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the Obsidian Files - So Much Noise!!!
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n7188u

Joined: 15 Nov 2015
Posts: 395

Posted: Fri Mar 17, 2023 10:29 am Post subject: Re: the Obsidian Files - So Much Noise!!!

I used the Noico material since I already had it (purchased a while back but never used due to the weight). And now it is installed. Unfortunately we have a few days of bad weather and wind coming so may not be able to test but will try just before sunset today if the wind dies down a little.



Regarding the material to use on the firewall, flammability of the material needs to be considered. I did a vertical flame test on the material I used in the airbox and the flame did not propagate. After the starting flame is removed it will burn for a short time but not support it (quits). I don't think it is any worse than the resin the airbox is made of but not 100% sure of course. And even wonder if it was the adhesive that was burning. But if there is a fuel or oil fire in the engine compartment this stuff will certainly not help. And the fumes may be another problem. So I would not consider this material unless more info on this becomes available.



I will do a more aggressive test later this weekend using a torch. Want to make sure the material will not support s flame a I saw earlier today.

BTW, I still think this stuff is way too heavy. But heck, if I can be as impressed as A51 is, I'll go on a diet to compensate 



Here are some pictures of the install. Looking forward to testing!



Best,

Chris

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Area-51

Joined: 03 May 2021
Posts: 372

Posted: Fri Mar 17, 2023 11:02 am Post subject: Re: the Obsidian Files - So Much Noise!!!

Nice job; you don't muck around!



Your airbox is slightly different and looks like your engine is possibly set forward?? 🤷🏼‍♂️



Technical data on the butyl is provided on the DCI website in support pdf document; or via emailing them. Be mindful though it is shielded by the foil so this prevents air supporting flammability, but at the right temperature butyl will liquify into a tar type goo. If the situation is getting that hot you will likely be having bigger issues to worry about than the butyl igniting.



Looking forward to hearing the results...

n7188u

Joined: 15 Nov 2015
Posts: 395

Posted: Fri Mar 17, 2023 11:36 am Post subject: Re: the Obsidian Files - So Much Noise!!!

A51,



I didn't muck around on this one because I am so desperate to see if this work and make my airplane quieter that this morning I woke up before the sun came up and couldn't sleep anymore so went to the hangar and got to work  Your excitement was certainly contagious and I am very curious to see if it works on mine as well as it did on yours.



Regarding the different airbox, my kit was a mix of bits and pieces scattered all over the country (and world). The airbox was missing but a kind soul in England send me an old one which is what you see here. However, when I tried to install it the thing's size was so off that the top cowling would not fit by a lot! I had to Frankenstein the airbox and chop about an inch of the top and glass it all up again. So yes, it is certainly custom.



Chris

Area-51

Joined: 03 May 2021
Posts: 372

Posted: Fri Mar 17, 2023 4:52 pm Post subject: Re: the Obsidian Files - So Much Noise!!!

Ahhh, that makes sense.. yes, i fitted some buffer pads under mine and had to remove them back to none because the top cowl was standing up too proud after...



Today i flew again for 75min and noticed that since installing this mod i am no longer feeling fatigued after i shutdown and exit the cockpit... prior after about 45min flight time i would start thinking about wanting to land and rest.. last three flights i have not had any thought about landing to rest... feeling a lot more connected to the aircraft now.



Rolled out on 23 as usual, popped the flaps and undercarriage away and rotated at 90kt into a steep climb at 5000rpm holding at 72kt; watched the dial hit 1000' in a blink, then cruised around at 75kt again... figured, hey i got nowhere to actually go, why burn all that fuel and load everything up?!🤷🏼‍♂️ Did a few more steep climbs up to 2500' then 4000', flew the inbound leg on 72kt simulated engine failure, made the circuit entry at 1000' on a 600fpm decent

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clivesutton

Joined: 23 Jan 2013
Posts: 187
Location: KENILWORTH

Posted: Sat Mar 18, 2023 2:48 am Post subject: Re: the Obsidian Files - So Much Noise!!!

