Silent Computer - Advice

"dorothy.bradbury" wrote:
o A reduction of 10dB(A) is a 50% reduction in sound energy
---- 3dB(A) is the margin most people can detect
---- however this too depends on the noise level re 20s or 70s


In the audio world, 1 dB is the accepted limit of human
perception (if they're listening for it). 3 dB may be the
limit for casual (or "unprepared") listening.

As long as you're referring to human perception by adding
the "(A)" to your dB figures, you might as well add
"apparent" to "sound energy" terminology.

And "db" is a measure of power, not energy; and instead
of "energy", you ought to be saying "intensity" for perception
effects. In short, you mix your terms from the physics basket
and from the psychoacoustics basket. In the physics (and
engineering) world, 10dB equals one Bell - a change in
power by a factor of 10, not a factor of 2. A factor of 2
entails a change in power of 3dB.


o Each 180-degree turn you make sound go thro reduces
it 6dB(A)


groan And if you spin around 10 times the sound will
reduce by 120dB! If you merely mean "fluctuate" by 6dB(A),
you have some explaining to do about how that sound does
or doesn't reflects of walls, refracts around the edges of the
ears, how it conducts through flesh and bone of the ear/head,
etc., instead of that blanket one-size-fits-all statement. In
college, conclusions stated without any comment on the
methodology was called "hand waving".



*TimDaniels*

Timothy Daniels wrote:
In college, conclusions stated without any comment on the
methodology was called "hand waving".

In engineering "rule of thumb" is adequate in many places, although I
wouldn't expect a physicist to be happy with that :-)

Timothy Daniels wrote:

"dorothy.bradbury" wrote:
o A reduction of 10dB(A) is a 50% reduction in sound energy
---- 3dB(A) is the margin most people can detect
---- however this too depends on the noise level re 20s or 70s

In the audio world, 1 dB is the accepted limit of human
perception (if they're listening for it). 3 dB may be the
limit for casual (or "unprepared") listening.

As long as you're referring to human perception by adding
the "(A)" to your dB figures, you might as well add
"apparent" to "sound energy" terminology.

And "db" is a measure of power, not energy; and instead
of "energy", you ought to be saying "intensity" for perception
effects. In short, you mix your terms from the physics basket
and from the psychoacoustics basket. In the physics (and
engineering) world, 10dB equals one Bell - a change in
power by a factor of 10, not a factor of 2. A factor of 2
entails a change in power of 3dB.

Yes, I already knew that it is strictly sound power which is rate of energy.
However, to simplify I also wrote 'sound energy', assuming everybody would
understand this the right way. But in usenet there is always someone needing
to nitpick (myself included).

"Rob Morley" pointed out:
Timothy Daniels wrote:
In college, conclusions stated without any comment on the
methodology was called "hand waving".

In engineering "rule of thumb" is adequate in many places,
although I wouldn't expect a physicist to be happy with that
:-)

Nor a mathematician. :-)
BTW, my "rule of thumb" is: If it don't work the first time,
press harder.


*TimDaniels*

-
Rob Morley stood up at show-n-tell, in
, and said:

Timothy Daniels wrote:
In college, conclusions stated without any comment on the
methodology was called "hand waving".

In engineering "rule of thumb" is adequate in many places, although I
wouldn't expect a physicist to be happy with that :-)

We call that 'fudge factor', in Chemistry and PCHem.


--
Strontium

"It's no surprise, to me. I am my own worst enemy. `Cause every
now, and then, I kick the livin' **** `outta me." - Lit

"Johannes H Andersen" wrote:
Yes, I already knew that it is strictly sound power
which is rate of energy. However, to simplify I also
wrote 'sound energy', assuming everybody would
understand this the right way. But in usenet there is
always someone needing to nitpick (myself included).

Hey! Nitpicking is one of the invaluable services
provided by Usenet. It's like primate grooming -
you pick my nits and I'll pick yours and together
we'll be nitwits, er... nit-free. Or whatever.


*TimDaniels*

"Timothy Daniels" writes:

And "db" is a measure of power, not energy; and instead

"dB" is a measure of many quantities (actually on ratios of
quantities) including power, energy, intensity, pressure, voltage, and
impedance. Originally, it was the amount a signal dropped in (power?)
level through one mile of telephone wire.

"dorothy.bradbury" writes:

Human Perception:
o 10dB increase in noise level is Perceived as a doubling in noise level
o 10dB decrease in noise level is Perceived as a halving in noise level
....
The 10dB figure is rough, but standard practice - some prefer to use 6.
Note this is a *Subjective* Human-Perception issue not an absolute.

