Phase Reverse?

**Rocker298** · 02-16-2004, 01:54 PM

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.

**cafn8** · 02-16-2004, 05:24 PM

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.

**dominic** · 02-16-2004, 05:35 PM

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

**MikeHunt79** · 02-21-2004, 06:17 PM

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.

**droswell** · 02-22-2004, 02:07 AM

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

**Superduck** · 02-22-2004, 02:43 AM

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

**aultl** · 02-22-2004, 11:59 AM

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.

**cafn8** · 02-22-2004, 12:25 PM

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.

**Rocker298** · 02-22-2004, 09:43 PM

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

**Superduck** · 02-22-2004, 10:55 PM

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