Hi Area-51,  I follow your posts with much interest - not least because you have a handle on the engineering physics as evidenced by your simulation images.



I was going to say that whilst adding mass coating is one route to improvement (Bergers Law 1911), isn't a set of noise-cancelling headphones a more optimum or at least, initial, route to achieve the same thing - and at less weight gain?.



Not a criticism, just interested to know why the additional mass route.  It will work if you add enough mass, but the carb rubber wear/unknown fire risk increase and general weight penalty does seem a bit onerous.



That said, the tinkering/problem-solving with these things is almost as much fun as the flying, of course !

Area-51

Joined: 03 May 2021
Posts: 372

Posted: Sat Mar 18, 2023 5:48 am Post subject: Re: the Obsidian Files - So Much Noise!!!

These are great discussion points, so let's break them down a bit...



The weight penalty factor v's noise cancelling headphones:



I've added 1100g stationed 50mm ahead of the firewall; my W&B has been marginally affected... The Bose Active X headsets weigh 0.485kg (1.1kg with carry case), the Clarity Aloft weigh 0.135kg (0.320kg with carry case)... The AUW penalty is now the added mass less the gain; we are now at around 0.4kg weight penalty... but hang on a minute, let's now fly two up! Bose headsets with carry cases 2.2kg, Airbox mod with  Clarity Aloft headsets and carry cases 1.75kg... just lost 0.45kg!! 🤷🏼‍♂️... thats two  more $100 hamburgers!! 😀



What about the carbi mount rubbers??? 181 is a Classic Mono, the air box is supported by the firewall, space frame, and the upper engine cowl panel... Additional load on carbi mount rubbers "Zero"... 🤷🏼‍♂️



Ok, now lets consider energy factors.. no more batteries required for headsets; saving the planet two batteries at a time... 🤔🤨



"Fire Factor", Risk = Low, Impact Occurrence = Medium to High... Survivable? Maybe 🤷🏼‍♂️



Now "Comfort Factor"... hmmm.. chalk and cheese; leaving it to personal preference... personally i like cheese over chalk unless i'm using a plumb bob... i like to eat cheese when its relatively quiet; and now i feel it will be a great feature to eat cheese while i am flying in the relatively much more comfortable situation of having the noise canceling dealt with at the source end rather than the user experience end.



Now "Pilot Performance Factor"... firstly i now exit the cockpit after 90min feeling like i could of just kept on flying the aircraft. Secondly my concentration level and situational awareness has expanded due to the reduction in ambient noise distraction. I am hearing new noises in the machine that were previously drowned out by the noise of the induction system; which since purchasing 181 has been my biggest source of discomfort.



Now "Wow Factor", when i pull the lid off the engine bay everyone will see the shiny silver air box; humans are distracted by and like shiny things, they will fail to notice all the wear & tear scratches and marks around the motor... they will walk away after thinking "wowwwwww".



Noise canceling headphones are a compensation for the inability, or probably just shear laziness, to affectively deal with the issue; unless of course the user loves to revel in the audial regime of the race track. Thanks, but no longer the regime for me; been there, done that, no longer wish to be there....



Its kind of a bit ironic but since doing this mod when ever i have selected for noise cancelling i turn it off after about 10 seconds thinking to myself "that sounds horrible" 😊🤔🤔🤷🏼‍♂️

clivesutton

Joined: 23 Jan 2013
Posts: 187
Location: KENILWORTH

Posted: Sat Mar 18, 2023 9:25 am Post subject: Re: the Obsidian Files - So Much Noise!!!

Thanks for your reply and i do understand your reasoning.



