# Stupid Idea: noise cancelation **URL:** \ **Category:** Factual Questions\ **Created:** [December 5, 2004, 10:10pm UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269 "2004-12-05T22:10:44Z")\ **Posts on this page:** 11\ **Page:** 2
**Author:** ![Harmonix](https://avatars.discourse-cdn.com/v4/letter/h/3ec8ea/32.png) [@Harmonix](https://boards.straightdope.com/u/Harmonix)\ **Post date:** [December 8, 2004, 10:49am UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269/21 "2004-12-08T10:49:32Z")
> [@don't ask](#): > > In sound recording noise cancellation is often used when recording someone who doesn’t want to listen to foldback through headphones. You provide two speakers wired out of phase and position the mic where the maximum cancellation effect occurs. It is easy to find by ear. You can test the same effect yourself if your stereo lets you change the leads around. Simply reverse positive and negative on one and stick your head between the speakers and notice how the sound level drops particularly on the bass. If it doesn’t and sounds better (which is common) you had your stereo wired out of phase anyway and that’s why it’s so gutless. Could you explain this further? Is this the same as reversing channels in software? Anything to get my speakers to sound better! ---
**Author:** ![don\_t\_ask](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@don\_t\_ask](https://boards.straightdope.com/u/don_t_ask)\ **Post date:** [December 8, 2004, 11:05am UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269/22 "2004-12-08T11:05:28Z")
As I said it only improves the sound if they are wrongly wired. If your cables have ordinary speaker cable connected at either end into push pins or other sockets and you set them up so that the negative terminal is different on each one, they will be out of phase. If the connectors are already made or use plugs it shouldn’t be possible. ---
**Author:** ![don\_t\_ask](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@don\_t\_ask](https://boards.straightdope.com/u/don_t_ask)\ **Post date:** [December 8, 2004, 11:11am UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269/23 "2004-12-08T11:11:53Z")
I should be clearer. Say on the back of your amp each speaker has a connector labellled + and - and say your speaker cabinets have connectors that are red and black. You are meant to connect say + to black and - to red on **both** boxes. If you connect + to red and - to black on one it will be out of phase. Many people set up their stereos without ever thinking that it makes any difference. I have seen home stereos with the bass cranked right up and amazed the owner by swapping the leads on one speaker and turniong the bass down. ---
**Author:** ![jovan](https://avatars.discourse-cdn.com/v4/letter/j/0ea827/32.png) [@jovan](https://boards.straightdope.com/u/jovan)\ **Post date:** [December 8, 2004, 11:23am UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269/24 "2004-12-08T11:23:41Z")
> [@Harmonix](#): > > Could you explain this further? Is this the same as reversing channels in software? > Anything to get my speakers to sound better! It’s not going to make your speakers sound better. Imagine that instead of continuous sound waves, numbers come out of your speakers. You have two speakers and you send them the same signal: -2, -1, 0, 1, 2, 1, 0, -1, -2… Sound travels at a certain speed. If a listener is at the _exact_ same distance from both speakers, he will receive two identical signals, which are summed up to produce: -4, -2, 0, 2, 4, 2, 0, -2, -4 Essentially the same thing but louder. You might have noticed that speaker cables have a positive and a negative ends. If you don’t plug those the same in both speakers, you’ll get the following signals: -2, -1, 0, 1, 2, 1, 0, -1, -2… 2, 1, 0, -1, -2, -1, 0, 1, 2… So, if you’re still in the same spot, when you add those two signals, you get nothing but zeros. In the real world, what happens is that whenever one wave’s pressure goes up, the other one goes down by the exact same amount. Now, imagine you’re slightly closer to one speaker than another. One signal is going to arrive a bit earlier, so that you get: 0, 1, 2, 1, 0, -1, -2, -1, 0 2, 1, 0, -1, -2, -1, 0, 1, 2… Add those together and you get: 2, 2, 2, 0, -2, -2, -2, 0, 2 Not silence anymore. In the real world, for the above effect to work, the distance must be static, you can’t have the listening point move around. With a human listener, rather than a microphone, there is another problem in that one of your two ears is always going to be close to one of the speakers. You can try it out to see what it actually sounds like. Unless you’re close to the “sweet spot” you probably won’t hear much of a difference but it sounds very weird if you’re in the right spot. ---
