Noise Elimination

 

Noise is always around us. We need sounds to warn us of danger, so plugging our ears with uncomfortable earplugs is not always practical. Traditional passive techniques for noise control include isolation, such as done in recording studios, and shields around noise sources. Industrial equipment is a prime source of unwanted noise, and new designs can reduce fan noise without heavy shielding. We are always looking at ways to reduce noise, such as with jet aircraft designs and barrier walls for road noise. As discussed later, noise reduction techniques even increase auto fuel economy!

Many times unwanted sounds are generated inside machinery such as a heating system. Using trial-and-error techniques to isolate the sound source can take months to find. A new method, called a Sound Camera, analyses all sounds emanating from the device. The original method used one microphone as a fixed reference, while another microphone scanned the object using a robotic system, but the scan time was too long. Adding more microphones reduced the scan time, but a full system of microphones would cost well over one million dollars! By using an array of 1,024 low cost hearing aid microphones, the Sound Camera quickly finds noise sources when tested in an anechoic chamber.

In addition to passive sound reduction techniques such as shielding, there are active noise control techniques to reduce noise. By feeding an opposite polarity signal to another sound generator (Anti Noise in the diagram on the right) the sound is canceled because of wave interference. In 1878, a British physicist and engineer used two Bell telephones to experiment with sound cancellation. For many years after 1878, noise cancellation was known, but practical devices were difficult to build. In 1933, Paul Lueg received a German patent for active noise cancellation. In the 1950’s, this technique was successfully demonstrated in rooms, ducts, and headsets. Even today, engineering students build noise-canceling (analog) headsets, but their usefulness is limited to low audio frequencies, along with many adjustments to make it work. Luckily, Digital Signal Processing technology advanced enough for what we use today.

In 1978, Dr. Amar Bose, founder of Bose Corporation, was flying from the USA to Switzerland and was offered headphones to listen to music. He could barely hear the music over the airplane engine noise. He scribbled on a paper napkin equations for his first headphone design that could remove this noise. Like with many simultaneous inventions, both Bose and the German Sennheiser company developed aircraft noise-canceling headphones in the 1980’s. On December 14, 1986, the Voyager aircraft flew for 9 days to become the first airplane to fly around-the-world aircraft without stopping or refueling. Being concerned about hearing damage with such a long noise exposure, the two pilots wore prototype Bose noise-canceling headphones. Today there are many noise-canceling headphones available for both aircraft and music consumers.

Automotive Noise Cancellation was introduced in the 2011 Chevrolet Equinox crossover, resulting in 32 highway mpg vs. 28 mpg (Honda CR-V, Toyota RAV4, and Ford Escape) in the competitors. Between 1,000 and 1,500 rpm, the engine is more fuel-efficient but it produces vibrations and a low-frequency rumble. Two cabin microphones measure the amplitude (and phase) of the engine noise, and the noise cancellation electronics send correction signals to the front door speakers and the subwoofer in the rear. The electronics also use engine speed to calculate the frequencies that need to be canceled. *

As the world develops more technology that adds noise to our life, new noise reduction designs are being developed to help ameliorate such disturbances.

*Geek talk – Unlike the 1-dimensional system of noise-cancelling headphones, cabin noise canceling is a 3-dimensional system. Each ear is directional and hears different sound levels, especially from the side. Since the engine develops periodic noise, a Fast Fourier transform makes noise cancellation easier to apply, as it can adjust to variations within the cabin.

There are 18 comments.

  1. Saint Augustine Member

    Vectorman:

    As the world develops more technology that adds noise to our life, new noise reduction designs are being developed to help ameliorate such disturbances.

    And yet the noise from people just being loud is still a problem.

    Paging @JayNordlinger.

    • #1
    • November 18, 2018, at 6:02 PM PDT
    • 3 likes
  2. Muleskinner, Weasel Wrangler Member

    Vectorman: As the world develops more technology that adds noise to our life, new noise reduction designs are being developed to help ameliorate such disturbances.

    As in the “open office” design?

    • #2
    • November 18, 2018, at 6:53 PM PDT
    • 2 likes
  3. Vectorman Thatcher
    Vectorman Post author

    Muleskinner (View Comment):
    As in the “open office” design?