In the Automotive world (based upon ~20+ years in hands-on NVH development there), the other issues were 1) moving the resonating cavity/intake openings as far away as possible from the operators ear - usually to the front of the engine bay, as, like reductions from mass damping, distance makes about the same difference in attenuation (double the distance, halve the sound power), and 2) working to try and tune the intake pipework so that it does not resonate at the typical cruise engine RPM.  Most cars have intake resonators that take this problem away - some of them are even closed-loop, measuring the tone and then adjusting the effectiveness of the anti-resonator to minimise it.



I wish we had the freedom to tinker here in the UK that you have - changing a nut and bolt can invoke cries for the mandatory need for a scrutinised MOD required, here.



And I dearly wish i still had access to the rather expensive test equipment i could get my hands on back then, a Campbells diagram [Sound level or Vibration amplitude vs both RPM and frequency scales - for both engine Vibration and emitted noise vs RPM for e.g. a Rotax engine installation in a Europa], could tell us all so much - why exhaust springs break so easily, why the noise in the cabin is what it is, and what the optimum engine RPMs would be to minimise those etc.

Area-51

Joined: 03 May 2021
Posts: 372

Posted: Sat Mar 18, 2023 2:45 pm Post subject: Re: the Obsidian Files - So Much Noise!!!

Oh woww!! Excellent... I totally agree with everything you have imparted... The job is yours, you're hired; report back in month's time 😂😂😂



Where can we liberate this said test equipment from???? We need to know where to put the Helmoltz resonator!!



Flew again today; the results are very consistent.. next stage is an internal baffle for the air box... an extended forward running induction tube was considered however this would end with the inlet port facing forward instead of upward; so any noise emissions would be directed out front like an air horn as apposed to an upwards direction away from the ground below. However so far i am totally satisfied with the result as is, but will take it further with the internal baffle and hear what happens 🤷🏼‍♂️



Any technical comments toward the "shut that noise up" project is helpful for Europans..

budyerly@msn.com

Joined: 05 Oct 2019
Posts: 282
Location: Florida USA

Posted: Sat Mar 18, 2023 6:07 pm Post subject: the Obsidian Files - So Much Noise!!!

As Clive alluded to, resonance in the opposed 4 banger exhaust is an issue as well as intake.  The thin fiberglass footwell directly under the feet of the pilot or passenger, especially in the Classic with the exhaust pipe exiting directly aft under the pilots feet, resonates something fierce and sounds just like the carb inlet.  Creighton Smith's 96EG underwent a surgery of the heavy Classic Exhaust and the aft lobe was removed, and a straight pipe added.  What a racket that made initially.  We changed the tuning length and put in an expansion pipe (nothing sophisticated, just cut the pipe and put a wider pipe on the end to change the note).  Creighton was a motorcycle racer, and he did an excellent job softening the sound.  What a difference.  Still louder than my 914 but it didn't bring the airport to a stop on ground runs any more.



Prop pressure pulses across the aircraft windscreen in the Europa are not that bad, so I have not been concerned with that as in other light aircraft.  However, any looseness in the doors hinges, latches and the D panel in the back are very large drum skins and the low frequency drone is annoying enough and fatiguing, but the rattle is irritating beyond description.  Prop power pulses as I call them, is normally not bad on a three blade that is tapered.  The two blade wide chord blades can definitely be felt and heard as even the XS nose tends to transmit some of that back to the cockpit.



Personally, having a 914 and leather interior makes most of my aircraft remarkably quiet on the inside.  The 912S and 912 versions I tend to prefer a rework of the inlet air geometry to break up the intake noise and as far as the exhaust note, I leave it to Robin at CKT Engineering for the 912S exhaust note.  It is amazing what a bad baffle in the exhaust sounds like.  Many don't realize they have a bad silencer baffle.  CKT has done a nice, but expensive, Rotax 912 down pipe exhaust and side exit that makes the little fart can engine sound very quiet.  My opinion for what that is worth.



