Big Brother Posted June 9, 2003 Share Posted June 9, 2003 Is it just packing - or is there a "hidden" compartment? I recently had an interesting chat with an acoustic engineer. His specialist subject is designing silencers for a variety of applications (inc Jet engine test cells 😬) Having discussed "free flow" type silencer that work by absorbing sound waves into "rockwool" type packing around a perforated tube, he went on to mention "resonance" type silencers. It seems these can be included into the design of the above and can be tuned to remove certain frequencies. These in simple terms (well as I decyphered from the technical stuff ) are a seperate bit of the silencer that is left empty. The sound waves (at a pre determined frequency) enter the resonant chamber and are free to expand - this in turn helps to cancel out some of the remaining sound. Apparently the end result is an overall quieter exhaust with a negligable increase in back pressure. Out of interest - do any of the currently available silencers offer this - or is it not practical? I guess as the silencer needs to be "tuned" to the engine/application, that it would not be a "one size fits all" product. Still, its got me thinking Steve Se7en-Up! Less is more! Eagles may soar, but weasels don't get sucked into jet engines! Link to comment Share on other sites More sharing options...
puremalt Posted June 9, 2003 Share Posted June 9, 2003 Haven't a clue, but MZ had designed a killer exhaust for their 1950's 2 stroke that varied the gas velocity, acting as an efficient exhaust vacuum, ensuring that there was no 'dirty' combustion within the cylinder. Bears no relevance to you but I wanted to show that I knew something! A paltry 99.9dB Link to comment Share on other sites More sharing options...
Big Brother Posted June 9, 2003 Author Share Posted June 9, 2003 😬 😬 😬 Steve Se7en-Up! Less is more! Eagles may soar, but weasels don't get sucked into jet engines! Link to comment Share on other sites More sharing options...
Mike Bees Posted June 10, 2003 Share Posted June 10, 2003 Most (if not all) 7 silencers are simply a can with a perforated tube running down the middle and rockwool (or similar) packing in between the two. Mike Link to comment Share on other sites More sharing options...
michaelw Posted June 10, 2003 Share Posted June 10, 2003 I have just ordered packing for my SBD exhaust and have recieved what I can only described as a blanket of wadding. I don't know whether this will be better than before. My exhaust had been packed too tight I think as there is a too much of the wire wool around the perforated tube and the rockwool has hardly been touched even though it pops and bangs and everybody I speak to was convinced the wadding was missing as it was so loud. Is it possible to pack it too tightly and can you put anything in teh perforated tube to "encourage the exhaust gases into the wadding if that makes any sense? Link to comment Share on other sites More sharing options...
Big Brother Posted June 10, 2003 Author Share Posted June 10, 2003 AFAIK the wire wool is there purely to protect the wadding from being "blown out" of the exhaust. Michaelw, when you had your exhaust apart - did the wadding completely fill the silencer can or was there a seperate "compartment" that is left empty? Steve Se7en-Up! Less is more! Eagles may soar, but weasels don't get sucked into jet engines! Edited by - steve motts on 10 Jun 2003 10:46:06 Link to comment Share on other sites More sharing options...
Big Brother Posted June 10, 2003 Author Share Posted June 10, 2003 Somebody must have repacked a silencer recently. (or unpacked 😬 Andy) Is the repackable type silencer filled completely or is there a seperate bits that left empty? Very interested now - and need the info to impress my friend at the pub tonight Steve Se7en-Up! Less is more! Eagles may soar, but weasels don't get sucked into jet engines! Link to comment Share on other sites More sharing options...
michaelw Posted June 10, 2003 Share Posted June 10, 2003 No it was just crammed full of wadding. I guess my point was I thought the wire wool was so dense no exhaust gases would ever reach the wadding and it would effectively act like a solid tube within the silencer. My exhaust is either 3" or 3.5" ID I can't remember. Going on the compartment idea, my understanding was that one way to silence a car is to slow the exhaust gases down to reduce the energy within the gases. If this is the case, does the larger bore mean quieter as the gases would be moving more slowly? This doesn't sound right to me but the large diameter may be act like a compartment. All this does boil down to the fact that I want to go on 100db track days and at the moment I am running at 102db. I have spoken to powerspeed and the guy very kindly said that he couldn't see any reason why my silencer should be so loud and not to buy a new one off him until I had re-packed mine - very unusual to turn down business and reasurring. Michael. Link to comment Share on other sites More sharing options...