    I still dislike that concept, but understand the flexibility of being able to change the space. At my last job before retiring, I had a high (> 6 foot) office wall with a door for some privacy, with little in the way of sound reduction.

    • #3
    • November 18, 2018, at 7:04 PM PDT
    • 1 like
  4. OldDanRhody Member

    Vectorman: As discussed later, noise reduction techniques even increase auto fuel economy! 

    From the perspective that noise production is wasted energy, I would expect this to be a field of vigorous research.

    • #4
    • November 18, 2018, at 7:09 PM PDT
    • 4 likes
  5. RightAngles Member

    Quiet, I’m tryin to read this thing

    • #5
    • November 18, 2018, at 7:35 PM PDT
    • 15 likes
  6. cirby Member

    I spent about $130 a month or so back to buy liquid cooling systems for my two main computers, partly to make them a quieter. Cut the background noise by at least half (and changed it from white-noise type fan sounds to lower-frequency pumps and slower fans).

     

     

    • #6
    • November 18, 2018, at 8:01 PM PDT
    • 4 likes
  7. Clifford A. Brown Contributor

    Reading this very informative post with my passive noise elimination Etymotic HF3 earphones, fitted with triple-flange ear-plugs around the earphones. I’ve worn earplugs in every high noise environment, including loud dance halls and movie theaters, my entire adult life, so I’m not yelling at the youngsters to turn up the volume or stop talking so I can understand.


    This conversation is part of our Group Writing Series under November’s theme of Elimination. There are plenty of dates still available. Perhaps someone will even offer a page from the diary of a hitman, purely fictional of course. Or maybe we will read about eliminating excess inventory. Hmm, inventory control specialist by day, hitman by night? Sounds like a TV drama? What about those ads? You know what I’m talking about—even the Charmin bears! The possibilities are endless, Ricochet cool cats! Why not tell us about it and start a conversation. Our schedule and sign-up sheet awaits. Caveat: Given the theme, please keep in mind the basic rules of R>. As you polish ryour little masterpiece, do ensure that it stays within the refined edge of tacky. As a heads’ up, our December theme will be Veneration. I’ll post the sign-up sheet mid-month.

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    • #7
    • November 18, 2018, at 8:33 PM PDT
    • 2 likes
  8. Vectorman Thatcher
    Vectorman Post author

    Clifford A. Brown (View Comment):
    Reading this very informative post with my passive noise elimination Etymotic HF3 earphones, fitted with triple-flange ear-plugs around the earphones.

    The good news about your earphones: – 35 dB to – 42 dB of noise isolation.

    The potential bad news: Maximum Output (SPL) = 120 dB

    The 120 dB maximum might cause hearing damage after a short time, not that many would crank the volume that high.

    Geek talk: The hearing damage chart is based on dBA, which adjusts the Sound Pressure Level (SPL) by using an A-weighting curve. Hearing damage naturally focuses on (300 to 4,000 Hz) speech, but music has a much broader frequency range.

    • #8
    • November 19, 2018, at 9:30 AM PDT
    • 2 likes
  9. EJHill Podcaster

    No, Max, please! Not the “Cone of Silence!”

    • #9
    • November 19, 2018, at 11:09 AM PDT
    • 7 likes
  10. Misthiocracy secretly Member

    • #10
    • November 19, 2018, at 11:24 AM PDT
    • 2 likes
  11. Aaron Miller Member

    Now drivers are not only blind to others waving them through an intersection because of tinted windows. They can’t hear sirens either. 

    • #11
    • November 19, 2018, at 11:38 AM PDT
    • 2 likes
  12. Vectorman Thatcher
    Vectorman Post author

    Aaron Miller (View Comment):
    Now drivers are not only blind to others waving them through an intersection because of tinted windows. They can’t hear sirens either.

    The Chevy Equinox noise reduction only subtracts engine noise. Sounds outside the cabin, speech between the passengers, entertainment, and hands free cell phone use are not affected. However, the latter three are the primarily reasons for not hearing sirens.

    • #12
    • November 19, 2018, at 11:51 AM PDT
    • 4 likes
  13. RushBabe49 Thatcher

    I do not remember where I found this aphorism, but I sure remember it well.

    “You can look away from pictures, but you cannot “listen away” from sounds.”

