Well, I'm working on getting the intake noise silenced, and certainly more quiet than it currently is on my car at the moment. Yes, I'm a wimp, and I get BS about it from my dad constantly.

Anyways, I've been throwing around the idea of somehow creating a common plenum style air filter box, similar to that of a 911 engine. Then came my physics class.. We're currently going in depth about standing sound waves, noise cancellation, and acoustics. Amongst this, the topic of Helmholtz resonators came up. Huh, well, that sounds familiar... "used to create destructive interference of standing waves in an internal combustion engine intake system"

The wonderful thing about Helmholtz resonators is that you are not sacrificing ANY air flow through the intake to silence the noise. The box is simply creating a resonating soundwaves with an exactly inverse wave that acts as active noise cancellation. Kinda like this:


For those of you that are not familiar with these on new cars, a Helmholtz resonator is this odd looking box sticking out the side of the intake tube:


The plan right now is to create a common plenum with two pipes leading to a Smooth T in the middle, and then the center pipe having a helmholtz resonator attatched to it. There was also some discussion with my professor that the distance between the carb throats to the resonator might be too great for the sound waves to cancel effectively. The idea of having 2 separate resonators on each connecting pipe is also being considered.

I won't get into the calculations on all of this, for the size geometry, and design of the box, but If I can get this to work, and really silence the intake noise, I will be manufacturing, and selling these systems, for FAR less than the only other common plenum intake system (790 Euros)

This will show the development and design of this system for those that are interested.

Cool

Interesting project, I'd just go the easy route and try to find some 912 airboxes...but I'm lazy

What you just posted is noise cancellation. If you have two sources of the same frequency and flip the polarity of one of the signals 180*, the apparent volume of the sound will decrease. If you combine two identical signals, in phase with one another you will effectively increase the apparent volume. If the signals are the same but do not reach the ears at the same time, you will have phase cancellation. If there are multiple frequencies at the same time, which is more natural than a single frequency, you will have comb filtering.

I think that is right...been 5 years since I've recorded an album of steeped into a studio.

Man I'm getting old, CRS is creeping in.

EDIT: Sorry Ethan, I just scanned your post before replying...you already knew wjhat you were posting about.

Cool Ethan, I'll be watching. I've been working on something similar, albeit without any kid of engineering or noise cancellation in mind. Good luck with it and keep us updated!

take a look at this.
Very popular in Germany to comply with strict sound pollution regulations.

http://www.csp-shop.com/cgi-bin/cshop2/fro...sort=0&all=

Thanks for all the support guys!

That CSP intake box was the one that I was talking about for being so expensive.. it sure is nice looking though. For my case (Broke ass college student) it's way out of my price range. I figure if I can put my head where my wallet would normally be, I might be able to come up with something that works.


Right now, I have a couple sheets of scratched out calculations for size/ etc. My real issue now is going to be measuring the frequency of the sound currently propagating from each weber. My issue is, as Black22 mentioned, is a multitude of frequencies. My plan is to aim for the cancellation of the most apperant frequencies. There's no way to completely cancel the sound, but I'm hoping to significantly decrease it's noticeability.

Well, came across some more information. Apperantly you can run a resonating chamber that is basically a one sided tube along the intake runner. The length of the tube should be 1/4 of the wavelength of the standing sound wave. When it reflects off of the closed end, and returns back into the intake plenum, it is exactl 180 degrees out of phase. Im picking up a sound frequency meter from a friend tomorrow, so i'll start taking data on my drive home friday.

The science is correct, the problem is the application. Maybe if you had a subwoofer you could create enough sound to cancel. The problem is that you don't have a single consistent tone it has variation in pitch and amplitude. The only way it would work is the wear a noise canceling headset.
We have an ANVS system on the Q400. It works, for the most part. However, it requires maintenance on a regular basis. Microphones go bad due to vibration, the shaker units become un-calibrated, and when there are pitch changes, you can tell the lag, which is irritating. In an aircraft, power changes are minimal, take off, climb, cruse and decent. In a car it will be much more pronounced. Your best bet is to buy a nice cancelling head set, strip out the mic, mounting it about ear level, and find a way to amplify the signal to accommodate an output great enough to have an effect. You'd be out about $300 for a headset+ cost for the speaker set up and other misc items.

http://q400nextgen.com/en/#/q400/technology/anvs/

Just a thought, but wouldn't ducting some intake tubes into the rear wheel wells provide enough distance from the passenger compartment to reduce the level of noise?

