> Recall that 'dB' is a relative logarithmic measure that is useless withou=
t some reference level to refer to. Again, dB is always relative!
> So when you say 'dBA' or 'dBanything it is always relative to some absol=
ute reference value.
Gregory,
This is just laziness when what is meant is dB(SPL) which is usually taken=
to mean as referred to 20 =C2=B5Pa (rms) as you said.
> Oh, let's now address the thermal noise of the air.
> I suspect that in absolute measurement terms, the thermal noise of the ai=
r is very much lower than the absolute value of 20 uPA. Like, way lower.
>
> Can someone please help out and provide the equations to calculate the 't=
hermal noise of the air' in absolute terms, i.e., in uPa?
> Please give a good description as to what all of this means, too.
The general thermal noise equation is:
P =3D kB T Delta f
P =3D power
kB =3D Boltzmann's constant
T =3D absolute temperature
Delta f =3D frequency bandwidth
My physics is 50 years old and I'm struggling to turn this into dB(SPL) but=
note that any thermal noise is proportional to frequency at the rate of 3dB=
per octave. This means that a weighting curve must be specified to turn thi=
s
into as audible noise threshold. Unfortunately no suitable curve is
recognised at 0 dB(SPL). In particular the phon is defined with the
40dB(SPL) weighting curve which is nothing like the threshold curve. See:
https://en.wikipedia.org/wiki/Equalloudness_contours
Note in particular, the threshold curve which goes down to 6dB(SPL) so kee=
n
ears should hear thermal noise.
I claim to have heard the thermal noise in air in my youth when I could hea=
r
very high frequencies. At some frequency the rising thermal noise met my
falling frequency response and I heard a hiss. It was in an exceedingly
quitet grassland in the mountains of South India and I could hear my heart=
thumping away noisily, the blood coursing through my ears, and faitly in th=
e
background, a high pitched hiss.
David Brinicombe