    I work in a factory, and in 2015 we had a major transformation of the production floor. In the interest of “flow”, our big metal-stamping and screw machines were brought out from behind a wall and integrated into the production line. That increased the ambient noise level dramatically, especially for the production folks right next to them. Everyone complained, and we were told “wear headphones”. We did, and they did not always help. Earlier this year, management gave in, and moved some machines back behind a wall. But they did it with no notice and no fanfare. Obviously, our management has a hard time admitting that they were wrong. It’s still noisy, and I still wear my Bose noise-canceling headphones, while I listen to my Rush Limbaugh podcast.

    • #13
    • November 19, 2018, at 12:04 PM PDT
    • 10 likes
  14. Hank Rhody-Badenphipps Esq Contributor

    RushBabe49 (View Comment):
    In the interest of “flow”, our big metal-stamping and screw machines were brought out from behind a wall and integrated into the production line.

    Figures. The most important part of any machine is the guy babe running it. I’m sure the IEs had spaghetti charts showing exactly how much wasted effort they saved by moving those machines the first time though.

    • #14
    • November 19, 2018, at 5:55 PM PDT
    • 1 like
  15. RushBabe49 Thatcher

    Hank Rhody, Red Hunter (View Comment):

    RushBabe49 (View Comment):
    In the interest of “flow”, our big metal-stamping and screw machines were brought out from behind a wall and integrated into the production line.

    Figures. The most important part of any machine is the guy babe running it. I’m sure the IEs had spaghetti charts showing exactly how much wasted effort they saved by moving those machines the first time though.

    The smarties at corporate also forgot that the majority of the metal parts we make in house have outside processing, like coating and heat-treating, so they weren’t really in-line anyway. We have some items that have three different outside processes. This is the result of top-down policies that are instituted with NO input from those directly affected. There were a bunch of that kind of error made that later had to be rescinded. Unfortunately, before that happened some of the best employees quit in disgust.

    • #15
    • November 19, 2018, at 7:00 PM PDT
    • 4 likes
  16. Clifford A. Brown Contributor

    Vectorman (View Comment):

    Clifford A. Brown (View Comment):
    Reading this very informative post with my passive noise elimination Etymotic HF3 earphones, fitted with triple-flange ear-plugs around the earphones.

    The good news about your earphones: – 35 dB to – 42 dB of noise isolation.

    The potential bad news: Maximum Output (SPL) = 120 dB

    The 120 dB maximum might cause hearing damage after a short time, not that many would crank the volume that high.

    Geek talk: The hearing damage chart is based on dBA, which adjusts the Sound Pressure Level (SPL) by using an A-weighting curve. Hearing damage naturally focuses on (300 to 4,000 Hz) speech, but music has a much broader frequency range.

    The point is to never need to crank the volume to harmful levels, as the environmental noise is lowered enough and the speaker is well inside the ear canal.

    • #16
    • November 20, 2018, at 11:42 AM PDT
    • 1 like
  17. OkieSailor Member

    Vectorman (View Comment):

    Aaron Miller (View Comment):
    Now drivers are not only blind to others waving them through an intersection because of tinted windows. They can’t hear sirens either.

    The Chevy Equinox noise reduction only subtracts engine noise. Sounds outside the cabin, speech between the passengers, entertainment, and hands free cell phone use are not affected. However, the latter three are the primarily reasons for not hearing sirens.

    Or having a bus full of kids pumped about going home from school. I recently crossed a 4 lane highway in town when, after the light turned green, looked both ways to make sure no one was running the red as I always do, then about 1 second after I cleared the intersection I heard and saw in my mirrors an ambulance transiting that intersection through the still red light at about 20mph over the limit. I didn’t see them because of the curve and couldn’t have heard due to ambient noise which is constant on school buses. That was too close for me.

    • #17
    • November 21, 2018, at 5:08 AM PDT
    • 4 likes
  18. Pony Convertible Member

    My son has a high performance sedan. The exhaust produced an annoying hum at cruising RPM. He T’d into the exhaust a dead end exhaust pipe that is 1/4 the wave length of the hum. The sound goes down the pipe, reflects off the end cap and comes back 180 degrees out of phase of the hum canceling it. It works great. 

    • #18
    • November 21, 2018, at 5:17 AM PDT
    • 4 likes