**Author:** ![Harmonix](https://avatars.discourse-cdn.com/v4/letter/h/3ec8ea/32.png) [@Harmonix](https://boards.straightdope.com/u/Harmonix)\ **Post date:** [December 8, 2004, 11:42am UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269/25 "2004-12-08T11:42:20Z")
Er ok. thanks. My speakers have plugs so I guess it’s moot for me. My next planned(read: if i’m ever not broke) set will have normal speaker wires. I’ll keep this in mind, thanks. ---
**Author:** ![jjimm](https://avatars.discourse-cdn.com/v4/letter/j/ba8739/32.png) [@jjimm](https://boards.straightdope.com/u/jjimm)\ **Post date:** [December 8, 2004, 11:51am UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269/26 "2004-12-08T11:51:31Z")
> [@Loopydude](#): > > If fan noise really all white noise? My PowerMac G4 a.k.a. Quicksilver was nicknamed “the wind tunnel” by irate purchasers when it first came out. For good reason, too, because it’s louder than hell when the fans really get going. I wish to hell you hadn’t mentioned this, because I’ve got one of those just by my right ear, and reading your post has got me noticing the horrendous fan noise and now I can’t get it out of my head! MAKE IT STOP. ---
**Author:** ![Julius\_Henry](https://avatars.discourse-cdn.com/v4/letter/j/8797f3/32.png) [@Julius\_Henry](https://boards.straightdope.com/u/Julius_Henry)\ **Post date:** [December 8, 2004, 12:47pm UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269/27 "2004-12-08T12:47:54Z")
For a fictional account of your new idea, read Arthur C. Clarke’s \*Silence Please \* in his collection **Tales from the White Hart**. That particular story was written in 1954. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [December 8, 2004, 1:13pm UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269/28 "2004-12-08T13:13:03Z")
> [@Loopydude](#): > > If fan noise really all white noise? Not really. A major chunk of fan noise comes from harmonics of the blade passage frequency. As an example [here](http://www.safetyline.wa.gov.au/pagebin/noisgenl0050.htm)’s a rather neat page on using active noise control to reduce noise from a fan. Figure 3 shows the before and after spectrum analysis of the fan noise. ---
**Author:** ![Harmonix](https://avatars.discourse-cdn.com/v4/letter/h/3ec8ea/32.png) [@Harmonix](https://boards.straightdope.com/u/Harmonix)\ **Post date:** [December 8, 2004, 5:16pm UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269/29 "2004-12-08T17:16:51Z")
> [@zut](#): > > Not really. A major chunk of fan noise comes from harmonics of the blade passage frequency. As an example [here](http://www.safetyline.wa.gov.au/pagebin/noisgenl0050.htm)’s a rather neat page on using active noise control to reduce noise from a fan. Figure 3 shows the before and after spectrum analysis of the fan noise. haha yes! that was exactly what I was thinking. I wonder where I could get those kind of components… ---
**Author:** ![Harmonix](https://avatars.discourse-cdn.com/v4/letter/h/3ec8ea/32.png) [@Harmonix](https://boards.straightdope.com/u/Harmonix)\ **Post date:** [December 8, 2004, 5:28pm UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269/30 "2004-12-08T17:28:38Z")
Heh, i’m going to make sure i’m not wooshing myself. That works because the anti-noise is generated from the same approximate location as the fan noise correct? ---
**Author:** ![Harmonix](https://avatars.discourse-cdn.com/v4/letter/h/3ec8ea/32.png) [@Harmonix](https://boards.straightdope.com/u/Harmonix)\ **Post date:** [December 8, 2004, 5:30pm UTC](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269/31 "2004-12-08T17:30:15Z")
Heh, i’m going to make sure i’m not wooshing myself. That works because the anti-noise is generated from the same approximate location as the fan noise correct? Er and thus it doesn’t matter where you hear it from. [Previous page](https://boards.straightdope.com/t/stupid-idea-noise-cancelation/278269.md?page=1)