I really don't understand that. I don't even know what the term
"noise level" means, and I certainly don't have my ears calibrated to
measure changes in it.

The best I could manage is to hope that I could guess when one sound
sounds like another sound at half or twice the distance. That rule
would still be plenty subjective, because it doesn't address the
variation of the sound with distance, but I can't imagine a case that
would give a figure of 10 dB or even one larger than 6 dB.


Maybe my rule is what people tend to use, but just tend to get it
wrong by at least 4 dB, on average, but in any case, asking
experimental subjects to judge "half as loud" seems like a really bad
way to define technical terms.

Johannes H Andersen wrote:

small audible reduction of 3 dB require a 50% reduction in sound
energy.
....
A doubling of absolute signal power represents an increase in dB of 3dB.
Reference http://ccrma-www.stanford.edu/~oded/M151/decibels.html ,
but this is also generally known from any textbook.
....
Human Perception:
o 10dB increase in noise level is Perceived as a doubling in noise level
o 10dB decrease in noise level is Perceived as a halving in noise level
This sounds correct, but you said in your earlier post:
"o A reduction of 10dB(A) is a 50% reduction in sound energy"
So you mixed up 'sound energy' with 'Percieved noise level' .
Strictly, as 3dB reduction corresponds to 50% reduction in sound energy,
....

It seems to me that JHH is mixing up two different "dB" units and is
also mixing up sound energy with sound intensity or power. JHH seems
to be using a Sound Power Level "dB" while he quotes someone who is
obviously using a Sound Pressure Level "dB". And sound energy seems
inappropriate here since we don't care about that and have no
reference energy to form a ratio with in the "dB" calculation. I
suspect that he meant sound power or intensity (power per unit area)
which both have the 50% = 3 dB characteristic.


The unit "dBA" (AKA "dB(A)") always (AFAIK) corresponds to something
called a Sound Pressure Level (SPL), which is neither an energy or a
power or even a sound pressure; it is a non-linear function of an
average (RMS) sound pressure, based on one definition of "Bel" (the
common (base 10) logarithm of the ratio of a power and a reference
power). Since (see Note 1) the power transmitted by a pressure wave
(through a fixed area?) is proportional to the square of the RMS
pressure (i.e., it's proportional to the average of the square of the
pressure), we have

SPL = 10 * log( Power1/Power0 )
= 10 * log( (k*Pressure1^2)/(k*Pressure0^2) )
= 20 * log( Pressure1/Pressure0 )

where the pressures are measured at a standard distance. SPL is
favored because it is easily measured with a calibrated microphone.
(See Note 2.)

(The following all assumes a omni-directional (AKA isotropic) source
of sound in free air; a source transmitting constant power.)

From the above plus some even more basic physics, one can see that
doubling the distance quarters the power through a fixed area, which
can be expressed as a power (per unit area) reduction of 6 dB or (from
the above equations) a SPL reduction of 6 dBA. Note that for a
circular sound dispersal pattern (a disk), these figures are 3 dB and
3 dbA and for an (imaginary) linear "dispersal" pattern, they are 0 db
and 0 dBA.

I think that the doubling/halfing one's distance from an isotropic
sound source in free air should *define* the meaning of "half/twice as
loud", but it seem that sound experts have agreed that these terms
should be associated with a SPL change of 10 dBA, based (foolishly,
IMO) on what "half/twice as loud" meant to some experimental subjects,
approximately.

Note 1:

I've not seen a good discussion of the reason, but it apparently
stems from the fact that the power of a pressure wave is proportional
to the square of the maximum displacements of the air molecules which
is proportional to the maximum pressure. (I suspect that this is
because an increase in displacement causes an increase in the force
required to compress the air (F = k * D), so the work done, W = F * D
= k * D * D = k * D^2, where k is some constant.)

Note 2:

The "A" in "dBA" implies that the frequency response of the microphone
which measures pressure is roughly that of a human ear, so that
inaudible sounds are ignored, roughly speaking. It should also imply
that the measurement is made 3 feet from the source in a particular
test enclosure per some ANSI standard, but fan makers often use 1
meter from a source suspended by springs in an anechoic chamber
because it gives them better numbers. The 3ft/1m difference is about
0.8 dB and the suspension avoids enclosure vibrations.

"Gary W. Swearingen" wrote:

"dB" is a measure of many quantities (actually on ratios
of quantities) including power, energy, intensity, pressure,
voltage, and impedance. Originally, it was the amount a
signal dropped in (power?)
level through one mile of telephone wire.


hee, hee Good joke.


*TimDaniels*