Hi all, very interesting discussion, however I feel that the thermal noise
is mostly related with wires and electronics. There the thermal noise can
be a more problematic issue than the thermal noise of air.
About silence, just recently I had a wonderful experience in the abandoned
tunnels (a 12 km network) of a chalk cave. Absolute silence improved by
soft sound absorbing chalk powder and very irregular tunnel surfaces. Next
time I go there with my microphones and recorders for doing some tests. In
some larger tunnels we found bats colonies, their social calls were
fascinating, but the most impressive noise was those of the wings of the
bats flying when disturbed. I did some recording with a phonometer but its
noise floor was higher than the ambience noise, probably I'd need the GRAS
microphone to make measures there.
Next time I'll go there with my usual microphones and recorders for doing
2014-06-14 10:03 GMT+02:00 Eric Benjamin
> > Greg O. wrote
> > I need to have a firm grasp on the concept of the thermal noise of air
> and how it affects microphones.
> I now have about a 12" stack of acoustic texts on my desk. Beranek's
> "Acoustics" was most helpful. Morse and Ingaard's "Theoretical Acoustics=
> gave me a 3-aspirin headache.
> Beranek gives calculated values for the real part of the impedance seen b=
> a disc on the end of a cylinder. That sounds like a microphone to me. T=
> real part of the impedance is the resistance, and that's what we need to
> calculate the noise. For a 1-inch microphone diaphragm, I calculate that
> the noise amounts to about -20 dB SPL. That is, the noise of the air is 2=
> dB quieter than an Audio Technica 4032 (but the 3032 diaphragm is smaller
> than 1 inch) and about 15 dB quieter than a Bruel and Kjaer 4179. But th=
> doesn't do much to point us towards making a microphone that can measure
> the thermal noise of the air. It's the noise from the air that is trappe=
> _behind_ the diaphragm in a conventional microphone that limits how quiet
> it can be. Along with all of the other noise sources.
> I hope that helps. I can show the details of the calculation if anyone
> would like to see it. This venue doesn't seem like the place!
> On Friday, June 13, 2014 11:10 PM, "Gregory O'Drobinak
> [naturerecordists]" <
> > wrote:
> What a lovely memory to have, being there in the mountains of India!
> i wish we could not only hear the recording of that soundscape as you
> perceived it, but also to see a good picture of what the space was like.
> Perhaps in the future, we can look forward to such things [Mike Rooke, yo=
> have much work to do. ++].
> Please take a look at the Gras microphone that i posted and its data shee=
> It claims that the lowest measurable SPL level is limited by the thermal
> noise of the microphone, which is spec'd at -2 dB SPL.
> This implies that one can break the 0 dB SPL barrier as Miike has propose=
> But it will certainly take quite a lot of creative thinking and work to
> make this happen.
> Gras claims to have it already, but we need an affordable solution that i=
> compact and adaptable to a large variety of recording arrays.
> I'm not sure that I understand the idea of random thermal noise causing a
> 3dB hit on the self noise of a mic.
> Eric B., can you please help us out here?
> Also, I need to have a firm grasp on the concept of the thermal noise of
> air and how it affects microphones.
> Somehow there must be a way to convert thermal noise power into dB SPL at
> the microphone diaphragm.
> I suspect that this involves some integration of the noise power over som=
> distance as a function of the area and compliance of the microphone
> Can anyone with a solid background in acoustics help us out here?
> BTW, sorry to burden all of the nature recordists out there with all of
> this crazy tech.
> But, ultimately this will benefit all of us, assuming that there will
> still be some pristine natural habitats left to record.
> Thanks & BR,
> Greg O.
> *From:* " [naturerecordists]" <
> *Sent:* Friday, June 13, 2014 6:15 PM
> *Subject:* [Nature Recordists] Re: 3032 capsules on PIP
> > Recall that 'dB' is a relative logarithmic measure that is useless
> without 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
> absolute reference value.
> This is just laziness when what is meant is dB(SPL) which is usually take=
> 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
> air 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
> 'thermal 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)
> note that any thermal noise is proportional to frequency at the rate of
> per octave. This means that a weighting curve must be specified to turn
> 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
> ears should hear thermal noise.
> I claim to have heard the thermal noise in air in my youth when I could
> 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 hear=
> thumping away noisily, the blood coursing through my ears, and faitly in
> background, a high pitched hiss.
> David Brinicombe
Centro Interdisciplinare di Bioacustica e Ricerche Ambientali
Universit=C3=A0 degli Studi di Pavia
Via Taramelli 24, 27100 Pavia