Noise is fatiguing.  ANR helps the ears, but the vibration of the various noises hitting the body is fatiguing.  The Europa has that phenolic type firewall that is very thick and I do not do any heat or soundproofing on it.  I do add a thin stainless steel plate some 4-6mm proud of the pilots footwell on some of the 914s as this cuts the heat on the feet.  Proper air venting is needed down here in the South.  I will not do a painted only interior because it is loud and sound bangs around all through the cockpit with nothing to absorb it.  I do not do any firewall sound proofing on a trigear because I build in a firewall aft of the nose gear covered with firewall 2000 (excellent fire and sound proofing).  The mono is a different animal.  I find the metal firewall an excellent sound transporter and amplifier.  The airflow through the slots is also a bugger.  I have tried the soft and hard Lord mounts and frankly didn't see any change in apparent vibration (purely measured by feel)  Props make a difference as does exhaust plumbing and support.  Nothing worse than an exhaust system in contact or firmly attached to the frame on a 912S.  The 914, you don't have a choice on the mounting.  But in its 15-20 years it has had a few turbo support cracks repaired.  



One of the reasons I went with the stock kit.  Experimenting to fix issues is time consuming/expensive and well, an experiment, sometimes it works, sometimes it doesn't and hopefully, it doesn't add weight.



Keep up the good work.



Bud Yerly



--

Area-51

Joined: 03 May 2021
Posts: 372

Posted: Tue Mar 21, 2023 1:24 pm Post subject: Re: the Obsidian Files - So Much Noise!!!

Yep, that D panel at the back of the cockpit along with its bulk head sure sounds like a brazilian festival drum... that got attended to not that long ago by fitting a 1/4" ply brace for the luggage ties to the reverse face of bulkhead; made a noticeable difference.



181 has a CTK exhaust fitted and i find its performance quite acceptable; DCI SheetHot 5000 had to be fitted to the lower cowl as it was burning and getting pretty close to igniting. The footwell area does not get that warm.



I have been receiving some pm's regarding the air box mod; its turning out a common grievance for many other europans. Regulations in the UK may possibly be mitigated by reviewing the mod as a "surface treatment" much like painting, as the box itself is not being modified; just my thoughts here not recommendations to depart from any regulations, especially any fire risk prevention regulations.

Area-51

Joined: 03 May 2021
Posts: 372

Posted: Wed Mar 22, 2023 3:54 am Post subject: Re: the Obsidian Files - So Much Noise!!!

Experiments and discoveries are moving forward over the past few days with some entertaining results.



A rudimentary study of some baffle concepts were drafted and run through the simulator; the variations happened by accident due to forgetting to half the density consumption to rpm speed; so the elongated porting was developed and employed on these over calculated equations of x2 actual rpm; once recalibrated the results were pleasing on all the variations.



Volumetric mass flow analysis through the alternate air boxes was applied at 6000, 5500, 5000, 4000 rpm at 1013.25 atmosphere at 20.05C. Filtering for Static Pressure primarily, and Acoustic Energy and Flow Path as added features.



Red represents Ambient Atmospheric Pressure which indicates zero restriction to air flow within the air box at the test engine rpm.



Testing revealed the carburettor porting tubes are slightly undersized causing a pressure drop in these regions at engine speeds above 4000rpm on normally aspirated setups. At 6000rpm the pressure drop equates to approx 600' altitude increase, and at 5500 rpm approx 300' altitude increase, and at 5000rpm around 80' altitude increase. I would not be overly critical of this restriction as there will be many other factors left unconsidered such as inlet manifold pressure pulses, additional pressure flow from the naca vent on the engine cowling, and others...

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n7188u

Joined: 15 Nov 2015
Posts: 395

Posted: Wed Mar 22, 2023 6:00 am Post subject: Re: the Obsidian Files - So Much Noise!!!

Very interesting conversation guys. Part of what makes owning an Europa fun is this type of interaction.



