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  • Phase Reverse?

    ok, so i had an idea, but correct me if it wont work, what if one was to record the noise one's engine makes, then have some software (tho i dont know what) to phase reverse that frequencies that the engine makes, and output that through the car's speakers, the only problem is i have no idea if it would interfere with the playing of music, and it would prolly cut off a bit of the music also. another thought is if it does work, one could hook it up to one's accerlorator, so it would get louder as it accelerates. i dont know if this would have any practical application, just a thoght.
    1997 Ford Crown Victoria Police Interceptor...
    Via m10000 Mobo w/ ITPS & pw-70a
    7" Lilliput Touch

  • #2
    Interesting idea, but I think if the idea is just to reduce electrical noise, the best thing to do is to treat the cause instead of the symptom. By that I mean instead of trying to cover up the noise by perfectly replicating it with the phase reversed through software, you would be better off eliminating noise before it gets to the signal. My favorite design cliche is K.I.S.S. (keep it simple stupid). People have been eliminating noise from car stereos for many years by adding filters, fixing grounds, separating power and signal, etc. I think that's your best bet.

    Don't get me wrong. I'd love to see that work if you know how. I think it's more complicated than recording the noise, though, since most electrical noises I've experienced in car stereos go up and down in pitch with RPM. Probably would have to be generated in real time. It would take somebody much smarter than me to do that.

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    • #3
      i think that rocker meant to use a microphone to record the engine and road noise in real time, and produce the inverse of the noise to make the inside of the car quiet.

      i think that there would be problems with trying this in suce a large space (the inside of a car), as i have only seen it used in headsets like this:
      Bose Headphones.....

      but it is definatly a good idea..... i might see if it is easy to do

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      • #4
        Ahh, a silent car would be lovely, wouldn't it?

        What you are talking about is called destructive interference, and it can work very well for certain applications. Something that drones (like factory machinery) can be made to be quieter using this method.

        The car is much more challenging, however. The main problem is that your noise cancelling source (eg, your speaker) has to be as close as possible to your noise source (eg, your engine), plus the speaker need to be able to reproduce the inverted signal as faithfully as possible... I guess that the speaker might have trouble with the lower frequencies, but this is only a guess... Your best bet is to try it, tbh. I could imagine tyre noise might be a bit easier to eliminate, maybe. Oh, and post your results, since I for one would be interested to know how well this works.
        My Setup

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        • #5
          You will need one manly DSP to process that real-time.

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          • #6
            I read about this in a Popular Science (Mechanics?) several years ago.
            They were talking about using it on planes to quiet them down.
            Noise cancellation has been around for a while, but is very difficult to get to work properly, especially in mobile environments. A pretty cool idea though. Stealth cars would be good for those with loud cars in quiet neighborhoods. (Like mine)

            Let us know if you figure it out.

            Cheers,

            Kris
            How Much Horsepower Can I have And Still Go To Heaven?

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            • #7
              Originally posted by Superduck
              I read about this in a Popular Science (Mechanics?) several years ago.
              Cheers,
              Kris
              Since you brought it up, I have a copy of the Electronics Now! magazine (Late 90's??) that shows how to build a set of noise canceling headphones. I just have to fing the damn thing. I was looking to implement this same thing by modifying the circuit shown so I could just cancel the engine noise whenever I wanted. I will have to dig through my stuff and see if I can find it. If I recall correctly the circuit costs >$50 in parts to build. The publishers website for Electronics Now! (http://www.gernsback.com) seems to be down at the moment.
              Aultl
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              • #8
                That sounds like just the thing. Seems like you could either cancel the noise at each source (maybe at the intake and exhaust) or as close to the occupants' ears as possible. I think Pontiac Fieros (or at least some of them) came with speakers in their headrests (Is that true Superduck?) A set of these from a junkyard might work. Just thinking out loud here.

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                • #9
                  i have thought abut some things, and have yet to find a phase revesring circuit/switch, or even plans for one online, but i figured it wouldnt even have to be a part of the computer, just get a small amp, and some speakers, and a microphone. there are some things i dont know much about, such as if one was to mount speakers inside of the engine compartment, and have one (or several) microphones also inside the compartment, what that would do, would that decrease the ammount of engine noise inside the car? i dont know, but if anyone knows were to find a phase reversing switch or circuit or anything let me know
                  1997 Ford Crown Victoria Police Interceptor...
                  Via m10000 Mobo w/ ITPS & pw-70a
                  7" Lilliput Touch

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                  • #10
                    Originally posted by cafn8
                    That sounds like just the thing. Seems like you could either cancel the noise at each source (maybe at the intake and exhaust) or as close to the occupants' ears as possible. I think Pontiac Fieros (or at least some of them) came with speakers in their headrests (Is that true Superduck?) A set of these from a junkyard might work. Just thinking out loud here.
                    The earlier models (84-85) had speakers in the headrests. The later ones got smart and mounted them in the rear pillars. The speaker seats (as I call them) sound like you're wearing a pair of headphones. That's not ideal for me.

                    As far as I remember, the problem with noise cancellation is the mic has to be close to the speaker. It will cancel all noise it receives, including your stereo, if mounted inside the car. I could be wrong though.
                    I'm not sure I can ASCII this, but I'll try...

                    /\/\/\/\-> | >|
                    Sound wave mic speaker
                    The mic picks up the sound, then inverts it, and shoots the inverted noise back towards the direction the sound is coming from. The opposite sound wave cancels the original one.
                    I think you need a directional sound, like what comes out the tailpipe, not ambient sound like engine noise.
                    I know that having two subs sharing an enclosure that are 180 degree out of phase will cancel eachother out. I'm not sure how to make this work in an open environment though.

                    Any audio engineers here that can correct me?

                    Kris
                    How Much Horsepower Can I have And Still Go To Heaven?

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                    • #11
                      I think it's already been done in the 90's by one of the exotic European car manufacturer. It was a prototype high dollar sports car. It has no muffler, just a straight pipe. The idea was to have speakers powered by an computer controlled amplifier. It has mics through out the car. The mics pick up the sound at the exhaust and the interior, then process the noise and then reproduce an out of phase sound to cancel out the road/exhaust noise. It has a muffler setting for ultra quite to intimedating rour. The benifit was theres no restricted muffler to cut down the performance and also to let the driver select what type of exhaust s/he wants.
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                      • #12
                        thats neat, i'd still like to give it a try, but i cant find any phase reversing anything
                        1997 Ford Crown Victoria Police Interceptor...
                        Via m10000 Mobo w/ ITPS & pw-70a
                        7" Lilliput Touch

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                        • #13
                          Yes it has been done before, but for the life of me i can't remember where. Anyways the story I read was that the microphone and the speaker both were hooked to the exhaust somehow. and the sound coming from the speaker was inside the pipe. I'l try and dig up info on it, it made the exhaust inaudible. Then it would be some road noise to worry about.

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                          • #14
                            I did a Google search and found these plans for noise cancelling headphones. Maybe with a bit more horsepower on the output and some full range speakers this circuit would do the trick.

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                            • #15
                              guys none of this can work. In an automotive enviroment it is not a case of simply playing an inverted signal through the car speakers. Oh if only it was

                              Basically if you are interested google-> filtered X-LMS algorithm.

                              she's not too robust, but she'll get you started. Basically you have to make an approximation of your mic -> speaker transfter function first (system identificaiton), and then run a FX-LMS algoritm on this assumption. The algorithm uses a least squares routine to estimate correct filter weights to apply to the microphone signal to output to the car speakers.

                              Youll be a lot better off running bose speakers on your head - unless you like playing with these things

                              HTH

                              Stu

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