Re: what is white noise?

Brian,

There is much misunderstanding regarding white noise.  Only one thing 
qualifies as white noise, and that is described by britannica.com.

Britannica states white noise is: "the effect of the complete range of 
audible sound-wave frequencies heard simultaneously, analogous to white 
light which contains all the frequencies of the light spectrum... White 
noise is aperiodic sound (that is, its wave pattern is not uniform). It's 
constituent frequencies are of random amplitude and occur at random 
intervals."  They explain it pretty well so please forgive my cut and 
paste here.  See the actual reference at: 
 http://www.britannica.com/bcom/eb/article/3/0,5716,78873+1+76834,00.html?
query=white%20noise

Britannica.com is an exceptional source of technical information which 
mirrors and in some cases exceeds what I have been taught in the classroom 
on many subjects - I strongly recommend it as a reference. (I have been 
surprised by how good!)

While I can't vouch for the following company, I can ascertain that the 
reference information they give is good on white and pink noise as they 
apply to sound masking (they don't apply) and their description of the 
uses of white and pink noise are very good. (I am not really familiar 
with "fixed percentage bandwidth filters" so I can neither confirm nor 
deny that particular info however...)
 
 http://www.ivie.com/pages/white_vs_masking_noise.html

So regarding your question, city noise may approach white noise under 
random circumstances but would require considerable analysis to verify and 
then would only qualify probably for seconds or milliseconds at a time. 
(Actually I am only surmising this - the best way to know how close of an 
approximation city noise is would be to run a sound level - time - 
frequency analysis and see the actual nature of the noise. My experience 
in cities would suggest that while the "background noise" could 
approximate this very thing, the intensity of certain frequencies from 
time to time such as honking horns, whistles, and buses accelerating etc 
would minimize the amount of time the approximation would be good.)  The 
best place to find white noise would be to generate it in a lab.  By its 
very definition, only white noise as described above would qualify as 
white noise (britannica describes a couple of musical instruments that 
demonstrate some white noise characteristics, but white noise is white 
noise.)

 I hope this helped.

Sincerely,
Steven Miller
smiller@kahuna.sdsu.edu
Undergrad - Mechanical Engineering
San Diego State University