Adapting a centralized filter unit then routing to the carbs with something like these?

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Just make sure that your sampling frequency is at leaset double that of the frequency being measured or else you will have aliasing. Essentially your output frequency will be distorted and "appear" to have a longer period (you will be skipping some of the peaks and valleys.) Haha, this is what we are learning in my theory and design of mechanical measurements class right now! If you ever want any help with the designs or calculations im open MoWeFr and am in the library a lot of that time, and i am always willing to help out if i can. I like projects like this, they are fun and help apply some of the shit we learn in our classes. I am not extremely familiar with taking/analyzing these types of measurements but would love to learn as well!

I dont know what type of frequency meter you will be using, or what kind of option settings it has (if any), but if you want accurate calculations, these are some other things to consider.

look for the parts you may need, or you can make them from fiberglass with foam molds.

using a foam mold, shape it as you like, cover it with fiberglass, then carve out the foam when the shape is dry.

Consider using (for a home built copy of CSP) the turbo hats from CB performance, and a stock beetle airbox from an aircooled beetle.

Rich

Ya foam molds are always a good cheep (depending on the foam you use) way to get the job done! only problem is there is soooo much foam to clean up after, so have a shop vac near! And you can either do a wet lay-up or if the resources are available, vacum bag it.
Another alternative you can take advantage of is having the mold printed on one of the few 3D printers on campus... the requirements to be able to use it are not very lengthy and it is very inexpensive compared to what it would cost to have it done outside of school. I dont remember what professor you would have to speak to, but if you wanted to go that route later on i am sure i could get a name for you.

if you go the foam route, use the florest foam, the green stuff.
Its available at michaels or AC moore, or a florist.

It holds up to the resin, its easy to shape, and its easy to remove at the end.

rich

It's been done, low-tech and cheap, and more than a few times. Those turbo pressure covers are from CB Performance, and if I recall correctly were connected with silicone turbo hosing to an airbox/filter assy from some Japanese car picked up for a few bucks at a pick-n-pull. It all costs less than $100 and is said to work pretty well. Some enterprising soul should put together kits....

what about the goldwing air boxes that Ron invented last month
I think he bought two boxes and significantly improved his noise problem without cancellation, but with simple isolation

all for 100 bucks or something as I recall.

Seems like a lot of work to make a beutiful sound go away.

Maybe I missed it, but is your car a 6 or a 4?

A while ago posted about using a "silencer" on a 6 to make sound at Laguna Seca. Interesting project.

http://www.914world.com/bbs2/index.php?sho...amp;hl=silencer

Hey Elliot, did I ever thank you for those ear plugs you were handing out at WCR08? They worked great!

Yeah. They do work great. I use them mostly for the road noise. The sweet engine sound still comes through. My wife uses them when she sleeps. I'm not sure why?

Just use mine.

http://www.914world.com/bbs2/index.php?sho...=148543&hl=

huh? WHAT?

Silly question...

If you tune it for a certain frequency, how will you keep it from amplifying other frequencies?

Remember, the motor operates over a range of frequencies. If you tune it to cancel a certain frequency, you will also get other frequencies that it amplifying.

Could you use a low-pass filter to accommodate for that. Anything above XX Hz is filtered out...?

If you can make one that works at a reasonable cost...I am in. Old ears do not like lots of noise.

Do you have access to a portable spectrum analyzer to do your sound testing?

I picked up a couple of them from ebay sellers ... about $50 or so for both. Unfortunately, they sit pretty tall in the engine bay ... don't want to give up my rain tray. They look like they'd be great for those running without

For now, my phone has an app for spectrum analysis that gives me a fairly accurate reading for frequency. From that, i can calculate wavelength hopefully.

Right now, im considering whether or not a true noise cancellation setup is really needed.. if i can get something to work as a kit without it, that will be devloped..

Not sure how you would accomplish that.

