<b>SHHH... SHHH... SHHH</b><br>"Oh just a very weird truck de-pressurizing something or whatever it is nothing to worry about"<br><b>Gets hit by truck</b>,(reply>), @John Arktor I'm not sure that endless graunchy Hissing is much of an improvement. The local bin lorries sound like a Barn Owl with a Loudhailer.,(reply>), If it doesn't save lives of construction workers - which it certainely does - it will certainely save nearby dwellers'sanity.<br>Try living near 3 construction sites for two years. Just try one week... If you don't end up with suicidal or murder ideas, then you're not human.,(reply>), @Zhong Ping - Having spent 15 years doing commercial construction sites, I can say that I would have no idea that this was supposed to be warning me of anything...,(reply>), I immediately think of Air Brakes too XD.<br>(the Buses &amp; Lorries hiss like that all the time here in the midlands; whilst most definitely NOT in reverse),(reply>), @The King in Yellow Coming from somebody who understands a fair bit of acoustics and psychoacoustics, it's really not that counterintuitive. Take an example from nature. Step out into a forest, and listen to birds. Try to find and locate one particular bird among several by its call. I doubt you could do it, unless you had practiced. Now, if somebody comes from any direction outside of your view and crunches on some leaves, chances are you would be able to intuitively locate them fairly quickly. This is the difference between a tone and a noise.<br><br>The thing about "echoing" and what it does to sound: the way we perceive a sound depends a lot on its spectra, or the frequencies it is composed of, and echo does weird harmonic things to sounds that consist of single tones. Phase, reverberation, etc. Disrupting that delicate balance of harmonics can disrupt where our brain thinks a sound is coming from, because again, the sound has changed. The difference with white noise, is that the sound already consists of every pitch and every harmonic at once at equal volume. That's the definition of white noise. So the effect of echo on white noise, and its resultant disorienting effect on us, is significantly negated.