The Science Behind Quiet Exhausts (and Airliners)

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Loveless
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The Science Behind Quiet Exhausts (and Airliners)

Post by Loveless » Wed Oct 24, 2012 12:37 am

So I'm driving home one night in my S14 (recently sold) from Living Dreams on 28S towards 66. I stop at the light to turn onto 66E and i notice there's a police cruiser in the adjacent turn lane. I know my car is loudish so I do the obvious and when the light turns green I baby it like any other 240 guy would do. Turns out that was a waste of effort, got pulled over and ticketed for my dual N1 exhaust.... Frustration ensued. This was the 2nd time in a few months that I got bagged for an exhaust ticket.

This got me thinking, a performance oriented exhaust that doesn't sacrifice too too much performance. An oxymoron I know, most people in the car world know that a super quiet exhaust usually is more restrictive and heavier with dampening material, therefore a performance handcuff. This lead me to purchase the Megan Type II. It's a standard catback setup for the S-chassis engineered nicely, given it's relatively cheap price, to give an OEM exhaust look and sound but still provide the flow and added response that a performance exhaust should. I bought it, installed it, and right away it was noticeably quieter at lower rpm's and at cruise rpm's (25-3200). So I was satisfied. A few weeks later making the same trip down 28S to 66E I decided to give it some beans around an on ramp, not sliding it but just a little quicker and with more lateral-G. Was in 5th, dropped it to 3rd and started around the on ramp, long story short a police officer was in 8 or so cars back and well... pulled me for exhaust. *facepalm*

Rack one up for the po-po.. Here I had myself thinking I had a quiet enough exhaust not to get noticed (naive thinking I know) and still got bagged for the noise levels..

Now this is where my curiosity came in. Being a pilot and bit of a scientist I set out looking into what make sounds quieter. So I looked into research Boeing is doing on the new 787 "dreamliner" airliner. This aircraft is the most modern, efficient and quietest in it's class, but why the quietest?

Drawing from NASA's Glenn Research Center project of the early 2000's called the Advanced Subsonic Technology (AST) engineers and scientists from Boeing, the Air Force and NASA set out to create a quieter jet engine design that still gave them the same or better performance and economy.

More Info on the AST project found here:
http://www.nasa.gov/centers/glenn/about/fs03grc.html

For many years, engineers have known that bulk acoustic liners provide better noise reduction over a range of frequencies. However, materials currently available are not well-suited for use in the harsh environment within a jet engine or an automotive exhaust. Previous attempts at providing acoustic liners that could withstand these harsh conditions resulted in materials that reduced aerodynamic performance and could not hold up against rotor-tip rub, making them unsuitable for use in the region over the fan rotor.

Researchers at NASA Glenn and NASA Langley have acoustically tested the Haynes 25 metallic foam, which significantly reduces noise over a range of frequencies and fan speeds. Besides having these favorable bulk acoustic liner properties, the material can withstand the harsh engine environment and can be designed to minimize aerodynamic losses. The new metallic foam acoustic liner material has been shown to have a long life in an oxidizing environment, withstanding a temperature of 1000°C in a burner rig for 30 minutes. The foam does not readily absorb fluids, such as hydraulic fluid or oil, that reduce the material’s efficacy. Nor have the freeze-thaw cycles, inherent in moving between high altitudes and ground level, resulted in the structural concerns associated with bulk acoustic materials currently on the market. Stress tests revealed that the metallic foam can withstand expected mechanical loads, having held up well under compression, bending, and tensile stress.
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The Haynes 25 metallic foam can double as a rotor-tip rub strip. Replacing the rub-strip layer in the containment system surrounding the fan rotors with this acoustic metallic foam brings a noise-reduction material into this region. The material properties of the foam, such as temperature, density, porosity, and weight can also be tailored to suit a specific application. While the current work is intended to reduce turbofan noise, the method is applicable to other applications such as ground power systems and ventilating fans.

All of this R&D lead to a hypothesis that is fairly simple in theory but hard in execution. "If you can cool exhaust gases, then the gasses themselves become a denser gas and have less sound wave conductivity."

So where does this lead the daily drifter? Well if you live our in the country it may not be an issue to you, but if you live in a more suburban or city environment, pay close attention.


=Low Acoustic Conductivity Automotive Exhaust Fabrication and Manufacture=

So here's how to do it for all you fabricators and gearheads with welders.

The longer distance you can make the exhaust gasses travel, the less conductive they are to sound waves. Reason being is that the gasses cool and become more dense during the time that they are expelled through to exhaust plumbing. Now you're saying, "hey, a long exhaust is a heavy exhaust, and it should cause too much back-pressure!!"