Simos Posted June 10, 2003 Share Posted June 10, 2003 Hi Steve, mine was also packed full of wadding around the central perforated tube. The tube was wrapped with steel wool although I would describe it as sparse. Obviously enough to stop the wadding being damaged and so if it is being described as 'lots' then it is probably too much or at best more than is required. Empty chamber to absorb a specific frequency is fine but would not be applicable to a road car where the revs and thus frequency are changing the whole time. Sounds more like it would be suitable to static engines like generators etc which run at constant revs for long periods. Just my view. There would of course be an application to get you paast scrutineering at 6k rpm on a track day as long as they didn't check you on a drive-by Link to comment Share on other sites More sharing options...
Big Brother Posted June 10, 2003 Author Share Posted June 10, 2003 This thought had crossed my mind Steve Se7en-Up! Less is more! Eagles may soar, but weasels don't get sucked into jet engines! Link to comment Share on other sites More sharing options...
AMMO Posted June 10, 2003 Share Posted June 10, 2003 Steve The stock Caterham 5" silencer has a resonator chamber in it at the back of the silencer (I know because I poked a hole in one to have a look). Mike Riley and I did discuss the possibility of incorporating one in our silencer. We came to the conclusion that it was better to have as much length of perforated core exposed to the packing rather than a resonator. Ease of manufacture and cost was another motive not to have one. No repackable silencer I have ever seen has had a resonator chamber in it. Is that enough to impress your friend or do you need more info? 😬 AMMO Link to comment Share on other sites More sharing options...
Big Brother Posted June 10, 2003 Author Share Posted June 10, 2003 Ammo, I suppose to get the chamber idea to work as efficiently as possible, the sound output of the car/bike it was to be fitted to would need to be analysed and the chamber "tuned" to the individual frequency that was loudest. I am interested in some of the comments about the amount of steel wool and packing material used. Is there any rule of thumb guidelines for the density/amount used. My friend is under the impression that a very high density (almost a solid block) of packing would be most efficient. This is based on his experience of building acoustic panels for noise reduction in jet engine test cells. I guess that a car silencer will be a bit different as its located a lot closed to the source of the noise – but then again I’m no acoustic engineer. This was plainly obvious when he started going into the theory of how silencers work 😬 Steve Se7en-Up! Less is more! Eagles may soar, but weasels don't get sucked into jet engines! Link to comment Share on other sites More sharing options...
I.Mupferit Posted June 10, 2003 Share Posted June 10, 2003 Steve, Whether or not the same principle applies I don't know but, my company makes attenuators for the air conditioning/ventilation industry as part of our product range and as your friend suggests the norm is to use rockwool that is as dense as possible (just like our employees 😬). We normally specify RW65 (65 kgs/m2) but, of course, you don't get variable airspeeds so these are only designed for a relatively small frquency range. Errr.......that doesn't really help much does it 🤔 Brent Link to comment Share on other sites More sharing options...
michaelw Posted June 10, 2003 Share Posted June 10, 2003 I would agree that more packing means more efficent as the air that gets into the wadding (if you see what I mean) would be slowed more but if it was too tight then no air would then permeate into the wadding at all and hence no silencing. Going off on a tangent is there any way you could have a butterfly valve half way up the perforated tube to direct more air into the wadding and thus have a silencer that has an adjustable ammount silencing. If this is a great idea than I am patening it. If, as I suspect, this is a bad idea I can only say I am the most dangerous of engineers i.e. one that has gone to college on an engineering HND and then done nothing with it. Link to comment Share on other sites More sharing options...
IanT Posted June 10, 2003 Share Posted June 10, 2003 Going off on a tangent is there any way you could have a butterfly valve half way up the perforated tube to direct more air into the wadding and thus have a silencer that has an adjustable ammount silencing. Friend of mine at work has a brand new 911 with the "Sports Exhaust" option. Sounds extremely fruity and, at a Bedford trackday a couple of weeks ago, he only just passed the static noise test (which, for a standard production car, is loud). The really good bit is that a simple button-press in the cockpit makes the exhaust whisper-quiet. Clever. You should sue Porsche immediately Ian. Link to comment Share on other sites More sharing options...
michaelw Posted June 10, 2003 Share Posted June 10, 2003 Hmmm not sur eabout that, but I did see a silencer years ago with a similar arrangement but it directed air through a chamber instead looked hideously in-efficent though. I think this effectievly what the Ferrari 360 system does as well - not sure though as I think that is to seperate chambers. Link to comment Share on other sites More sharing options...
