Re: what is a 'sone' - the bath fan sound measurement

Mark,

This is an interesting question.  One of my favorite sources on the 
internet for "definition" type questions is britannica.com from the folks 
at Encyclopedia Britannica.  Their definition of a 'sone' can be found 
here:
 http://www.britannica.com/bcom/eb/article/9/0,5716,70469+1+68689,00.h
tml?
query=sone

The problem with the sone measurement is, as britannica says, it is a 
subjective measurement.  That means it is based on individual human 
judgement, as opposed to the decibel where we actually measure sound 
pressure or power differences scientifically.  Scientists can adjust the 
decibel scale to match more precisely what humans hear by "A-weighting" 
their decibel measurements, which involves excluding certain portions of 
the measurement that are known to have a smaller effect on human hearing.  
A-weighted measurements (dBA) are generally lower in value than the 
original unweighted decibel measurement (dB).  It would be very difficult 
(based on the definition of 1 sone = 1000Hz tone at 40 dB)to relate the 
sone to decibels.  If I had to approximate (very loosely), I would guess 
that if 1 sone = 40 dB, and we add three decibels every time we double the 
amount of sound, we would get (very) roughly the following:

1 sone  = 40 dB
2 sones = 43 dB
3 sones = 44.7 dB
4 sones = 46 dB

HOWEVER

This would be a starting point only!! If I were testing I would run tests 
myself to determine the actual sound pressure or power in dB with a sound 
meter.  The conversions above would almost surely be inaccurate for the 
following reasons:

Fan noise contains more than just a 1000 Hz tone. (Humans are very aware 
of tones. A fan which is quiet but emits all its sound in a single tone is 
usually considered 'louder' than a louder fan distributing its 'noise' 
over a wide range of frequencies.

Humans may or may not perceive twice as much sound pressure as twice as 
loud (most likely you would not).

As an engineer I am very cautious whenever we consider human feelings in 
an experiment.  Even when an engineer structures an experiment, she/he 
must be very careful not to incorporate their own bias into the tests.  
For example, if someone wanted a quiet fan and had to pick from two very 
similiar fans in sound output, they may say that the fan which is better 
looking is the quietest when in fact the ugly one may be slightly quieter.

Additionally, when we make scientific measurements of sound we usually do 
so to a 'standard'.  A standard is an accepted way of conducting the tests 
so that if you and I both conduct the same tests to the same standard in 
different times and places we should get approximately the same result.  I 
am unaware of a standard for sone measurements.  It would be interesting 
to note if the fan manufacturers state something like "3 sones using XYZ 
standards" or some such thing.  Without a standard comparisons don't mean 
much.

So after all this, the bottom line is use the sone for approximation only, 
and use a sound meter to determine the actual output in decibels.  There 
really isn't a conversion or correlation between the two, except at 1 sone 
= 1000 Hz @ 40 dB.

Good Luck!
Steven Miller
smiller@kahuna.sdsu.edu
Undergrad - Mechanical Engineering
San Diego State University