I have not been able to fly the airplane enough to reach a conclusion on this most recent mod to my plane. I only flew around 20 minutes last Friday, was windy and bumpy so had other worries in my mind. Tomorrow AM looks nice so will do a short local before work but really want to do a longer x-country to be able to provide an objective conclusion (on the subject of fatigue).



But I hate to report that based on that short flight last Friday I did not find that the mod made as large a difference as I was hoping for based on A51's  encouraging results. But again, the jury is still out. 



I think I do detect a slight difference. Maybe a little softer on the high frequency. And I think it is worth keeping the change. But I was hoping for more. Will report again after I get a few longer flights in.



Please do keep in mind that I have a paint only interior and, as Bud indicated, I too agree this is a good reason why my Europa is loud.



Clive, I still need to go through all the tech discussion in detail (I am at work dealing with other tech discussions) but I did buy a new Bose A 20 when I started flying my Europa. It is incredibly effective at cutting all low frequency noise (prop) but all the high frequency ones seem to go right through making it VERY annoying. In fact, I sometimes switch to my non-ANR headsets and find the low frequency drone much more pleasant (but yes, it is louder). 



So I will continue evaluating and will provide some final thoughts later on. And all feedback on this subject is greatly appreciated.



I think that if I try something in the near future it will be lighter noise insulation on the metal firewall, some quick prototype level interior paneling to see if I can see a change before commuting to that route, and I really wanted to try a wheel pant I have for the mono, maybe in hopes of deflecting air away from the wheel well and reducing turbulence in that area. 



Chris

peterz(at)zutrasoft.com
Guest

Posted: Wed Mar 22, 2023 6:24 am Post subject: the Obsidian Files - So Much Noise!!!

Fwiw,



Classic, mono, no fairings, panted interior, 912s, stock exhaust, factory cold air intake plenum.



My biggliest bang was the softer lords swapout. Before with the stock rubbers the gear whine and harsh compression pulses excited the cabin surfaces and the engine was the most overbearing noise.  Post rubbers, the gear whine is below the engine, and the engine is below the 130mph cruise air noise. e-props unique sound signature is more noticable than the WDs when pulling throttle back. I doubt i would have heard that with the stock mounts. High freq engine noise is far below the wind noise.  Bose do a fantastic job now. 



E-props reduced the large start stop excursions (and obvious gearbox dogging) experienced with the WD, nullifying my previously required exhaust and oil reservoir clearancing efforts with the softer lords.



Unhappy with bose high freq atten? Try the lightspeeds for comparison lol.



I also get a noticeable buffet from my unfared mono wheel, but havent got around to fairing it…..watching with interest if you do.



Cheers,

PeteZ

 	  Quote: 	
		   On Mar 22, 2023, at 10:09 AM, n7188u <chmgarb(at)gmail.com> wrote:

 

 

 

 Very interesting conversation guys. Part of what makes owning an Europa fun is this type of interaction.

 

 I have not been able to fly the airplane enough to reach a conclusion on this most recent mod to my plane. I only flew around 20 minutes last Friday, was windy and bumpy so had other worries in my mind. Tomorrow AM looks nice so will do a short local before work but really want to do a longer x-country to be able to provide an objective conclusion (on the subject of fatigue).

 

 But I hate to report that based on that short flight last Friday I did not find that the mod made as large a difference as I was hoping for based on A51's  encouraging results. But again, the jury is still out. 

 

 I think I do detect a slight difference. Maybe a little softer on the high frequency. And I think it is worth keeping the change. But I was hoping for more. Will report again after I get a few longer flights in.

 

 Please do keep in mind that I have a paint only interior and, as Bud indicated, I too agree this is a good reason why my Europa is loud.

 

 Clive, I still need to go through all the tech discussion in detail (I am at work dealing with other tech discussions) but I did buy a new Bose A 20 when I started flying my Europa. It is incredibly effective at cutting all low frequency noise (prop) but all the high frequency ones seem to go right through making it VERY annoying. In fact, I sometimes switch to my non-ANR headsets and find the low frequency drone much more pleasant (but yes, it is louder). 

