Ha, for real!

Has anyone ever heard of this technology? Apparently born in
European vehicles (Germany)?

I was having a chat with my buddy who's a mad scientist when it
comes to engine tech.

He explained that acoustic energy is pumped into the exhaust to
reduce sound levels.

I can understand how this works, the acoustic wave would be
180 degrees out of phase with the exhaust pressure waves.

The negative pulses would cancel out the positive pulses and
reduce most of the exhaust noise.

I didn't have much time to ask what sort of mechanism was used
to produce the audio, or where it was placed.

Not sure about many things, but I'd be interested in links or a discussion
on the system.

Has this technology ever hit production?

Also noteworthy in our little talk - muffler design to improve aerodynamics.

Shaping and placing the exhaust system out of the air stream
to prevent turbulence at the rear of the car.

Sometimes I wish I could talk for hours with this man with a tall
cold drink in hand!

 

I think I would talk to him a little more. Sounds pretty interesting.

 

I've heard that when runnong dual turndowns under the car to try to point them at each other to reduce exhaust noise. Same premise I think.

 

Basically this is referring to active noise cancellation where an equal but 180deg out of phase signal is produced to offset the actual sound.

Unfortunately the wattage required is rather high to offset a 130+ decibal exhaust. Check out the power required to make 130db with your car stereo. Also any type of speaker that could withstand the exhaust environment would have to be rather strong & therefore expensive.

This technology is used in some high end headsets used in the aircraft industry.

It is a rather interesting theory & could in fact work very well. I just never saw it used to quiet the exhaust of a car.

 

No doubt about the amplitude needed to efficiently cancel exhaust
noise.

I would really like to hear one of these system in action (active and
off).

It would be freaky to be driving 100 MPH and not hear the rumble
of our V8's!

I'm thinking it would be almost impossible to make a "noise reduction"
system function properly in an open enviroment where sound waves
are boucing off every known surface.

Seems like too much work for what it's worth? From the talk I was
having, these "speakers" didn't seem to be very big as they fit
within the frames of the car. Maybe the speakers he was referring
to were tranducers, or something of that nature?

 

The answer is yes, the technology is in production, on buses and in some aviation cases. We did a TON of work on this for passenger cars years ago. It can work reasonably well. The idea is simple, but the devil is in the details as they say. You put a microphone in the exhaust stream, use a signal processor to analyze the signal, break it down and feed the signal 180 degrees out of phase with the original to the speakers. The noise is cancelled out and it's quiet. No restriction, no power loss, just exhaust with a huge reduction in noise level.

No manufacturers want to pay for it. Here are some of the problems:
Have you ever thought about putting a microphone in an exhaust stream? They don't live long. You end up trying to get the sound measured while not exposing the 'phone to the stream. It's tough getting them to live a few thousand miles in the required environment (think not just exhaust, but also road salt, changing temperatures and humidity), let alone 80,000 or 100,000.

Next, try putting speakers next to an exhaust environment and getting them to live very long, (similar to the microphone problems). Speakers add to the problem because you need an enclosure large enough to allow for cancellation of low frequencies. The lower the attenuation frequency, the larger the enclosure volume required. Packaging can become a nightmare, and overall weight can be higher than the original system.

Now, consider a performance engine. Under WOT, you need to sample, analyze, and generate a new sound, in time for the sound to be attenuated. Think how quickly the rotation (frequency) is changing and you get an idea of the challenge with how fast you need to compute, transform the signal, and amplify. Not a trivial undertaking. Diesel buses don't change frequencies nearly as quickly, nor through as great a range, so it's a little easier.

Incidentally, this technology is used in turboprop aircraft extensively and effectively with the microphones and speakers inside the cabin.

The problems might be mitigated with the right materials and technology, but cost is going to be maybe an order of magnitude over what the OEM pays for a passive system right now. That's what kills the deal at this point. In the OEM world, cost is king. If performance is acceptable now, there is no need to push the envelope at ten times the cost, even if it works.

Now about those aerodynamic mufflers. For the domestic OEM's, it seems as though the exhaust design is sometimes an afterthought. You put the exhaust system where there is space available. The traditional muffler is a low-cost capped elliptical design. Effective, but not aerodynamic. A more form-fitting, stamped design muffer is possible and is in production on quite a few vehicles. They can be significantly more expensive, depending on complexity. A stamped muffler can cost a ton of money to tool up and if you want to make a change in the design, it costs again. Totally smooth, flat surfaces are out due to the steel vibrating loudly. Elliptical shells don't rap as much, and stamped designs always have ribbing stamped into the shells to give rigidity and keep the shell from vibrating. Kills the aerodynamics a little, but keeps NVH livable.

This just scratches the surface on this topic...

 

The 'speakers' are placed along side the exhaust under the car?

THis would cancel ambient noise, or passenger compartment noise correct - Nothing is fed back directly into the exhaust pipe?

I can see why the system would be rare. Find room to build
speaker enclosures under the car between the frame rails would
be a task in itself.

Give up power to weight ratio for a reduction in exhaust noise.
Not a good trade off for the performance community, but an
interesting concept otherwise.

 

The speakers are placed in an enclosure that sits towards the end of the exhaust system. The sound is directed into the exiting exhaust stream. You are sampling only exhaust noise with the microphones and targeting only exhaust noise with the speakers.

It was really an interesting thing to hear. I heard dyno runs where it was rigged with a switch to turn the system off and on mid-run. The difference was dramatic.