Join 3,377 readers in helping fund MetaFilter (Hide)

How do White Noise machines work? Are white-noise sleep machines really made up of a mosaic of tiny repeating patterns?
February 22, 2011 5:26 PM   Subscribe

How do White Noise machines work? Are white-noise sleep machines really made up of a mosaic of tiny repeating patterns?

I am one of them folks whut cant sleep without some sort of "fan noise"
I simply cannot get to sleep in complete silence.
Therefore I have a noise machine that makes whale noises, rain, wind, all that kind of stuff though I really only ever use it in the WHITE NOISE mode.

Something I have noticed is that my ear will pick up a pattern in the noise. Almost like a repetitive kind of soft techno rhythm. Once I hear it my brain locks onto it and my brain sort of grooves along on it. It doesnt affect my sleep either. In fact it tends to help.

What's INTERESTING however is that its often a DIFFERENT pattern/beat. And it has switched up on me many different nights.

So I guess what Im asking IS...

• Does anyone have any idea how this noise is generated?

• Is it simply layers of these kinds of grooves and in aggregate they make a white noise wall?

• Is it a random groove generated each time?
posted by Senor Cardgage to Health & Fitness (11 answers total) 3 users marked this as a favorite
White noise usually uses a noise source that's then filtered to give it the "white" property. In electronics, it's probably cheapest and easiest to just use something that generates quantumly random electrical signals. Here's a PIC hack for it.

So, no, it's probably not generating any sort of patterns that are then being put together.

The patterns are coming from your brain.
posted by Netzapper at 5:33 PM on February 22, 2011 [1 favorite]

How much does the pattern change? I had a similar machine that was clearly using recorded, sampled loops rather than generating the noises. Could it be the same sound with your mind locking onto different parts of it, so to speak?
posted by kyrademon at 5:44 PM on February 22, 2011

Yes, digital white noise can often have a recognizable pattern. It's generated using a pseudorandom noise generator. These days they're made with a PIC or similar kind of programmable microprocessor. It's interesting that you can pick up a pattern, as that was a common complaint when they became common in electronic musical instruments, using things like discrete shift registers and/or multiple randomly tuned rectangle wave generators summed together. Often, the repeating pseudorandom sequences were simply not very long, and a pattern could be discerned on careful listen. With these older designs a random "groove" might be generated each time. It may also be matter of perception, depending where your brain picks out a "beginning" point of reference. I suspect the same may apply with some software based pseudorandom noise generators, depending on what scheme they got going to generate the pseudorandom noise.
posted by 2N2222 at 6:11 PM on February 22, 2011

Usually they're doing something like amplifying thermal noise. No, there is no pattern.

What Netzapper means by "filtering to give it the white property" is that "White Noise" is defined as being equal amplitude at all frequencies. (As opposed to "Pink noise", which is equal energy.)

No, there are no patterns. But the human brain is a pattern-finding device, and sometimes it will seem to find patterns which aren't really there. Which is why you can sometimes see faces in a speckled ceiling, or animals in clouds, or an image of Elvis on a piece of toast. And it wouldn't surprise me if some people think they hear repeating patterns in thermally-generated white noise, even though there aren't any.
posted by Chocolate Pickle at 6:20 PM on February 22, 2011

1) There are noise generating chips (as mentioned in Netzapper's link), as well as ways of generating pseudo-random 'white' noise programmatically; the 'traditional' way was to use a zener diode or reverse-biased transistor & amplify the junction noise.
2 & 3) Possibly, if a generator chip is being used - many early ones, at least the noise generators used in early or cheap digital synths, didn't include a random or pseudo-random noise source but instead relied on a pseudo-random pattern 'burnt' into them.

But yeah, the brain is basically a giant pattern-finding machine - and if there's no pattern to find, it'll damned well invent one!
posted by Pinback at 6:20 PM on February 22, 2011

There's a visual analog to this, which is probably going to be experienced by fewer and fewer people as TV goes digital and clever. If you tune an old analog TV between channels, its automatic gain control will crank right up and the display will turn whatever internal electrical noise the tuner stages generate into random on-screen "snow" (you also get roughly pink noise through the speakers). Stare fixedly at one spot on the screen while it's doing that, and your brain will shortly start to organize it into patterns of whirling drifts. Some drugs make this effect much stronger.
posted by flabdablet at 6:47 PM on February 22, 2011 [1 favorite]

Your brain is basically evolutionarily programmed to look for patterns, even where there aren't any. This is called apophenia and/or pareidolia. It's entirely possibly the patterns you perceive are just your brain trying desperately to add meaning to the noise.
posted by axiom at 7:46 PM on February 22, 2011

Some white noise machines, like the SleepMate, are actually mechanical (there's a fan inside) and produce a much more random noise. I have trouble with the electronic ones due to the pattern issues that everyone has mentioned, and I love my SleepMate. Regretting that I left it at home during this trip, actually.
posted by cabingirl at 8:01 PM on February 22, 2011 [1 favorite]

There was an article in Byte in the 1980 volume IIRC that detailed a way to build a white noise generator on the Apple II by using a 9 bit Maximal Linear Feedback Shift Register. The amount of code was trivial by today's standards and it worked pretty well. It essentially slammed through 9 bits, or a 512 value sequence using that value to determine whether or not to pop the speaker. In that day, you could implement the whole thing in a few LSI logic chips for a few bucks.

It wouldn't surprise me if your device has sampled white noise that it plays back - after all, it already has everything in it to play other samples, so why not put in white noise? Who cares if it repeats now and again?
posted by plinth at 8:24 PM on February 22, 2011

You might try downloading Audacity (it's free) and generating a 79-minute track of white noise (it's easy), then burning it to a CD and setting it on repeat. I include a fade-in and fade-out so that the abrupt change in noise doesn't wake me after I've fallen asleep already. The only catch is you have to have a CD player with a "disc repeat" option (or you could probably use an iPod connected to speakers). MeMail me if you try this and need a few more hints.

As for the pattern thing you mention, I've never noticed it with my disc.
posted by AugieAugustus at 6:57 AM on February 23, 2011 [2 favorites]

Senor Cardgage, I have definitely experienced the same phenomena with my white noise machine - I can pick up patterns and they are different from time to time. My solution has been to turn it off and then back on and the pattern is no longer noticeable.

To those people who are saying that it must be in our minds, my SO and I have often heard a pattern at the same time, so I really do think it's an effect of the particular machine rather than our brain waves our freakishly similar. I chalk it up to the fact that I bought a noise machine for $9.99.
posted by FreezBoy at 8:13 AM on February 23, 2011

« Older In a few days I'm moving to Ca...   |  Have you ever tried one of the... Newer »
This thread is closed to new comments.