 

 So I will continue evaluating and will provide some final thoughts later on. And all feedback on this subject is greatly appreciated.

 

 I think that if I try something in the near future it will be lighter noise insulation on the metal firewall, some quick prototype level interior paneling to see if I can see a change before commuting to that route, and I really wanted to try a wheel pant I have for the mono, maybe in hopes of deflecting air away from the wheel well and reducing turbulence in that area. 

 

 Chris

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=510476#510476

n7188u

Joined: 15 Nov 2015
Posts: 395

Posted: Wed Mar 22, 2023 6:34 am Post subject: Re: the Obsidian Files - So Much Noise!!!

Interesting Pete, 



Since I will probably be removing the engine for painting in the next couple of weeks, does anyone have any dimensions for the new spacers needed to retrofit the Lord mounts? I remember Erich telling me a while back that getting the engine to rest in the correct position took a bit of effort.



I could give this a try. 



Chris

n7188u

Joined: 15 Nov 2015
Posts: 395

Posted: Wed Mar 22, 2023 6:51 am Post subject: Re: the Obsidian Files - So Much Noise!!!

to add one interesting anecdote regarding the noise in my Europa, people tell be that when I fly overhead to enter the pattern (midfield downwind entry is common at my airport due to a Class C at one end of the field) my airplane makes no sound. They can only hear what they think is just wind noise and makes them think it is an electric airplane. Good job CKT.



But my airport is riddled with very noisy airplanes (RVs, SX300s, Bonanzas, T34s, P51s, AT-6s, etc) so I think that even if I had no muffler people would think my airplane is quiet 



But inside .... another story. Although I once took a friend up for a ride and he thought I was crazy and that the airplane was not that noisy inside. But he has an F1 Rocket so he may not be a good judge of what a small Rotax powered composite airplane should sound like 



Chris

peterz(at)zutrasoft.com
Guest

Posted: Wed Mar 22, 2023 6:54 am Post subject: the Obsidian Files - So Much Noise!!!

From a past Erich to me thread:

“the new spacers are .063” shorter than the originals.  Final length of the spacer is 1.592”.  As you noted the ID of the Lord mounts is larger at 1/2” vs 7/16”. To keep the engine in the same location relative to the cowl I installed a stock AN washer (.063”) between the engine mount and cup washer of the Lord isolator.”

Note Erich has the low compression 914, vs my 12S so his start stop shake is less.

Also, my classic mount and tight exhaust clearances meant i had to tighten them up at just over 1.5” when running the heavy WD, in addition to lowering the oil can through the cowl, and clearancing the footwells.

Cheers,

PeteZ



 	  Quote: 	
		  On Mar 22, 2023, at 10:43 AM, n7188u <chmgarb(at)gmail.com> wrote:



--> Europa-List message posted by: "n7188u" <chmgarb(at)gmail.com>



Interesting Pete, 



Since I will probably be removing the engine for painting in the next couple of weeks, does anyone have any dimensions for the new spacers needed to retrofit the Lord mounts? I remember Erich telling me a while back that getting the engine to rest in the correct position took a bit of effort.



I could give this a try. 



Chris





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=510478#510478

n7188u

Joined: 15 Nov 2015
Posts: 395

Posted: Wed Mar 22, 2023 7:23 am Post subject: Re: the Obsidian Files - So Much Noise!!!

Thanks Pete. Good info.

Chris

Area-51

Joined: 03 May 2021
Posts: 372

Posted: Wed Mar 22, 2023 2:58 pm Post subject: Re: the Obsidian Files - So Much Noise!!!

Choosing not to fly today; instead thought toward identifying drumming areas of the fuselage; details attached along with required acoustic patch sizes. Some areas of hatches and footwell floor are only accessible for treating during building; footwell floor void could probably be filled with expander foam though. Total added mass can be estimated from sizings. Sizings can be reduced up to 50% in strip width and still remain affective. Total added mass is around 12kg if using the more dense 2.3mm butyl sheeting... about the same mass if using the lighter thicker 5.7mm polyester sheeting at 100% sizing.