Big Brother Posted June 10, 2003 Author Share Posted June 10, 2003 Demon Tweeks 2003 catalogue - page 215 Supertune Adjuster Looks just like a butterfly that is fitted within the exhaust. It can be adjusted externaly to "tune power and noise output" After reading the recent post on backpressure, I'm not sure this would be a good idea. From what I can see (I'm NO expert) this product reduces noise by increasing back pressure - inturn reducing power. A bit like a Supertrap. Also - messing with the exhaust back pressure = messing with the intake mixture (on a carb car at least) I guess you get nothing for nothing 😬 Steve Se7en-Up! Less is more! Eagles may soar, but weasels don't get sucked into jet engines! Link to comment Share on other sites More sharing options...
michaelw Posted June 11, 2003 Share Posted June 11, 2003 I have got a supertrap but I am worried about the mixture problem. I would like to do 100db track days and I don't really mind losing a bit of power temporarily to achieve this but I am worried about the engine. I suppose this is an age old problem and will invariably end up leading to an expensive purchase of a quieter silencer unless you can reduce noise without having to adjust the mixture. Michael. Link to comment Share on other sites More sharing options...
Simos Posted June 11, 2003 Share Posted June 11, 2003 'packing as dense as possible...' Taken to its logical conclusion you'd end up with just a steel tube and no silencer... or not ? Methinks a jet engine cell may be be tuned to a small range or frequencies also. Afterall turbines run in a small range of revs, nominal revs is often 90% 0f or = to max revs. They are very unsuited to varying revs or load. To look at it another way if you discount the jet engine cell and look at a recording studio where the walls are coated in sound deadening stuff to prevent echoes ruining the recording - you will see tiles of hardish foam but shaped into lots of pyramids each facing a different direction. The idea is the foam absorbs the sound energy eventually but more importantly the flat surfaces reflect the sound in different directions so you do not get a clear echo and interference can reduce it still further. My thoughts are that in the silencer the pressure pulse from the exhaust cycle whizzes down the tube and splits up passing through the purforations and wire wool (hence just enough is best) and into the rockwool. The rockwool has a very high surface area and complex topology giving lots of opportunity for the gases to slow down and give up their energy before making their way back through the purforations and out of the exit. The gas flow is clearly a constant for a given rpm but the high energy spikes describing the individual cylinder detonations have been smoothed yielding a lower peak energy at your ear and thus quieter overal sound. If I'm not talking bollocks (yeah right) then in addition to the usual increasing diameter, increasing the length and perhaps tapering the bore casing (larger) toward the exit would optimise the pressure drop. I do realise that techcraft et al just make the biggest you can fit down the side of the seven so the above is irrelevent but we're just discussing arn't we ? As a further thought, if we're taking pressurised gas into the rockwool to slow it down; I wonder if varying the density of the wool down the silencer would have more effect than a single density. Like a four stage step down. Think of wrapping the tube with say 4 full-depth wraps of different density with the lightest closest to the exit. This is sort of a hi-jack of the initial compartment idea but perhaps better suited to a wide frequency range. I seem to recal Rolls Royces having 4 silencers between headers and exit but separated by tube. [/bull****] Link to comment Share on other sites More sharing options...
