The problem is that nature sounds are irregular. To be most effective, white noise should be steady and constant. Tatt's what your brain is best at adjusting to and tuning out. Irregular sounds, although nice to listen to just don't work as well.

The other weakness of this type is that they often can't be turned up loud enough to mask sleep disturbing sounds. I guess that's because at greater volume Shamu, the killer whale, sounds frightening rather than relaxing.

I'm familiar with several white noise machines. There is one that I think is superior to the rest.

To learn which one it is and why, please visit: www.pilot-pauls-travel-accessories.com/white-noise-machine.htm

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