Have tried attaching an pdf study paper on induction noise reduction theory.



Mid to forward L/R of centre upper cowl - 4"x10"



Mid to forward L/R of centre lower cowl - 3"x12"



Naca vent slider door rattle



Behind naca vent area to L/E of wing - 6"x10"



Fuselage, forward foot well floor step area - 4"x4"



Forward hatch fuselage area - 2"x8"



Mid centre line to rear hatch hinge - 2"x14"



Vert stab rib to fuselage fillet - 4"x30"



Vert stab trim bar port area - 4"x8" upper 2"x8" lower



Fuselage upper lower seam, fuel filler to L/E of h'stab - 3"x50"



Fuselage upper 1/8 quadrant, fuel filler to L/E of h'stab - 4"x45"



Fuselage belly, flap hinge covers to static port - 6"x40"



Hatches, lower frame 1"x 12" and rear corner sections 6"x8"x8" triangle

Induction_IJRESM_V1_I10_115.pdf

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Area-51

Joined: 03 May 2021
Posts: 372

Posted: Sat Mar 25, 2023 6:39 pm Post subject: Re: the Obsidian Files - So Much Noise!!!

Today is Sunday and the conditions are useless again for flying.



So its back into "Shut That Noise Up" project...



In this post 181's air box is getting analysed for harmonic resonance. The test passes frequencies from 1hz to 20,000hz through the 1.5mm fibreglass component to see where points of dynamic distortion will be at specific frequencies; blue is zero red is maximum.



Four frequencies were isolated from the pack to review what is going on at 5600 / 5200 / 4000 / 3600 rpm. The data provided correlates to where the air box body is "resonating" at specific rpm. The purpose of this is to identify focused points of resonance that can be individually attenuated for specific rpm ranges if desired as opposed to wrapping the entire box.... image order correlates to rpm list above.



The tests reveal that all areas of the air box resonate at one or more of the applied engine rpm frequencies; so complete attenuation of the air box is acceptable.



Further acoustic analysis shows the stock air box is generating 67db of sound at about 4900rpm; image attached below. this volume gradient is fairly linear as rpm increases and diminishes with distance from the source; volume gradient falls off from 5000-7000rpm. Placing a microphone probe 200mm above the air filter port reads 25db, while placing the mic probe off the rear wall reads 21db with volume on most everything below 4000rpm dropping off sharply.



This test is only the response of the air box itself to a frequency. It is not yet being subjected to induction generated acoustic pressure waves.



From these tests we can ascertain how to manipulate the stock air box to absorb induction noise frequencies from the carburettors.



Each version of modified air box will require the same analysis procedure to determine which design is performing the best.

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n7188u

Joined: 15 Nov 2015
Posts: 395

Posted: Sun Mar 26, 2023 5:04 pm Post subject: Re: the Obsidian Files - So Much Noise!!!

Today I finally had a chance to get a longer flight in. It's hard to say how much improvement the install of the sound deadening material on the air box has on the internal noise. But there is certainly improvement!! The flight today was very pleasant and more enjoyable. But what really struck me is that, as A51 experienced, I had to lower the intercom volume quite a bit since it felt too loud. Same thing when I was playing music as I generally do. So, even though I can still hear relatively high frequency sound, but maybe less, it seems that overall there is a reduction in noise.



But, I have to admit this is all just based on perception. It is indeed hard to quantify the change. But I have to say that despite the increase in weight, it is a keeper.



On a different note: I finished rebuilding my spray booth and getting closer to starting the painting process. I will hold off taking the airplane apart this week to see if I fly it to Sun n Fun but by next weekend it will come apart so no more flying until it is done. I also have a couple of pending jobs I want to perform on the airframe.



Chris

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