Big Brother Posted June 11, 2003 Author Share Posted June 11, 2003 Simos, I read you post with interest and showed it to the guy I am working with (the acoustic engineer) He said to point out the following "very common misconception" As the exhaust gas flows through along the exhaust silencers perforated tube, there is no reason for it (the gas) to pass through the perforations. Just like electricity – the gas will always try to take the path of least resistance. As the gas has velocity it will simply travel through the pipe (as long as there are no restrictions or severe bends) and come out the open tailpipe end. However, the sound waves generated by the movement of these gasses will pass through the perforated tube and be "absorbed" by the packing. It is true that the denser the packing, the higher the resistance to gas flowing through it. But this is apparently not the same for sound waves. He also recons that solid steel tube packing may be a bit on the heavy side for use on a Seven 😬 😬 😬 He mentioned that (as Ammo and yourself concluded) normally "bigger is better" when it comes to silencers but apparently only up to a certain point. The more absorbent material you can expose the sound waves to - the greater the drop in "noise". Hence longish silencers such as "the Goodwood". There are however acoustic limitations in the size of the packing chamber surrounding the perferorated pipe. When you get up to a certain diameter of can - any bigger will have no improvement in sound attenuation.....but making the silencer longer...that’s another matter. I was showed the theory/maths behind this...and it was bloody scary *tongue* He also said the reason for the wire wool is there is not to provide protection from heat. Its there to stop the packing fibres from being sucked out by the exhaust gasses as they travel through the silencer. Apparently there has been quite a bit of talk of manufacturers experimenting with Nomex cloth to perform this roll. This would provide quite a high resistance to the gasses but acousticaly offer virtually no resistance to the sound waves. Finally he said that he sort of agreed with your theory about "layering" the packing although for a completely different reason. I will "try" to explain his thinking (and probably make a total pigs ear it 😬) Imagine a standard “free flow” silencer. "noise" enters the silencer one end at 110db As it moves along the silencer, some of its energy is absorbed into the packing material. By the time the “noise” reaches halfway down the silencer its down to say 90db. When the “noise” exits the silencer it has dropped to around 80db (Goodwood here we come 😬) Now to silence 110db – you would need XXX amount of absorbive packing surrounding the perforated tube. But to silence 90dbdb – you would only need XX amount. And to silence 80db – you would only need X amount. Putting this in real terms means that you could design a cone shaped silencer that would still be as efficient as a standard shape one but would weigh less 😳 (Would look a bit funny though) Another thing I almost forgot.....goes back to what Michaelw was saying about directing more sound into the packing. In industrial silencers (seen a pic - VERY BIG silencers) sometimes a "bullet" is used. This is a bullet shaped.."shape" made out of perforated metal and fixed using vanes in the centre of the perforated tube. The gasses just pass through/around it, but the sound waves are bounced off it and are deflected into the perf outer tube and then into the packing material. Going to stop now as I just said "perf" and am beginning to sound like I know what I'm talking about. Now thats scary! 😬 Steve Se7en-Up! Less is more! Eagles may soar, but weasels don't get sucked into jet engines! Edited by - steve motts on 11 Jun 2003 18:27:01 Link to comment Share on other sites More sharing options...
AMMO Posted June 11, 2003 Share Posted June 11, 2003 Steve There are a couple of design features incorporated in the Raceco silencers that nobody has touched upon. I'm not going to tell you what they are. 😬 The bigger diameter being better up to a point is correct. The 7.25" silencer is quieter than the 6.25", that is a fact. I don't thing we will ever make anything bigger as it would be plain ugly. We are unlikely to find out what the maximum diameter for quiteness is going to be. You will reach a point were making the overall diameter of the silencer bigger is not going to make it any quieter. The length is very important. Our silencers are as long as we could possibly accomodate. They were designed as long silencers from the word go. The "Goodwood" silencer came out after ours. They obviously came to the same conclusion as us. I received some new high temperature blanket today and a woven tube thing. It is a small diameter tube that abrubtly turns into a large diameter tube. The idea is to pull the small tube over the core, pack loose material around it and then pull the large diameter tube of woven material over the whole thing and pop it into the silencer as one thing. Don't think we will use this as it seems a bit fiddly. The high 800 / 900 degrees C. blanket is three times the price of the 600 degrees C. blanket. This we will probably only use it on silencers that prove to be problematic due to their continous high speed use. We have experimented with taper cores on one of the prototype Caterham silencers. It didn't work so we never adopted it. It worked very well on an oval titanium Aprilia silencer I designed. This made more power and was quieter than any other brand of competition silencer we flowbench and dyno tested(Termignoni, Leo Vinci, Aprilia and Akropovic). Now discontinued as in the fashion conscious world of motorcyling people preferred to see Akrapovic on their silencer rather than Raceco. 🙆🏻 Michael Don't use a Supertrapp if you can avoid it. They kill power, screw up the fuelling and can make things overheat. One poor guy melted the back of his Elise when he fitted one to his car to get through noise testing at Hethel. AMMO Link to comment Share on other sites More sharing options...
