[Top] [All Lists]

Re: 3032 capsules on PIP

Subject: Re: 3032 capsules on PIP
From: madl74
Date: Fri Jun 13, 2014 4:15 pm ((PDT))
> 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 'dB-anything it is always relative to some absol=
ute reference value.


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=
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:
Note in particular, the threshold curve which goes down to -6dB(SPL) so kee=
ears should hear thermal noise.

I claim to have heard the thermal noise in air in my youth when I could hea=
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=
background, a high pitched hiss.

David Brinicombe

<Prev in Thread] Current Thread [Next in Thread>

The University of NSW School of Computer and Engineering takes no responsibility for the contents of this archive. It is purely a compilation of material sent by many people to the naturerecordists mailing list. It has not been checked for accuracy nor its content verified in any way. If you wish to get material removed from the archive or have other queries about the archive e-mail Andrew Taylor at this address: andrewt@cse.unsw.EDU.AU