but they say they cannot relate that to the 'standard' static noise test. I have yet to be tempted by thier 'turn up and see if we black flag you' approach.

I suspect that rear engined cars fare worse on the static noise test because you have the engine noise added to the exhaust noise, but I don't have any measurements to back that up!

In practical terms getting below 105dbA is a 'must'
Getting below 102dbA gives access to more choice of tracks & dates
Getting below 98dbA involves 'wishfull thinking' and
Getting below 95dbA involves taking the plugs out, draining the fuel tanks to save weigth and running round on the starter motor.

I am just under 105dbA with the original GTD 'silencers' packed with Acoustafil

Dave

 

[Keith]

The TESLA meter I have offered here is a professional unit with on board calibration and is accepted by all Local Authorities for Entertainment and Noise At Work regulations. As such it is also capable of complex LaEq measurements.

Not a Radio Shack fishing weight or iPhone App by any means.

Putting the record straight. Available to borrow FOC from Hampshire - handy for Thruxton, Goodwood or the scenic route to Castle Coombe.

 

[Dave Bilyk]

I finally found time to repack the silencer with Acoustafil. I had to cut a hatch in the top, weld on some steel strip, drilled and tapped to attach a cover plate. Freshly packed I got a larger reduction than I expected from 107dBA at 4750rpm to 98dBA at 4750rpm, so I expect that any change after a few miles will not exceed the 105dBA limit.
At 3750 rpm it was 93dBA and at tickover 85dBA.
I guess that the big reduction was because there was no significant packing left when I opened the silencer up.

Oh, I used a Bruel and Kjaer 2260 precision sound level meter.

regards
Dave

 

[Dave Garner]

Wow Dave, 9dbA is a serious reduction!

When I checked back I got a 5dbA reduction using Acoustafil in my (empty of packing) GTD silencers.

Don't forget the surprisingly low max rpm for the GT40...........(and the 'honest face' when saying it)..................

Croft are doing more 105 dbA track days this year - if you fancy meeting up now you are 'noise compliant'.............. and I NEARLY have a working clutch!

Dave

 

[Dave Bilyk]

Dave, at Goodwood on monday 101dB 3750rpm although I did overshoot to 4000ish so a good result. Keep me informed if you are considering going to Croft.
DaveB