Big Brother Posted June 11, 2003 Author Share Posted June 11, 2003 Ammo, Having never seen the inside of one of you silencers - but having read the description on your website I would guess there is possibly something "special" about the design of the perf tubing you use. Am I close 🤔 One idea my friend has come up to improve on the standard silencer with is to have 2 perf tubes in a single silencer body. The tubes would be a slightly smaller diameter to the inlet pipe and need a Y piece at each end.This would have the effect of acousticaly making the silencer appear longer - improving the sound attenuation. The down side is that the packing in between the 2 tubes would need to be of a similar thickness to the packing surrounding the outside of the tubes. To work properly the silencer can would need to be oval (when viewed from the end). It should work very well accousticaly but would look a little strange. Come to think of it, I've seen something that looks similar strapped to the side of a 1957 Lotus 7 😬 By blanket - do you mean the actual packing? Having never packed a silencer myself - or seen it done, I guess its just a case of stuffing the wadding in until its full. If its not a "trade secret" 😬, how densily (sp?) do you recommend your silencers are packed? Also, have you experimented with the types of packing used? Talking to my friend - he is still under the impression that the more dense the packing used, the better it would work. He agreed with Brent and recommends about 65 kgs/m2. He assures me this is a lot denser than the types of packing shown in the DT catalogue. Steve Se7en-Up! Less is more! Eagles may soar, but weasels don't get sucked into jet engines! Link to comment Share on other sites More sharing options...
michaelw Posted June 12, 2003 Share Posted June 12, 2003 Just as I thought I'm way out of my depth technically here - a little knowledge is a dangerous thing. I had assumed the packing slowed the gasses down but this can't happen as the same ammount of air has to exit the exhaust I suppose - ignoring the effect of back pressure of course. Steve - yes the "blanket" I was talking about is the packing on the SBD silencer. I appreciate it is almost impossible to tell but should 100db be achievable with most large silencers (6.5" O/D, 3" I/D about 2 foot long) or am I dreaming. There is another post from Marius I think that talks about the SBD silencer not getting below 105db, but I suspect he is running wild cams high compression etc whereas my VX is an SBD "standard" build. The information I have been given varies wildly. Ammo, thanks for the advice on supertrap, I only bought it as a back up going on the assumption I would try to get the silencer quiet enough on its own and take the supertrapp along just in case. But it looks so restrictive even with all the disks in I am tempted not to bother for the sake of my engine. Michael. Link to comment Share on other sites More sharing options...
AMMO Posted June 12, 2003 Share Posted June 12, 2003 Steve Not telling you if you are close or not 😬 We use 1.5 kilos of packing in the 6.25" / 160 mm diameter silencer which is 29.5 " / 750 mm long. I don't think we are approaching anywhere near 65 kg / M2. The blanket is the same stuff as the loose packing but in blanket form. Available in different thicknesses. The idea is tha you have packing material in contact with the whole of the core. If when packing the silencer you should leave a void or a less densely packed area this is where the material will start to burn. Using a blanket avoids this happening. Michael 24" is a bit short. AMMO Link to comment Share on other sites More sharing options...
Big Brother Posted June 12, 2003 Author Share Posted June 12, 2003 Just as I thought I'm way out of my depth technically here - a little knowledge is a dangerous thing. I had assumed the packing slowed the gasses down but this can't happen as the same ammount of air has to exit the exhaust I suppose - ignoring the effect of back pressure of course. I had assumed the same too....untill I was "shown the error of my ways" 😬 The exhaust gasses will pass straight through a "free flow" silencer and out the tail pipe. The packing is not there to absorb the gasses - its there to absorb the sound waves these gasses generate as they pass through the silencer. I was given a book to look at yesterday thats a primer for acuostics and noise control. There are more numbers in this book than letters! Its a very complex subject - one I know bugger all about 😬 Ammo, Not telling you if you are close or not 😬 Ill take that as close 😬 When you were developing your silencers, did you ever try to vary the density/type of packing used? Only asking as I was shown some test results for a 4" thick accoustic panel. The density and type of packing had a huge effect on the performance of the panel. I'm not sure how this would relate to a silencer on a car/bike but I have been assured the principles are the same. Steve Se7en-Up! Less is more! Eagles may soar, but weasels don't get sucked into jet engines! Link to comment Share on other sites More sharing options...
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