Read this... http://library.thinkquest.org/19537/Physics5.html

Mark. Bring your GW air filters to Rt 66 if you. Want to sell them.
Mike

Hey Mike - I just went back to my ebay history ... they cost $60. including shipping for both of them.

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Not sure if I'm gonna make the RT66 this year (yet) ... but maybe I could entice a local from here to transport them if you're interested

There must be value in the intake resonators even though there are lots of sound frequencies to deal with because most new cars include them.

The big car companies must have this figured out, maybe you can find out what they are doing.

Maybe try cutting one off an intake from a 2 liter/4 cylinder car in a junk yard and graft it into your intake and do some sound level measurements.

John

Or go back to a stock plenum and add EFI.

or 912 air cleaners. Nearly bolt on.
Or beetle air cleaners.
There have been several threads for folks that have done this. I was looking at doin it a few years back but went for EFI and ditched the carbs. I know most like the noise of screaming IDFs at 4k, but I like to finish a 50 mile drive without a headache.

Do it. Make a kit you WILL sell them.
Zach

I built a cheesy intake a year or so ago to see if it would quiet things down and then tried to measure it.

This is the idle measurement with ITG foam filters. The ITG filters are really well made but by far the loudest I tried. For autox, DE, racing they seem like a good way to go.

Click to view attachment

Here is the intake I built so I could get some measurements. This is just welded up from 18 gauge. The round tube is 3" steel exhaust pipe from the flaps. I eventually wrapped the air boxes in dynamat.

Click to view attachment

(Air box tops are off in this picture).

Click to view attachment

Here is the idle measurement with the ugly box:

Click to view attachment

The RPM numbers on my timing light are not accurate in the picture because of the strobe effect of the old fashion LED digits. The last reading looks like 640 but it could easily be 880 but the camera did not capture the other segments lit.

It seems from my cheap sound meter that it is 6 db quieter. I photographed 6 or 7 measurements each way and these are about the averages.

Driving the car it does not seem that much quieter. The raspy edge you get when the throttles are open is gone, but the engine as a whole just makes a lot of noise. I think having some kind of insulated rain tray that blocks more of the noise from the engine would be more effective in making it quieter. I keep planning to build a thin rain tray and then line it with dynamat. Of course you need enough air opening to keep the engine cool and fed.

One improvement this air box makes is that your idle is smoother and stronger, I think because you have better air velocity at low RPM. I could not measure this effect but it was noticable in improved drivability at low RPM. I am sure the high RPM performance was hurt but I rarely rev past 5000 anyway.

I ended up making a nicer version that I need to take a picture of.

John

i love watching you all crack out on this stuff. i wish i was a little bit more advanced when it comes to these details. that is ok "a long journey starts with one step." haha

Your idea works nice in 2 dimensions. When you go 3d everything goes wrong and here are some simple issues you'll have ro deal with:
1. Your microphone is dirrectional and doesnt have the capacity to detect the sound wave propagation. All you can do is build a canceling wave that will cancel the noise at the mic.
2. Your speakers are dirrectional so if you want to cancel the sound in one place, you don't have much control over the other locations.

Mostly for the road noise...

--DD

I understand the inverted sine wave (negative noise) theory/practice to cancel out primary sound waves. MANY years ago we used 'negative noise amplification' technology for video duplication, but not sure why all the electronics are required for an automotive application, unless it's more of an engineering exercise?

Please continue with your study as I'm really interested in seeing the results ... I'm just leaning more towards a mechanical solution for my application.

Ya think?

Oh man!!! are you people talking about??

Any updates, Ethan?

Would love to see something like this for the IDF's with the addition of pre-heat as well. Both for the 914 & 72-79 bus application.

Man, I totally forgot about this..

My ideas were thrown in the garbage bin when this came along...

I just didn't need to worry about it anymore..

But I have had a few ideas that would fairly accurately mimic the design/shape of the factory air box for 6's, and could include a pre-heat if necessary.

Ideally, it would use the same airfilter as the 911.



Right now, I don't think this will be going much further, at least until next quarter whn I have some down time between classes to throw some ideas together. The only problem, is that I no longer have a carbed 4 to test fit things...haha

I do ... and it could be a guinea pig if need be

... or you can just make me a really cool carbon-fiber version of a stock '6' air filter housing, sized for two 40 IDFs