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Now hold on there pard, using the theory and principle of fluid motion explained by a Mr. Bernoulli, deriving straight from Newton's 2nd law, we can create an exhaust with minimal back-pressure and weight.

Image

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Here we see the principle in action and how it affects ans curve ball after it is thrown by a pitcher.
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Using this formula we can ascertain a given pressure and acoustic conductivity that we hope to attain and the exhaust tip, this gives us pressures we need to replicate in various sections of the exhaust to match our desired decibel reading at the tip. We can do this by using the Venturi effect.
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When air is moving through a choke point with to areas of equal volume and pressure on either side the air in the choke point moves faster to maintain the same flow, thus decreasing pressure and cooling. This is illustrated in this video of a Boeing 777 airliner at takeoff. Notice the vapor forming in the intake, when air drops in pressure it cools, thus loosing the ability for it to remain hydrated (hold moisture). All of the moisture condenses into a visible could and is ingested into the engine.



This naturally occurring phenomenon actually acts as natures muffler, the moisture and decreased air pressure in and around the engine actually dampens 3-8 dB's of noise. In your design, incorporate three things a centrally located choke point with bypass (see Figure 1) and porous surfaces (resonators with dimpled or meshed interiors) inside the exhaust to increase the number paths for gasses to take, thus decreasing gas temperature. Avoid flat surfaces inside the plumbing and angles inside the plumbing where gasses converge of about 20 degrees. This will cancel out any echo effect.

So in closing I'll leave you with this, if you want quiet, think temperature, pressure, and porous.

*Figure 1*
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Jonathan Loveless
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Re: The Science Behind Quiet Exhausts (and Airliners)

Post by Yoshi » Wed Oct 24, 2012 1:19 am

whut?

Can you explain it easier to me? Cuz I need to do this soon and I don't want to buy 4 more mufflers.
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Re: The Science Behind Quiet Exhausts (and Airliners)

Post by Loveless » Wed Oct 24, 2012 1:29 am

The moral of the story is, don't buy cheap mufflers and resonators. The stuff that Vibrant produces uses some of these concepts i've outlined in their products.

Specifically these: http://vibrantperformance.com/catalog/p ... ucts_id=47

The farther the exhaust gases travel the quieter the sound. If you employ a venturi system somewhere than the sound will decrease by i'de say 10%, paired with some nice Vibrant resonators that have Haynes 25 incorporated in them you'll have a nice flowing exhaust without a bunch of racket.
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Re: The Science Behind Quiet Exhausts (and Airliners)

Post by Loveless » Wed Oct 24, 2012 1:35 am

An interesting concept would be the addition of a ceramic coating along the inside of exhaust tubing to negated the absorption of heat into plumbing, thus lowering gas temps and lowering sound.
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Re: The Science Behind Quiet Exhausts (and Airliners)

Post by Yoshi » Wed Oct 24, 2012 11:26 am

So what did you wind up going with?
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Re: The Science Behind Quiet Exhausts (and Airliners)

Post by Loveless » Wed Oct 24, 2012 10:48 pm

I sold the car with the Megan type 2. The plan was to mod the Megan exhaust by adding two vibrant resonators back to back in the mid pipe. I calculated it out and adding just the two resonators would add almost 500 more cubic inches to the inner volume and about 5 sq feet of surface area to the inside if the exhaust.
Jonathan Loveless
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Re: The Science Behind Quiet Exhausts (and Airliners)

Post by Jazpin » Thu Oct 25, 2012 12:30 am

Holy wall of text batman! Good read though man, awesome stuff.
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Re: The Science Behind Quiet Exhausts (and Airliners)

Post by Loveless » Sat Mar 30, 2013 9:31 pm

These would help too. But good luck finding one in the US already or getting one here economically. It's a muffler made by a company in Australia called XForce. It's their Varex line of mufflers. They use a remote control to actuate a butterfly valve in teh muffler itself. On loud settings it is essentially a straight through setup, but when you close the valve it directs gasses around the baffles and honeycombing in the muffler. Sometimes this results in stock db levels or even quieter.

http://www.xforce.com.au/about-varex-re ... uffler.php

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Re: The Science Behind Quiet Exhausts (and Airliners)

Post by Barnie3 » Sun Mar 31, 2013 12:18 am

I have the megan type 2 exhaust, and welded in a vibrant super quiet resonator and got a cusco cat. My sr still is kinda loud! Pisses me off. The vibrant resonator barely did anything.
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Re: The Science Behind Quiet Exhausts (and Airliners)

Post by Yoshi » Sun Mar 31, 2013 2:13 pm

IIRC BMW does something similar but it's vacuum actuated.
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